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QUARTERLY REVIEWS ORGANOMETALLIC COMPOUNDS OF THE! FIRST THREE PERIODIC GROUPS By G. E. COATES M.A. B.Sc. (THE UNIVERSITY BRISTOL) THE study of organometallic compounds which dates from Frankland’s preparation of diethylzinc in 1849-just a century ago-has resulted in the development of a number of valuable reagents for organic syntheses the most notable being the Grignard reagents and the organo-lithium and -cadmium compounds In addition it has provided experimental material on which advances in valency theory have been based. This is a particular feature both of the earliest work by Frankland which included the first clear exposition of a system of valency and of the most recent discoveries about the properties and structures of metal alkyls which like the problems raised by’ the boron hydrides and related compounds require significant extensions to current valency theory.The latter aspect is emphasised in this Review which is restricted to the elements of the first three groups of the Periodic Table since the organo-compounds of some of these elements provide structural and valency problems of a type not common among organo-derivatives of other groups. The alkyls of these elements include compounds with the properties of salts e.g. methylpotassium K+CH3- and covalent molecular compounds like dimethylmercury HgMe, neither type presenting any serious problem. But there are also a number of compounds like ethyl-lithium dimethyl- beryllium and trimethylaluminium the properties of which exclude formu- lation as salts ; these are now known to be associated. Trimethylaluminium for example is dimeric and should be represented as Al,Me,.In the metal alkyls that are associated the number of available valency electrons is less than twice the minimum number of covalent bonds which must be present in the molecule ; this requires that some of the bonds must have an order of less than one since there are not enough electrons to form electron-pair single bonds. Such molecules have been termed “ electron deficient ”. Generally they occur among the compounds of elements (of the f%st three groups and to some extent the transition metals) having more stable available low-energy orbitals than vdcncy electrons ; e.g. beryllium has available two three or four low-energy orbitals (the sp sp2 and sp3 groups) but only two valency electrons and aluminium which can form a stable sp3 low-energy set of tetrahedral orbitals has only three valency electrons (see R.E. Rundle 34). 217 218 QUBRTERLY REVIEWS In the molecule AlzMes it is probable that the aluminium atoms make use of their four stable orbitals but with the inevitable result that some bonds in the molecule must have less than two electrons each. This type of compound cannot occur when there is a very large electronegativity difference between the bonded atoms since the structure then becomes ionic. Thus all the alkali metals except lithium the least electropositive form ionic alkyls and they all form ionic halides (but the ether-solubility of LiBr and LiI is noteworthy). Electron-deficient molecules are found predominantly among the com- pounds of lit;hium beryllium magnesium boron aluminium and to some extent gallium.Preparative Methods Three general types of reaction are available for the preparation of organometallic compounds. The most direct method reaction between a metal and an organic halide frequently leads to metal organo-halides e.g. Mg + EtBr + EtMgBr (a typical Grignard compound) Li + Me1 -+ LiMe + LiI A1 + Me1 -+ Al,Me + AI,Me,I + AI,Me,I + A1216 (*I Si + MeCI -+ SiMe,CI + SiMe2C1 + SiMeCI The last example finds industrial application in the manufacture of silicone polymers but the silicon has to be activated by alloying with copper (see copper below). Another industrial modification is the use of a lead- sodium alloy for the manufacture of tetraethyl-lead Pb + 4Na + 4EtCl -+ PbEt + 4NaCl The other two general preparative methods are less direct since they both involve the conversion of one organometallic compound into another.The reaction between an organometallic compound and a metal e.g. HgMe + 2Na + Hg + 2NaMe 3HgMe2 + 2Ga -+ 3Hg + 2GaMe3 usually proceeds in such a direction as to form a more reactive from a less reactive compound. It is thus particularly useful for the preparation of highly reactive organo-compounds by the use of the relatively unreactive and easily manipulated mercury dialkyls and diaryls. On the other hand the reaction between an organometallic compound and a halide usually leads to the formation of a less reactive product from a more reactive starting material e.g. 4EtMgBr + SnC1 -+ SnEt + BMgBr + 2MgC1 2MeMgI + HgCl -+ HgMe + MgI + MgC1 This reaction is useful for the preparation of organometallic compounds from the rather highly reactive but easily accessible Grignard and organo- lithium reagents.The Alkali Xetals The true organo-derivatives of the alkali metals compounds in which the metal is bonded to carbon provide some of the most reactive compounds COATES ORGANOMETALLItY COMPOUNDS 219 known to Chemistry. The alkyls of sodium and potassium react with all organic compounds with which they have been tested with the exception of the paraffin hydrocarbons. The lithium derivatives are less reactive and have found extensive synthetic use. Pbical Properties.-The organo-alkali compounds fall into two classes those with salt-like and those with covalent properties.1 The alkyls and aryls of sodium potassium rubidium and czesium are colourless solids insoluble in paraffin hydrocarbons and benzene ; they cannot be vaporised without decomposition and give conducting solutions in dimethylzinc.They are commonly described as salts e,g. K+Et-. Their conducting solutions in dimethylzinc diethylzinc and trimethylaluminium probably contain compounds of the type Mz+[<ZnEt2R,]= M+[AlMe,R]- ; the com- pound RbZnEt has been isolated (colourless crystals m.p. 70-75"). The conductivity of these solutions is low but of the order to be expected for salts dissolved in solvents of low dielectric constant in which the salt would exist mainly in the ion-pair form. Some of the salt-like COD- pounds containing benzyl triphenylmethyl and similar groups are highly coloured (usually orange or red) and are frequently quite soluble in organic solvents. On the other hand the lithium derivatives with the exception of methyl- lithium behave as covalent compounds.They are colourless and dissolve in non-polar solvents ; they may be distilled or sublimed in a high vacuum but their vapour pressures are remarkably low. Ethyl-lithium m.p. 95" which may be crystallised from benzene is non-conducting when fused and although it would be expected from its molecular weight (36) to boil at a very low temperature yet it can be distilled or sublimed only with some difficulty and would appear to be highly associated. Measurements of molecular weight by the freezing point of solutions in benzene indicate about a six-fold degree of association. n-Butyl-lithium has a vapour pres- sure of - 4.5 x lo-* mm. at 60" and - 2 x loe3 mm. at 70" (corresponding to a latent heat of sublimation of 33 kcals./mole).3 The dipole moment in benzene solution 0.97 ~ 4 would seem inadequate to allow this low volatility to be attributed to dipole association although dipole clusters may have been present in the solutions used for these measurements in which case the dipole moment of a single n-butyl-lithium molecule may be greater.The association may be of a predominantly chemical nature similar to that which causes the dimerisation of BH, AlMe, and other " electron- deficient '' molecules. Preparation and Chemical Properties.-The lithium alkyls and aryls are generally prepared by the reaction between lithium metal and the appro- priate halide. Ether may be used as solvent in the preparation of methyl- and phenyl-lithium but since it slowly reacts with other lithium alkyls light petroleum is preferred for such cases.With the exception of methyl iodide 1 K. Ziegler F. Crossman H. Kleiner and 0. Schiifsr Annulen 1929 4'93 I. 2 A. v. Grosse Ber. 1926 59 2653. 4A. Young and 1511. T. Rogers 6. A w r . Chem. Xoc. 1946 68 2748. E. Warhurst Paraday Xoc. Discussions 1947 2 239. 220 QUARTERLY REVIEWS chlorides and bromides give better yields than iodides since the iodides undergo the Wiirtz reaction more readily resulting in loss of product e.g. LiC,H + C,H,I -+ C8H, + LiI For synthetic use lithium derivatives are never isolated but are allowed to react further in the solution in which they are prepared (as is usual with Grignard reagents). When it is necessary to prepare pure solvent-free organo-lithium compounds the reaction between lithium and the appro- priate organo-mercury compound is more suitable 2Li + HgEt -+ 2LiEt + Hg Methyl-lithium is not satisfactorily obtained in this way but may be pre- pared by taking advantage of its insolubility in benzene or light petroleum in which it is precipitated in a double decomposition 2Li + C,H,I -+ LiC,H + LiI 2LiEt(sol.) + HgMe,(sol.) -+ 2LiMe $ (insol.) + HgEt,(sol.) Organo-derivatives of the other alkali metals are prepared by the organo-mercury method with light petroleum as solvent for the mercury compound ; a suspension of the organo-alkali compound appears on con- tinued shaking of the reaction mixture.The salt-like nature of the very reactive alkali-metal alkyls implies that they may be regarded as derived from acids namely the corresponding hydrocarbons. Thus ethylsodium may be regarded as the sodium salt of the very weak acid ethane.Consequently one might expect ethane to be formed from ethylsodium by the action of any stronger acid; this occurs with benzene NaEt + Ph*H + NaPh + C2H which should therefore be regarded as a stronger acid than ethane. React- ions of this type provide about the only way of comparing the relative acidities of such exceedingly weak Toluene is a stronger acid than benzene since it forms benzylsodium and benzene from phenylsodium NaPh + PhMe -+ Na*CH,Ph + C,H This is in accordance with the direction of the dipole moment of In this way an order of hydrocarbon acidities -€I3 may be drawn up and linked with those of the more C,H < C,H < CH,Ph < CH,Ph < CHPh < xanthen < aniline < fluorene < phenylacetylene < alcohols < phenol Hydrogen (H,) would appear to behave as a stronger acid than benzene since it displaces benzene from the alkali-metal phenyls K*c,H + H -+ KH + C,H This is one of the very few non-catalysed reactions of molecular hydrogen at toluene (I).familiar weak acids (1.1 J. B. Conarit atid G. W. JVhelai?tl J . Asner. Ghent. SOC. 1932 54 1212. W. K. McEwcn ibid. 1936 58 1124. H. Gilman A. L. Jacoby and (Miss) H. Ludeman ibid. 1938 60 2336. COATES ORGANOMETALLIC COMPOUNDS 22 1 room temperature. The relative rates of this reaction illustrate the order of reactivities of the alkali-metal aryls since the reaction rate decreases in the order Cs Rb K Na Li. The replacement of hydrogen bg an alkali-metal atom by reaction of an aromatic compound with an alkali-metal alkyl is called metalation.Thus as mentioned above toluene is metalated in the side chain by ethyl- sodium. The orientation of nuclear metalation is sometimes rather abnor- mal ; while aryl ethers thioethers and tertiary amines are metalated by n-butyl-lithium in the ortho-position triphenylamine is metalated in the meta-position. The orientation of aromatic metalation is readily determined by subsequent reaction with carbon dioxide and identification of the resulting carboxylic acid. The metalation of a partly reduced aromatic system sometimes results in dehydrogenation with the formation of the fully aromatic ring. For example 1 4-dihydronaphthalene is metalated by n-butyl-lithium a t - 15" but splits off lithium hydride at about + 15" on continued stirring in ether A typical addition reaction is the synthesis of 2-aryl-pyridines by the reaction between pyridine and lithium aryls.Phenyl-lithium adds to pyridine and on being heated in boiling toluene for several hours splits off lithium hydride giving 2-phenylpyridine 8 This is rather similar to the Tschitschibabin reaction between pyridine and sodamide which requires similar experimental conditions The synthetic applications of organo-lithium compounds have recently been reviewed by G. Wittig.g Copper Silver and Gold Grignard reagents readily react with Group Ib halides and by working at sufficiently low temperatures it is sometimes possible to isolate organo- metallic compounds. Usually however these decompose very readily to give the free metal and a hydrocarbon 2CU*C6H = 2cU f c6H6*c8H5 * Org.SynM. Coll. Vol. 11 p. 517 New York 1943. " Newer Methods of Preparative Organic Chemistry " Interscience New York 1948. 222 QUARTERLY REVIEWS Treatment of a Grignard reagent RMgX with anhydrous cupric chloride has been used as a method for preparing compounds R,. Sometimes it is possible to add cupric chloride and the organic halide to magnesium and ether without preparing the Grignard solution as a separate stage. Phenyl- copper and -silver have been isolated but decompose rapidly on storage. The reaction between Grignard reagents and auric bromide (in ether) gives derivatives of the type R,AuBr which have been studied comprehensively by C. S. Gibson and his collaborators.l* Trimethylgold has very recently been prepared in ethereal solution at - 65" from methyl-lithium and a suspension of auric bromide.ll It probably exists as an etherate Me,Au-OEt, and decomposes above - 35" to - 40" to give gold methane and ethane.Stabler compounds capable of recrystallisation at room temperature were obtained by the addition of electron-donor molecules e.g. ethylenediamine benzylamine or a-aminopyridine + - - 4 - - + + - ZMe3Au-OEt + NH,.CH,*CH,*NH2 + Me,Au*NH,*CH,-CH,*NH,*AuMe An ethereal solution of trimethylgold reacts rapidly with ethereal hydrogen chloride giving the relatively stable dimethylauric chloride Ms CI Me - + \ / \ / / \ / \ ZMe3Au-OEtZ + ZHC1 -j. Au Au + 2CH4 + 2Et,0 MB c1 Cuprous and silver methyls and ethyls are formed in an interesting reac- tion which has been the subject of recent kinetic studies.12 Addition of a solution of silver nitrate in methyl or ethyl alcohol to a solution of tetra- methyl-lead at temperatures of - 10" to - 60" gives a precipitate of methyl- silver AgNO + PbMe + AgMe + PbMe,NO or a solution which decomposes into silver and ethane according to the first- order law.It is remarkable that the methyl radicals which are probably the primary decomposition product do not form methane by reaction with solvent molecules. Similarly the addition of alcoholic cupric nitrate to tetramethyl-lead precipitates primrose-coloured methylcopper which does give methane on subsequent decomposition and reaction of CH,* with solvent molecules. The ratio of ethane to methane over the complete course of formation and decomposition was 1 2 in accordance with the scheme Stage I. Cu++ + PbMe -+ Cu+ + PbMe,+ + CH,.Stags 11. Cu+ + PbMc -+ CuMe + PbMe,+ CuMo + solvent -+ CH Ns 2CH,* -.+ C,H l o Reviewed by Sir G. T. Morgan and F. H. Burstall " Inorganic Chemistry A 11H. Gilman and L. A. Woods J . AWT. Chem. SOC. 1948 70 550. Survey of Modern Developments " Heffer 1936 pp. 373-375. C. E. H Bawn and F. J . Whitby Faradap SOC. DiSCW8iOn8 1947 2 228, COATES ORCANOMETALUU COMPOUNDS 223 These solid copper and silver methyls are polymeric compounds of some kind and may be salts; even if they are salts containing CH anions it would still be possible for methyl radicals to be formed on their decomposition. Evidence for the reverse type of reaction vix. the formation of an organo-compound from a metal and a radical has been obtained by W. M. Whaley and E. B. StarkeyY13 who found that a benzene suspension of phenyl- diazonium borofluoride decomposed on boiling with copper powder giving a solution the reactions of which were those to be expected of phenylcopper Ph*N,+ BP,- -+ C,H,* + CuPh This reaction is somewhat similar to the formation of organo-derivatives of mercury arsenic and antimony by the decomposition of aryldiazonium halides in acetone or ethyl acetate in the presence of these elements.l* There is convincing evidence for the transitory formation of methylcopper in the reaction between copper and methyl chloride at 350"; l5 2Cu +MeCl + CuCl + CuMe The methylcopper was detected by its decomposition into copper and a gas which removed a lead mirror ; at 250" the half-life of the methylcopper was about 0-002 second.This discovery was a result of an investigation into the mechanism of the direct formation of methylsilicon chlorides (for the manu- facture of silicone polymers) from methyl chloride and a copper-silicon alloy; the presence of copper is necessary for a satisfactory reaction.All attempts to prepare organo-derivatives of copper silver and gold by reaction between the metal and mercury alkyls or aryls have resulted in the formation of hydrocarbons but none of the desired product. feu) Beryllium and blagnesium In contrast to the very extensive investigations which have been carried out on Grignard reagents the simple organo-derivatives MgR and the corresponding beryllium compounds BeR have received little attention. Dimethylberyllium may be prepared by refluxing a mixture of beryllium metal and dimethylmercury with a little mercuric chloride as catalyst in an inert atmosphere.Vacuum-sublimation gives dimethylberyllium as white needles. The same compound may also be prepared from methyl- magnesium iodide and the etherate of beryllium chloride BeCI,,ZEt,O + 2MeMgI -+ BeMe +MgI + MgCIa + 2Et,0 The product is separated from the reaction mixture by " ether-distillation " ; ether is allowed to drip on the hot reaction mixture whereupon it rapidly evaporates and carries some methylberyllium into the receiver. By keeping the receiver warm the beryllium compound is condensed together with some ether excess of which evaporates is condensed and allowed again to fall l3 P. A. Bolth W. M. Whalcy and E. 73. Starkey J . Amer. Chem. Xoc. 1943 65 14 W. A. Waters "The Chemistry of Free Radicals" Oxford 1946 Chap.8. 15D. T. Hurd and E. G. Rochow J . Amer. Chem. SOC. 1945 67 1057. 18 H. G h a n and F. Schulze J. 1927 2663. 1456; W. M. Whaley and E. B. Starkey ibid. 1946 68 793. 224 QUARTERLY REVIEWS on the reaction mixture. Dimethylberyllium is much more volatile in the presence of ether than when pure ; its melting point has not been observed since it sublimes without melting. The few other known beryllium dialkyls (ethyl propyl and n-butyl) are liquid at room temperature. Grignard-like compounds have been prepared from the metal and methyl (or ethyl) iodide the reaction requiring catalysis by a little mercuric chloride l7 e .g . Be + Me1 -+ Me*BeI All these compounds are highly reactive to oxygen and moisture and the dialkyls react rapidly with carbon dioxide (dimethylberyllium inflames in this gas).The beryllium dialkyls present a structural problem similar to that of the lithium alkyls (excepting lithium-methyl) since their evidently covalent charactertheir solubility in non-polar solvents such as benzene and their volatility-excludes a polar formulation e.g. Be++(Me-),. Their vapour pressures are however much lower than might be expected from a con- sideration of their molecular weights. Dimethylberyllium ( M 39) has a vapour pressure l8 of 1 mm. at 108" and 30.5 mm. at 158.6"; its latent heat of sublimation is 22 kcals./mole. The monomeric form of a dialkylberyl- lium should have a linear (sp) configuration and zero dipole moment so the relative lack of volatility cannot very well be due to dipole association. Since the general chemistry of beryllium indicates a strong tendency for the element to form four-covalent bonds it would appear probable that the alkyls are associated in such a way as to render this possible.As the element has only two valency electrons the alkyls must if they associate behave as " electron-deficient molecules " in the same sense as diborane B,H, and trimethylaluminium Al,Me, are classed as electron-deficient. The increase in the volatility of dimethylberyllium brought about by ether (an electron-donor molecule) is then due to the formation of a com- pound of the type Me,Be(OEt,), analogous to the known dietherate of beryllium chloride C1,Be (OEt 1,. Tapour-density measurements l8 on dimethylberyllium indicate a considerable degree of association in the gas phase Temp. OK. . . . . . 450 456 462 468 Mol. wt.. . . . . . Degree of association . . . The intensely reactive dialkyls and diaryls of magnesium which may be prepared from the metal and the appropriate mercury derivative are much more salt-like in their physical properties being non-fusible and non-volatile solids. They may however be very slowly ether-distilled and are soluble in the non-polar solvents benzene and dioxan. When heated 17H. Gilman and F. Schulze J . drner. Chem. SOC. 1927 49 2904. G. E. Coates and N. D. Euck unpublished. COATES ORCANOMETALLIC COMPOUNDS 225 they decompose without melting or vaporisation giving an olefin and mag- nesium hydride MgEt -+ MgH + 2C,H Some of the Grignard compounds decompose in a similar way 170-200° ZEtMgI MgI + MgH + 2C2H vacuum The magnesium hydride produced in these pyrolyses is insoluble in those organic solvents with which it does not react ; it was found to reduce benzo- phenone to diphenylcarbinol .The organo-derivatives of calcium strontium and barium have received little attention ; phenylcalcium iodide from calcium and iodobenzene in ether is somewhat more reactive than the corresponding Grignard reagent giving for example triphenylhydrazine with azobenzene in contrast to reduction to hydrazobenzene by phenylmagnesium iodide. Zinc Cadmium and Mercury The simple organo-derivatives of this group are normal covalent com- pounds the volatility of which corresponds to their molecular weight. The molecules are linear the metal forming two collinear sp bonds. The compounds show a striking change of reactivity which diminishes from zinc to mercury ; dimethylzinc is spontaneously inflammable and reacts violently with water whereas dimethylcadmium is slowly oxidised by air and hydro- lysed by water and dimethylmercury is inactive.Zinc alkyls are readily prepared from dry zinc-copper couple and the alkyl iodide ; the alkylzinc iodide disproportionates on heating RI + Z n -+ RZnI ZRZnI --+ ZnR + ZnI Most of the organo-derivatives of the group may be conveniently obtained from the anhydrous metal halide and a Grignard reagent. Once used extensively for synthetic purposes the inconveniently in- flammable organo-zinc compounds have been almost entirely superseded by the Grignard reagents; zinc alkyls still find application however in the peparation of quaternary carbon compounds,20 e.g. 2Me,CCl + ZnMe -+ 2Me& + ZnC1 The relatively low reactivity of organo-cadmium compounds has been turned to advantage in a recently developed method for the preparation of ketones from acid chlorides 21 2RGOC1 + CdR’ + 2R.CO.R’ + CdC1 The resulting ketone which would react further with Grignard reagents does not react with the organo-cadmium compound.The latter is not for l9 D. B. Clapp and R. B. Woodward J . Amer. Chem. SOC. 1938 60 1019; 2o C. R. Noller J. Amer. Chem. SOC. 1929 51 594. a1 J. Cason Chern. Reviews; 1947 40 15. P. Jolibois Compt. rend. 1912 155 353. 226 QUARTERLY REVIEWS this purpose isolated but is prepared in ethereal solution by the addition of dry powdered cadmium chloride to the appropriate Grignard solution. In addition to the simple derivatives such as the dimethyl which are so useful in the preparation of organo-derivatives of other metals mercury forms a very large number of organic compounds.The mercurution of aromatic compounds by reaction with mercuric acetate is a well-known reaction Organo-mercuric compounds of the type RHgX undergo two noteworthy reactions ; on treatment with nitrosyl chloride nitroso- compounds are formed 22 RHgX + NOCl -+ RNO + HgXCl Electrolysis of RHgX in liquid ammonia solution results in the deposition on the cathode of a conducting solid of metallic appe~rance,~3 which decomposes at room temperature into equimolar proportions of Hg and HgR2. This substance is easiest to obtain when R = Me and the products become increasingly unstable as R becomes more complex. The cathode deposit is probably a metal consisting in the product from methylmercuric compounds of Hgf-Me ions in a metallic lattice together with the equivalent number of “ free ” electrons i.e.an organic metal. Electrolysis of aqueous solutions (preferably containing pyridine to increase solubility) of methyl- mercuric acetate gives a good yield of dimethylmercury by the dispropor- tionation reaction Hg+-Me + E -+ Hg*-Me + 8Hg +&HgMe Numerous mercury derivatives of aldehydes ketones and carboxylic acids have been prepared but the structure of few of the compounds has been determined. The remarkable compound known as ethane hexamer- carbide 24 is obtained as a yellowish insoluble powder unattacked by per- manganate chromic acid hypochlorites hydroxylamine or hydrazine by the prolonged action of yellow mercuric oxide and boiling aqueous alkali on ethyl propyl allyl or amyl alcohol acetaldehyde cellulose starch or sucrose.The compound has the empirical formula C,O,Hg,H[, and since it forms salts C,02Hg,X with acids (also insoluble) it has been given the structure (11). This structure is not compatible with the normally col- linear bonds formed by bicovalent mercury and considering the extreme insolubility of the substance it would appear more likely to have a poly- meric structure (111). The compound behaves as a base-exchange resin ; 2 OH O-Hg Hg-0 I I I I HO-Hg Hg-OH Hg-c-7-Hg I (11.) 23 L. I. Smith and F. L. Taylor J . Amer. Chem. SOC. 1935 57 2460. 23 C. A. Kraus ibid. 1913 35 1732. 24 K. A. Hofmann Ber. 1898 31 1904; 1900 33 1328. 26 Unpublished observations by the Reviewer. COATES ORGANOMETALW[C COMPOUNDS 227 on being shaken with aqueous potassium chloride the solid takes up C1- in exchange for OH- ions the effect being more marked with potassium bromide and still more with the iodide.Ions such as NO,- which do not so readily form covalent bonds are not exchanged in this way but form salts [e.g. C,O,Hg,(NO,),] only by the action of the free acid on the base. Boron The simple alkyls and aryls of boron BR, are normal covalent compounds with none of the tendency to associate shown by its neighbours beryllium and aluminium. The lower members of the series e.g. BMe, are rather slowly hydrolysed by water but are very sensitive to oxygen spontaneously inflammable in air and burn with a characteristic green flame. The chemis- try of boron is dominated by the tendency for the element to become four- covalent by acquiring the use of extra valency electrons.Thus although the simple alkyls do not associate to achieve this end as the hydride does (BH + B2H6)y they very readily show their electron deficiency by com- bining with a variety of electron-donor molecules e.g. ammonia trimethyl- amine and the phosphines. The relative stabilities of compounds of the type Me,B-NR have been measured in order to compare the basic strengths of amines in a non-aqueous system,26 and to examine the influence of steric effects on basic strength. Thus although triethylamine forms a very much less stable compound with methylboron than does trimethylamine yet quinuclidine forms a com- pound of exceptional stability - + + - + - /cH2-cH2 \+ - Et,N-BMe < Me,N-BMe < CH-CHa-CHa-N-BMe \ / CHa-CH In the last compound the carbon atoms of the quinuclidine molecule are held back from the nitrogen and exert no steric hindrance on reaction with another molecule.Disproportionation reactions which are quite common among the boron hydrides (e.g. 6B2H,Cl -+ 5B2H + 2BC1,) also occur among the alkyl derivatives ; thus treatment of chlorobisdimethylaminoborine with di- methylzinc does not give the expected methyl compound but a mixture of the dimethyl derivative and trisdimethylaminoborine 27 (MeaN),BCL + ZnMe (+ (Me,N),BMe) Me2N-E3Me + (IMe,N ),B 26 H. C. Brown M. D. Taylor and M. Gerstsin J. Amer. Ckm. Soc. 1944 66 431 ; H. C. Brown and M. D. Taylor ibid. 1941 69 1332 ; H. C. Brown and Sei Sujishi ibid. 1948 70 2878. a? Unpublished observations by the Reviewer. 228 QUARTERLY XEVIEWS The reactive and very easily hydrolysable liquid Me,N*BCl is remarkable in that it slowly changes into a crystalline dimer of probable structure (IV) which is not attacked by prolonged boiling with water or dilute acids or alkalis.28 The methyl derivative Mie,WBMe does not dimerise whereas the hydride Me,N-BH dimerises but very readily dissociates to the monomeric form.A study of these e0mpounds2~ shows that dimerisation is promoted when electron-attracting groups (e.g. chlorine) are attached to the boron and electron-repelling groups (e.g. methyl) to the nitrogen ; in this way the charges in the ring system become more evenly distributed over the molecule. Although many arylboronic acids R*B(OH), have been prepared and studied particularly as regards their acid strengths and orientation on nitration the simplest alkylboronic acids have only recently been isolated ; they form anhydrides with surprising facility.Monomethylboronie acid 30 is dehydrated by passage over CaSO,,~H,O giving a cyclic anhydride (V) g 2 /-\ Me,N+ + M e B c12 (IV.) \-/ BMe 0 (M.P. - 38"; / \ \ / 0 I I 0 MeB BMe b.p. 79".) 3Me*B(OH) -3 (V.1 Similarly the volatile dimethylborinic acid Me,B*OH (v.P. 36 mm. at 0") is readily dehydrated with phosphoric oxide to the anhydride Me,B*O*BMe (b.p. 43"). These anhydrides react with boron trifluoride giving the pre- viously unknown methylboron fluorides (BOMe) + 2BF -+ 3Me*BF + B,O 3Me,B*O*BMe + 2BF -+ 6Me,*BF + B,O The very volatile methyl ester of dimethylborinic acid (b.p. 25") may be prepared by the following reactions :31 Me,N-BMe d Me,NH*BMe,*OMe M00H f Me,NH,& + Me,B*OMe Aluminium The aluminium alkyls and aryls are very reactive and show many of the characteristics of electron-deficient molecules.Although they may be 28 E. Wiberg and K. Schuster 2. anorg. Chern. 1933 213 89. 2s E. Wiberg FIAT Review of German Science (1939--46) Inorganic Chemistry 30 A. B. Burg J. Arner. Chem. SOC. 1940 62 2228. 31 Unpublished observations by the Reviewer. Vol. 1 1948. COATES ORQANOMETALLIC COMPOUNDS 229 prepared from aluminium by the dialkyl- (or diaryl-) mercury method a new and more satisfactory method for larger-scale preparations has recently been described,S2 by which methyl iodide is heated under reflux with alu- minium. After about twelve hours all the methyl iodide is consumed and the reaction vessel contains an equilibrium mixture of the methylaluminium iodides and trimethylaluminium A1 + Me1 -+ (AlMeI,) + (AlMe,I) + (AlMe,) The pressure is then reduced and trimethylaluminium is slowly taken off a t high reflux ratio from a fractionating column; the equilibrium is thus disturbed and about one-half of the theoretical methyl can be taken off as trimethylaluminium .Trimethylaluminium m.p. 15.1" b.p. 130° has the properties of a typical covalent compound but is dimeric 32 both in benzene solution and as vapour. It is one of the very few associated electron-deficient alkyls of which the (geometrical) structure has been determined ; the infra-red absorption spectrum 33 indicates that the molecule has a bridge structure (VI). Electron- diffraction data do not clearly distinguish between an ethane-like and a bridge This result together with others connected with different electron-deficient molecules (e.g.diborane B2H6 and tetramethylplatinum Me2A1 AIM@ {PtMe,),) has presented quite a serious problem to valency theory. A bridge structure implies an unusual to form five covalent bonds. One interpretation of this and related problems is due to R. E. Rundle,34 who suggests that a single carbon 2p atomic orbital may form one molecular orbital which binds two atoms by half-bonds. Thus a non-hybridised p orbital may form covalent bonds in the following ways The positions of the hydrogen atoms are not revealed. /cT \ / CH (VI. 1 covalency for the bridge carbon atoms which appear (1) a normal CT single bond as in hydrogen chloride H(ls)-C1(3p) ; (2) a normal TC bond forming the second component of a double bond as in ethylene (3) each lobe of a p orbital may form a half-bond.In each case the orbital holds two electrons of opposite spin in accordance with Pauli's exclusion principle. Two reasonable sets of orbitals have been suggested for the trimethylaluminium molecule ; in each set a single C 2p orbital forms one molecular orbital which binds two aluminium atoms by half- bonds. 32 R. X. Pitzer and K. S. Gutowsky J . Amer. Chem. Xoc. 1946 68 2204. 33 K. X. Pitzer and R. K. Sheline J. Chem. Physics 1948 16 552. 84 N. R. Davidson J. C. Hugill H. A. Skinner and L. 33. Sutton Trans. Faraday SOC. 1940 36 1212; L. Brockway and N. Davidson J. Amer. Chem. SOC. 1941 63 3287 ; H. A. Skinner and L. E. Sutton Nature 1945 156 601 ; R. E. Rundle J . Amer. Chm.SOC. 1947 69 1327. R 230 QUARTERLY REVIEWS (a) The aluminium and the end carbon atoms are believed to be in the tetrahedral state the bridge carbon atoms being in the trigonal state as in ethylene. Let all the bonds except (for the moment) those involved in the carbon bridge be supplied with two electrons each as for normal single bonds. Of the 16 electrons left let 12 be supplied to the six bonds formed by the trigonal bridge carbon atoms (two C-H and four GA1 bonds) allow- ing these each to form three single bonds (VII). Perpendicular to the plane H I SP2 H Is02 H (VII.) (VIII.) (IW of the sp2 orbitals is 8 non-hybridised 2p orbital (VIII). There are now four electrons left over to supply both of the bridge carbon atoms each of which can accommodate two electrons of opposed spin in its 2p orbital.This orbital combines with the two H( 1s) orbitals to give one H( 1s) + C(2p) + H(1s) molecular orbital which holds two electrons and binds two hydrogen atoms (IX). In this way each of the four hydrogen atoms concerned is held to a carbon atom by a half-bond. There is a fundamental difference between this type of bond and the " singlet link " which was at one time invoked to explain the boron hydrides. The latter involves the occupancy of a molecular orbital by one electron only and would give a paramagnetic molecule contrary to experimental fact while according to the above view one molecular orbital which is doubly occupied binds two atoms with half-bonds. It is of some interest to note that this sp2 + 2p set of orbitals with the Zp orbital forming two half-bonds is identical with the calculated transition state in reactions of the type This type of reaction includes the Walden inversion.Quantum-mechanical calculations show that the transition state (X) in the reaction between COATES ORaANOMETALLIC UOMPOUNDS 231 hydrogen atoms and methane has a potential minimum of the order of 20 k~als./mole.~~ It may be significant that this is of the same magnitude H as the heat of dissociation of A&.&fe,. The above formulation of AI$f.e imparts a negative charge I to each of the aluminium atoms and half a positive charge to H-C-H each of the four hydrogen atoms held by half-bonds. It could very reasonably be objected that the relatively electropositive aluminium atoms could not hold a negative charge when bound but it must be realised that this charge distribution corresponds to complete absence of polarity in all the covalent bonds.A small dipole moment in the expected direction Al-C of about 2-2-5 D. in each Al-C bond would entirely remove the negative charges from the aluminium atoms and in a similar way the fractional positive charges on the bridge hydrogen atoms would also be reduced. ( b ) According to an alternative formulation also due to R. E. Rundle,34 the various atoms are in similar orbital states but the half-bonds are confined to the ring (XI). The difference between the two models is very / \ H to much more electronegative carbon atoms ; (X.1 -1- I H slight. The second model does not involve any charge inequalities in the fully covalent form but it is likely to involve slightly greater steric strain.It is particularly interesting to note that triethyl- and tri-n-propyl-aluminium are dimeric while the triisopropyl compound is mon0meric.3~ This is to be expected on either model since more than one alkyl group attached to the bridge carbon atom would introduce serious steric strain. In agreement with its electron-deficient character trimethylaluminiurn reacts with it number of eleetron-donor molecules. These reactions illus- E. Gorin W. Kauunann J. Wdter and H. Eyring J . Chem. Physics 1939 7 633. 232 QUARTERLY REMEWS trated below result in the formation of normal molecules which can be formulated according to well-established valency theory - + - + Al,Rils + 2NMc3 -+ 2Me3A1-NIMe3 A1,Me6 + 20Me --f ZMe,Al-OMe The formation of etherates prevents the preparation of aluminium alkyls by the Grignard method.Some rather striking compounds 36 are formed by reaction with electron-donor molecules containing reactive hydrogen atoms ; for example with dimethylamine the addition compound Me3A1-NHMe is first formed but when heated above its melting point it loses methane and gives a substance (Me,.Al*NMe,), which is dimeric as shown by vapour-density measurements. A ring structure (XII) has been assigned to this compound. I n this structure the aluminium atoms satisfy their - + Me I O-AIMe /+ -\ Me,Al- +OMe (XII.) Me (XIII.) tendency to form four-covalent bonds ; thus the substance is relatively unreactive and no longer combines with trimethylamine. The correspond- ing boron compound Me,B*NMe is monomeric; this provides a further illustration of the greater electron-deficient behaviour of aluminium ; e.g.trimethylboron and boron trichloride are monomeric while the aluminium compounds are dimeric ; similarly boron hydride (B,H,) is dimeric while aluminium hydride (Al€€3)n is polymeric. A similar reaction between trimethylaluminium and methyl alcohol gives a trimeric six-membered cyclic compound (XIII). It is of some interest to note that the alkoxides of aluminium have been shown (by cryo- scopic measurements in naphthalene) to be tetr~meric,~~ like the alkoxides of thallium (TlOR),.38 Very little is known about organo-derivatives of the Group IIIA elements ; the compound ScEt,-OEt and the fltrium analogue have been de~cribed,3~ having been prepared by the Grignard method. Gallium Indium and Thallium The tendency of the third-group elements to form four-covalent bonds is well illustrated by the chemical behaviour of their organo-compounds.36 H. C. Brown and N. R. Davidson J. Anzer. Chem. Xoc. 1942 64 316. 37 R. A. Robinson and D. A. Peak J . Physical Chem. 1935 39 1125. 38 N. V. Sidgwick and L. E. Sutton J . 1930 1461. 39V. Plets Compt. rend. Acad. Sci. U.R.S.S. 1938 20 27 (Chem. Abs. 1939 33 2108). COATES ORCIANOMETALLIC COMPOUNDS 233 Their electron-deficient character is much less marked than with the alumi- nium compounds and leads to association only in the case of gallium tri- chloride the methylgallium hydrides and digallane (Ga,H,). Trimethylgallium (b.p. 55") which is monomeric in the vapour state,40 may be prepared from the chloride and dimethylzinc 41 or by heating metallic gallium with dimethylmer~ury.~~ The compound is very reactive inflaming in the presence of air even at - B O O and is rapidly hydrolysed.Stable compounds are formed with electron-donor molecules. The etherate (b.p. 98.3") is obtained by direct combination or by the reaction between gallium trichloride and ethereal methylmagnesium iodide ; 41 it is exten- sively dissociated as vapour but is much less reactive to air and moisture than the trimethyl. The ammine Me,Ga-NH and the similar compounds Me,Ga-NMe and Me,Ga-NEt are stable to dry air. Mixed alkyl halides are formed by the reaction between trimethylgallium or its ammine and ethereal hydrogen chloride ; 41 the reaction with the ammine takes the following course 4- - - 4 - - + (1) Me,Ga,NH + HC1 + Et20 -+ GaMe,,OEt + NH,C1 (2) Me,Ga,OEt + NH,Cl -+ Me2GaC1,NH + OEt + CH The monoammine of dimethylgallium chloride is a solid m.p.54" soluble in ether and appreciably volatile in a vacuum. The diammine on the other hand formed from the monoammine by absorption of ammonia at room temperature is non-volatile soluble in the polar solvents alcohol acetone and liquid ammonia but insoluble in ether; it is probably a salt NH,+[Me,Ga(NH,)Cl]-. The dichloride MeGaCl results from the further action of hydrogen chloride and also forms ammines. Some interesting compounds result from the reduction of the ammines of trimethylgallium and dimethylgallium chloride by means of sodium in liquid ammonia.41 In these reactions sodium acts as an electron donor and displaces ammonia leaving the gallium with a negative charge and thereby still able to form four-covalent bonds.The reduction may be carried to two stages - + - 4 - - (1) Na + 2Me3Ga-NH -j. Me,Ga-NH,-GaMe + Na+ + 4Hg + NH (2) 2Na + 2Me3Ga-NH +I- Me,Ga-G$Ms + 2Na+ + 2NH - - + - Addition of ammonium bromide to the product of the second reaction re- forms trimethylgallium ammine. These compounds are analogous to some of the derivatives of diborane NH,+[BH,-NH,-BH,] and Na,++[B,H,]-. The reduction of dimethylgallium chloride results in the appearance of a bright orange colour which may be due to the Me,Ga' radical - + - Me,GaCl + N a -j. NaCl + Mc,Ga* 40 A. W. Laubengayer and W. F. Gilliam J. Arner. Chem. Xoc. 1941 63 477. 41 C. A. Kraus and F. E. Toonder ibitl. 1933,55 3547 ; Proc. Nat. Acad. Sci. 1933 4 2 E. Wiberg T. Johannsen and 0. Stecher 2. anorg. Chem, 1943 251 114.19 292. 234 QUARTERLY REVIEWS but the product obtained on evaporation of the ammonia (solvent) is the amide Me,Ga*NH, which may be sublimed in a vacuum + -. Me,Ga-NH + Me,Ga*NH + &H2 The dimeric methyl hydrides of gallium which like those of aluminium are formed by the action of a glow discharge on a mixture of the trimethyl and hydrogen are of particular interest in their disproportionation reaction with trimethyl- or (more conveniently) triethyl-amine to give the volatile (extrapolated b.p. 139") dimeric hydride digallane 4 3 I- _. 3Ga,H,Me + 4NEt -+ Ga,H + 4Me3Ga-NEt The organo-derivatives of indium have received much less attention than those of -gallium. The trimethyl which may be prepared from the metal and dirnethylmerc~ry,~~ is a beautifully refracting crystalline solid m.p.88-4" b.p. 135.8" ; its vapour like that of trimethylgallium is mono- meric45 Electron diffraction 46 shows that the molecule is planar the indium atom making use of the trigonal set of sp2 orbitals. The electron- deficient character of trimethylindium is very slight since it is reported to form neither an ammine nor an etherate. Although the compound is spon- taneously inflammable at room temperature in air at atmospheric pressure very slow oxidation at - 78" appears to follow the rather remarkable course 44 itInMe + 0 -+ Z(InMe,),O + 2C,H Triphenylindi~m,~~ m.p. 291 " (decomp.) is readily oxidised to amorphous phenylindium oxides of indefinite composition but gives the mixed bromides InPh,Br InPhBr, and finally InBr on treatment with bromine in benzene solution.The mixed bromides appear to have a salt-like character since they do not melt below 300". Although the chemistry of organo-thallium compounds has been inten- sively studied it is only relatively recently that the simple TIR derivatives have been prepared. Reaction between thallic chloride and Grignard reagents goes readily as far as the sfage R2T1X which are true salts of the [R,Tl]+ cations. The latter are very stable and isoelectronic with the stable dialkyl- and diaryl-mercury compounds. The hydroxides prepared by the action of silver oxide on the halides are strong bases their aqueous solutions absorbing carbon dioxide from the atmosphere. The general tendency of Group IIIB elements to form four-covalent bonds is still shown to some degree by thallium since stable compounds (XIV) and (XV) are formed from dimethylthallium hydroxide and acetylacetone or 43 E.Wiberg and I'. Johannsen Angew. Chem. 1942 55 38 ; Naturwiss. 1941 29 4 4 L. M. Dennis R. W. Work E. G. Rochow and E. M. Chamot J . Arner. Chem. 46 A. W. Laubengayer and W. F. Gilliam ibid. 1941 63 477. 48 L. Pauling and A. W. Laubengayer ibid. p. 480. 47 W. C. Schumb and H. I. Crane ibid. 1938 60 306. 320. SOC. 1934 56 1047. COATES ORGANOMETALLIC COMPOUNDS 235 These covalent compounds are soluble in non-polar solvents salicylaldehyde. and may readily be sublimed.48 Triethylthallium may be prepared from diethylthallium chloride and ethyl-lithi~m.~~ It is a bright yellow liquid b.p. (extrapolated) 192" which readily decomposes above-about 100". Reactive in comparison with the chemically inert diethylthallium salts the triethyl readily reverts to the latter by reaction with hydroxylic compounds hydrogen halides etc.TlEt + H,O + TlEt,*OH + C,H Although soluble in ether it does not form an etherate and no compound with ammonia or trimethylamine has been described. Triphenylthallium undergoes a particularly interesting reaction when boiled in xylene solution 50 since the reactive monophenylthallium is formed TIPhB + TlPh + C,H,*C,Hs and fairly rapidly disproportionates 3TIPh -j. 2T1 + TlPh 48 R. C. Menzies N. V. Sidgwick E. F. Cutcliffe and J. M. C. Fox J . 1928 1288. 4D H. P. A. Groll J . Amer. Chem. SOC. 1930 52,2998 ; S. F. Birch J. 1934 1132 ; 6o H. Gilman and R. G. Jones ibid. 1940 62 2367. E. G. Rochow and L. M. Dennis J . Amer. Chern. Soc. 1935 57 486.

ISSN:0009-2681    

DOI:10.1039/QR9500400217

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

LIGHT ABSORPTION AND PHOTOCHEMISTRY (including photo-polymerisation and the effects of light on dyes) By E. J. BOWEN M.A. D.Sc. F.R.S. (FELLOW OF UNIVERSITY COLLEGE OXFORD) To advance the subject of light absorption by molecules chemists need (A) a pictorial standpoint to understand the phenomena qualitatively and (B) means of calculating from simple principles the wave-lengths of absorp- tion bands the band intensities their polarisations whether a band corresponds to photodissociation or to a vibrationally stable excited state the coupling of nuclear vibrations to the excitation and the contour of the band the relations between absorption and fluorescence or phosphorescence and the effect of solvent medium on absorption. A number of reviews have dealt with some of these matters.lWs Progress towards solutions of these problems has consisted first in a clear understanding of the application of wave-mechanical principles to the spectra of the hydrogen atom and the molecule H, and secondly in attempts to relate more complicated molecular examples to hydrogen by successive approximations from simplified initial assumptions.Round the nucleus of a hydrogen atom a series of wave-patterns or orbitals are theoretically assumed whose amplitude squares give the charge distribution of an electron " occupying " the orbital. The magnetic properties of the electron require each orbital to have two forms of opposite magnetic direction or " spin " and no more than one electron can occupy any one magnetic form of any one orbital (Pauli principle). The different energy levels of the atom are represented by the electron occupying different orbitals.The orbitals resemble the three-dimensional wave-patterns of vibrations in a medium about a point in space; the fundamental Is of lowest energy being a spherical-shaped wave with maximum amplitude a t the centre. First harmonics have a nodal surface spherical for 2s and planar for 2p. Second harmonics have two nodes ; besides the 3s and 3p types there are 3d with two nodal planes. The process of light absorption is conceived as in Fig. 1. The energy of the light here causes a 1s orbital to change to a 2p orbital and the hour-glass shape of the latter with its separation of negative charge into two halves may be visualised as due to the directional action of the electric vector of the light. The intensity of the transition is measured by the square of the " dipole moment of transition " which may here be pictured as measured by the separation distance d of the " centres of gravity " 1 W.C. Price Ann. Reports 1939 36 47. 2 G . N. Lewis and M. Calvin Ciwn. Reviews 1939 25 273. 3 It. A. Morton Anm. Reports 1941 38 7. 4 E. J. Bowen ibid. 1943 40 12. ci E. A. Braude ibid. 1945 42 105. A. Maccoll ibid. p. 16. C. A. Coulson Quart. Reviews 1947 1 144. L. N. Ferguson Chem. Reviews 1948 43 385. 236 BOWEN LIGHT ABSORPTION AND PHOTOCHEMISTRY 237 of the two halves of the p orbital. Is -+ 2s transitions are (‘ forbidden ” because no (‘ transition moment ” is involved and 1s -+ 3d because two new nodes would have to be simultaneously developed which would need ‘( quadrupole ” radiation. The energy levels of the H atom are accurately calculable from the Schrodinger equation.S P FIG. 1 Light absorption by a hydrogen atom. The hydrogen molecule is then treated as a two-centre atom ; spherical s orbitals become elongated o orbitals rotationally symmetrical about the molecular axis.6 Two electrons in a as orbital form the ground state. There are two first harmonics with one nodal plane corresponding to atomic p orbitals o with the plane crossing the axis and nu containing the axis FIG. 2 Absorption by a hydrogen molecule. (Note u orbitals are rotationally symmetrical about the molecular axis JZ orbitals have a nodal plane through the atomic nuclei. The suffixes 7 and u mean that if a straight line is drawn in any direction through the centre of the molecule the wave-function has the same or the opposite sign respectively as the centre is passed.On light absorption one electron passes to an orbital having an extra node Le. surfke a t which the wave-function changes sign which is a t right-angles to the electric vector direction of the incoming plane- polarised light.) the latter not being rotationally symmetrical. The absorption bands of hydrogen at 1000 and 900 A. arise from “permitted” transitions of one electron to these orbitals and are highly directional since the new node is developed at right-angles to the electric vector (see Fig. 2) (i.e. transitions are polarised along and across the molecular axis respectively). No change 238 QUARTERLY REVIEWS of spin normally occurs on light absorption by undisturbed atoms. Triplet levels of H reached by transitions similar to the above with change of spin of the electron occur at lower energies than the singlet levels and theoretic- ally might be observed as excessively weak absorption bands at longer wave-lengths in hydrogen molecules whose symmetry is distorted by collisions etc.The principles on which to find answers for many of the queries above are therefore all illustrated by the molecule H,. Other queries depend on the shapes of the potential-energy curves of the molecule. Electronic excitation does not immediately alter inter-atomic distances (Franck-Condon principle). The excited molecule is thus usually formed in a compressed condition and begins to vibrate. Vibrations may also be coupled with electronic excitation because of symmetry requirements. In organic chemistry the first molecule of interest from the point of view of light absorption is ethylene H,C=CH,.The double bond may be regarded as composed of two electrons in an essentially o,-shaped orbital (Fig. 2) and two in one resembling the nu of H,. The longest wave-band at 1750 A. is treated as the transition of one electron from the 3t to an " antibonding " ng orbital having an additional nodal plane crossing the molecular axis. For light absorption to occur most efficiently therefore the electric vector must lie along the C-C link. This has been experiment- ally verified for C=C- and N=N-containing molecules oriented in crystals. To obtain quantitative predictions about absorption bands the direct route would be to treat the molecule as a two- or more centred system of positive nuclei surrounded by electrons whose orbital energies are to be calculated.This being impossibly dificult the problem is broken down by attempting to approximate to the molecular orbitals by combining atomic orbitals which themselves are approximations for atoms other than hydrogen. Two ways of combining atomic orbitals have been developed by different schools the valence bond (V.B.) and the linear combination (L.C.A.O.) methods. The details of these methods both involving great mathematical complexity have been given elsewhere.6* Judged by their quantitative capabilities there is little to choose between them. The present purpose of the Reviewer is to compare them from the " pictorial " point of view. In ethylenic and azo-type molecules the L.C.A.O. method proceeds as outlined above representing light absorption as a T G ~ -+ zg transition.The V.B. method regards ethylene as a " resonance hybrid " of valency forms CH2=CH2 and CH2-CH2. The energy levels of the molecule are derived by mathematical treatment giving a series of " combinations " of the above structures in varying proportions. Depending on the proportions each level is " stabilised " or lowered in energy by what is called " resonance energy ". To a first approximation however the ground level is mostly CH2=CH and the excited level CH2-CH,. The fact that azo-compounds absorb at longer wave-lengths than ethylenic is attributed to a relatively smaller resonance energy of the former in the ground state and it larger in the excited state diminishing the energy difference of the fevels.2 A similar explanation can be given to the fact that compounds of type ==C?-C= f - + - BOWEN LIGHT ABSORPTION AND PHOTOCHEMISTRY 239 absorb at longer wave-lengths than those of type =N-N= Here the excited states are mostly composed of the resonance forms -C=C- and -N=N-- and the relatively greater resonance energy of the former brings its energy level down towards the ground state.Similarly the longer-wave absorption of fulvene (I) an unusually large lowering of the energy of the ground level of the latter because of resonance between Kekulh and Dewar structures of nearly equal energy. Further comparisons have been made of absorption band positions between compounds whose ‘‘ resonance ” either in the ground or in the excited state is increased or diminished by strain steric effects etc.* Although this use of the V.B.standpoint applied in a non-quantitative way to explain trends in light absorption is simple and successful up to a point it does not seem to lend itself to wide development. It is therefore important to see how far interpretations based on the L.C.A.O. method can be employed ; for although this method takes one away from the ‘( valency ” picture it more closely follows the fundamental concepts of wave-mechanics in treating electrons as wave-patterns. On the molecular-orbital picture every electron is in principle distributed over the molecule as a whole; some may be treated as almost entirely localised as lone pairs on atoms or in particular bonds ; others as the n electrons of conjugated systems as largely delocalised. The differences in the absorption band positions of the carbon and nitrogen compounds discussed above is then referred to differ- ences of extents of ‘‘ localisation ’’ of electrons involved in the transitions “ delocalisation ” being analogous to “ resonance ” and lowering energy levels.For pictorial use the molecular orbital standpoint seems to have special advantages in affording clearer insight into the light absorption of highly conjugated molecules into the directional properties of electronic transitions into triplet levels and into transitions involving non-bonding electrons. The following discussion is intended to bring out these points. The V.B. method relies for its appeal to chemists (as distinct from its use by mathematicians) on its representation of molecules by valency structures. The precision of this attempt however fades as the molecule becomes more conjugated.Naphthacene for example has 5 Kekule structures 110 Dewar- like structures and 649 doubly excited structures. Its ground state must be conceived as a resonance hybrid of 40% of the Dewar forms 60% of the doubly excited forms with a negligible contribution from the Kekulb forms.9 The molecular orbital concept applied to benzene treats the atomic skeleton as a multi-nuclear atom and assigns the six ‘( aromatic ” electrons in pairs to wave-patterns with one node two nodes etc.10 These patterns are shown in Fig. 3 on the top line all (being n-type electrons) having a nodal plane coincident with the plane of the ring and 1 having no additional node 2 and 3 having a node crossing the ring (electron waves of opposite phase on the two halves) 4 and 5 with two nodes.A sixth not shown H-c=cH compared with that of benzene may be attributed to 1 >=CH* H-C=CH (1.1 A. and B. Pullman La Revue ScientiJique 1946 145. 10 K. F. Herzfeld Chem. Reviews 1947 41 233. 240 QUARTERLY REVIEWS would have three crossing nodes and therefore opposite wave phases on consecutive carbon atoms. The ground level has orbitals 1 2 and 3 filled as shown by. the dots (black and white circles indicating opposite spins). Observed absorption bands correspond to the electron transitions given in the figure. The orbital energies increase from 1 to 5 and the levels increase in energies downwards. The strongest band at 1800 A. arises from a per- mitted transition resembling a 1s -+ 2p atomic transition. The 2600 A 4 5 3 A 1-1 v 1 0 2 (> Symmetry symbol of level.Absorption I band A. (Ground level) a 0 a 0 a 0 a 0 a a 0 a (Unobserved triplet level) 3J% * O a a 0 a - -. a a 0 (Unobserved triplet level) 3400 Very weak triplet level a 0 a __ _ _ _ ~ 0 0 2600 a 0 2000 1800 Very strong a 0 a a 0 0 FIG. 3 Orbitals of benzene. band the “ benzene band ” usually observed in ultra-violet spectrographs is weak since it is “ allowed ” only when the molecular symmetry is distorted by a particular type of nuclear vibration. At 3400 A. is an extremely weak band which has been recognised only from phosphorescence effects the excitation being normally “forbidden” because of change of spin of the electron. Since ordinary instruments permit measurements on the 2600 A. band and on part of a band at 2000 A. only comparisons of absorption-band BOWEN LIGHT ABSORPTION AND PHOTOUHEMISTRY 241 data on benzene and its derivatives have been confined to these wave- lengths.Recent work has shown a much greater regularity of the effects of substituents on the 2000 A. band than on that at 2600 ~ . l l This clearly points the way to an extension of the work to the 1800 A. band where the complications due to the largely " forbidden " nature of the others particu- larly in respect of band intensity may be minimised. Anthracene has absorption bands in the near ultra-violet at 3700 A. and at 2500 A. The fluorescence excited in either band corresponds with the transition from the 3700 A. level to the ground state. The molecule excited at 2500 A. must therefore revert in a time short compared with a vibration period to the 3700 A.level and radiate from thence. This is remarkable in view of recent work on the directional nature of these transitions.12 Molecular orbitals of anthracene are shown in Fig. 4. The 14 " aromatic " electrons in the The former has a vibrational structure the latter not. FIG. 4 Molecular orbitals of anthracene. ground level are disposed in the 7 lower wave-patterns while above are shown (dotted) the unfilled orbitals to which electrons pass on excitation. The transitions are such that the electric vector of the light must lie along the short axis of the molecule for the 3700 A. band and along the long axis for the 2500 A. band. Anthracene crystals thus possess strong dichroism in the ultra-violet. This treatment of the problem is attractive because of its relations to molecular symmetry.Just as the V.B. method however becomes blurred for highly conjugated molecules so the molecular-orbital method encounters similar difficulties when developed mathematically. The original L.C.A.O. method of constructing molecular orbitals bas to be refined by allowance for electron interaction which results from simple superposition of electron orbitals and further by the possibilities of inter- actions of configurations of electrons of the same symmetry.13 This is l l L . Doub and 2. M. Vandenbelt J . Amer. Chem. Xoc. 1947 69 2714. 12 C. A. Godson Proc. Physical SOC. 1948 60 257. J. Jacobs ibid. 1949 62 710; D. P. Craig Proc. Roy. SOC. 1950 200 474. 242 QUARTERLY REVIEWS virtually equivalent to a concept of (‘ resonance ” of electrons between orbitals in the representation of a molecular energy level.In spite of this complication the molecular-orbital picture is useful for visualising triplet levels of conjugated molecules (see Fig. 3) which are now recognised as responsible for phosphorescence bands and for some weak absorption bands. l4 Considerations of band strength are now as important as those of band wave-length. The latter gives the energy of the orbital transition and the former defined by the band area 1 E . dcu where E = molar extinction coefficient of a dilute solution and c1) = wave-number of centre of band gives the mean life z of the excited level ; all the excited molecules being assumed to revert by radiation to the ground level we have (approxi- mately) l5 Weak bands therefore correspond to large z values i.e. to transitions forbidden in either direction.(Actual mean lives of excited levels may be shorter because of internal or collisional energy degradation.) It is not easy however to decide whether a weak band arises from a singlet-singlet transition forbidden by symmetry principles or from a singlet-triplet transition. The weak band at 3000 A. shown by aldehydes and ketones is attributed to the former; l6 the red bands of nitroso-compounds to the latter.15 Magnetic measurements can be used to decide in favourable cases,17 but more useful may be a study of the effect of substituents in changing the band wave-length and intensity together with theoretical expectations mostly as yet undeveloped. A complication in these matters is the effect of one absorption band on another. Weak bands may ‘‘ borrow ” intensity from neighbouring strong bands and may also (( borrow ” polarisation directions as well destroying the sharpness of some of the above considerations.18 For an electronic transition where every molecule absorbs liglit when its “ target area ” is struck the maximum possible extinction co- efficient can be calculated from geometrical principles alone.x~ These give E, = 0.87 x 102*a where a = “ chromophore area ” which can be approximated to from bond dimensions. It is interesting to compare this. with the approximate expression E, = 1.24 x 1020d2 which may be derived from the theory given above for an absorption band at 4000 A. with a “ half-width ” of 5000 cm.-l where d = distance electronic charge is transferred during the absorption (cf. Fig. 1 and discussion).For an elongated conjugated molecule a is a product of a large length and a small width while d appears as a square. Unfortunately detailed comparisons of the significance of these quantities and their extended quantitative application are limited by “ intensity borrowing ” between absorption l / ~ = 2.88 x l O - @ ~ ~ f E.dW l4 M. Kasha Chem. Reviews 1947 41 401. l6 G;. N. Lewis and M. Kasha J . Amer. Chem. Xoc. 1945 67 994. l6 H. L. McMurry J . Chem. Physics 1941 9 231 241. l7 G. N. Lewis and M. Calvin J . Amer. Chem. Xoc. 1945 67 1232. R. S . Mulliken and C. A. Ricke Rep. Prog. Physics 1941 8 231. le E. A. Braude J. 1950 379. BOWEN LIGHT ABSORPTION AND PEOTOUHEMISTRY 243 bands and non-fulfilment of the condition of fully allowed transitions in most examples. A simplified method of calculating both the wave-lengths and the intensities as well as determining the orbital symmetries for aromatic hydrocarbons and dyes has recently been developed.20 The n electrons of the conjugated system are treated mathematically as a problem of ‘‘ elec- trons in a box ” 21 which has been successfully used in the theory of metals.Such calculations avoid the use of individual atomic orbitals in constructing the molecular ones and are relatively easy to understand and work with. Electron Transfer Spectra.-The brown solutions of iodine in aromatic hydrocarbons alcohol etc. show an intense ultra-violet absorption band closely resembling that of the ion 13-,22 The effect is favoured by substitu- tion of methyl groups in the ring which tend to increase the n-electron donating power.The interaction of the molecules must be by polarised complex formation held together by van der Waals forces and the strong band intensity indicates that light absorption causes a considerable shift of electron density distribution within the complex. Whether the iodine molecule is oriented parallel or perpendicular to the hydrocarbon ring and the direction of the charge displacement on light absorption is not yet known. The intense absorption bands of ferric ion complexes have been inter- preted as electron-transfer spectra.23 The ferric ion has five d electrons which occupy three of the 3d orbitals in Complexes the remaining two 3d one 48 and three 41 orbitals being filled by electrons supplied by the co-ordinating groups. Very weak bands a t 5500 A. and 7000 A. are ascribed to forbidden d -+ d transitions among the five atomic d electrons.Intense bands whose wave-lengths depend on the electron-donating powers of the co-ordinate groups are found for complexes in the ultra-violet or visible region e.g. at 5000 A. for ferric thiocyanate. Excitation has been attributed to the passage of an electron from one of the co-ordinate links to an atomic d orbital giving a ferrous ion and a radical e.g. SCN with thiocyanate. The further reactions of such radicals account for the light-sensitiveness of ferric complexes particularly in presence of reducing or oxidising agents. The photochemical decompositions of ferric cobaltic and manganic oxalates tartrates etc. may be imagined to be of this nature C03+(C20,2-) + hv + C02+(C,0,2-)~ + C20,- Proof of the reality of such electron transfers has been found from the effects of the radicals ininitiating polymerisation.47 What may be queried however is the special use of the term “ electron-transfer spectra ” for certain absorp- tion bands.The bands may be compared with those of the molecule HI H. K- J . Chem. Physics 1949,17 1198 ; J. R. Platt and H. 13. Klevens ibid. 2C20,- -+ C,O,2- + 2 c 0 pp. 470 481 484. 21 See ref. 24 p. 61. 2a H. A. Benesi and J. H. Hildebrand J . Amer. Chem. Soc. 1949 71 2703. E. Rabinowitch and W. H. Stockmayer ibid. 1942 64 335 ; Rev. Mod. Physics 1942 14 112. 244 QUARTERLY REVIBIWS or the other hydrogen halides. Here the excited state is anti-bonding and photo-dissociates into atoms.24 The ground state is only partly ionic however and to attribute the absorption band to an electron-transfer mechanism would be an exaggeration particularly for hydrogen iodide.The linkages in the complexes must also be partly covalent and partly ionic. Secondly there is the important question whether the long-wave limit of the so-called electron-transfer band provides a value for the energy of the electron transference. We have to distinguish between bands of this kind characterised by high ektinction coeffticients because of the large ‘‘ dipole moment of transition ” and bands due to atomic d --+ d transitions of the metallic ion which being “ forbidden ” must be weak. In ferric complexes the distinction is experimentally very clear. Other evidence is available for chromic complexes. Chromium is the only transitional element whose spark spectrum has been analysed to give the ground level of the ion (CrS+) in the gaseous state.Its fwo lowest d+d forbidden transitions are 4P + 2CJ a t about 6700 A. and 4F + 2H a t about 4900 A. Prmtically all chromic complexes in solution have absorption bands near these positions e.g. chrome alum 6400-5300 and 4700-3700 A. and chromitartrates maxima at 6100 and 3750 A. It is reasonable to conclude that these rdther weak bands in solution correspond to the atomic d + d transitions and this accords with the low light sensitivity of chromic complexes. Cobalt salts however show features which complicate the simple picture. Solutions of cobaltioxalate have two fairly weak bands at 6050 and 4260 A. and a strong one at 2500 A. The last would be identified as the electron-tranBfer band and the first two as &+d transitions.Although the solutions are stable under 6050-A. illumination however they decompose with a quantum efficiency of about unity at 4260 A . ~ S Here the reduction of the cobaltic ion to cobaltous bg electron transfer appears to be a secondary change following an initial d-d orbital shift the transfer occurring at a much lower energy than the concept of the 2500 A. band as the “ electron transfer band ” would indicate. The time seems ripe now for a systematic exploration of inorganic complex-ion spectra and a clearer insight into the problem awaits new measurements. Photochemistry in Relation to Textiles The photochemical behaviour of dyes on fabrics has been discussed at a recent symposium.2s The observed effects may be regarded from two aspects that of the practical dyer and that of the physical chemist.The subject also divides itself into two parts the fading or changes of colour of dyes on long illumination and the “ tendering ” of fabrics particularly cotton found when certain dyeings are exposed to light. The practical man requires data corresponding closely to the behaviour of dyes under conditions of use evaluated by practical judgments. He is interested not so much in the extent of photochemical action as in what the changes in 2 4 E. J. Bowen “ Chemical Aspects of Light ” Clarendon Press 1846 p. 94. 26 J. Vrmek 2. Ekckrochem. 1917 23 336. 2e J . SOC. Dyere Col. 1949 66 685-788. BOWEN LIGHT ABSORPTION AND PHOTOCHEMISTRY 245 appearance or strength are. Considerable fading of some shades is less objectionable than a little fading in others.In attempting to define resist- ance to the effect of light the difliculty is met that in some cases the observed change is slow in the early stages and accelerates later while in others a small initial rapid change quickly slows down with time.2' Changes of humidity,28 temperature accessibility of dye to air presence of other dyes traces of impurities such as copper or iron differences in spectral distribution between testing lamp and sunlight,29 and other factors all contribute to make the practical asskssment of the effect of light on dyes a very complex matter. 30 The physical chemistry of the photoreactions of dyes is now in process of taking shape as a coherent picture. It was first shown for basic fluorescent dyes,31$ 32 and later for azo 3 3 9 34 and vat dyes,35 that excitation of the dye molecule by light may lead to the formation of a reduced dye radical and oxidation of some substance present.This process competes with loss of excitation energy by fluorescence internal degradation or collisional degradation. Fluorescent dyes by reason of their relatively long life in the excited state have a greater opportunity of reaction unless very long- life triplet levels of the dye are formed.36 The possible part played by triplet levels arising from partial degradation of the excitation energy is still obscure. If D represents a dye molecule we have D + h v + D* (singlet excited level) . * (1) D* -j. D + hv' (fluorescence) . (2) D* + D (degradation of energy to heat) . (3) and possibly the change of D* into a triplet excited level (not distinguished below from D*).If a substance AH capable of being oxidised is present in the system the following reaction may occur giving a pair of reactive radicals D*+AH -- D H + A or D-+AH+ . (4) Factors influencing the efficiency of this reaction will be the redox potentials of D* and AH the rate of encounter of D* and AH and the extent of reverse change after dissipation of the excitation energy. The rate of encounter will depend on whether D* and AH are linked as a complex or have to diffuse together ; if the latter rates of diffusion and the mean life 27 T. H. Morton J . Xoc. Dyers Col. 1949 65 597 ; T. Vickerstaff and D. Tough 28 W. L. Lead ibid. p. 723. as B. S. Cooper and F. S. Hawkins ibid. p. 586. 30 S. Burgess ibid. p. 732 ; G. Nordhammar and N. Gralen ibid. p. 741.31 J. Frauck and F. Haber Sitzungsber. Preuss. AEad. Wbs. Berlin 1931 13 250 ; F. Haber and J. Weiss Proc. Roy. SOC. 1934 A 142 332. 33 J. Weiss Trans. Paraday SOC. 1939 35 48; 3s B. E. Blaisdell J . SOC. Dyers Col. 1949 65 618. 34 E. Atherton and I. Seltzer ibid. p. 629 ; N. F. Desai and C. H. Giles ibid. p. 639. 36 C. H. Bamford and M. J. S. Dewar ibid. p. 674. 36 M. Kasha Chem. Reviews 1947 41 401 ; E. J. Bowen J . Soc. Dyers Col. 1949 ibid. p. 606. 1946 42 133. 85 613. s 246 QUARTERLY REVIEWS of D* are important. Hemming in of the reactants by solvent molecules so that they cannot drift apart before they react again in the reverse direction (having lost the original excitation energy) (Pranck-Rabinowitch " cage " effect) usually makes the measurable efficiency of reaction (4) very small.A decision between the possibilities of DH + A or D- + AH+ in reaction (4) is not easy to make ; each has been championed by different investigator~.3~ 3' It is probably true to say that experimental observa- tions favour the first possibility of a hydrogen-atom transfer where the reactants collide bimolecularly and favour the electron-transfer mechanism where the reactants are initially bound in a complex (cf. cobaltioxalates). If A is a ferric or similar ion the electron transfer would seem more probable. The production of radicals by reaction (4) leads to complicated secondary changes which are gradually becoming ~nravelled.~~ Many of these com- plexities are due to the effects of molecular oxygen. Dyes are relatively stable to light and oxygen in non-oxidisable solvents such as water; in presence of organic compounds a number of types of effect may be observed some of which are illustrated below.Azo-dyes in organic solutions free from oxygen are reduced by light to the hydrazine derivatives and later to substituted anilines.33 Here AH in reaction (4) is the solvent which is dehydrogenated in reactions (4) and (5) DH + AH --+ DH (hydrazine) + A . ( 5 ) The solvent radicals mutually interact A + A -+ non-reactive products (6) In presence of oxygen the dye remains undecomposed and the solvent is photo-oxidised. It is probable that the DH radicals react thus D H + O -j. D f H O . (7) The solvent is then oxidised by a complex radical reaction involving the attack of 330 on AH and of 0 on the radicals A. A very complete study of the photosensitised oxidation of tetralin by certain vat dyes indicates a similar mechanism.37 Here it has been possible to interpret the chain process of oxidation as A + 0 -+ AO .' (8) AO,+AH + AO*OH+A . * (9) with chain-ending reactions A + AO -+ non-reactive products . (10) In addition it was found that molecular oxygen deactivated the excited dye molecule D * + O -+ D + 0 . * 6. * (11) Reaction (1 1) may possibly involve the production of less reactive triplet levels of D and 0,. Some dyes of a reactive nature are photo-bleached irreversibly by oxygen. 37 C. H. Bamford and M. J. S . Dewar Proc. Roy. Soc. 1949 A 198 252. 38 R. Livingston J . Xoc. Dyers CoE. 1949 65 781. AO A + A +AO -+ +I BOWEN LIGHT ABSORPTION AND PHOTOCHEMISTRY 247 The effect of traces of reducing agents in accelerating the reaction is significant.39 This indicates that reactions (7) and (8) provide intermediate peroxide radicals which in some circumstances can oxidise and destroy the original dye.Examples are known of reversible photo-reductions of dye molecules by other molecules e.g. the effect of phenylhydrazine sulphate on illuminated methylene-blue 40 Reaction (12) reverses itself in the dark. It must take place as a two-stage reduction beginning with reaction (a) both stages being reversible when the excitation energy is dissipated. The same general scheme of reactions is capable of explaining in outline the photosensitised change of one dye in presence of another.41 The inter- action of radicals with ions of variable valency affords a qualitative reason for the effect of inorganic impurities on the stability of dyed materials to light.An important type of photo-reaction of dyes on fabric especially cotton is " tendering ".42 43 Certain vat dyes particularly yellow ones behave in this way. After exposure to light the cotton is depolymerised and oxidised so that it falls to pieces while the dye remains essentially unchanged. The effect is not explicable by reaction (4) where AH is cellulose but has been shown to result from the intermediate formation of hydrogen peroxide.42 The effect requires oxygen and is enhanced by moisture and by alkalinity. It has therefore been suggested that the first reaction after excitation of the dye by light is 35 D* +OH- -+ D-+OH or DH+O- . (13) followed by reaction (7) giving HO radicals. The OH radicals then oxidise the cellulose giving some carbon dioxide in the neighbourhood of the dye while the HO radicals dismute to give hydrogen peroxide which diffuses into the fabric and oxidises it to give oxycellulose.Under dry conditions reaction (4) may play a part followed by (8) and (11) the slowness of the reaction being ascribed to the reversibility of (4). It is an observed fact that yellow vat dyes are usually much more active photo-tenderers than blue ones. A probable reason is that reaction (13) is bound to be largely reversed by the Franck-Rabinowitch effect. Yellow dyes absorb short-wave visible quanta of greater energy than those absorbed by blue dyes and this may supply the local thermal energy required for some of the resultants of (13) to break loose from their molecular " cage ". These effects of dyes in " tendering " cotton are quite distinct from the direct action of ultra-violet light on cellulose.44 Under 2537-~. illumination D* + Ph*mNH,+ -.+ DH + Ph.N:NH,+ . (12) 39 K. Weber Ber. 1936 69 1026. 40 G. Holst 2. physikal. Chem. 1937 A 179 172; 180 161 ; 1938 182 321. 41 F. Scholefield and C. Patel J . SOC. Dyers CoZ. 1928 44 268 ; F. Scholefield 4 2 G. S. Egerton J . Soc. Dyers Col. 1949 65 764. 43 D. Ashton D. Clibbens andM. E. Probert ibid. p. 650 ; A. Landolt &id. p. 659. u H. F. Lamer and W. K. Wileon J . Amer. Chem. Soc. 1949 71 958. andH. Turner J . ITeztiZeItwt. 1933,24,130 ; J. Bohi Helv. Chim. Acta 1929,12,121. 248 QUARTERLY REvfEWS cellulose is degraded by C-C and C-0 bond splitting moisture but not oxygen being necessary. At longer wave-lengths some photo-oxidation occurs but the rates are low because of the small light absorption.Reversible colour changes in dyed materials exposed to intense illumina- tion are sometimes observed. 45 This phenomenon called phototropism is probably due to cis-trans-changes in the molecular structures of the dye molecules. Photosensitised Polymerisations Vinyl-type polymerisations are chain reactions initiated by free radicals. A convenient way of introducing radicals into the system is by their photo- chemical production in situ either by opening an ethylene bond to give a diradical or by the photo-dissociation of benzoyl peroxide benzoin diacetyl and similar organic molecules which give mono-radicals. 46 Ferric com- plexes in aqueous solution such as Fe3+F- Fe3+Cl- and Fe3+OH- which yield on illumination ferrous ions and a free radical are also effective.47 The rates of photochemically initiated polymerisations are usually propor- tional to the square root of the intensity of the light owing to the chain- termination mechanism of bimolecular radical combination.It is now recognised that chain-transfer processes occur during polymerisation ; so that four velocity constants at least are necessary to interpret the rate those of initiation propagation transfer and termination. For styrene and methyl methacrylate polymerisations the steps may be represented as 48 2M -j. ZX or D2 El (Initiation) X + M -+ X E2 (Propagation) D,+M -+ P 2 + R D2+M + D l + R R + M -j. P + R k3 D 2 + X -+ D + Q R + X -j. P i + & k'4 2X -+ ZQ k* D,= 1 7 1 7 7 5 , one end R = growing transfer polymer P = " dead " initial polymer P = " dead " transfer polymer X = any active centre Q = any '' dead " centre.'k} (Chain transfer) 'k} (Chain termination) D + X + P 2 + Q where D = initial polymer growing at both ends 46E. Stearns J . Opt. SOC. Amer. 1942 32 282. 48 EL. W. Melville Proc. Roy. SOC. 1937 A 163,511 ; T. T. Jones and H. W. Melville ibid. 1940 A 175 392 ; G. M. Burnett and H. W. Melville ibid. 1947 A 189 456 ; idem Nature 1945 156 661 ; P. D. Bartlett and C. G. Swain J . Amer. Chem. SOC. 1945,67,2273 ; 1946 88 2377 2381 ; R. B. Whyte and H. W. Melville J. SOC. Dyers Col. 1949 65 703. 47 M. G. Evans and N. Uri ibid. p. 709 ; Nature 1949 164 404. 48 C. H. Bamford and M. J. S . Dewar Proc. Roy. SOC. 1948 A 192,309 329 ; 1949 A 197 356. BOWEN LIGHT ABSORPTION BND PHOTOCHEmSTRY 249 These four constants can be evaluated by measuring the rate of polymer- isation with different initial rates of initiation (light intensity) by the use of a rotating sector or a shutter to '' chop " the light beam and by following the " after-effect " or continuation of the reaction after the illumination is cut off.The photochemical after-effect in vinyl polymerisations is very much longer than those of atom reactions such as the hydrogen and bromine combination. 49 Miscellaneous Photo-reactions The quantum efficiency of oxidation of rubber solutions by oxygen in ultra-violet light has been shown to be initially unity.s0 This indicates the formation of a hydroperoxide by direct addition. Only when the concentra- tion of this substance rises high enough for it to absorb light and be itself photochemically decomposed to radicals does an oxidation chain reaction of high photo-sensitivity set in.Zinc tetraphenylchlorin has been shown to react in light with o-quinones losing two atoms of hydrogen to the latter to give zinc tetraphenylp~rphin.~~ The rate of reaction is independent of the quinone concentration to such high dilutions that clear evidence is provided to show that a long-life triplet level of the zinc compound is involved. This can be associated with its phosphorescence of mean life 8 x lom3 see. The photolysis of gaseous hydrogen peroxide at 2537 A. proceeds by the mechanism H202 + hv + 20H OH + H 2 0 2 + HZO + HO2 ZHO -+ H202 + 0 with an experimental quantum efficiency of 1-7 (theoretical maximum 2),52 indicating very little back reaction.The photolysis of persulphate solutions is more complicated.53 No OH radicals seem to play a part but other intermediates must be present in the decomposition to sulphate because of the observed effects on the rate of H,O+ OH- and C1-. In spite of careful and accurate work the mechanism is still obscure. Nitrobenzene vapour in the short-wave ultra-violet decomposes to give nitrosobenzene and p-nitrophenol 54 C,H,.NO + hv --+ C,H,*NO + 0 C,H,*N02 + 0 -+ HO*C,H,*NO tert.-Butyl nitrite vapour exposed to ultra-violet light gives acetone and the hitherto unprepared nitrosomethane 55 (CH,),C-O-N=O + hv + (CH,),CO + CH,*NO 49 F. Briers D. L. Chapman and E. Walters J. 1926 562. 6o E. J. Hart and M. S . Matheson J . Arner. Chem. SOC. 1948 70 784. 61 M. Calvin and G. D.Dorough ibid. p. 699. 6 2 D. H. Volman J. Chem. Physics 1949 17 947. 63 L. J. Heidt J. B. Mann and H. R. Schneider J. Arner. Chem. SOC. 1948 70 64 S. H. Haatings and F. A. Matsen ibid. p. 3514. s6 C. S. Coe and T. F. Doumani W. p. 1616. 3011. 250 QUARTERLY REVIEWS The latter substance often invoked to explain the chain-stopping power of nitric oxide in hydrocarbon reactions readily dimerises and gives formald- oxime on heating. A fundamental difficulty in interpreting experimental results of photo- dissociations involving radicals in gaseous systems is caused by the diffusion of radicals to the vessel walls. This has been the subject of theoretical treatment and applied to ketone decompositions. 56 When exposed to ordinary intense light sources reversibly photo- dissociating gases such as nitrogen dioxide or chlorine give small stationary atom or radical concentrations only because of the high rates of recombina- tion.A new arrangement has been described whereby up to 10,000 Joules of electrical energy stored in condensers can be discharged in a few thousandths of a second through a column of rare gas giving intensities of light lo4 times greater than steady sources.57 Very high photo-stationary radical concentrations can thus be obtained and under these conditions new final products may be formed by inter-radical reactions. Much work at present is concentrated on photo-changes induced by high-velocity particles or very short waves. This has been recently sum- mari~ed.~* The importance of one short-wave photo-reaction seems to have escaped general notice.Geologists have for some time maintained that the free oxygen of the air was wholly derived from photosynthesis by plants. It now appears that the major source was the photo-reaction of water vapour in very short ultra-violet light. 59 Water vapour and carbon dioxide react photochemically in short-wave light to give oxygen and formaldehyde and this reaction is suggestive as a possible source of living matter on the earth. Recent developments in the production of lithium fluoride plates make work in the region 1800-900 A. more practicable and doubtless new papers will soon appear on this littIe explored field. 66T. L. Hill J . Chem. Physics 1949 17 1125. 57 R. G. W. Norrish and G. Porter Nature 1949 164 658; Proc. Roy. Soc. 1950 6* F. S . Dainton Ann. Reports 1948 45 5. 59 W. Groth 2. Elektrochem. 1939 45 262. A 200 284.

ISSN:0009-2681    

DOI:10.1039/QR9500400236

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

SOME ORGANIC PEROXIDES AND THEIF& RBACTIONS By E. G. E. HAWKINS B.Sc. PE.D. A.R.I.C. (THE DISTILLERS’ COMPANY LTD. RESEARCH AND DEVELOPMENT DEPT.) IT has been known for a considerable time that certain classes of organic compound especially ethers and olefins peroxidise on storage and that such peroxides cause danger during distillation. This danger has generally been avoided by catalytic decomposition of the peroxide or by the addition of an antioxidant to the compound concerned so as to prevent the formation of peroxidic products. During recent years peroxides have been isolated from a considerable number of compounds and purified to varying degrees ; in some cases the chemical reactions of these peroxides have been studied. More extensive study of their reactions allied to a solution of the problem of their safe production and control on a large scale may well lead to their taking their place with other classes of compound as valuable industrial intermediates.Whether this proves true or not there is no doubt that many oxidation reactions utilised in present-day processes proceed through a peroxide stage. In this Review no mention is made of diacyl peroxides or of the catalytic effect of peroxides in telomerisation and reactions involving addition of halo- gen compounds and esters to olefins so adequately studied by Kharasch. An attempt has been made to summarise the methods of formation and chemical reactions of certain classes of organic peroxide ; the production and interaction of radicals play a large part in peroxide chemistry. The only collected information previously available in this field has been due to A.Rieche,l although a review of reactions of per-acids has recently appeared.3 Two further reviews have discussed peroxide formation and decomposition under a variety of conditions in the light of the possible mechanisms for such reactions. 1. Aliphatic Peroxides (A) Alkyl Hydroperoxides (RO*OH).-Alkyl hydroperoxides have been produced by two main methods (i) by reaction of hydrogen peroxide with alkyl sulphates (alcohol + sulphuric acid) in the presence of acid or alkaline catalysts and (ii) by controlled oxidation of a hydrocarbon. The first method is of greater general application since the required alcohol can often be made available whilst the corresponding hydrocarbon may not conve- niently be oxidised to the hydroperoxide or only in very poor yield ; isolation may also be more difficult from the mixture of products given by the second 1 “ Alkylperoxyde und Ozonide ” Steinkopff 1931.2 “ Die Bsdeutung d0r Organischon Peroxyde fiir die chernische Wissenschaft u. 3D. Swern Chem. Reviews 1949 45 1. a J. E. Leffler ibid. p. 386; C. E. Frank ibid. 1960 46 156. 251 Technik ” ; Sammlung chemischer u. ehem.-technischer Vortriige 1936 34. 252 QUARTERLY REVIEWS method. However hydroperoxide concentrates have been isolated from the oxidation of kerosene fractions in the presence of magnesium oxide,5 and oxidised 2 7-dimethyloctane yields a 2-hydroperoxide and a 2 3-dihydro- peroxide although one might have expected a 2 7-dihydroperoxide.6 Rather ill-defined peroxides in fairly low concentration have been re- ported by oxidation of pentane hexane and octane,' heptane and iso- heptaneY8 and n-he~adecane,~ whilst tertiary alkyl (especially tert.-butyl) hydroperoxides together with the ditertiary alkyl peroxides and other oxygenated products have been made by oxidation of branched-chain hydrocarbons in the presence of traces of hydrogen bromide as catalyst.10 Many hydroperoxides have been prepared by method (i) in a state of reasonable purity ; they include the hydroperoxides of methy1,ls 11 ethyl,l 1 2 13 1 4 propyl isopropyl 14 sec.-butyl,l4 tert. - butyl,14 16-19 tert.-amyl,l41 2O triethylmethy1,l9 1 1 2 2-tetrarnethylethy1,lS penta- methylethyl,19 ethylene glycol and ethers of glycols,l4 whilst 2 5-dihydro- peroxy-2 5-dimethylhexane and 2 7-dihydroperoxy-2 7-dimethyloctane have been similarly prepared.l9 The lower members of the series are explosive and susceptible to shock sharp surfaces and warming but as the C 0 ratio increases these peroxides become less explosive although thermal decomposition is vigorous with evolution of gas and steam. Thermal decomposition brings about fission of the molecule and the types of product given are dependent on the radicals formed. Methyl hydroperoxide gives rise to methanol formaldehyde formic acid dimethyl ether oxides of carbon and water. 21 tert.-Butyl hydroperoxide (at 250") yields acetone methanol ted-butanol and w5tterY1* although the proportion of decomposition products appears to vary with the temperature and at 100" the main product is tert.-butanol. The peroxides from C1 to C, hydrocarbons yield ketones when heated.22 ti U.S.P. 2,447,794 (Union Oil (20.). 8K. I. Ivanov V. K. Savinova and V. P. Zhakhovskaya Doklady Akad. Nauk. 7P. Mondain-Monval and B. Quanquin Gompt. rend. 1930 191 299. 8 K. I. Ivanov Actu Physicochim. U.S.S.R. 1938 9 421. Q C. Kroger and A. Kaller Oel u. Rohle 1943 39 669. 10 U.S.P. 2,395,523 2,403,758 2,403,771 2,403,772 2,434,888 2,446,797 (Shell 11 A. Rieche and F. Hitz Ber. 1929 62 2458. 12A. Baeyer and V. Villiger ibid. 1901 34 738. 13 A. Rieche and F. Hitz ibid. 1929 62 2478. l6 S. S. Medvedev and E. Alekseeva Ber. 1932 a 133 ; J. Gem. Chem. Russia 16N. A. Milas U.S.P. 2,223,807. l 7 N. A. Milas and S. A. Harris J. Arner. Chem. SOC. 1938 60 2434. Is R. Criegee and H. Dietrich Annalen 1948 560 135. 20 N. A. Milas and D. M. Surgenor J. Amer. Chmn.SOC. 1946 68 643. 22 P. George E. K. Rideal and A. Robertson Nature 1942 149 601. S.S.S.R. 1948 59 703. Dev. C O . ) . N. A. Milas U.S.P. 2,176,407. 1931 1 1193 1200. N. A. Milas and D. M. Surgenor ibid. 1946 68 205. S. S. Medvedev and A. Podyapolskaya Acta Physicochim. U.S.S.R. 1935,2 487. HAWKINS ORGANIC PEROXIDES AND THEIR REACTIONS 253 Catalytic reduction leads to the formation of the corresponding alcohol by loss of one atom of oxygen. Sodium sulphite is in general a much slower reducing agent whilst the effect of ferrous sulphate is probably partly that of a reducing agent and partly catalytic. This class of peroxide is relatively stable under mild acid conditions whilst cold dilute alkali gives rise to the alkali-metal salt of the hydro- peroxide. In the presence of concentrated or hot alkali however these hydroperoxides are unstable and decompose to aldehydes ketones and acids.15 Reaction with a further molecule of an alcohol in the presence of sulphuric acid is the normal route to the synthesis of a dialkyl peroxide 23 ROOOH + R'OH -+ RO-OR' + H,O Thus tert.-butyl hydroperoxide undergoes a smooth reaction at < 10" with ethyl sulphate and potassium hydroxide to yield ethyl tert.-butyl peroxide.24 Alternatively the metal salt of the hydroperoxide may be made to react with an alkyl halide in an inert solvent in which the metal halide formed is insoluble. 25 Organic acid chlorides react with alkyl hydroperoxides in a basic solvent (e.g. pyridine) to yield per-esters ; under mild hydrolytic conditions the hydroperoxide and acid can be regenerated.Many esters of tent.-butyl hydroperoxide have been prepared in this manner,20 and the crystalline p-nitrobenzoates have recently been used by R. Criegee and H. Dietrich 26 for characterising several hydroperoxides ; these authors state that esteri- fication of fed.-alkyl hydroperoxides proceeds much more easily than with tertiary alcohols. These per-esters liberate iodine from acidic potassium iodide much more slowly than do the original hydroperoxides. Aldehydes on reaction with hydroperoxides readily provide mono- hydroxydialkyl peroxides R'CH( OH)*O*OR. Mineral-acid catalysts are generally used except in the case of formaldehyde which is sufficiently reao- tive without the presence of a catalyst ; the reaction may simply be carried out in ether 27 or water 28 RO*OH + R'CHO + R'CH(OH)*O*OR Ketones react in a similar fashion but the product may with an additional molecule of hydroperoxide yield a diperoxy-compound 29 +RO-OH RODOH + R'COR" + RO*O*CR'R"*OH _____+ CR'R"(0-OR) This reaction has been extended to keto-acids ; for instance hvulic acid gives CH,*C(OH)(O-OR) -CH,*CH,*CO,H although one might have expected lactonisation to follow yielding the y-lactone.23 (a) A. Rieche Ber. 1929 62 218 ; ( b ) W. Bockmuller and L. Pfeuffer Annalen 1939 537 178 ; (c) N. A. Milas and L. H. Perry J. Amer. Chem. Soc. 1946 68 1938. 24 U.S.P. 2,403,758 2,414,769 (Shell Dev. Co.). 26 U.S.P. 2,403,709 (Shell Dev. Co.). 26 Annalen 1948 560 135. 27 A. Rieche Ber. 1930 $3 2642. 28 U.S.P. 2,400,041 (Shell Dev. Co.) 2e U.S.P. 2,455,569 (Shell Dev. Co.) ; F. H. Dickey et al.Ind. Eng. Chem. 1949 41 1673; Q.T. Wiles et al. ibid. p. 1679. 254 QUARTERLY REVIEWS Mention has recently been made in the patent literature of the addition of tert.-butyl hydroperoxide to activated double bondsY3O e.g. acrylonitrile or methyl acrylate CH :CH*CN + CMe,*O-OH -+ CMe,*O*O*CH,*CH,*CN CH :CHCO,Mo + CMe,*O*OH + CMe,*O*O*CH,*CH,*CO,Me (B) Hydroxyalkyl Hydroperoxides [RCH( OH)*O*OH].-These compounds have been prepared by reaction of hydrogen peroxide with aliphatic aldehydes preferably in an anhydrous medium (e.g. ether). In aqueous solution the tendency is to the formation of bishydroxyalkyl peroxides RCH( OH)*O*O*CHR*OH ; hydroxymethyl hydroperoxide could only be prepared from anhydrous formaldehyde.31 The C to C, homologues are solids with melting points ranging from 40” to 67°.32 Ketomalonic ester on treatment with hydrogen peroxide in ether similarly gives a mixture of hydroxydicarbethoxymethyl hydroper- oxide C( OH)(CO,Et),O*OH and di(hydroxydicarbethoxymethy1) peroxide [C( OH) (CO,Et),O*],.33 The alkyl hydroxy-hydroperoxides are insoluble in water but are slowly decomposed by it to give bishydroxyalkyl peroxides + RCHO + water RCHO + H,O + RCH(OH)*O*OH - RCH(OH)*O*O*CHR.OH Hydroxymethyl hydroperoxide explodes violently when heated in a flame but the higher homologues on fusion or heating in acetic acid are con- verted into the acid corresponding to the aldehyde used in their preparation. Spath Pailer and Schmid 32b had hoped to obtain the alcohol with one carbon atom less than the hydroxy-hydroperoxide by thermal treatment but found that the acid was the main product in every case.Alkali also leads to formation of the corresponding acid NaOH RCH(OH)*O*OH 4 RC0,Na + H,O Reaction of a hydroxy-hydroperoxide RCH( OH)*O*OH with an alde- hyde R’CHO leads to the unexpected result that the unsymmetrical bis- hydroxyalkyl peroxide RCH( OH)*O*O*CHR’*OH? is not formed ; rearrange- ment leads to the production of two symmetrical compounds 32a 2RCH(OH)*O*OH -j- 2R’CHO --+ RCH(OH)*O-O*CHR*OH + R’CH (OH) *O*O*CHR’* OH The lower homologues are converted into mixtures of aldehydes and acids by ferrous s~lphate.3~7 32u An interesting reaction involves dehydration of lower hydroxy-hydro- peroxides with phosphoric oxide ; very explosive polymeric alkylidene peroxides are giten ; the higher homologues under similar conditions provide a ~ i d s .3 ~ ~ (C) Dialkyl Peroxides (RO*OR‘).-The lower members of this series 30 B.P. Appln. 16,783/48 (N.V. de Bataafsehs P.M.). 31 A. Rieehe and R. Meister Ber. 1935 68 1465. 32 (a) A. Rieche ibid. 1931 64 2328 ; (b) E. Spath M. Pailer and M. Schmid 33 N. A. Milas and P. C. Panagiotakos J. Amer. Chern. SOC. 1946 $8 633 ibid. 1941 74 1552. HAWKINS ORGANIC PEROXIDES AND THEIR REACTIONS 255 have been known for over twenty years and were prepared by treatment of hydrogen peroxide (or a hydroperoxide) with an alkyl sulphate in the presence of alkali.34- Dimethyl peroxide is a gas at room temperature (b.p. 13.5") and is so explosive that only about 2 g. can safely be prepared at a time. The higher homologues have been prepared in the laboratory by the above method with or without alkali and by the reaction of alkyl halides with metal salts of hydroperoxides as mentioned previously 25 RO-OM + R'X + ROOOR' + MX Experiments carried out by the Shell Development Company have shown that branched-chain hydrocarbons (e.g.isobutane and isobutene) when oxidised in the presence of hydrogen bromide yield tert.-alkyl peroxides and hydroperoxides amongst the products. lo The peroxides have been separated from the hydroperoxides by azeotropic distillation with water. 35 The lowest members are unstable and explosive and liberate little iodine from acidified potassium iodide solution. Passage of the dialkyl peroxide through a heated tube causes formation of radicals e.g. CMe;O*O*CMe + 2CMe,*O* -+ Me* + COMe Me* + CMe,*O*O-CMe + C,H + COMe + CMe,*O* The alkyl radicals may then undergo one of three reactions (a) They may dimerise to yield paraffins; e.g.di-tert.-butyl peroxide yields ethane (and acetone) whilst di-tert.-amyl peroxide provides butane (and acetone) ; 18 2o similarly di-(3-ethylpent-3-yl) peroxide gives mainly butane and diethyl ketone on heating and tert.-butyl 1 1 2 2-tetramethyl-n-propyl peroxide yields neopentane and acetone under comparable conditions. 23G (b) They may react with a different radical formed in the same de- composition ; e.g. methylcyclohexyl tert .-butyl peroxide provides ethane acetone 3-methylheptan-2-one (I) and 3 4-dibutylhexane-2 &&one (11) the suggested mechanism being 23c Me Me ( 7 . O . ' " " - Me - + COMe Me Me Me Me Me 34 A. Rieche and W. Brumahagen Ber. 1928 61 951 ; A. Baeyer and V.Villiger ibid. 1900 33 3387; A. Rieche and F. Hitz ibid. 1929 62 218. 86 U.S.P. 2,383,919 (Shell Dev. & CO.). 256 QUARTERLY REVIEWS It does not appear obvious however why the diketone (11) should be formed in preference to tetradecane-2 13-dione by end-to-end addition of the radicals (111). (6) They may further react with other compounds introduced simul- taneously e.g. paraffins or olehs for instance CHMe:CH + Me* -+ *CHMe.CH,Me followed by (i) *CHMe*CH,Me + CMe,*O*O*CMe + CHMe,*CH,Me + 2COMe + Me' (ii) CHMe :CH + *CHMeGH,Me -++ *CHMo*CH,*CHMe*CH,Me or Thus propene gives but-1-ene but-2-ene pentanes and pentenes whilst isobutene forms neohexane isopentane and tertiary pentenes ; ally1 chloride yields 2-chlorobut-l-ene l-chlorobut-2-ene and chloropentanes. The paraffins propane and isobutane lead to formation of isobutene and neopentane.36 Reaction of peroxides with benzaldehyde similarly leads to formation of the radical PhCO*O*CHPh* which dimerises to s-diphenylethylene dibenz- oate.37 It has been found that di-tert.-butyl peroxide decomposes in various solvents (cumene and tert.-butylbenzene) in a similar fashion to the thermal decomposition to give a mixture of tert.-butanol and acetone but in a tertiary amine (tributylamine) the product consists almost entirely of the alcohol.38 In the above reactions the alkyl (e.g.methyl) radical when liberated has been able to react with another compound or radical present by addition substitution or removal of a hydrogen atom. In the liquid phase thermal decomposition of di-tert.-butyl peroxide where no other compound is present the methyl radical liberated abstracts hydrogen from a further molecule of the peroxide to leave a new radical which provides isobutene epoxide on decomposition 39 (i) CMe,*O*O*CMe + Me* + CH + CMe,*O*O*CMe,*CH,* (ii) CMe,*O-O-CMe,*CH,* + CMe,*O* + CMea*CH \ / 0 Under these conditions of decomposition the reaction products contain more epoxide than acetone or tert.-butanol. Reduction of dialkyl peroxides where possible gives rise to the corre- sponding alcohols. Zinc and acid have been used for diethyl peroxide and Raney nickel under pressure for di-tert.- butyl peroxide. Treatment of dialkyl peroxides with a mixture of sulphur dioxide and chlorine (or bromine) at 0-20" in ultra-violet light leads to production of a sulphonyl halide.41 Di-tert.-butyl peroxide on reaction at 0" with hydrogen bromide yields up to 64% of 1 2-dibromo-2-methylpropane.40 W.E. Vaughan J. Anzer. Chem. Soc. 1948 70 95. 313 U.S.P. 2,396,206 2,396,217 (Shell Dev. C o . ) ; F. F. Rust F. H. Seubold and 37 Idem ibid. p. 3258. 38 J. H. Raley I?. F. Rust and W. E. Vaughan ibid. p. 1336. 39 E. R. Bell F. F. Rust and W. E. Vaughan ibid. 1950 '92 337. 40 N. A. Milas and C. N. Winnick ibid. 1949 71 748. 41B.P. Appln. 31,756/48 (N.V. de Bctttwfsche P.M.). HAWIUMS OWANICY PEROXIDES AND THEIR REACTIONS 257 (D) Monohydroxydialkyl Peroxides [RCH( OH)*O-OR] .-This class of compound is usually prepared by reaction of hydroperoxides with aldehydes or ketones ; ether or an excess of the carbonyl component may be used as solvent ROBOH + R'COR" --+ RO.0-CR'R"*OH In general an acidic catalyst has been used,27 28 29 42 although formal- dehyde readily reacts in the absence of catalyst and in this respect there is a similarity with the reaction of the corresponding carbonyl compounds with hydrogen peroxide to yield hydroxy-hydroperoxides.An excess of the hydroperoxide in this reaction leads to formation of diperoxy-compounds as mentioned on p. 253. Hydroxy-dialkyl peroxides are distillable liquids rather less explosive than dialkyl peroxides and more stable than non-peroxidic semiacetals. Acid iodide only leads to liberation of ca. 50% of the iodine to be expected from the active 0xygen.l Little information is available concerning the products of thermal decomposition of these compounds except that it has been stated that for- maldehyde and other carbonyl compounds are formed.An unexpected reaction is that with alkali it is claimed that atomic hydrogen is produced.27 28 Methyl hydroxymethyl peroxide also yields formic acid and methanol whilst hydroxymethyl teyt.-alkyl peroxides provide methane and hydrogen. With ferrous sulphate an exothermic catalytic reaction takes place to yield an acid and an alcohol EtO*O*CH,*OH -+ EtOH + HC0,H (E) Bishydroxyalkyl Peroxides [RCH( OH)*O*O*C€€R*OH] .-Like the hydroxyalkyl hydroperoxides these compounds are produced by reaction of hydrogen peroxide with aldehydes or ketones ; a second molecule of carbonyl compound may also be added to a hydroxyalkyl hydroperoxide H30 + RCHO + RCH(0H)OOH + RCHO $ RCH(OH)*O*OCE€R*OH Since these peroxides are more stable than the corresponding hydroperoxides the above reactions tend to go to completion to the right in the presence of water.Bishydroxymethyl peroxide has been produced apart from by the normal route by the decomposition of ethylene ozonide 43 and from the action of ozone on dimethyl ether.44 The higher homologues up to bis- hydroxydodecyl peroxide,l and halogenated peroxides e.g. from have been prepared. The first member of the series CH,(OH)*O*O*CH,(OH) is a solid m.p. 62-64' but the next higher homologues are liquids. Bishydroxyheptyl peroxide (m.p. 69 ') and further members are solids-bishydroxydodecyl peroxide has m.p. 84O.l They are non-explosive compounds-except the 4 2 B.P. 444,544 (N.V. de Bataafsche P.M.). 43 E. Briner and P. Schnorf Helv. Chka. Acta 1929 12 154. 4 4 F. G. Fischer Annakn 1929 4'96 244.45 A. Baeyer and V. Villiger Ber. 1900 33 2481. 258 QUARTERLY REVIEWS lowest member-and liberate iodine quantitatively from acid iodide solution. Thermal decomposition of this series of compound leads to formation of a mixture of acid and aldehyde RCH(OH)*O*O*CHR*OH -+ RCHO + RCO,H + H,O Warming with water leads to reconversion into aldehydes and hydrogen peroxide ; the aldehydes may often be removed from the reaction mixture in steam. Warm dilute acid leads to a similar but faster reaction and warm alkali also provides aldehyde and hydrogen peroxide except with the two lowest members of the series which yield the acid in additioml? 46 As with the hydroxyalkyl hydroperoxides it has been found that phos- phoric oxide causes dehydration and production of the highly explosive alkylidene peroxides (see below) related in structure to ozonides.Thus bishydroxyethyl peroxide actually gives a mixture of the monomeric and dimeric ozonides of b ~ t - 2 - e n e ~ ~ ~ ~ 47 as proved by their decomposition with acid and alkali.47 Several of these alkylidene peroxides have been isolated from different members of the series and all appear to be very dangerous compounds sensitive to shock. Bishydroxymethyl peroxide forms derivatives with ammonia and other nitrogen compounds (urea and hydrazine) ; however di-( l-hydroxy-8- carboxyoctyl) peroxide on treatment with semicarbazide in ethanol yields the semicarbazone of the aldehydo-a~id.~~~ (F) Alkylidene Peroxides and Ether Peroxides.-The formation of ethyl- idene peroxide (-CHMe*O*O*)2 from hydroxyethyl hydroperoxide by treat- ment with phosphoric oxide has already been mentioned (p.254) and this reaction can be extended to the corresponding propyl derivative. Ethyl- idene peroxide has also been found in the products of auto-oxidation of ether and has been formed by heating butene ozonide in wacuo.4* These compounds are exceedingly explosive and sensitive to friction and are decomposed by acid and alkali to the correspond- ing aldehyde and acid respectively. Dimeric acetone peroxide has been produced by reaction of solid Caro's reagents with acetone in ether 49 and from the ozonides of compounds with a terminal isopropylidene group (e.g. mesityl oxide citral and The inset structure has been suggested for this compound ; it Reduc- Acetone also forms a trimeric peroxide by the action of hydrogen It has m.p.98" and reacts with acids to regen- 0.0 / \ 0.0 squalene).l has m.p. 132" and on catalytic decomposition affords acetic acid. tion with zinc and alkali leads to acetone. peroxide and mineral acid. erate acetone and hydrogen peroxide. M0,C \ / 46 (a) G. King J. 1942 218 ; (6) A. Rieche R. Moister H. Sauthoff and H. Pfeiffer Annalen 1942 553 187. 47 A. Rieche and R. Meister Ber. 1932 65 1274. 48fdem ibid. 1931 64 2335; 1939 72 1933. 48 A. Baeyer and V. Villiger ibid. 1899 32 3628 3632; 1900 33 124. HAWKINS ORGANIC PEROXIDES AND THEIR REACTIONS 259 A mixture of both peroxides is produced during the autoxidation of diisopropyl ether. One mechanism suggested 5O for such formation is H,O (i) CHMe,*O*CHMe + 0 -j. HO*O*CMe,*O*CHMe -+ HO*O*CMe,*OH COMe + H,O -1 .1 CMe,*O*O* f CHMe,*OH I pol ymerises (ii) CHMe,*O*CHMe + 20 -+ H20 HO.O*CMe,*O.C~l/ls,*O.OIT + 2HO-O*CNe2-OH COMe + H,O 4 4 2CMe,*O*O* + H,O I polyrnerises Higher ketones also yield polymeric peroxides on treatment with hydrogen peroxide and sulphuric acid 61 or with Caro's acid.It has also been claimed that peroxides are formed from l=vulic acid and mesityl oxide,l although little has been done to study these compounds in detail. (a) Alkenyl Peroxides.-( 1) Mono-oEeJins.-The fact that ole fins form peroxides in the presence of air has been known for a considerable time ; peroxides from pentene trimethylethylene and hexene have been reported and a polymeric keten peroxide R,C*CO which decomposed t o the ketone COR and carbon dioxide has also been described.53 A straight-chain olefin e.g. hex-l-ene has been found to give a con- version into hydroperoxide of only I.*5-2% even in the presence of a cata- lyst (e.g.copper chloride) but mixed hexenes from dehydrated 2-methyl- pentan-1-01 gave up to 10% of h~droperoxide.~~ This seems to indicate that branching of the chain leads to greater ease of peroxide formation or greater stability of the peroxide compared with the corresponding straighl- chain compound. Peroxides from the lower olefins (propene but-l- and -2-ene isobutene pent-l- and -2-ene 2-methylbut-2-ene hex-2-ene and hept-3-ene) have been stated to be obtained 55 in 5% yield (+ 10% of other oxygenated products) by liquid-phase oxidation at - 50" to 150". It was suggested by J. L. Bolland 56 that the first stage in the oxidation of all olefins is the formation of a hydroperoxide at the a-methylene group and only in the case of a diene is a cyclic peroxide considered to be possible.56b The work of E.H. Farmer and H. P. Koch also leads to such a viewpoint. I I 0.0 6o A. Rieche and K. Koch Ber. 1942 75 1016 ; K. I. Ivanov V. K. Savinova and E. G. Mikhailova J . Gen. Chem. Russia 1946 16 65 1003 1015. G1 W. Uilthey M. Inckel and H. Stephan J . pr. Chem. 1940 154 219. 62 C. Engler Ber. 1900 33 1094. 63 J. d'Ans and W. Frey ibid. 1912 45 1848. 64 H. Hock and A. Neuwirth ibid. 1939 72 1562. 66 B.P. 614,456 (Petrocarbon). 66 ( a ) Quart. Reviews 1949 3 1 ; (b) Bolland and H. Hughes J. 1949 492. 260 QUARTERLY REVIEWS However C. Paquot 57 has suggested that two types of peroxide are formed by oxidation of olefins i.e. hydroperoxides and cyclic peroxides ; these would both be expected to provide the different classes of decomposition products actually found.For instance the olefin CK,R*CR’:CHR” would give RCH-CR’ :CHR ” and RCH;CR ’*CHR” I 0 :OH v 0:O decomposes decomposes 1 I * RCHCR’ :CHR” * RCH,*CR’*CHR’’ -v 0 -j. glycols and I OH ketones + RC0CR’:CHR” + acids etc. + aldehydes and ketones by scission of chain A similar type of cyclic peroxide has been suggested to explain the oxidation products of anethole (see p. 268). Recently a preliminary study of the oxidation of various phenyl-substi- tuted olefins has been carried and by restricting the uptake of oxygen to & mole per mole of olefin in one case a product containing up to 65% of peroxide was obtained. At the same time the unsaturation decreased by about 43% suggesting that cyclic or polymeric peroxides were formed epoxide formation might well have partly accounted for this.Further oxidation led to a breakdown of the molecule to acids carbon dioxide and water. Presumably such cyclic peroxides would be very unstable and may not be isolable from a simple olefin even if it should exist. However whatever the mechanism of epoxide formation might be the presence of such com- pounds amongst olefin oxidation products has been shown by Paquot ; 59 a patent for the production of epoxides by oxidation of olefins in the presence of basic substances has been granted.60 In a similar way with the peroxides from unsaturated fatty acids and esters a hydroperoxide has been isolated from methyl oleate 61 and methyl elaidate,62 but it has been suggested 63 that a type of ring peroxide may be formed as an intermediate *CH:CH*CH,* -+ *CH*CH*CH,* -+ *CH*CH:CH* .. * . . . 0-0 0-OH 67 Bull. SOC. chirn. 1945 12 120. 68 J. W. Lawrence and J. R. Shelton Ind. Eng. Chem. 1950 42 136. 68 Thesis Fac Sci. Univ. Paris 1943. 6O U.S.P. 2,052,195 (Standard Oil (20.). 61 ( a ) E. H. Farmer and D. A. Sutton J. 1943 119 ; (b) C. E. Swift F. G. Dollear and R. T. O’Connor Oil & Xoup 1946 23 355. 6 2 D. A. Sutton J. 1944 242. 63 F. D. Gunstone and T. P. Hilditch J. 1946 1022. HAWKINS ORaANIC PEROXIDES AND THEIR REACTIONS 261 During autoxidation of oleic acid elaidic acid and methyl oleate an increase in ultra-violet absorption has been observed probably through secondary formation of conjugated unsaturated ketones.64 Methyl methacrylate and vinyl acetate yield polymeric peroxides with oxygen.65 Reduction of that from methyl methacrylate affords methyl a/?-dihydroxyisobutyrate suggesting that the peroxide system involved the carbon atoms originally carrying the double bond.Pent-3-en-2-one on oxidation gives little peroxide ; the main reaction leads to fission of the molecule and production of acetaldehyde.66 Very little information is available on the reactions of olefin peroxides. Reduction of hex-1 -ene hydroperoxide with sodium sulphite yielded hex- 1 - en-3-01 indicating that the a-methylene group was that originally attacked.5* The reduction of the hydroperoxides of methyl oleate and methyl elaidate 6x9 62 by a variety of means (aluminium amalgam hydriodic acid catalytic) has yielded methyl hydroxyoleate and then methyl hydroxy- stearate.Hydroxgoleic acid is also formed by action of alcoholic potash on the hydroperoxide,61b and permanganate oxidation leads to a mixture of octoic pelargonic suberic and azelaic acids. Hydroperoxides normally react with lead tetra-acetate to yield oxygen and the corresponding ketone.6' It has been variously claimed that un- saturated olefin peroxides do 61b and do not give oxygen and ketones with the tetra-acetate ; the structure of this class of peroxide needs further resolving on the evidence of this reaction. (2) PoZyolefins.-The structure of many of the polyolefin peroxides is still in considerable doubt although suggestions have been made for some of the simpler ones. 2 3-Dimethylbutadiene and isoprene yield peroxides which on hydrogenation provide formaldehyde and a mixture of other decompo- sition products.69 2 4-Dimethylpenta-2 4-diene gives only a crude per- oxide; on heating at 80" this forms a dimer CI4Hz4 but at 100-120" a violent decomposition takes place to give formaldehyde formic acid and acetone.The structure proposed for the peroxide was ' 0 Me,C*CH I :CMe*CH I or [-2g"" 0 0 a This may be compared with the diperoxide of squalene which appears to contain a hydroperoxide group and a O*OH cyclic peroxide in the system shown inset ; reduction leads to a trio1 con- I I I -CH,*CMe :CH*CHCH2*CMebCH*CH,* taining an a/3-glycol system.56b 0 -0 Peroxides from hepta-2 4-dien-6-one mostly dimeric have been made.66 64 R. T. Holman W. 0. Lundberg W. M. Lauer and G. 0. Burr J . Amer. Chem. 65 C . E. Barnes R. M. Elofson and G.D. Jones ibid. 1950 72 210. 66 H. Albers and W. Schmidt J. pr. Chem. 1943 162 91. 67 R. Criegee H. Pilz and H. Flygare Ber. 1939 72 1799. 'O R. Jacquemain Cmpt. rerzd. 1942 215 200. Soc, 1945 67 1285. VV. Treibs ibid. 1944 77 69. 6s K. Bodendorf Arch. Pharm. 1933 271 1. T 262 QUARTERLY REVIEWS Autoxidation of methyl linoleate gradually takes place at 37" accom- panied by increased conjugation of the double bonds. Two peroxides are probably formed ; on hydrogenation one gives methyl 9- and 13-hydroxy- stearate (Geneva numbering) suggesting that the point of oxidation is at the methylene group between the double bonds followed by rearrangement to a conjugated system and the other peroxide yields a dihydroxy- stearic acid.71 These results indicate that a cyclic peroxide as well as hydro-peroxide might well be produced during the oxidation.The peroxide of methyl eleostearate on hydrogenation gave (a) lower aldehydes ; ( b ) methyl m-formyloctoate OCH*[CH,J,*CO,Me ; (c) a glycol C1gH3804 not an a-glycol ; (d) a trace of ester C,,H3,0 ; and ( e ) a sodium carbonate-soluble saturated compound CI9H3,O8 which appeared to be a stable peroxide. The major products were (b) and (c) the latter probably being methyl 9 12-dihydro~ystearate.~~ Autoxidation of the system *C:C*C*C:C*C*C:C* gave an increase in con- jugation and the extent of double-bond displacement seemed related to the degree of peroxidation.73 2. Alicyclic Peroxides (A) Cyclic Paraffin Peroxides.-In general hydroperoxides of this class have been made by oxidation of the corresponding hydrocarbon although the reaction of tertiary alcohols with hydrogen peroxide and sulphuric acid has also been utili~ed.~3~~ 26 Criegee has shown that the products are iden- tical when prepared by both methods.Oxidation of cydohexane normally gives rise to cydohexanone cyclo- hexanol and adipic acid but it is claimed 8,74 that at 316-328" this hydro- carbon yields two non-volatile peroxides C4H804 and C,H,,O (possibly [HO*O],*C,H9*OoO~CH2*oH). Recent patents 75 suggest that hydroperoxides of naphthenes may be obtained in concentrations up to 10% by oxidation in the presence of sodium carbonate or hydrogen carbonate ; presumably the carbonates remove acids formed during the oxidation and the presence of these acids would bring about decomposition of the peroxides. Some of the hydroperoxides so formed have been purified by concentration through their metal salts and 1 -methylcycZopent yl 1 -methylcycZohex yl 26 9 and 1 -ethylcycZohexyl hydroperoxide 26 have recently been prepared in a pure state.During autoxidation of cis-decalin a Walden inversion takes place with the formation of the trans-9-hydroperoxide. 78 79 71 S. Bergstrom Nature 1945 156 717 ; Arkiv Kemi Min. Qeol. 1945 21 A 7 2 W. Treibs Ber. 1943 '26 670. 73 J. L. Bolland and H. P. Koch J. 1945 445 ; E. H. Farmer H. P. Koch and 7 4 K. I. Ivanov J . Gem. Chem. Russia 1936 6 470. 76 U.S.P. 2,430,864 2,430,865 (Union Oil). 78 E. J. Gasson E. G. E. Hawkins A. F. Millidge and D. C. Quin in the press. 77 K. I. Ivanov and V. K. Savinova DokZady Akad. Nauk. S.S.S.R. 1948 59 493. 7a Idem Compt. rend. U.X.S.R. 1945 48 31. 79 R.Criegee Ber. 1944 77 22. No. 14 1. D. A. Sutton J. 1943 541. HATV"S ORGANIC PEROXIDES AND THEIR REACTIONS 263 A difficulty connected with the formation of this class of hydroperoxide is that after a fairly low optimum concentration is reached its decomposition to acids ketones and alcohols proceeds as rapidly as its formation.76 Chavanne and his co-workers80 studied the oxidation products of many substituted cyclohexanes and cyclopentanes and found that in general fission had occurred at the points of substitution to yield open-chain oxygenated compounds as well as cyclic alcohols These compounds must have arisen from the initially formed hydroperoxides although these were not isolated by Chavanne. With alkali these hydro- peroxides yield metal salts which may be used for their concentration but concentrated warm alkali brings about decomposition in most cases.The corresponding carbinols are produced smoothly by reduction with sodium sulphite or under catalytic conditions. 79 A study of some of the reactions of methylcyclopentyl and methylcyclo- hexyl hydroperoxides has recently been carried out .*l From methylcyclo- pentyl hydroperoxide the decomposition products given under a variety of conditions axe mainly 1 -methyleyelopentan- 1-01 methyl butyl ketone and 6-hydroxyhexan-2-one. A most surprising reaction is that of these two hydroperoxides with ferrous sulphate ; diketones with double the number of carbon atoms of the original hydroperoxide are formed in each case 82 2 &.'* - 2 &. - 2 ECH2 a Esters may be formed in the normal way by reaction with acid chlorides in the presence of pyridine and tertiary hydroperoxides are esterified more easily than tertiary alcohols.p-Nitrobenzoates have been used as crystal- line derivatives for the characterisation of several hydroperoxides.26 An interesting rearrangement of the esters of decalin hydroperoxide in various solvents causes formation of an isomeric hemiacetal ; this on hydrolysis yields 6-hydroxycyclodecan-1-one 83 fie&-& 0 Criegee has recently pointed out that the ease of rearrangement depends on G. Chavanne Bull. Xci. Acad. roy. Belge 1926 12 105 ; G. Chavanne Bull. SOC. chim. Belge 1927,36,206 ; G. Chavanne and 0. Miller ibid. 1930,39,287 ; G. Chavanne and E. Bode ibid. p. 206 ; G. Chavanne and 0. Miller ibid. 1931,40,611 ; G. Chavanne Mme. Pahlavouni and Mlle.Katzenstein ibid. p. 626 ; 1932 41 209 ; G. Chavanne and G. Tock ibid. p. 630; P. Dupont and G. Chavanne ibid. 1933 42 537. 81 E. G. E. Hawkins in the press. 8 2 E. G. E. Hawkins and D. P. Young in the press. 83 R. Criegee Ber. 1944 77 722 ; Annalen 1948 560 127. 264 QUARTERLY REWEWS (i) the strength of acid used in esterification (ii) hydroperoxide structure and (iii) the solvent. Hydroxycgcbal kyl hydroperoxides and di( hydroxycycloalkyl) peroxides are formed by reaction of cyclic ketones with hydrogen peroxide similarly to the comparable reaction with the open-chain carbonyl compounds ; 84 e.g. 8 ___+. polymers These hydroxy-hydroperoxides are highly explosive and on dehydration yield polymeric peroxides. However in a recent paper R. Criegee W. Schnorrenberg and J.Becke g5 have shown that the peroxides formed from cyclohexanone and hydrogen peroxide are actually H OoO-OoOH H O . O o W o O H HO. OfiO-OfiO‘ 0 H and On condensing (VI) with acetone and (VII) and (VIII) respectively are given cycbhexanone the compounds From the ease of formation of these compounds the authors conclude that the cyclic formula (IX) for the trimeric acetone peroxide is probably correct. In it recent communication Criegee 26 claims to have obtained the 1 1 6 6-tetrahydroperoxide (X) of cyclodecane by treatment of cyclo- decane-1 6-&one with hydrogen peroxide. (B) Cyclic Olefin Peroxides.-These peroxides have been produced by treatment of the cyclic olefins with air or oxygen followed by final separation from the other products and unchanged olefin. cycbPenteny1 hydroper- oxide has been prepared by this means,67 but much more study has been 8 4 N.A. Milas S . A. Harris and P. C. Panagiotakos J . Amer. Chem. SOG. 1939 61 2430 ; U.S.P. 2,298,405 (Research Corp.). Annulen 1949 5435 7. HAWKINS ORGANIC PEROXIDES AND THEIR REACTIONS 265 devoted to cyclohexenyl hydroperoxide.67~ 86 Hydroperoxides of l-methyl- cyclohexene 679 8Ge,87 1 2-dimethylcycZohexene,8~~ 87 menthene,88 a-phellandr- ene cyclohexadiene and or-terpinene 69 have been similarly prepared and it has been stated that dimethyl- methylethyl- and phenylmethyl-fulvenes yield diperoxides.89 It has been suggested that the keto-alcohol obtained by oxidation of menthofuran 90 arises via a cyclic peroxide (XI) or a hydro- peroxide (XII). HO.0 O.OH H O . 0 c3 O.GH Me go G.OH (=S Ozonolysisof 1 2 3 4 5 6 7 8-octahydronaphthalenehasgivencom- plex peroxides,91 the actual peroxide formed depending on the solvent used for the ozonolysis.0 3 .+. . (methanol) 0.1 (acetic acid) the acetoxy-homologua 06 (m) Thermal decomposition of cyclohexenyl hydroperoxide at 70-80 * gives cycluhexenol but when it is heated with water cyclopentenealdehyde is also produced as well as small quantities of cyclohexanetriol and acid.8Ge H. Hock and 0. Schrader 86b state that dilute sulphuric acid reacts with cyclohexenyl hydroperoxide to yield cis-cyclohexane-1 2401 and cyclo- pentenealdehyde. However a careful study of the decomposition of this hydroperoxide and those from various substituted cyclohexenes with acid by Farmer et aZ.8ee has shown it to give the corresponding trio1 as well as the unsaturated alcohol and a little cyclopentenecarbonyl derivative.These products may possibly arise by the sequence of reactions shown on the next page. Reaction (iv) to give cyclopent-l-en-l-a1 is parallel to that of the acid decomposition of the hydroperoxide of d e ~ a l i n . ~ ~ Alkali reacts with these hydroperoxides to give as main product the corresponding unsaturated alcohol and in addition some acids.86 A study 86 ( a ) H. N. Stephens J. Amer. C k m . Soc. 1928 50 568; (b) H. Hock and 0. Schrader Naturwiss. 1936 24 159 ; ( c ) idem 2. angew. Chem. 1936 49 565 ; (a) H. Hock and K. Glinicks Ber. 1935,7l 1430 ; (e) E. H. Farmer and A. Sundralingam J. 1942 121 ; (f) N. D. Zelinski and P. P. Borisov Ber. 1930 63 2362. 87 E. H. Farmer and D. A. Sutton J.1946 10. 88 H. Hock and S. Lang Ber. 1942 75 300. 80 C. Engler and W. Frankenstein ibid. 1901 34 2933. Bo R. B. Woodward and R. H. Eastman J . Amr. Chem. SOC. 1950 '72 399. B1 R. Criegee and G. Wenner Annalen 1949 564 9. 266 QUARTERLY REVIEWS of the acid products from cyclohexenyl hydroperoxide showed the presence of formic acetic glutaric adipic (mainly) a-hydroxyadipic and higher- boiling acids. Many of the shorter-chain acids must have arisen through degradation but the a-hydroxyadipic acid may well have come by fission of a G-C link (Q (ii) ( i i i ) . - of the triol. On catalytic reduction the expected saturated alcohol is given. cyclo- Hexenyl hydroperoxide gives cyclohexanol and 1 -methylcycZohexenyl hydro- peroxide provides both 3- and 2-methylcycEohexano1 (owing to the presence of the 6- and the 3-hydroperoxide).a-Terpinene and a-phellandrene per- oxides both yield diols on hydrogenation and cyclohexadiene peroxide is reduced slowly to cis- and trans-quinitol. Sodium sulphite brings about reduction of cyclic olefin hydroperoxides to the corresponding unsaturated alcohols ; the hydrogen sulphite (pyro- sulphite) reacts rather more vigorously and with rnenthene hydroperoxide it gives rise to mentha-1 3-diene in addition to the unsaturated alcohol.92 Catalysts such as ferrous phthalocyanine lead to an exothermic decom- position to yield the enones andenols.86e Lead tetra-acetate causes liberation of oxygen.67 cycZoHexene and menthene hydroperoxides have been methylated with methyl sulphate and sodium hydroxide at controlled pH ; yields of 40-70% of the methyl peroxide are recorded.88 Diazomethane cannot be used for the methylation of these compounds.(C) Peroxides from Partly Reduced Aromatic Hydrocarbons.-Prepara- tion of these hydroperoxides has again been limited to the method of controlled air oxidation of the hydrocarbon. The most extensively studied member of this series is the hydroperoxide of tetralin ; 933 94 the 9 2 H. Hock and S. Lang Rer. 1942 75 313. O3 M. Hartmann and M. Seibcrth HeZv. Chim. Acta 1932,15,1390 ; U.S.P. 1,924,786 ; B.P. 396,351. O4 (a) H. Hock and W. Susemihl Ber. 1933 66 61 ; ( b f W. Nussle G. W. Perkins and G. Toennies Amer. J . Pharm. 1935 107 29; (c) I(. I. Ivanov V. K. Savinova and E. G Mikhailova Compt. rend. Acad. Xci. U.R.S.S. 1939 25 34 ; ( d ) A. Robertson and W. A. Waters J. 1948 1574 1578 1585.HAWKINS ORGANIC PEROXIDES AND THEIR REACTIONS 267 hydroperoxides of indane 95 and octahydroanthracene S6 have also been isolated. The presence of tetralone and ditetralyl peroxide was detected during the oxidation of tetralin ; 97 the ketone and tetralol have also been found in the products of thermal decomposition of the hydropero~ide.~~ Other products obtained by thermal treatment were dihydronaphthalene y-o- hydroxyphenylbutyraldehyde y-o-hydroxyphenylbutyric acid and B-o- carboxyphenylpropionic acid ; 94d the first acid has also been isolated in small quantities from the acid-treated hydropero~ide.~~ The aldehyde and the acids may well be produced by a mechanism similar to that suggested by Criegee for decalin peroxide -+ hydroxycyclodecanone. Alkali reacts in the normal way with these hydroperoxides ; with cold alkali the sodium salts are given but on heating the hydroperoxide is con- verted into ketone or alcohol or a mixture of both.94a 95 Tetralyl hydro- peroxide provides mainly tetralone (?Ox) whereas the indane compound gives indan-1-01 (55%) and a little indan-l-one (5%).Reduction with sodium sulphite yields the corresponding alcohol in every case ; 92 the pyrosulphite however causes some dehydration of a-tetralol to di-a- tet ral yl ether and dihydronapht halene. Permanganate oxidation of tetralyl hydroperoxide gives rise to B-o-carb- oxyphenylpropionic acid,94a and free oxygen brings about decomposition of octahydroanthryl hydroperoxide to the ketone.96 Reaction of a benzene solution of formaldehyde with tetralyl hydro- peroxide affords tetralyl hydroxymethyl peroxide ; the normal ether solvent used in this type of reaction gives no satisfactory product.99 As in the case of the cyclic o l e h hydroperoxides methylation by means of methyl sulphate and alkali proceeds smoothly.88p95 All these hydroperoxides lose water on catalytic treatment (ferrous sulphate ; 93,959 9i3 lead tetra-acetate ; 95 metal phthalocyanines 100) to give good yields of the corresponding ketones.Tetralyl hydroperoxide has been used as an oxidising agent for various olefins in the presence of an osmium tetroxide catalyst ; lo1 for instance 1 2-dimethylcyclohexene was converted into octane-:! 7-dione. 96 Idem ibid. 1943 76 1130. 95 H. Hock and S. Lang Ber. 1942 75 1051. 97 S. S. Medvedev Acta Physicochim. U.S.S.R. 1938 9 405.98 H. Hock and S. Lang Ber. 1944 77 257. gg K. I. Ivanov V. K. Savinova and E. G. Nikhdova J . @en. Chem. Russia loo J. H. Helberger and D. B. Hever Ber. 1939 72 11. lol T. Cosciug Ann. sci. Univ. Jmsy See. I 1941 27 303. 1938 8 51. 268 QUARTERLY REVIEWS (D) Aralkyl Peroxides.-( 1) Hydroperoxides. -.Air oxidation of hydro- carbons is again the normal mode of formation although triphenylmethyl hydroperoxide at least appears to have been formed by reaction of triphenyl- methyl chloride with hydrogen peroxide. lo2 Various alkylbenzenes oxidised at 100-102" gave peroxides in quantity corresponding to 60-S0~0 of the oxygen consumed. 22 Passage of oxygen through the heated hydrocarbon either as a homogeneous phase or in emulsion has been used to prepare the hydroperoxides of cymene,lo3 p-xylene,lo4 diphenylmethane,Qs isopropyl- benzene,99 lo5 and sec.-butylbenzene,lo6 and the dihydroperoxides of diiso- propylbenzene (m- and p-isomers).lo' It has been found that oxidation of various substituted benzenes (ethyl- see.-butyl- see.-amyl- see.-hexyl-) yielded acetophenone whilst others (as- diphenylethane diphenylmethane and triphenylmethane) similarly gave benzophenone all presumably through the hydroperoxides. lo8 The oxidation of various alkylbenzenes at higher temperatures (100- 650") has been studied and it is stated that both isopropyl- and tert.- butyl-benzene yield phenol and acetone. log Anethole on oxidation yields a mixture of anisaldehyde anisic acid acetic acid a ketol and a glycol as well as dimeric autoxidation products. The mechanism suggested 110 for the production of these compounds involves the reaction of an intermediate cyclic peroxide similar to those mentioned by Paquot for aliphatic olefins M e o a C H .C H M e - M e o O E H O + CH3-CO2H L0-J ( i ) CO,H + CH,.CHO \ M e 0 0 M e o a y H . y H M e + peroxide ' - Me a C 0 . CHzMe OH O*COR Io2 H. Wieland and J. Maier Rer. 1931 64 1205. Io3 J. H. Helberger A. Rebay and H. Fettback ibid. 1939 72 1643. Io4 H. Hock and S. Lang ibid. 1943 76 169. Io5 B.P. 610,293 629,637 630,286 (Distillers Co. Ltd.) ; G. P. Armstrong R. H. Hall and D. C. Quin hTature 1949 164 834 ; J. 1950 66. (a) K. I. Ivanov V. K. Savinova and V. P. Zhakhovskaya Doklady Akad. Nauk. S.S.S.R. 1948 59 905; ( b ) E. G. E. Hawkins J. 1949 2076; ( c ) E. G. E. Hawkins in the press ; (d) I. M. Kolthoff and A. I.Medalia J. Amer. Chem. SOC. 1949 '71 3789. HAWEINS ORGANIC PEROXIDES AND THEIR REAUTIONS 269 The products of oxidation also include compounds formed by demethyl- ation nuclear oxidation to phenols and nuclear degradation. Acid leads to production of a phenol and a carbonyl compound from those hydroperoxides studied possibly through the hemiacetal rearrangement c RR'.OH CR R'.O.OH b OH For instance diphenylmethyl hydroperoxide yields phenol and benzal- dehyde whereas phenol and acetone are formed from aa-dimethylbenzyl hydroperoxide (from isopropylbenzene) phenol and methyl ethyl ketone from the sec.-butylbenzene homologue and quinol and acetone from p-aaa'a' t etramethylxyl ylene dih ydroperoxide (from p -diisopropyl benzene). Alkali appears to give rise to the corresponding carbonyl derivative although the metal salt of the hydroperoxide is first formed.It has been stated 103 that the hydroperoxide from cymene yields cuminaldehyde although this infers that oxidation has taken place at the primary rather than the expected tertiary carbon atom. That from p-xylene gives p-tolualde- hyde which by a Cannizzaro reaction is converted into tolylcarbinol and toluic acid. lo4 All the hydroperoxides of this class are smoothly reduced to the alcohol by sodium sulphite. Reduction has also been carried out by hydrogena- tion or by the use of alkali metal sulphides.lll Decomposition of the hydroperoxides of ethylbenzene isopropylbenzene and see.-butylbenzene with ferrous sulphate leads in each case to aceto- phen0ne.10~~ 106 The physical chemistry of the reaction of the ferrous ion with the hydroperoxide of cumene has been studied in detail.106d Thermal and catalytic decomposition of aa-dimethylbenzyl hydroperoxide has been shown to yield a mixture of acetophenone 2-phenylpropan-2-01 and a-methyl- styrene.Triphenylmethyl hydroperoxide is unaffected by ferrous sulphate ; vi ith benzoyl chloride it yields an isomer of the expected benzoate probably Ph,C( OPh)OBz since with alcoholic potash phenol benzoic acid and benzo- phenone are obtained.lo2 This is yet another case of rearrangement of a hydroperoxide or its ester to a hemiacetal. It has been shown 106c that aa-dimethylbenzyl hydroperoxide may act as a mild oxidising agent especially in the presence of catalysts ; aldehydes may be oxidised to acids secondary alcohols to ketones and even olefins con- verted into various oxygenated products.( 2 ) Peroxides (ROOOR).-Many of these have been described in the E. G. E. Hawkins and I). C. Quin unpublished. lo8 H. N. Stephens J. Arner. C'hern. SOC. 1926 48 2920 ; lo9 S. Tonomura Bull. Inst. Phys. Chem. Res. (Tokio) 1942 21 774. 110 L. Schulz and VV. Treibs Ber. 1944 7'7 377. ll1 U.S.P. 2,491,926 2,484,841 (Hercules Powder (20.). H. N. Stephens and F. L. Roduta ibid. 1935 57 2380. 270 QUARTERLY REVIEWS literature and no attempt is made here to list them. In many cases how- ever they have been prepared but their reactions not studied. One of the procedures most frequently adopted for their preparation has been that of treating a tertiary halide with finely divided metal (e.g. zinc silver mercury) in the presence of air or oxygen.These peroxides have been prepared by it variety of workers although Marvel and his co-workers (193544) in America and Bowden et at. (1939-40) in Wales have contributed to a large extent in this field. Several diary1 ketone peroxides have been prepared by ozonolysis of unsaturated systems e.g. Ph,C:CH -+ benzophenone peroxide and (p-M[eC,H4),C:CH~e -+ di-p-tolyl ketone peroxide.l12 3. Peroxides of Heterocyclic Compounds It has been known 113 for a considerable time that the condensation product of urea and methylglyoxal formed a peroxide on treatment with hydrogen peroxide and ferrous sulphafe. This peroxide underwent the following reactions HO-CMe *NH HO O*CMe*NH i >O HO*O*CH-NH or I >co HO-CH - f i ~ i . 0,WCH :C-NH MeCO NH 1 >.o CO*NH I >O 1 CO*NH MeCO NII Me*CO*NH NHCMeSNH Like open-chain ethers cyclic acetals and ketals will peroxidise in air,114 and peroxides of dioxans and dioxolans have been made in dilute solution for use as polymerisation catalysts.115 The dimerisation product of methyl vinyl ketone (probably 6-acetyl- 2-methyl4 6-dihy'dro-1 4-pyran) forms a peroxide on treatment with hydro- gen peroxide and acetic acid.l16 Peroxides of tetrahydrofuran and its alkyl-substituted homologues are formed by low-temperature autoxidation (25-40") ; 117 peroxides of furan 112 Inter alii C. S. Marvel and V. E. Nichols J . Amer. Chem. SOC. 1938 60 1455 ; J. Org. Chem. 1941 6 296. lI3L. Seekles Rec. Trav. chim. 1927 46 77. lx4M. Kuhn J. pr. Chem. 1940 156 103. 116 B.P. 586,146 (Usines de Melle). 116K. Alder H. Offermanns and E. Ruden Ber.1941 74 905. 117 (a) B.P. 532,158 614,392 (Usines de Melle) ; (b) A. Robertson Nature 1948 162 153. HAWKINS ORGANIC PEROXIDES AND THEIR REACTIONS 27 1 and 2 5-dimethylfuran l18 have been prepared similarly the speed of oxida- tion being increased by illumination or addition of catalysts (CaCl, FeSO, NiSO, MnSO,). Recently it has been found that tetrahydrocarbazole and its homologues yield peroxides on autoxidation ; 119 these are probably of the type shown inset. y-Butyrolactone may be prepared by direct oxidation of tetra- hydrofuran in the presence of catalysts l20 and it is also a product of the decomposition of tetrahydrofuryl hydroperoxide.lf7& On the other hand it has also been claimed l15 117a that this hydroperoxide and its alkyl homo- lopes are decomposed by acid or alkali to afford a y-aldol or y-ketol and hydrogen peroxide.Presumably butyrolactone is formed as the result of further reaction of the hydrogen peroxide with the hydroxybutanal followed by dehydration of the hydroxybutyric acid & Fury1 peroxide resinifies on storage and reduction over palladium- barium sulphate gives 30-40% of succindialdehyde. 2 5-Dimethylfuryl peroxide is similarly reduced to hexane-2 5-di0ne.l~~ The autoxidation products of 2-methylfuran and tetraphenylfuran have been studied in a similar way.121 Autoxidised 2 5-dimethylfuran when cooled to - 80" deposits crystals of hex-3-ene-2 5-dione and when the speed of oxidation is increased by use of one of the catalysts mentioned above the yield of diketone is increased to 50 yo. In general 22 118 G. 0. Schenck Naturwiss. 1943 31 387 ; Ber. 1944 77 661. ll9 R. J. S. Beer L. McGrath A. Robertson and A. €3. Woodier Nadure 1949 164 l Z o B.P. 608,530 (I.C.I.). 121G. 0. Schenck Angew. Chem. 1948 60 244. l Z 2 Idem C'hern. Ber. 1947 80 289. 363.

ISSN:0009-2681    

DOI:10.1039/QR9500400251

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

STRUCTURE AND ACTIVITY IN SYNTHETIC INSECTICIDES By W. A. SEXTON B.Sc. PH.D. F.R.I.C. (RESEARCH LABORATORIES IMTERIAL CHEMICAL INDUSTRIES LIMITED DYESTUFFS DIVISION BLACKLEY MANCHESTER) General Considerations.-In considering relations between chemical constitution and biological activity it is often difficult to correlate structure with effect when the measured biological response is the death of the organism. Death may result from many causes and the action of the poison may be indirect with death ultimately resulting from the interruption of a complex dynamic system of delicately balanced biochemical reactions. Such difficulties increase with the complexity of the organism and insects are much more complex than micro-organisms such as bacteria or protozoa. It is therefore not yet possible to correlate constitution and activity throughout the whole field of insecticides where the chemical types vary from inorganic arsenic to complex molecules of diverse type such as rotenone the pyrethrins the chlorinated hydrocarbons and the new phosphoric acid derivatives.A further complicating factor is the effect of the mode of presentation on the toxicity of the insecticide. Insecticides may be pre- sented as fumigants contact poisons or stomach poisons. Contact poisons are absorbed by passage through the insect cuticle and sometimes also through the spiracles. Stomach poisons are so-called because they are eaten. A stomach poison may have relatively little activity as a contact insecticide. Furthermore with contact insecticides for example the physical form may have a very great effect on the toxicity; factors such as the use and nature of a solvent or emulsifying agent and the particle size of the dispersion greatly influence the result.There is also the question of insect specificity. These factors often render difficult the comparison of results obtained by different workers. In spite of the diEculties outlined above there are however certain general principles which have emerged particularly in relation to contact insecticides and these will be briefly discussed before proceeding to the treatment of the different chemical types. It is a first essential of insect poisons as indeed of all substances possessing biological activity that they must be capable of absorption and translocation to a point a t which the chemical reactions responsible for the toxic response may be initiated.(It should be said at this point that consideration is being given to genuine toxic effects ruling out what might be termed the mechanical killing of insects such as might occur by “ drowning ” through blockage of the spiracles.) Physicochemical properties may therefore play a dominant rGle and there may be a direct quantitative relation between toxicity and some physical constant. In such cases there appears what J. Ferguson 1 has classified as “ physical ” toxicity the outstanding charac- teristics of which in contact and fumigant insecticides are (i) the peak activity Proc. Roy. SOC. B 1939 127 387. 272 SEXTON STRUCTURE AND ACTIVITY IN INSECTICIDES 273 as an homologous series is ascended and (ii) the equal molar toxicity of compounds of different chemical classes provided that they have no very pronounced chemical reactivity.This may be illustrated by (a) the approximately equal molar toxicities to the grain weevil of vapours of toluene ethylene chloride and methyl alcohol and ( b ) the approximately equal contact insecticidal action on blowfiies of 2-hexylthiobenzthiazole ethyl phenylmethylcarbamate and dimethylaminoacetonitrile in each chemical class physical properties play an important rGle. Where physical mechanisms predominate two consequences follow. The first is that the toxicity may be greatly affected by the presence of a second organic sub- stance which may be an insecticidally inert solvent. This may either depress the toxicity or enhance it and the latter effect is commonly referred to as synergy and the substances producing it are called synergists.This will be referred to again later. The second consequence is that the toxicity may be highly specific-either specific to insects as distinct from other biological types or even affecting some insects and not others ; for Perguson has pointed out that whereas in " physical " toxicants the chemical poten- tials (considering a particular biological response) of individual molecules lie very close together the order of magnitude of the chemical potential is a characteristic of the organism. Specificity of course may result from other causes. Metabolite antagonism may be one of these and it is possible that phenomena analogous to those encountered in the study of inhibition of bacterial growth may ultimately be shown to be concerned with the action of insecticides.It has been postulated as a working hypothesis that in addition to the physical criteria mentioned above a biologically active compound e.g. an insecticide must be capable of combining with a vital cell constituent .2 The union may be a firm one involving covalent bonds and having only a limited degree of reversibility. At the other extreme it may be a very loose union such as that due to hydrogen bonding easily reversible and yet capable of disturbing the delicate balance of the normal biochemical reactions (e.g. certain narcotics and other " physical " toxicants). There is a whole range of unions of varying strength between these two extremes and it is necessary to include the unions of macrornolecular species through a multiplicity of points of attachment by van der Waals forces electrodent links hydrogen bonds etc.which through stereochemical " fit " may add up to a considerable force. The two factors physical and chemical will be interdependent. Any particular molecular alteration may affect both the physical properties and the capacity for combination with all con- stituents and such dual effects may be either mutually supporting or antagonistic so far as the final toxic effect is concerned. The nature of the cell constituent or constituents with which the insecticide combines is not likely to be the same for all types of insecticides and its elucidation in individual instances can come only as the result of much more detailed biochemical investigations than have been attempted heretofore. * W. A. Sexton SOC. Exp. Biol.Symposium 111 1949 p. 1. 274 QUARTERLY REVIEWS P. Lauger H. Martin and P. Muller have put forward the general view that the activity of contact insecticides is associated with a grouping which is responsible for the toxic effect and a grouping which confers solubility in lipoids and they have analysed the structures of a wide variety of contact insecticides from this standpoint. Although this theory is probably basically sound it appears to the Reviewer to be an over-simplification and to require modification (Ref. 4 p. 320). The view that one group in the molecule more than any other may be responsible for the particular chemical reaction within the organism which is the prime cause of its death may well be valid in quite a number of instances. This is the concept of the toxiphoric group.It is possible however that there may be other groups within the molecule which can enter into significant unions with cell constituents and that such unions hinder the access of the molecule to the site a t which the toxi- phoric group would operate. Furthermore the chemical reactivity of the toxiphoric group may be modified by molecular variation at other points. Thus although a series of insecticidal compounds may be seen to possess a common toxiphoric group such as an unsaturated carbonyl system as exemplified by the Swiss workers it does not follow that all compounds containing this group will be insecticidal. With regard to the conception of the lipophilic group it is the lipophilic properties of the molecule as a whole which are relevant. These may on occasion be greatly modified by the introduction of a specific group as for example the introduction of a long fatty chain in an alkyl thiocyanate but it is not always possible to point to a specific group as being mainly responsible for fat solubility a case in point being D.D.T.It appears to the Reviewer that on present knowledge one could say of contact insecticides that they must contain a toxiphoric group which has rather a precise degree of chemical reactivity enabling them to combine with a particular constituent of the insect thereby disturbing the normal biochemical sequence or initiating new processes which result in death. If their reactivity is too great they may become immobilised by some reaction having no serious consequences for the insect before they can reach a vital centre.If they contain more than one such group it is possible though not of course certain that the rival attractions of different receptor points within the organism may result in a diminished affinity for either (Ref. 4 p. 95). The molecule must have a degree of lipoid solubility enabling it to penetrate the insect cuticle and possibly to pass other physical barriers before reaching the site of action. Although this article is primarily concerned with synthetic insecticides some discussion of two classes of natural products the pyrethrins and the nicotine alkaloids is included for two reasons. First although some of the compounds in these classes are of purely natural origin quite a number have actually been swthesised. Secondly when considering the mode of action of insecticides from the chemical standpoint one must not differentiate Helv.Chim. Acta 1944 27 892. W. A. Sexton " Chemical Constitution and Biological A4ctivity " E. & F. N. Spon London 1949. SEXTON STRUCTURE AND ACTIVITY IN INSECTICIDES 275 between natural and synthetic compounds. Certain points which are brought out in connection with the pyrethrins and the nicotine alkaloids will be seen to have a general bearing on the relation between constitution and biological activity of synthetic insecticides. Synthetic Analogues of Some Natural Products.-The natural pyrethrins are contact poisons which exhibit a characteristic paralysant action on insects. They consist of mixtures containing esters of chrysanthemum- carboxylic acid (I) and of the monomethyl ester (11) of chrysanthemumdi- carboxylic acid with the keto-alcoholic pyrethrolones and cinerolones.The pyrethrolones are stereoisomeric forms of (I11 ; R = CH,*CH:CH*CH:CH,) CH *CO H / \ Me,C-CH - CH CMe-C0,Me Co2H Me,C-CH.CH CMe (1.) (n.) and the cinerolones are stereoisomeric forms of (I11 ; R = CH,*CH:CH*CH,).S In these as in other complex molecules it is not easy to decide which part of the molecule is the most reactive and which is therefore to be considered in relation to combination with cell constituents. Lauger Martin and Muller have suggested that it is the cyczopropane ring since it behaves like an unsaturated structure conjugated with the ester-carbonyl group. If however the pyrethrins are subjected to catalytic hydrogenation it is C Me H,C'%C. C,H , I t H,C-CO C M e HO.HC~\'CR I ' H,C-cO (mS (m not the cycbpropane ring which is attacked for H.L. Haller and F. B. LaForge 6 obtained 3-methyl-2-n-amylcycZopent-2-en-l-one (IV) together with dihydro-derivatives of (I) and (11) thus hydrogenation splits the ester linkage. This is not surprising since the high reactivity of the system -X*CH,*CH,*CO- where X is a hetero-atom is manifest in the behaviour of such substances as aldols and Mannich bases. It seems likely therefore that this reactive group is responsible for a reaction or reactions with vital cell constituents which partly determine the insecticidal activity. The degree of reactivity may well be critical and this will undoubtedly be modified by the other parts of the molecule particularly by the unsaturation. Tetrahydropyrethrin I in which the C,H side chain has been reduced has much lower insecticidal a~tivity.~ Similarly a synthetic " pyrethrin " made from the natural ( + )-trans-chrysanthemum-monocarboxylic acid and the synthetic compound (I11 ; R = Bu) had low activity.On the other hand compounds having high activity of the characteristic pyrethrin type have been synthesised by using compounds of type (111) in which R may F. B. LaForge and S. B. Soloway J. Amer. Chem. SOC. 1947 69 186. J . Org. Chem. 1937 2 49. F. B. LaForge and W. F. Barthel ibid. 1947 12 199. * L. Crombie M. Elliott S. H. Harper and H. W. B. Reed Nature 1948 162 22. 276 QUARTERLY REVIEWS be CH,*CH,:CH2 CH,*CMe:CH, CH,*CH:CHMe or CH,*CH,*CH:CH,. The compound derived from the first of these substituents and the natural isomeride of chrysanthemum-monocarboxylic acid was considerably more active than natural pyrethrins.Again with (I11 ; R = CH,*CH:CH,) there was no difference in activity between the compounds obtained by esterification with the (&)-cis- and the (&)-trans-acid although both com- pounds were somewhat less active than that derived from the (+)-trans-acid. Stereoisomerism therefore as in the nicotine series plays a definite though not highly significant part. It would appear that the differences in activity amongst these synthetic compounds are a reflection of the part played by the physicochemical properties of the molecule which probably determine its access to the site of the toxic reaction. The synthetic work mentioned here is described in recent papers by M. S. Schlechter et and by W. A. Gersdorf .96 It may be remarked that the system -X*CH,*CH,*CO- is frequently found in natural compounds including some with characteristic biological activities.Cocaine y-pelletierine and lobeline may be exemplified. The idea may be further extended if the principle of vinylogy is included for -X*CH,*CH,*CH:CH*CO- will then be of equivalent reactivity and this is contained in such substances as parasorbic acid (V) kawain (VI; R = -CH:CHPh) and mandarenin (VI ; R = CH,*CH,Ph).-In this way ,!laps-unsaturated &lactones differ from unsaturated y-lactones. (=) (a Another feature of studies of pyrethrin derivatives deserves comment and supports the conception that it is not the cyclopropane ring which gives to the molecule its unique biological properties. Esters of chrysanthemum- carboxylic acid with normal aliphatic Cl%& alcohols are as toxic to aphids as are the pyrethrins.Nevertheless their mode of action is probably quite different for when these compounds are applied to cockroaches they do not produce the muscular paralysis which is the characteristic effect of pyre- thrins.l* Herein lies an illustration of the danger in comparing constitution with a biological response as vague as death. Similar difficulties obscure the consideration of nicotine. Various alkaloids related to nicotine (VII) occur in solanaceous plants and are all powerful contact insecticides. These include both (+)- and (-LJ ' ' M e (y2 \" H @ N' (JnrS (m (x.) 9 (a) M. S. Schlechter N. Green and F. B. LaForge J. Amer. Chem. Soc. 1949 10 E. K. Harvill Contr. Boyce Thompson Inst. 1939 10 143. 71 1517; ( b ) J . Econ.Entom. 1949 42 632. SEXTON STRUCTURE AND ACTIVITY M INSECTICIDES 277 (-)-forms of nornicotine (VIII) and (-)-anabasine (IX) the optically inactive form of which (neonicotine) was first obtained by synthesis. There are some differences between the activities of these three substances and their stereoisomers but their toxicities are of the same order of magnitude. The effect of structural variations has been studied by alteration of both rings. In the work of C. H. Richardson and H. H. Shepard,ll nicotyrine (X) and metanicotine (XI) had about one-tenth of the toxicity of nicotine to aphids whereas dihydrometanicotine and 3- 1 '-ethylaminoethylpyridine (XII) had one-hundredth of this activity while 3- 1'-methylaminobutyl- pyridine (XIII) and 2-phenylpyrrolidine were still less active.The bio- logical activity of the thiazole analogue (XIV) of nornicotine resembles that of nicotine thus providing an example of the equivalence of the thiazole and pyridine rings. In determining what part of the molecule may be responsible for com- bination with cell constituents it is of interest to consider some of the chemical reactions of nicotine and particularly (as was done with the pyrethrins) its behaviour on hydrogenation. Reduction of nicotine with sodium and alcohol or catalytically results in fission of the molecule at the point indicated in formula (VII) and formation of hexahydro- and octahydro- metttnicotine. l3 Treatment of nicotine with acylating agents such as acetic anhydride or benzoyl chloride results in a similar rupture.14 This would therefore appear to be the most reactive point of the nicotine molecule and a suitable cell constituent RH having an active hydrogen atom might give rise to a product of structure (XV).If a similar reaction occurred with the relatively inactive compounds (XI) and (XIII) it would lead to loss of ethylamine and methylamine respectively the adduct differing materially from the type (XV) by loss of the side-chain nitrogen atom. In dihydro- metanicotine also of low activity the molecule contains no such reactive point and such insecticidal activity as it possesses may be realised by an entirely different mechanism. In this connection it is of interest that l1 J . Agric. Res. 1930 40 1007. 12 H. Erlenmeyer and R. Marbet Helv. Chim. Acta 1946 29 1946. 13 F. Blau Ber. 1893 26 628 ; W. Windus and C. S. Marvel J.Amer. Chem. Xoc. 1930 52 2543 ; J. Overhoff and J. P. Wibaut Rec. Trav. chim. 1931 50 957. l4 A. Pinner Ber. 1894 27 1053 2961. U 278 QUARTERLY REVIEWS insecticidal activity is shown by alkyl- and benzyl-substituted pyridines and in the alkyl series there is maximum activity at propyl. In metanicotine (XI) the extracyclic double bond is so placed that an additive reaction with a hypothetical cell constituent RH might well result (through the electromeric changes indicated) in addition of the group R as with nicotine at the carbon atom adjacent to the pyridine ring. Nico- tyrine as a pyrrole derivative is highly reactive. Pyrrole derivatives have reactive hydrogen atoms in the a-positions as exemplified by the addition to maleic anhydride to give a-pyrrylsuccinic acids.l5 2-Phenylpyrrole affords a dimeride by linkage at the a(5)-position not bearing the phenyl group.16 Nicotyrine has been insufficiently studied from this aspect and the point at which it could combine with the hypothetical cell constituent RH under physiological conditions can hardly be foreseen. In the group of compounds under consideration physicochemical proper- ties undoubtedly make a n important contribution to the insecticidal effect. This might account for the large difference between nicotine and Z-phenyl- pyrrolidine the removal of a basic centre being critical. (Compare the well- known effects of the basic side chains in synthetic antimalarial drugs.) On the other hand there may well be in addition significant differences in the chemical reactivity of these two compounds.The basic strength is probably a significant factor in regulating the degree of activity in this series for L. C. Craig 1' found a relation between the dissociation constants of a series of pyrrolidine derivatives and their insecticidal activity the weaker bases being the most toxic. This may be concerned with membrane permeability. Organic Sulphur Compounds,-The insecticidal properties of the normal aliphatic thiocyanates axe of considerable importance from both the theoretical and the practical viewpoint. The lower members have fumigant but little contact action and as the series is ascended maximum contact activity is shown between C8 and C14 and usually at C, or C12. The dominant feature of the lower aliphatic thiocyanates is the high chemical reactivity of the -SCN group. It is not surprising to find that they are toxic also to animals to micro-organisms and to green plants.This toxicity has been attributed by W. F. von Oettingen and his collaborators 18 to the liberation of hydrocyanic acid for not only do animals poisoned by the thiocyanates exhibit symptoms of hydrocyanic acid poisoning but the compounds yield this acid when in contact with minced liver. It is con- ceivable that the more reactive lower members are immobilised by reaction with some constituent of the insect cuticle and hence cannot show contact insecticidal action whereas when they are used as fumigants absorption is through the spiracles and there is no such impediment to penetration. The higher members of the series are less reactive and do not show the general toxicity of the lower members.The characteristic reactivity is however not lost but only modified quantitatively and one reaction of thiocyanates l6 0. Diels and K. Alder Anruzlen 1931 486 211. l6 C. F. H. Allen M. R. Gilbert and D. M. Young J. Org. Chem. 1937 2 227. 17 Iowa State Coll. J. 1931 5 327. J . Id. Hyg. TOZiCOl. 1936 18 310- SEXTON STRUCTURE AND AO- IN LNSEC'I'IUlDES 279 which may be significant is their hydrolysis which is catalysed by the acetate of an organic base and proceeds according to the equation 2RSCN + H,O -j. RS*SR + HCN + HOCN It has been suggested (Ref. 4 p. 312) that the basic reaction on which the contact insecticidal activity of the higher fatty thiocyanates depends rnay be initiated by attachment of the -SCN group at one of the numerous salt-like linkages which are probably a feature of native proteins.Such an attachment might well serve to dislocate the balance of the biochemical processes even though it might not result in liberation of hydrocyanic acid at the point of attachment. The normal aliphatic chains of appropriate length probably adjust at the same time the degree of reactivity of the -SCN group and the lipoid solubility which governs absorption through the cuticle. Many molecular variants of dodecyl thiocyanate have been examined and it is interesting to note that it is permissible to interrupt the normal aliphatic chain with oxygen atoms or even with an appropriately placed benzene ring without loss of insecticidal activity. exemplified by C,H s*O*[ CH,] ,= 0. [CH,] ,*SCN and PhO *[ CH,] ,*SCN. This rnay be The aromatic thiocyanates have not been subjected to the same degree of critical investigation as the aliphatic compounds and although there are claims in the patent literature for activity of various aromatic compounds notably those which also contain an amino-group little can be found by way of quantitative data.A further feature of the aromatic thiocyanates is their phytocidal action which obviously restricts their practical utility as insecti- cides. For references to the original literature R. C. Roark and R. L. Busby's review 20 should be consulted. In the isothiocyanate series the lower members e.g. methyl ethyl are active as fumigants but apparently not as contact poisons. There is a con- trast with the thiocyanates however in the higher aliphatic region for dodecyl isothiocyanate has little or no contact activity.More attention has been paid to the aromatic isothiocyanates and although the literature contains insufficient data for the consideration of the effects of constitutional changes a-naphthyl isothiocyanate has been used commercially in fly-sprays. High stomach-poison activity has been shown by reduced thiadiazines of the type (XVI) which are generally referred to in the early literature as carbothialdines 21 They are readily obtained by reaction of a dithiocarbamic (=S (=S acid R'NH*CS,H with formaldehyde and a primary amine NH,R. 2-Thio- 3-phenyl-5-methyltetrahydro-1 3 5-thiadiazine (XVI ; R = Me R' = Ph) l9E. Hoggarth and W. A. Sexton J. 1937 815. 2o '' A List of Organic Sulphur Compounds used as Insecticides " U.S. Dept. Agric. 21 W. H. Davies and W. A. Sexton Biochem.J. 1948 43 461. Bur. Entomology 1935. 280 QUARTERLY REVIEWS is considerably more toxic in a poison bait to locusts than is sodium arsenite. These compounds readily break down to isothiocyanates and this has been suggested as being associated with their insecticidal activity. In other words a molecule of type (XVI) provides a method for presenting an $80- thiocyanate to an insect in what is in effect an altered physical form. The finding of insecticidal activity in both thiocyanates and isothio- cyanates directed attention to other readily accessible compounds containing the 4.C.N- sequence of atoms.21 2-Mercaptobenzthiazole (XVII or XVIII ; R = H) was the starting point since like thiocyanicacid it could be alkylated at either the sulphur or the nitrogen atom.Contact insecticidal activity was found in both 8- and N-alkylated derivatives and it was a characteristic in both series that the highest activity was found in the first three members the activity decreasing thereafter as the molecular weight increased. In other words the physical properties played a dominant r6le. It has already been pointed out that where physical properties play a dominant r6le there is likely to be considerable biological specificity. In conformity with this it was observed that whereas the 8-methyl compound (XVII ; R = Me) was quite highly toxic to blowflies the N-methyl isomer (XVIII ; R = Me) was inactive. Against aphids however there was less difference in susceptibility the tendency being towards greater toxicity with the N-methyl compound. It is of some interest to compare the chemical reactivities of three insecticidal compounds containing the -S*C*N- sequence.As already pointed out the significant reactivity in the thio- cyanates may be associated with the susceptibility to hydrolysis i.e. electron accession to the carbon atom of the toxiphoric group. The thiazole com- pound (XVII ; R = Me) is a weak base that is to say it tends to donate electrons ; reactions requiring electron accession e.g. hydrolytic fission of the thiazole ring require much more drastic conditions. The isomeric compound (XVIII ; R = Me) is neutral though the fact that it gives a quaternary salt with methyl iodide indicates a capacity for electron donation. In those three compounds therefore the reactivities of the supposed toxiphoric groups vary both in nature and degree.Since they are all toxic to aphids there are at least two possible inferences (i) that they act by different mechanisms and (ii) that provided that there is the requisite degree of reactivity in the toxiphoric group its nature may not be important. There is insuflicient evidence to decide between them. Although its main practical use is as an anthelmintic the biological activity of phenothiazine (XIX) was first noted in its toxicity to mosquito larva Later investigations showed that as a stomach poison it equalled lead arsenate in its toxicity to certain leaf-eating insects. It has no action its a contact spray against flies or aphids. In spite of its widespread use as an anthelmintic surprisingly little is known of its mode of action. cQ3 a0p 0t:m (=) (=.) (=S SEXTON STRUCTURE AND ACTMTY M INSECTICIDES 281 Chemically its outstanding characteristic is its susceptibility to oxidation.Aerial oxidation particularly in the presence of traces of iron or copper compounds gives a bright green colour probably a semiquinone form. Neutral or alkaline oxidising agents give the y-basic sulphoxide (XX) which is isomerised by acids to hydroxyphenothiazine the leuco-compound of phenothiazone (XXI). Further oxidation leads finally to the red dye thionol the leuco-compound of which is excreted in the urine of animals which have consumed phenothiazine. The insecticidal activities of pheno- thiazine have therefore been attributed by various workers to its oxidation products. J. W. ZukelyzZ for example suggested that the active agent is a conjugate of leucophenothiazone or leucothionol which may act through inhibition of the important respiratory enzyme cytochrome-oxidase.H. B. Collier 23 has observed the inhibition of various enzymic oxidation- reduction systems by the oxidation products though not by phenothiazine itself. The evidence for the oxidation theory of the mode of action of phenothiazine is scanty and further work is obviously required. It would be informative for example to examine the insecticidal properties of phenothiazine derivatives in which certain critical positions such as the hydrogen atom of the imino-group or the positions para to the imino-group were blocked by methyl. Amongst other organic sulphur compounds various types are mentioned in the papers by Lauger 24 and LBuger Martin and Muller.3 Dichloro- derivatives of diphenyl sulphide sulphoxide and sulphone particularly the 4 4’-compounds are stated to be active as stomach poisons but no quantita- tive data are presented.Possibly activity is associated with the reactivity of one or both chlorine atoms. Certain aromatic sulphuric esters and sulphonamides are also active as stomach poisons while the sulphonic acid group is made use of in various water-soluble textile-mothproofing agents in order to provide affinity for the wool fibre. Chlorinated Hydrocarbons.-Various chlorinated derivatives of the lower aliphatic hydrocarbons have been known for a long time as insecticidal fumigants. Their activity is entirely governed by physical considerations a fact which is brought out by Ferguson,l who quotes data relating to their toxicity to wireworms.These substances include chloroform carbon tetrachloride and trichloroethylene. Substances also included in the same category of “ physical ” toxicants are benzyl chloride chlorobenzene and o-dichlorobenzene as well as chlorine-free substances such as dimethylaniline pyridine and carbon disulphide. The practical significance of these materials is however exceeded by the modern chlorinated hydrocarbon contact insecticides the study of which started with the well-known sub- stance 1 1 l-trichloro-2 2-di-p-chlorophenylethane (D.D.T.) (XXII see Table I). suggested that the activity of D.D.T. was associated with the lipoid-solubilising properties of the CCl group and the toxic effect of the p-chlorophenyl residues. Quite apart from Lauger Martin and Muller 22 J. Econ. Entom.1944 37 796. s3 Canadian J. Res. B 1940 18 345 ; 1942 20 189 284. 44 Helv. Chim. Acta; 1944 22 71, 282 QUARTERLY REVIEWS the general point already made concerning the apportioning of the solubility properties to one substituent group in the molecule it is extremely doubtful whether the Swiss workers were correct in assigning the toxic r6le to the p-chlorophenyl groups. A different viewpoint was early expressed in a suggestion by H. Martin and R. L. Wain 25 that insecticidal activity was associated with the ability of the molecule to lose hydrogen chloride giving the unsaturated substance (XXIII) the hydrogen chloride liberated in situ being responsible for the toxic acfion. Since then a great deal of work has been carried out on compounds analogous to D.D.T. and it is clear that the position is not so simple as was first suggested.It has been pointed out by more than one investigator that the ability to liberate hydrogen chloride does not always run parallel with insecticidal efficacy nor are the compounds with highest lipoid solubility always the most insecticidal. Some of these difficulties are discussed by S. Kirkwood and P. H. Phillips 26 and by J. R. B u s ~ i n e . ~ ~ The results may be reviewed afresh in the light of the hypothesis that activity is due to the correct balance of two different but probably interdependent sets of factors the ability of the molecule to react with a vital cell constituent and the modification of this reactivity or the regulation of transport of the molecule by its physical properties. The contact insecticidal activities of some D.D.T.analogues are given in Table I the assessment of activity being based on the studies of various investigators who used several insect species. Although some of the compounds show marked species selectivity which is itself an indication of the importance of physicochemical factors the results are mainly taken from data obtained on flies 279 28 and they are therefore considered to provide a valid basis for considering the effects of structural variation. In the table " high " signifies an activity not less than that of D.D.T. '' moderate " - A The loss of hydrogen chloride on thermal decomposition or on hydrolysis points to the grouping CH*CCI as being the most reactive chemical centre of the molecule and therefore as the one most likely to constitute the point of attachment to or reaction with the hypothetical vital cell constituent.Table I also contains data obtained by S. J. Cristol 29 for the relative reactivities of certain of the compounds towards alcoholic alkali. The first five compounds contain the grouping CH-CCl and it will be seen that p-chloro-groups are not essential for activity. The differences in activity between the individual members of this group of five are consistent with what is to be expected from the variations of physical properties brought about by the substituent groups. This is further supported by the fact that in the alkoxy- and alkyl-substituted types (XXIV) and (XXV) activity is known to decrease as the homologous series is ascended. At the same time " low ' 1 -__. o and " very low " less than &. 26 Nature 1944 154 612.27 J. Soc. Chem. Ind. 1946 65 3561.. 28 X. Kirkwood and J. R. Dmey Canadian J . Em. B 1946 24 69 ; G. T. Barry and R. Boyer ibid. B 1948 26 511 ; R. C. Browning et aE. ibid. D 1948 26 282 ; R. Domenjoz Helv. CGm. Acta 1946 29 1317. 26 J. Pharmucol. 1946 87 375. 29 J. Amer. Chem. Soc. 1945 67 1494. SEXTON STRUCTURE AND ACTNITY IN INSECTICIDES 283 XXII XXIV XXV XXVI XXIII XXVII XXVIII XXIX xxx XXXI XXXII TABLE I Contact insecticidal activity of anabqaes of D,D.T. Insecticidal activity. High High (D.D.T.) Very low to flies; high to lice and bed-bugs High Moderate Very low very low High Moderate very low Low Moderate to low Low Moderate High Very low (moder- ate to lice) Relative rate constants as for alkaline hydrolysis at 70". 303 2480 3470 9.18 10.9 36.9 567 37.1 284 QUARTERLY REVIEWS it is to be noted that there are some big differences in chemical reactivity for example between (XXII) and (XXIV) which both have high insecticidal activity.The very low toxicity of 1 1 1-trichloro-1 2-diphenylethane (XXVI) is perhaps surprising but need not be excluded from consideration on a physical basis. Removal of one of the aliphatic chlorine atoms (in XXVII) still leaves high activity as the same capacity for chemical reactivity (manifested in the ability to liberate hydrogen chloride) remains though at a somewhat reduced level. The fact that 1 1-di-p-chlorophenylethane (XXVIII) shows moderate activity is difficult to explain as here the charac- teristic chemical reactivity has been completely removed. It is very difficult to assess the significance of a moderate degree of toxicity in an individual compound chosen from a large group because it is always possible that when the structure departs too far from the general pattern the mole- cule may show activity by virtue of some entirely different mechanism.All that can be said about (XXVIII) therefore is that more critical biological experiments are required in order to decide whether it should properly be considered as belonging mechanistically to the D.D.T. class. With the other three compounds in which the characteristic reactivity of the CHCCl system has been modified there is a very low level of activity. These are (XXIII) in which the reactivity of the chlorine atoms will be reduced by the unsaturation (XXIX) where replacement of the remaining aliphatic hydrogen atom eliminates reactivity typified by the ability to lose hydrogen chloride and (XXX) where the reactivity will be reduced by the substitution of fluorine for chlorine.I n these three compounds it must also be borne in mind that the structural alterations leading to reduced chemical reactivity will also affect physical properties which may contribute to the reduced toxicity. The substitution of CCl in D.D.T. by CBr should enhance reactivity rather than reduce it,29 yet the compound is relatively non-toxic. This may be a physical effect. On the other hand if the compound is too reactive it may be immobilised by reaction with some cell constituent before it can reach the site a t which it might initiate a toxic response. Four compounds having only one aromatic nucleus are included in the table and all retain the CH*CCl system.It may be considered significant therefore that one of these compounds (XXXI) shows high activity. It seems that in this molecule there have appeared physical properties which appropriately balance the chemical reactivity. It is clear that neither the lipoid-solubility theory nor the chemical- reactivity theory is sufficient to explain the behaviour of the D.D.T. analogues and that a combination of the two is necessary. What seems to be essential is the combination within the molecule of an appropriate degree of chemical reactivity not too low and not too high with the correct physical properties. In regard to the nature of the physical properties necessary to ensure activity it can be said with some degree of certainty that fat solubility is essential if only because of the necessity of penetrating the lipo-protein layers in the insect cuticle.Once this penetration is accomplished however there may be other factors which are significant and one of these may be stereochemical being concerned as suggested by Busvine 27 with association SEXTON STRUCTURE AND ACTIVITY IN INSECTICIDES 285 with some enzyme or other macromolecule. In this connection it is inter- esting to note 30 that flies which have through breeding become resistant to D.D.T. are also resistant to the stereochemically similar methoxy-analogue (XXIV) but not to chlorinated hydrocarbons of different molecular pattern such as y-benzene hexachloride (hexachlorocyclohexane) “ Chlordane ” or “ Toxaphene ”. The low activity of (XXVI) may perhaps in this sense be partly associated with the lack of p-substituents as also may be the relative inactivity of the D.D.T.isomer (XXXII) although both these substances as well as the highly insecticidal methoxy-compound (XXIV) have reduced reactivity compared with D.D.T. Since the commercial introduction of D.D.T. as an insecticide several other chlorinated hydrocarbons of comparable potency have been discovered. The first of these was obtained by the addition of chlorine to benzene and of the mixture of stereoisomers obtained the y-isomer alone was of outstanding effectiveness .31 This isomer was believed to have the same configuration as mesoinositol and it was suggested that the insecticide might owe its activity to interference with some vital reaction involving mesoinositol.Antagonism was indeed demonstrated by using certain micro-organi~ms,3~ but inositol did not have any effect on the toxicity of y-benzene hexachloride to greenhouse thrips .z3 Moreover X-ray investigations have recently shown that y-benzene hexachloride is not isomorphous with mesoinositol,3* and it would appear that the evidence for the significance of mesoinositol in the insecticidal action of y-benzene hexachloride is not substantial. The evi- dence quoted above on the susceptibility of D.D.T.-resistant flies to y-benzene hexachloride suggests that its mode of action may be different from that of D.D.T. By analogy it is well known in the field of trypanosomiasis that drug resistance acquired as a result of exposure of the organisms to an individual trypanocide is exhibited against trypanocides of the same chemical class but not against trypanocides of different types.This is generally considered to be a manifestation of the existence of more than one toxic mechanism. If a trypanosome has characteristics which render it resistant to arsenicals it is not necessarily resistant to the heterocyclic quaternary salt type which acts on different centres. Although there is no evidence of cross-resistance between D.D.T. and benzene hexachloride it is quite possible that the two compounds share certain though not all the charac- teristics necessary for insecticidal activity. They are alike in having lipoid solubility which probably facilitates passage through the cuticle the primary phase of absorption. They have the same type of chemical reactivity as manifested by their ability to lose hydrogen chloride and hence they have a similar capacity for combination with cell constituents.Their most 30 G. W. Barber and J. B. Schmitt J . Econ. Entom. 1949 42 287 ; J. Keiding and 31 R. E. Slede Chem. and Ind. 1945 314. 32 S. Kirkwood and P. H. Phillips J . Bid. Chem. 1946 163 251 ; H. W. Buston S . E. Jacobs and A. Goldstein Nature 1946 158 22. 33 R. L. Metcalf J . Econ. Entom. 1947 40 522. a4 G. W. van Vloten et al. Nature 1948 162 771. H. van Deurs Nature 1949 163 964. 286 QUARTERLY REVIEWS marked difference is in molecular shape which perhaps determines affinity for separate macromolecules. Much of what has been said about y-benzene hexachloride can be applied to two other products which have high contact insecticidal activity. " Chlordane ",35 obtained by the chlorination of the dimer of cyclopenta- diene may be represented by (XXXIII) the stereochemistry of which has not yet been II CCL I 1 (XXXRE) investigated." Toxaphene ",36 a chlorinated terpene derivative shares with " Chlordane " y-benzene hexachloride and D.D.T. the property of lipoid solubility and the ability to liberate hydrogen chloride on hydrolysis. Organic Derivatives of Phosphoric Acid,-Although some attention had been paid by Swiss workers 3 to organic phosphorus compounds in connection with the mothproofing of textiles the most important observations on their general insecticidal activity were made in Germany during the war. Detailed accounts of this work including preparative methods are to be found in the reports of allied missions to Germany,37 but it is convenient to summarise here the salient points and to include references to work carried out since the war.The starting point was apparently the strong fumigant activity of methanesulphonyl fluoride and systematic alteration of this molecule first by replacing the sulphur atom by other groupings led to the discovery that fluorobisdimethylaminophosphine oxide (XXXIV) and ethyl O-dimet hyl- amino O-acetoxyphosphonite (XXXV) had marked toxicity to aphids. The latter compound moreover had the property' of being absorbed by plants CL H ~ ~ c ~ ~ ~ ~ - C HCL cLc 1 fc ,CHCL EL A CH /OEt (Me,N) ,PO*F Me,WPO \OAC (XXXIV.) (XXXV.) and translocated thus rendering other parts of the plant toxic to insects. In other words it acted systemically as does selenium which can be absorbed by plants from the soil.Further research was directed towards obtaining the maximum insecticidal effect and reducing to a minimum the mammalian toxicity while at the same time improving the stability to water particularly in the presence of lime. The use of pyrophosphoric acid derivatives led to an increase in activity and bisdimethylaminophosphonous anhydride (XXXVI) proved to be very useful as a plant systemic (" chemo- therapeutic ") insecticide (see ref. 38 for details of tests recently conducted (XXXVI.) (XXXVII. ) (Me,N),PO*O*PO (NMe) (EtO),PO-0 *PO (OEt ) Qs C. W. Kearns L. Ingle and R. L. Metcalf J . Econ. Entom. 1946 38 661. 86 L. A. Steams et al. &id. 1947 40 79. 87 B.I.O.S. Final Reports 714 1095 1808. 88 W. E. Ripper R. M. Greenslade and L. A. Lickerish Nutare 1949 168 787.SEXTON STRUCTURE AND ACTIVITY IN INSECTICIDES 287 in England). Another very active contact insecticide was the so-called hexaethyl tetraphosphate a mixture in which the active ingredient is prob- ably tetraethyl pyrophosphate (XXXVII) (cf. J. W. Hansen 39). This compound also has systemic properties,40 as also has the compound (XL) referred to later.41 As a device to improve the stability to lime the group PS was substituted for PO both in the pyrophosphoric series mentioned above and in the phenol derivatives described below. The view has been expressed by G. Schrader 3' that the insecticidal activity is associated with an anhydride structure based on phosphoric acid the second molecule being either phosphoric acid or another acid such as hydrogen fluoride methanesulphonic or acetic acid.Marked activity is found where the second molecule is an enolic substance such as ethyl aceto- acetate [giving diethyl 1-carbethoxyprop-l-en-2-yl phosphate (XXXVIII)] and particularly when p-nitrophenol is employed. p-Nitrophenyl diethyl phosphate (XXXIX) was known as " E.600 " and the corresponding thio- phosphate (XL) as " E.605 ". The latter has been developed commercially (E tO ),PO *O *CMe :CH*CO,Et (EtO),PO*O@ - (XXXVIII.) (XXXIX.) (EtO),PS*O@ (XL.) both in England and America (where the name " Parathion " has. become attached to it). For a synthesis see J. H. Fletcher et aZ.42 Table 11 taken TABLE I1 Toxicity to aphids of ~ o m e phosphoric acid derivatives Concn. %. 0-2 0.005 0.005 0.2 0.2 0.05 0.2 0.05 0.02 0.2 0.05 0.05 0.2 0.001 Kill %. i0-8( 100 100 70 100 50 100 90 50 95 90 100 50 100 - EtO*PMeO-O*C,H,.. . E tO.PMeO *O*C,H,Me*NO a -P(-O) Et0*PMe0*0*C,H4C1-o . Concn. %. 0.2 0.05 0.005 0.05 0.005 0-05 0.2 0.05 0.005 0.05 0.02 ~ Kill %. 80 50 100 100 80 100 100 95 0 1 00 50 - - 39 J . Econ. Entom. 1947 40 600. 4o P. W.IZimmerman and A. Hartzell. 41 D. D. Questel and R. V. Connin J . Econ. Entom. 1947 40 914. Contr. Boyce Phomp8on Inst. 1947 15 11. J . Arner. Chm. Soc. 1948 70 3943. 288 QUARTERLY REVIEWS from data provided in ref. 37 illustrates some of the effects of structural variation on insecticidal activity. A polar influence of the substituents in the benzene ring is discernible for example in the contrast between the effect of m- versus o- and p-nitro-groups in the first four compounds. Sub- stitution of a dimethylamino- for an ethoxy-group evidently reduces the activity considerably but replacement of one ethoxy-group by methyl or of PO by PS does not materially affect the activity.For a proper assess- ment of the significance of structural variation however it would be necessary to examine more compounds and to secure more accurate biological data. The phosphoric ester insecticides appear to act on the nervous system and they share with the very poisonous diisopropyl fluorophosphonate the capacity for irreversible inhibition of the important enzyme cholinesterase which is intimately associated with the mechanism of nerve-impulse transmi~sion.~3~ 44 Miscellaneous Chemical Types.-Before the discovery of the chlorinated hydrocarbon and phosphoric ester types of contact insecticides a very wide search was made amongst organic compounds both naturally occurring and spthetic for substances which might replace existing insecticides.The latter had certain inherent disadvantages. The arsenic compounds and nicotine alkaloids were poisonous to mammals while the derris products and pyrethrins required a biological test for standardisation of quality. Many useful compounds were discovered some of which have already been described in the section dealing with organic sulphur compounds. Certain others will be mentioned here but first a few words must be said on the phenomenon generally known as synergy. It frequently happens that when two substances are employed in admix- ture in a contact insecticidal spray the toxic effect is greater than the sum of the contribution to be expected from the individuals.This has shown up particularly where one of the components is pyrethrin and the literature contains many examples of so-called synergists for pyrethrins. There are two possible explanations. The first is physical postulating an increase in permeability of the insect cuticle caused by the presence of the synergist which thus renders a greater proportion of the toxic material accessible to the body of the insect. This has been illustrated experimentally by H. Hurst,*5 who showed that blowfly larva were unaffected by either alcohol or kerosene but were rapidly killed by immersion in a mixture of the two solvents. The kerosene assisted penetration by the alcohol. This was confirmed and extended by V. B. Wigglesworth 46 to other pairs of solvents. The latter author found that penetration of pyrethrins was assisted by oleic and other fatty acids.Thus by this physical mechanism the effective activity of a contact insecticide can be improved by admixture with a second substance which is itself insecticidally inert. The second possible explana- 43 K. P. Dubois and G. H. Mangun Proc. SOC. Exp. Biol. Med. 1947 64 137. 4 4 K. B. Augustinson and D. Nachmansohn J. Biol. Chem. 1949 179 543. cs Trans. Paraday Soc. 1943 39 390. 48 Nature 1941 147 116 ; Bull. Entom. Res. 1942 33 205, SEXTON STRUCTURE AND ACTMTY M INSECTICIDES 289 tion is that if two insecticides having different modes of action are used in admixture insects which are only partly affected by the one may succumb to the effects of relatively small doses of the other. In this way a synergic effect may be apparent.The characteristic response of an insect to pyre- thrins is rapid paralysis (" knock-down ") from which the insect may recover if the dose is not too great. Most of the synergists developed for use with pyrethrins do not show this characteristic effect though they may have toxicity themselves. It is possible that if an insecticide mixture contains just sufficient pyrethrins to paralyse the flies the affected flies become more susceptible to the toxic action of the second component. One of the earliest of the commercial synergists for pyrethrins (for use in flg-sprays) was N-isobutylundecenamide (XLI). 47 It was later shown CH :CH*[CH,] G O *NHBuf (XLI.) CH,*CH :CHCH :CH*CH,*CH,*CH :CH*CO*NHBu* (XLII.) CH,*[CH,],*CH:CH*[CH,],*CH:CH*CO*NHBu~ (XLIII.) that certain related higher aliphatic amides having insecticidal properties similar to those of the ppethrins occur in plants.Thus the substance affinin (XLII) is obtained from the roots of a Mexican plant originally characterised as Erigeron dfinis D.C. but later defined as Heliopsis Zongipes (A. Gray) Blake.48 Related to this is herculin (XLIII) obtained from the bark of the Southern prickly ash ZanthoxyEum clava-hercuZis L. and also certain other naturally occurring compounds referred to by M. Jacobson.49 Affinin and herculin differ from N-isobutylundecenamide in that they have marked paralysant and toxic properties by themselves whereas the last substance is useful mainly as a synergist for pyrethrum.50 It is possible that the unique properties of the two natural products are associated with a special feature of their chemical reactivity such as might be provided by the unsaturation occurring G$ to the carbonyl group and the point calls for further investigation.Another amide (XLIV) having synergic properties with pyrethrum against houseflies has recently been developed commercially in America.51 Sesamin (XLV) the active principle of oil of sesame has little insecticidal action by itself but .CH,.CHEt.[CH,]; Me is a powerful synergist for pyrethrum. This co activity is apparently governed more by the (-) nature of the substituents in the benzene ring than by molecular shape for various stereoisomers of sesamin are equally effective while replacement of the methylenedioxy-group by two 47 A. Weed Soap 1935 14 No. 6 133. 4* F. Acree 3%. Jacobson and H.L. Haller J. Org. Chem. 1945 10 236 449 ; J . Amr. Chem. Soc. 1948 70 4234. 1947 12 731. A. Hartzell and H. I. Scudder J . Econ. Entom. 1942 35 428. 6 1 A. Hartzell Contr. Boyce Thompson Inst. 1949 15 337. 290 QUARTERLY REVIEWS methoxy-groups destroys the activity. 52 Piperine (XLVI) differs from sesamin in that it is markedly toxic to houseflies by itself besides showing synergy with The results with these two compounds have stimulated a wide investigation of compounds containing the methylene- dioxy-group many of which proved to be insecticidal either alone or by synergy with pyrethrum. Outstanding compounds appear to be (XLVII) and (XLVIII) which are powerful synergists.53 54 CO..Et Nitriles of a-amino-acids have been studied by A. D. Ainley and W. A. Sexton,55 who found in this class several substances having high contact insecticidal activity.There was marked specificity of action against different insects and the insecticidal properties were very largely governed PhMeN*CHR*CN (XLIX.) [-CH,*NMa*CHR*CN] (L.1 by the physical properties of the molecule. Among the most toxic to aphids were (XLIX) and (L ; R = n-hexyl) the latter being at least as active as n-dodecyl thiocyanate. Although these compounds can give rise to hydrogen cyanide under certain conditions of hydrolysis this is not believed to be the mode of action. Rather it has been suggested that they may react with a cell constituent bearing some such group as amino by addition at the cyano-group or by displacement of the substituted or-amino- group. The insecticidal properties of certain fluorine compounds were discovered in Germany during the war.37 Methanesulphonyl fluoride MeS0,F was found to have fumigant activity equivalent to that of ethylene oxide but raising the molecular weight in the aliphatic series or the use of aryl groups in place of methyl lowered the toxicity.Low activity was found also in aryl esters of fluorosulphonic acid F*SO,*OAr. The most interesting sub- stances emerged from a systematic evaluation of derivatives of 2-fluoroethyl alcohol. Certain esters and urethanes derived from this alcohol were very 6sE. 15. Harvill A. Hartzell and J. M. Arthur Contr. Boyce Thompson Inst. 1943 13 87. 6rM. E. Synerholm and A. Hartzell ibid. 1945 13 433; M. E. Synerholm A. Hartzell and J. M. Arthur ibid. 1945 14 7 9 ; H. Wachs Science 1947 105 531; H. 0. Schroeder H.A. Jones and A. W. Lindquist J. Econ. Entom. 1948 41 890. H. L. Haller F. B. LaForge and W. N. Sullivan J . Org. Chem. 1942 7 185. 66 Biocltem. J. 1948 43 468. SEXTON STRUCTURE AND ACTIVITY IN INSECTICIDES 291 toxic to aphids on spraying amongst the most active being the acetals (LI) and (LII). Not only were they very toxic to aphids but (LI) showed to a remarkable degree the property of being absorbed by the plant and trans- located thereby rendering all parts of the plant poisonous to insect pests. CH2 (O*C,H,F) 2 CH2(0 *C2H,*0 *C,H,F) (LI4 (LII.) There is nothing known concerning the mode of action of these substances as insecticides. B. C. Saunders 56 has discussed the relations between structure and mammalian toxicity in related fluorine derivatives and two points are worthy of note.First that in the most toxic compounds the fluorine atom is firmly bound and secondly that toxicity seems to be associated with the capacity for oxidation to fluoroacetic acid. The insecticidal and ovicidal properties of dinitro-o-cresol (LIII ; R = Me) have been known for over twenty years and this substance has had extensive practical use. More recently higher homologues have been investigated and the compound in which R = cycbhexyl has proved to be not only a more powerful insecticide than the cresol derivative but also apparently less toxic to animals and less phyt~cidal.~~ 58 J. F. Kagy 58 examined the toxicity to silkworm larvze (stomach-poison test) of the series in which R varied from C to C, including both n-alkyl groups and cyclopentyl and cyclohexyl groups in the n-series toxicity increased to a maximum at C or C, the C compound falling back to the level of the C compound.Although the cycbhexyl was less toxic than the n-hexyl compound it was still much more toxic than the methyl compound. The influence of physical properties is clear. No study has been made of the chemical mode of action of these dinitrophenols but it is possible that they have a mechanism related to that whereby 2 4-dinitrophenol exerts some of its biological effects though the latter is a less potent insecticide than dinitro-o-cresol. In this connection it is of interest to note recent evidence 59 that dinitrophenol prevents phosphate uptake in an enzymic oxidation of glutamine and apparently uncouples phosphorylation from oxidation. 66 J. 1949 1279. 67 A. M. Boyce et al. J . Econ. Entom. 1939 32 432. ss .?bid. 1941 34 660. W. F. Loomis and F. Lipmann J . Biol. Chm. 1948 173 807.

ISSN:0009-2681    

DOI:10.1039/QR9500400272

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

By G. M. BURNETT PH.D. (CHEMISTRY DEPARTMENT UNIVERSITY OF BIRMINGHAX) THE application of physicochemical methods to the elucidation of the mechanism of free radical reactions has in the past few years progressed to a point at which it is possible to place the whole problem of radical reactivity on a more fundamental basis. This is most necessary and desirable in order to obtain a better understanding of those reactions in which it is known that free radicals participate. The goal of this type of investigation must be in the long run to establish a relationship between the nature of the molecular architecture of the radical and its reactivity and so far only the fringe of this major problem has been touched. Two general types of radical reaction have now reached the stage of being capable of interpretation in terms of the fundamental rate constants of the reactions.Por oxidation processes involving hydrocarbons it has been established that the general scheme representing the course of reaction is Initiation Production of radicals by + radicals of the 1 type R- or R0,- benzoyl peroxide or R02H + hv or 2R0,H Propagation R- + 0 -j. R0,- Termination RO + R0,- ROg- +RH -+ RO,H+R- R- + R02- + non-radical products R--+R- I In the above scheme RH and R0,H represent the hydrocarbon and its hydroperoxide and R- and R0,- are the radicals. The present situation in this type of reaction i.e. the oxidation of olefin hydrocarbons has recently been reviewed,l so in this discussion the results of these investigations will not be considered in detail. Polymerisation reactions involving olefinic compounds afford a very powerful tool in the study of radical processes.Here the radical nature of the reaction has been established beyond all doubt. The mechanism involves the formation of free radicals which add on to the double bond of the olefin so that the radical is perpetuated this process being repeated until the free radical is finally destroyed usually by reaction with another of its own kind. Recently it has been found possible to evaluate the reaction velocity constants for these polymerisation processes. 2$ 3* * 1 J L. Bolland Quart. Reviews 1949 3 1. 2 G. M. Burnett and H. W. Melville Nature 1945 156 661 ; Proc. Roy. SOC. 1947 P. D. Bartlett and C. G. Swain J . Amer. Ghem. SOC. 1945 67 2273 ; 1946 68 4 C. H. Bamford and M. J. S. Dewar PFOC. Roy. SOC.1948 A 192 309. A 189 456. 2381. 292 BURNETT RADICAL POLYMERISATION REACTIONS 293 I. The Mechanism. of Polymerisation ReactionS* The separate reaction steps which take place in a polymerisation process may be formulated as follows for a general monomer CH,:CXY in which X and Y are substituents one of which is generally hydrogen Initiation Production of free radicals by ZCH :CXY (1) ki (2) I end -CH,*CXY- (3) or CH,:CXY + hv + radicals with free or R- + CH2:CXY Propagation Transfer to monomer Transfer t o polymer Termination MCH,*CXY- + CH,:CXY -+ -CH,*CXY*CH,*CXY- kp (4) -CH,CXY- + CH2:CXY -j. CHSCHXY + -CH:CXY kf ( 5 ) MCH,*CXY- + P -j. P- + mCH,*CHXV xc (6) 2 -CH,*CXY -j. inactive products kt (7) In general reaction-velocity constants are derived only for very low monomer conversions (usually of the order of l%) so the reaction involving transfer to polymer may be neglected for the present.Thus four constants have to be determined in order to characterise uniquely this type of free- radical reaction. The various steps of the reaction have now to be discussed in greater detail. P. J. Flory first postulated the bimolecular nature of all steps of the overall process ; thus the initiation reaction in purely thermal polymerisation must be by the interaction of two monomer radicals. This may give rise to either a single molecule possessing two unpaired electrons or two molecules each of which have a single free electron. Styrene being taken as an example this may be formulated as 2PhCH :CH + -CH,*CHPh*CHPh*CH,-- or 2PkCH:CH -+ Ph*CH2*CH2- + PkCH:CH- It is now considered most likely that the initiation step involves the forma- tion of a diradical rather than of two monoradicals.The evidence which most favours this postulation is derived from the inhibition of the polper- isation of styrene by p-benzoquinone. From this reaction an adduct whose molecular weight agrees with the addition of two molecules of styrene to one of quinone has been isolated.'? * It is thought that such an addition falls into the Diels-Alder type of reaction i.e. 0 Trans. Faraday Xoc. 1944 40 1. W. Kern and K. Fueurstein J . p. Chern. 1941 158 186. H. W. Melville and W. F. Watson Trans. Faruday SOC. 1948 44 886. 13 J . Amer. Chena. Soc. 1937 59 241. * Tho terminology adopted in this review is based on that of G. Gee and H. W. More recently tihere The corresponding nomen- ; Melville has been a tendency to employ numerical subscripts.clature of other workers is k for ki ; 4 k for kp ; 8 k6 for kf.8 and used in all subsequent work by Melville's school. k for either kf 6 or kt ; 8 k for kt X 294 QUARTERLY REVIEWS In photochemical initiation the absorption of a quantum of light energies raises the energy level of the n electrons of the double bond to such an extent as to create a free diradical. Although this will be true of almost all monomers studied exceptions can arise. showed that in the case of methyl vinyl ketone a second initiation process played a part in the reaction. This in essence was the normal photolysis of a ketone The third method of initiating polymerisation is by the use of a catalyst such as peroxides and certain azo-compounds or photosensitisers.In each case the compound first breaks down to give free radicals which are capable of adding on to the monomer molecule in order to give free-radical chain carriers. The propagation step is straightforward and requires no further explana- tion. In the transfer reaction shown in the above scheme the radical is assumed to abstract a hydrogen atom from the monomer which then acquires free-radical characteristics. There is however a certain duality in this reaction since it is possible that the hydrogen atom is donated by the radical to the monomer with a similar result i.e. It is thought that this may be the case in the polymerisation of styrene,lO and has been definitely shown to be true in the dimerisation of methallyl chloride in which a chlorine atom is donated by the monomer to the dimeric radical 11 T.T. Jones and H. W. Melville CH,*CO*CH:CH + CHS- + CH2:CH- + CO MCHgeCXY- + CH2:CXY -+ -CH:CXY + CH,*CXIY- CH,C1 CH3 CH,Cl I I 1 I I f I I I CH2CI CH,CI CH,Cl CH,Cl CH CH3 CH,*C*CH,-&*CH,Cl + CH :C*CH,CI -+ CH,*C*CH,-C:CH + CH,*G-. Although it is agreed that the termination does involve the simultaneous destruction of two active centres there is considerable controversy over the exact chemical nature of this reaction. The two radicals may lose their activity either by direct combination viz. or by disproportionation Although these two processes cannot be distinguished kinetically it is obvious that the precise nature of this step will have a profound effect on the molecular weight of the product ultimately formed.The evidence on either side is somewhat inconclusive although several significant points emerge from the data available. For combination it has been established that in the polymerisation of methyl methacrylate catalysed by hydroxyl radicals (derived from hydrogen peroxide in a redox system) the resulting lo C. H. Bamford Internation Colloquium on Macromolecules Amsterdam 1949. l1 F. R. Mayo K. E. Wilzbach and R. Van Meter J . Amer. Chem. SOC. 1948 hc*CH,*CXY- + -YXC*CH,M + MCH,*CXY*CXY*CHF MCH~*CXY- + -YXC*CHv + N.CH:CXY +-CH,*CHXY Proc. Roy. Soc. 1946 A 187 19. 70 4069. BURNETT RADICAL POLYMERISATION REACTIONS 295 polymer contains two hydroxyl groups per molecule.12 From this result it has been assumed that the growing radicals are terminated by a com- bination reaction but the introduction of a second hydroxyl group might arise from a transfer reaction with either hydrogen peroxide or water (used as a solvent) or by the termination of a growing chain by a hydroxyl radical.Many radicals are known to combine almost exclusively e.g. MeO*C,H4*QK.CH,*CH3 l 3 and p-xylyl,l4 although the latter disproportion- ates in the gas phase. C. C. Price 15 has pointed out that radicals of the type (CH,),C*CO,R combine exclusively and argues by analogy that polymethyl methacrylate radicals having a very similar structure ought to do likewise. In this type of work however argument by analogy is rather dangerous since many large radicals have been shown to disproportionate to a very great extent e.g. n-octyl radicals.16 In polymerisation reactions carried out under conditions in which the initial step is the formation of diradicals Le.thermal and photochemical initiation it can be shown that the molecular weight of the final polymer will be dependent of the rate of reaction if combination occurs and will be inversely proportional to the rate when dismutation is the terminating pr0cess.1~ This can readily be shown to be true in theory but thus far no such independence of molecular weight of rate of reaction has been established. Using the viscosity tech- nique Bamford and Dewar 4 have demonstrated that if there is dispropor- tionation the rate of change of viscosity will be dependent on I(l-a)/% whereas for combination the rate will be proportional to 1112 when the intensity of illumination I is high. They have not so far found the latter condition to be true.Further J. Weiss 18 has shown that for radicals which possess an appreciable dipole moment there is a greater tendency for the radicals to disproportionate. Since most polymer radicals do have a dipole moment the more conclusive evidence appears to be in favour of disproportionation. As was pointed out earlier however there is no certainty that diradicals are formed in either thermal or photochemical initiation so the evidence is by no means above criticism despite the fact that the bulk of experimental data does point to diradical initiation. A possible way of removing this impasse might be by the use of photosensitisers which give free diradicals as a result of photolysis. Such a photosensitiser cyclohexanone has already been shown to be effective in promoting the polymerisation of vinyl chloride.19 A photosensitiser of this type would effectively remove all ambiguity with regard to the exact nature of the initiation process and so the viscosity method or a direct measurement of the molecular weight as a function of rate could be readily applied with the object of obtaining a unique result.. l2 J. H. Baxendale S. Bywater and M. G. Evans J. Polymer Sci. 1946 1 237. l3 M. S. Kharasch H. C. McBay and W. Hi. Urry J. Org. Chem. 1945 10 401. l6 C. H. Bamford and R. G. W. Norrish J. 1938 1531. l7 See e.g. C. H. Bamford and M. J. S. Dewar Proc. Roy. SOC. 1949 A 197 356. lS L. Valentine Ph.D. Thesis Aberdeen 1949. l4 M. Swarc Faraday SOC. Discussions 1947 2 138. 16 Ibid. p. 402. Faraduy Xoc. D&cu98iom 1947 2 104. 296 QUARTERLY REVIEWS II.Rate Constants for the Polymerisation of Single Monomers The methods which have been employed in these investigations have been the technique of intermittent illumination and the viscosity method. From the kinetic scheme detailed in the previous section it can be shown that when the quasi-stationary state is reached i.e. when the rate of chain initiation equals the rate of termination the rate of polymerisation can be expressed as rate = kPkt-l/2IW[M] . - (8) where I is the rate of initiation and [-M] the monomer concentration. Also the degree of polymerisation (Le. the osmotically measurable chain length) of the polymer will be p = rats of conversion of monomer - kP rate of formation of polymer - 7cf + k+/2Plz/[M] which may be rewritten as Substitution of the value of 1 1 / 2 from ( 8 ) leads to (9) Hence if the reciprocal degree of polymerisation is plotted against the rate of conversion a linear relationship will be found the gradient of the line being rC,/kp2[M] so that the ratio of k p / k t / 2 may be found.At the same time the intercept of the straight line represented by equation (9) on the PI-1 axis will give ~ c f l l ~ . It now falls to determine a second ratio between propagation and termination constants which will enable all the reaction velocity coefficients to be determined uniquely. Such a quantity is the lifetime of the growing polymer chain which is defined by B-1 = kf:flkp + k$/21l/2/kp[M] . Ti-1 = +/kP + kt(rate)/kp2[~12 number of active centres per unit volume number of centres destroyed per unit volume per unit time t F- = h[P]-l = (kJ)-1/2 * (10) where [PI is the concentration of active centres which under steady state conditions will be (I/L,)1/2.The lifetime of the active centres is most readily obtained by means of the rotating sector technique. The theory of this experimental method has been fully set out by Burnett and Melville,2 and similar treatments of the problem have been published with certain exten- sions to take into account steady dark reactions.20y 21 This difference in treatment is most necessary in the case of oxidation processes when the thermal dark rate is appreciable but this condition can also arise in polymerisation reactions especially for those in which photosensitisers have been used to start chain growth. From equation (10) it is evident that z-1 = kt(rate)/kp[M] so that 8 plot of the reciprocal lifetime against rate of monomer conversion 20G.Gee and L. Bateman Proc. Roy. Sw. 1948 A 195 376. z1 M. S . Matheson E. E. Auer E. B. Bevilacqua, and E. J. Hart J . Amer. Chem. Soa. 1949 '71 497. BURNETT RADICAL POLYMERISATION REACTIONS 297 will give the value of kp/kt and hence a combination of the two series of measurements will yield kp kt and k,. The value of k$ if not determined otherwise can be found from the thermal rate of polymerisation which will be rate = k$/2kp[M]~/k$/2 since the rate of initiation is ki[M]2. As kp/k;/2 is known ki may be easily calculated. Several assumptions have been tacitly made in the foregoing description and since these are of vital importance it is necessary to comment on them more fully. The -most important probably is the fact that the analysis above assumes that the distribution of radicals throughout the reaction system is uniform.In using photochemical initiation this implies that the absorption of light must be very weak. It is a good criterion that for average size reaction vessels the extinction coefficient should not exceed 0.05 and should if possible be lower. Secondly it has been assumed that the reactivity of the radical is independent of molecular. size. It will be shown later that this assumption may constitute an oversimplification of the problem but as a first approximation at least it is valid. In the third place it has become fairly common practice to '' trigger " polymerisation with photosensitisers but the compounds used must not be chosen indis- criminately. The photosensitiser must as a molecule take no part in the reaction other than to act as a source of free radicals which must be capable of very rapid addition to the olefinic double bond and must not in any other way enter into the course of the reaction.If these conditions are not fulfilled the tendency will be for the rate of reaction to cease to be propor- tional to the square root of the light intensity and so the theory of inter- mittent illumination will no longer be applicable. It is evident from equation (9) that if the transfer velocity coefficient becomes large the value of the ratio kf/kp may become sufficiently high to bring about an apparent independence of molecular weight on the rate of polymerisation. In such instances it becomes necessary to determine the rate of chain initiation directly.This is most conveniently accomplished by the use of inhibitors or retarders.* It has been found that p-benzo- quinone has the ability to suppress completely certain polymerisation 8 z2 23 The time of complete inhibition is normally propor- tional to the quinone concentration and on the assumption that one quinone molecule stops one polymer chain it is possible to compute the rate of initiation. More recently a second method has been found which may prove to be more reliable. Certain azo-compounds notably a-azobisiso- butyronitrile (1 1'-dicyanoazopropane) have been found to react with very great efficiency with most monomers. Since these compounds are also good photosensitisers it follows that the rate of initiation can be found by a comparison of the rates of polymerisation under thermal and photochemical M.S. Matheson E. E. Auer E. B. Bevilacqua and E. J. Hart J . Arner. Chem. 22 S. Foord J. 1940 48. Soc. 1949 71 2610. * See Melville and Watson8 for distinction between these terms. 298 QUARTERLY REVIEWS conditions. This is possible since the first-order constant for the thermal decomposition of the initiating material can be determined with very great precision. 24 The viscosity method elaborated by Bamford and Dewar has the advantage that it requires only simple apparatus and from a series of simple measurements it is possible to determine with ease the rate con- stants for thermal initiation propagation termination and transfer. In essence the method makes use of the fact that the rate of a photochemical chain reactiondoes not fall to zero-or to the thermal dark rate-immediately the source of illumination is removed and also that the,maximum rate of reaction is not set up until an appreciable time after the light is switched on.The kinetic scheme used by the authors is as follows Initiation 2M -+ D or 2X Propagation M + X -+ X Transfer D + M -+ D + R R + M -+ P + R Termination D + X -+ D + Q R + X -+ P + Q 2X -+ 2Q D + M -j. P + R D + X -j. P i + & in which D is a free diradical D a free monoradical derived from an original diradical R a radical formed by the transfer reaction X any radical end PI and P polymers derived from diradicals and transfer radicals respectively and Q any dead centre. Houwink's viscosity equation being used in the form [q] == K(MoP)a where K and a are constants and allowance being made for the difference in viscosity behaviour between polymer derived from diradicals and from transfer radicals it can be shown that the rate of change of viscosity for thermal polymerisation is where 7 the ideal specific viscosity is [q]c c being the polymer concentration in base moles per litre and K' is related to K .For photochemical initiation where /I = kj(kikt)-1/2 and t = (1 + AI)l/?- A being a constant and 1 the relative light intensity. Measurement of dq/dt over a range of light inten- sities enables one to calculate /I (k,/kr,)(1fa)/zk~1-a)/2) and A . A further relationship is provided by the molecular weight of the thermal polymer which is given by The only further relationship required to enable the computation of the individual rate constants to be carried out is obtained by making use of the fact that the rate of polymerisation does not fall to the dark rate imme- 1" = 2Tcp(k&t)-l/2/(2p + 1) 24F.M. Lewis and M. S. Matheson J . Amer. Chem. SOC. 1949 '91 747. BURNETT RADICAL POLYMERISATION REACTIONS 209 diately the light is removed. The authors define the after-effect as the difference between the measured specific viscosity and that which would have been obtained had the rate of reaction fallen to the dark rate on switching off the light. The analysis for this is complex and requires for its solution the assumption that a can be expressed as m/n where m and n are small integers. Measurement of the after-effect allows Ic to be computed and so all the other velocity constants can be determined. The Values oE Rate Constants in Polymerisation Reactions.-Now that the methods of measurement have been detailed and bearing in mind the assumptions which have been made we are in a position to consider the results obtained for a number of representative investigations covering only the first stages of the reaction and carried out in the medium of the pure monomer.Vinyl Acetate.-The first monomer to be studied with a view to deter- mining the rate constants was vinyl acetate. Results on this particular compound are somewhat divergent probably owing in part at least to the very great difficulty of its purification. Hydrolysis of the ester leads to formation of acetaldehyde and acetic acid the former being described as both a retarder 25 and under certain conditions a photosensitiser as a result of photolysis of the aldehyde.A new technique was evolved by P. D. Bartlett and K. Nozaki26 whereby the monomer was partly polymerised in vacuo before the final distillation. This method seems to be very advantageous since all anomalous behaviour is avoided. The results of a number of independent investigations are shown in Table I. TABLE I Rate constants for vinyl acetate polymerisation Method. _________ Rot. sector do. do,* d0.O Viscosity Temp. Source. .__ (2) & (27) ( 3 ) (23P (29) 15.9" . . 25 . . 25 . . 25 . . 0 . * . %. (1. mole-1sec.- 1). lit5i ~ 39.2 2024 11.76 1954 11-78 2800 1 22 8 - - ca. 5a 0.23 0.14 (a) This measurement was made at 50". ( b ) With cr-azobisisobutyronitrile as photosensitiser. ( 0 ) With di-tert.-butyl peroxide as photosensitiser. (d) For the purposes of comparison the published results have been doubled since these authors have assumed combination to be the termination reaction whereas the others assumed disproportionation.* As was mentioned above one of the main differences in the study of vinyl acetate polymerisation is that the transfer constant is rather high so H.W. Melville and IF. R. Tuckett J. 1947 1201 1211. 26J. Amer. Chem. SOC. 1946 68 2377. * Throughout this review all results are compared on the basis of disproportion- ation i.e. all published American results have been doubled. 300 QUARTERLY REVIEWS that it is not generally possible to use the molecular-weight method to compute E p / E t / 2 . In view of this it is necessary to determine the rate of photochemical initiation directly an operation which must always be fraught with some degree of uncertainty.The inhibitor technique has been most usually 23p 28 the first two sets of workers using p-benzo- quinone which appears to be a highly efficient inhibitor of vinyl acetate no doubt on account of the exceptionally high activity of the growing radical and the relative inertness of the monomer. Bartlett and Kwart on the other hand have made use of duroquinone in their inhibition experiments. The values calculated by G. Dixon-Lewis 29 were obtained by using a method depending on Bamford and Dewar’s viscosity technique a modification being introduced to allow for the fact that the thermal polymerisation rate is zero. When it is considered that one cannot reasonably expect that the final results will be correct to much less than a factor of 2 on account of the rather involved experimental methods employed its well as the complexity of the reaction itself the results detailed in Table I will be seen to be on the whole in remarkably good agreement.It should be pointed out that the first results published by Burnett and Melville 2 have been corrected to allow for two points over which there has been much controversy. All the reaction constants in the table refer to single-radical ends whereas in the original paper diradicals alone were considered. This means that the original Ep and E values have been divided by 2 and 4 respectively. The k value has also been corrected to allow for a slightly non-uniform distribution of radicals in the reaction system.27 The photosensitisers used by the American investigators 23p 28 were examined to ensure that they took no part in the polymerisation other than that of starting the reaction chains.Furthermore Matheson and his co-workers have made due allowance for the considerable dark rate experienced at 25 * with the azo-type photosensitiser and Bartlett and Kwart arranged that their measured dark rate was less than 0.6% of the photo-rate under which conditions the effect of the dark rate is negligible. It is naturally more fundamental to compare the frequency factors and energies of reaction for the various steps of the chain process rather than to deal with the absolute values of the rate constants at definite temperatures. Since we are concerned with bimolecular reactions it is well known that the velocity coefficients can be expressed in the form I% = A exp(- E/RT) where A is the frequency factor (having a normal value of 1011-1012) and E is the energy of activation for the reaction.The following table shows the results obtained by such analyses Energies of activation and frequency factors for vinyl acetate 4 8.3 x 105 9.8 x 105 - 4-86 x lo8 BTJRNETT RADICAL POLYMERISATION REACTIONS 301 The agreement between the energies of activation in the first and the third set of results is within the expected experimental error of -J= 1 kcal. and the frequency factors also lie well within expected divergence. The very high valuesaf the activation energies and frequency factors given by Matheson et al.23 are surprising and may be due at least in part to.the method of determination of the rates of initiation Lea the use of p-benzo- quinone in the case of monoradical initiation.The very high value for the termination energy is all the more astonishing since it had been assumed that the termination reaction involved combination-a step which has usually been associated with zero or very low energies. In view of the wide discrepancies in the energy and frequency factors it would appear most desirable to conduct a reinvestigation of this phase of the reaction. On the whole the greatest attention to all possible sources of error appears to have been paid by Bartlett and Kwart but so far these workers have not attempted to investigate the effect of temperature on the velocity constants. ,Styrene.-The polperisation of styrene has been studied by numerous workers using both the accepted methods to determine the rate constants for the reaction steps.Again there is a certain amount of divergence of opinion but in certain respects agreement is closer than in the case of vinyl acetate. The values which have been determined are shown below. Temp. 0" . 25 . 30 . 30 . Rate constants for styrene polymerisation 4.51 x 10-18 1.32 x 10-18 (1. mole- 'see.- l). 6-91 1 1.83 x 106 18.7 i 2-79 x lo6 51-9 1 1.05 x lo7 135 1 1-7 X lo8 I 7.47 x 10-6 6.68 x 1-66 x 10-3 - Method. Viscosity do. Rot. sect.a d0.b (a) Photosensitised by benzoyl peroxide. (b) Photosensitised by bisazopropane but full details are not yet available. Here again there is some discrepancy but it is not yet possible to com- ment adequately on these results since the details of Matheson's work are not known. Even so the differences between the energies of activation in particular are too high to allow any great reliance to be placed on the figures of the last table.The Reviewer has made some measurements of the photochemical overall energy of activation for styrene. The justifiable assumption being made that the energy for the initiation step is zero this determination gives (E - &E$). The measured value of this quantity comes out at 5.50 kcals. G. M. Burnett H. W. Melville and L. Valentine Trans. Farachy Soc. 1949 45 960. 28 P. D. Bartlett and K. Kwart J. Arner. Chem. Soc. in the press. 2s Proc. Roy. SOC. 1949 A 198 510. s1 M. S. Matheson unpublished results. H. W. Melville and L. Valentine Trans. Faraday Soc. in the press, 302 QUARTERLY REVIEWS per mole which is in much closer agreement with Bamford and Dewar's results (5.1 kcals.) than with Matheson's (7.5 kcals.).From the energies of activation for the termination and transfer reactions a rather interesting point arises. If disproportionation is the operative chain-termination mechanism this implies the abstraction of a hydrogen atom from one radical to saturate the other i.e. there is a rupture of a C-H bond. In the same way the transfer reaction requires a C-H bond to be broken and yet the difference in the energies of activation necessary to bring this about is very high indeed so it must be deduced that there is some very fundamental difference between the nature of the two reactions. Temp. &. 23.6" - 30 - 50 _ _ _ _ ~ 0 6-82 x 10-l6 I - Energies of activation and frequency factors for styrene polymerisation 4. ~ I I ~ I ~ * ~ Ei. Ep. ~ 4. ~ Ef. ~ S;;.~ - - 1.23 x lof2 1.02 x lo6 3-07 x lo8 1.50 x lo' 37.0 6.5 2-8 14.2 9.0 3.0 - (31) 3.80 x los 2.4 x 1O1O - - __ -. 310 6.6 xlO' 1-46 x 10-31 Rot. sect.a 1 (33) 286 2.44 x lo7 1 7-50 x 3.35 x 1.17 x d0.a ~ (21) 41.6 Viscosity (32) 1-14 x 10-2 8.5 x 10-5 (34) 1 2-69 x lo6 6.13 x lo-* 5.11 x lo-* 1.28 x - b I - - Methyl MetkcryZate.-The polymerisation of methyl methacrylate has been the subject of a vast number of investigations because of the many abnormalities which even now have been only partly explained. The study of this reaction has been complicated by the fact that reproducibility of results has been difficult to obtain especially from worker to worker. Again this appears to be bound up with the question of purity of the monomer. C. H. Bamford and M. J. S. Dewar 32 have gone to considerable lengths to obtain exceptionally pure monomer their method giving high reproducibility in initial measurements but agreement with other workers is lacking so far.The results for the reaction velocity coefficients are set out below. If the expected discrepancy factor of 2 is borne in mind the results of this table are in quite good agreement. This is all the more Rate constants for methyl methacrylate polymerisation (a) Direct photo-initiation (b) Benzo yl peroxide- accelerated react ion. surprising on account of the very anomalous behaviour of methyl meth acrylate under normal polymerisation conditions. The results obtained by the rotating sector are in comparatively good agreement although the discrepancy in the termination constant might be considered to be somewhat 32 Proc.Roy. SOC. 1949 A 197 356. 33 H. W. Melville and N. H. Mackay Tram. FaTacky SOC. 1949 45 332. 84 G. V. Schulz and G. Harborth Makrml. Chem. 1947 1 106. BURNETT RADICAL POLYMERISATION REACTIONS 303 high. The main difference between the results may in fact be in the purification techniques. Although both Melville and Matheson state that reproducibility was obtained in their experiments the work of Bamford and Dewar was complicated by the fact that the rates observed were not wholly reproducible. They were forced then to modify their system of calculation to allow for this by always carrying their after-effect experiments to the same rate of reaction. They attribute this non-reproducibility to two factors (a) formation of it photocatalyst and (b) thermal production of an inhibitor.Chemically both these reactions appear possible by side reactions with elimination of either -water or methyl alcohol photosensdiser end catalyst and CH3 + o+O t 2CH3*OH ,L=CH CH30 OCH CH2=d o=c c=o - I CH3 CH3 retarder Since the other authors found no evidence of the thermal polymerisation which is higher than that of styrene under comparable conditions it may be that their monomer contained traces of inhibitor which would lead to higher values of kt. On account of the complexity. of the reaction a t temperatures higher than about lo” Bamford and Dewar did not determine the energies of activation for the steps of the reaction. Melville and Mackay report that the energy of activation of the propagation step is 4-4 kcals. per mole giving it frequency factor of 5-5 x lo5 while the termina- tion reaction does not have any measurable energy of activation but is probably of the order of 1 kcal./mole.This leads to a frequency factor > 108. Matheson and his collaborators by measuring the rate constants at four different temperatures find that Jcp = 10.26 x lo6 exp(- 6 3 1 0 / R T ) Ict = 2-72 x lo9 exp(- 2840/RT) ky = 5.50 x lo6 exp(- 12280/RT) Extrapolation of Schulz and Harborth’s results at two temperatures gives good agreement with Bamford and Dewar’s data. The only other investiga- tion of the thermal rate of polymerisation of methyl methacrylate is described in a short paper by C. Walling and E. R. B r i g g ~ . ~ ~ They compute the value of the initiation rate constant at three temperatures above 100” ki is 0.70 x 10-15 1. mol.-lsec.-l.The energy of activation for initiation is found to be 22 kcals. giving a frequency factor considerably less than unity (0.36). From the rates of thermal polymerisation it is evident that the purity of the monomer leaves much to be desired despite the care taken 35 J . Arner. Chem. Soc. 1946 68 1141. 304 QUARTERLY REVIEWS to avoid contamination by air. If Bamford and Dewar's hypothesis regard- ing the formation of inhibitor at high temperatures is accepted then the explanation of the low rates found by the Americans is obvious. It is therefore probable that these results can be disregarded. p-Methoxystyrene.-Extending the work of Bamford and Dewar R. W. E. Axford 36 has investigated the polymerisation of p-methoxystyrene with the object of determining the effect of the introduction of thep-methoxy- group into the styrene molecule.The general characteristics of the reaction were similar to those of styrene but the monomer polymerises much less rapidly. A comparison of the velocity constants at 0" for these two monomers is shown below. Except in the case of thermal initiation all Bate constants for styrene and p-methoxystyrene at 0" k( . . . . kp . . . . Jc . . . . Jcf . . . . Jcf/lCP . . . Styrene. 4.51 x 10-l8 6.91 1.83 x lo6 7-47 x 10-5 1-08 x 10-5 p-Methoxyst yrene 5.94 x 10-18 2.92 1-06 x lo6 1-98 x 10-6 5-78 x 10-5 the rate constants are considerably less than those for styrene. Axford estimates that the energy of activation for the initiation reaction is 30 kcals. per mole which as might be expected is less than that for styrene (38 kcals.).Thus it appears that the introduction of the methyl group has the effect of lowering the " polymerisation capacity " of styrene a fact which has been predicted from other consideration^.^^ This reduction in activity will be due either to a reduction in the reactivity of the radical by more resonance stabilisation or to reduction in monomer activity. The former seems to be more likely as it appears that the radical reactivity plays a major rcile in determining the rate of polymerisation (see below). Methyl Acrylate.-Methyl acrylate is a monomer of considerable theoretical interest because of its close resemblance to the methacrylate. Despite this similarity in chemical structure the vapour-phase polymerisation of these two monomers showed several very significant differences as was demonstrated by H.W. Melville.38 Thus it is of importance to see whether these anomalies are repeated in the liquid phase and to what the differences can be attributed. Using the sector technique M. S. Matheson 39 has been able to determine the propagation and termination velocity constants for a number of temperatures from which he has deduced that kp = 2.0 x 10' exp(- 7100/RT) 7cg = 5.60 x 1O1O exp(- 3100/RT) 38 Proc. Roy. SOC. 1949 A 197 374. s8 (a) Proc. Roy. Soc. 1937 A 163 511 ; (b) ibid. 1938 A 167 99. *# Unpublished results. P. P. Shorygin and N. V. Shorygina J . @en. Chem. Russia 1939 9 845. BURNETT RADICAL POLYMERISATION REACTIONS 305 It is advantageous to compare these figures with those obtained by the same author for methyl methacrylate for which lcp = 10.2 x f O S exp(- 630O/lt(T) lct = 2-60 x log exp(- 2800/RT) Here there is a very close similarity in the energies of activation for the two monomers so attention need be paid only to the frequency factors.From these results it is evident that the propagation reaction in the case of meth- acrylate is about 5 times more efficient than that of the acrylate but the efficiency of interaction of the radicals is reversed so that the termination rate of acrylate is very much greater than that of methacrylate. Since the resonance energy of both monomers and their derived radicals cannot be expected to differ by very much this difference in radical interaction efficiency must be attributable to the steric hindrance induced by the presence of the methyl group directly attached to the carbon atom of the double bond of methyl methacrylate.A similar explanation was invoked by Melville 38a to account for the differences in polymerisation characteristics in the gas phase. Butyl Acrylate.-A further ester of acrylic acid namely butyl acrylate has been investigated by A. F. Bickel and H. W. Melville.40 It would not be expected that the introduction of the butyl group in place of methyl would produce any profound effect on the polymerisation characteristics of this monomer since the butyl group is well separated from the radical end. Nevertheless the behaviour of this monomer is rather peculiar in many respects the most significant being the fact that large deviations from the theoretical intermittent illumination curve have been obtained. This natur- ally makes the evaluation of the lifetime somewhat uncertain but by working on the part of the experimental curve giving closest agreement with theory the following values of the propagation and termination velocity constants were found XC = 4-4 x 104 exp(- 2100/RT) kg = 1-8 x lo4 (with no energy of activation) In this case the frequency factors for both reactions are very low indeed implying that the reactions are both very inefficient.This is true more particularly of the termination step for it has been found that the most probable frequency factor is of the order of lo8. There appears to be no apparent reason for the very considerable differences between the constants found for butyl and those of methyl acrylate since it is evident that the presence of the butyl group will make little difference to the resonance energy of the molecule or radical.Cornparison of Radical Reuctivities.-The question now arises as to whether any deductions can be made as to the radical activities merely on the basis of the results which have been set out in the preceding discussion. Before undertaking such a comparison of reactivities it must be remembered that the rate constant for propagation and transfer reactions must depend 40 Pram. Far&# Soo. in the prese. 306 QUARTERLY REVIEWS on the reactivity of the monomer as well as that of the radical. As a very rough guide therefore it can be stated that as the radical reactivity increases that of the monomer molecule will decrease. As an example the styryl radical can exist in a number of resonance forms which tend to stabilise the radical while the effect of resonance on the molecule will tend to make the olefinic link more reactive.The following table shows the values of some ratios which make such a comparison possible. Rate constants at 25” Butyl acrylate 41 . . Styrene 30 . . . . Methyl methacrylate a1 . Vinyl acetate 27 . . Monomer. 13.1 0-134 - I 10.2 39.5 2-81 1.1 275 5.0 750 22.3 970a kf/kp x lo6. I 2.79 - I I (a) Calculated from the data of Dixon-Lewis.2B From this table which admittedly contains only a few of the monomers studied it will be seen that despite the great variation in the values of the individual constants the magnitude of kt/2/kj3 does not vary to the same extent. Indeed it appears that the reactivity of the radicals towards monomer and another radical of the same kind cannot be significantly different between different monomers.Also since these monomers are arranged in the order of their reactivities in copolymerisation the data give credence to the rough guide stated above as will be seen from the values of k:/2 (this is chosen as a better quantitative measure of the reactivity of the radical than E, which must refer to the interaction of two radicals). It is interesting that the order is preserved in the propagation constants which would suggest that it is the reactivity of the radical and not that of the monomer which is of major importance. The most surprising feature is the trend shown by the transfer constant. This one would assume would depend more on the strength of the C-H bond than on radical reactivity. As mentioned previously the reaction may be due to a donation of a hydrogen atom from the radical to monomer.If this were true one might reasonably expect the radical to be again of the greater importance. Variation of Rate Constants with Molecular Size.-Early workers in the field of polymerisation kinetics assumed that there was a variation in the value of the rate constants for each step of the reaction this usually being expressed as a function of molecular size. More recently the accepted view has been that the reactivity of the radical is independent of chain length. Some experiments have been carried out with the intention of verifying this feature. I n their original investigation of the polymerisation of vinyl acetate Burnett and Melville showed that doubling the chain length produced a negligible change in the rate constants from which it was deduced that for chains containing more than 100 monomer units the 41Results of M.S. Matheson communicated by Prof. H. Mark at International Colloquium on Macromolecules Amsterdam 1949. BURNETT RADICAL POLYMERISATION REACTIONS 307 velocity constants were independent of molecular size. On revising the figures of that paper with the aid of Burnett Melville and Valentine's data,g7 it is found that the ratio IC,/k; is 1.26 x 10-6 and 1.34 x lo-* for chain lengths of 176 and 352 respectively. Thus it will be seen that there is no proportionality between these values and the chain length and further- more the variation of 7% is well within experimental error. Some similar experiments in the case of methyl methacrylate and styrene show greater divergence as shown in Table II.44 The variation in methyl methacrylate TABLE I1 Variation of rate constants with molecuhr size I I I I Monomer.Degree of polymerisation. j kp/kt x los. 1 ~ Ratio. 4500 17000 I ii:; 1 '*09 Methyl methacrylate . . I I 1 Styrene . . . . . 390 1.24 I 1.41 I I I is again probably not significant but with styrene the difference appears to be sufficiently great to be outside probable experimental error so that in this case at least there seems to be a definite variation in the rate con- stants under the two conditions used. It will be noted that in these cases there is a decrease in the ratio for the higher chain lengths which would indicate there is either an increase in the efficiency of the termination reaction or a decrease in that of propagation. Variation in Rate Constants with Environment .-The course of polymerisa- tion of methyl methacrylate has been noticed by a number of workers to be subject to a self-accelerati~n.~~ The effect was shown to occur under isothermal conditions and to depend on the rate of initiation and the temperature.Since the higher the initiation rate and the higher the temperature the greater the amount of polymer formed before self-accelera- tion commenced it was evident that this effect could be correlated with the viscosity of the solution of polymer in monomer. This was further verified by increasing the initial viscosity of the monomer by the addition of cellulose tripropionate. Under these conditions it was found that the self-acceleration was apparent from the start of the reaction indicating that the necessary conditions for self-acceleration could be simulated in this way.It has been shown that this effect can be duplicated in solution in which the monomer solvent is either a poor solvent or a precipitant for the polymer. The same autocatalytic type of curve was 0btained.~3 At the same time as the increase in molecular weight takes place it was found that there was an increase in the molecular weight of the resulting polymer. This can be 4 2 See e.g. E. Trommsdorff Colloquium of High Polymers Freiburg 1944 (Re- printed in B.I.O.S. Report No. 363 Item No. 22). 43 R. R. Smith and R. a. W. Norrish Nature 1942 150 336. 308 QUARTERLY REVIEWS true only if either the rate of propagation increases or alternatively if the rate of termination decreases. It is difficult to see how the first alternative can be true since one would expect if anything a decrease in propagation due to a lowering of the mobility of the monomer.Thus it is almost neces- sary to -accept the second alternative. The reduction in rate of termination can be imagined as due to the fixation of the growing radicals in the viscous medium and so prevents those radicals from making contact. In the case of solution it seems to be most plausible to suppose that owing to the fact that the polymer molecule will tend to coil in poor solvents the active ends of the radicals will become buried and hence be inaccessible to the other radicals but at the same time the diffusion of the monomer to the active centre must not be seriously impeded. It is possible to interpret the phenomenon on a somewhat different basis. C.Walling 44 has suggested that in a poor solvent the monomer will be preferentially adsorbed on the polymer so causing a submicroscopic phase separation and effectively '' blanketing " the active radical end. Although the two explanations predict similar results there seems to be more support for the coiling of the molecule since it is also applicable to those cases in which the polymer is insoluble in its own medium there then generally being a violent self-acceleration e.g. acrylonitrile. 42 The polymerisation of vinyl acetate carried out in 12-hexane solution (which brings about precipitation of the polymer) has been investigated in order to ascertain which of the alternatives given above is correct.45 It was shown that whereas the propagation constant varied slightly-the variation was within experimental error and could be discounted-the value of the rate constant for illumination fell by a factor of 5.This substantiates the view that the rate of the termination reaction is lowered by the change in environment. This fact is itself somewhat peculiar since it had previously been considered that radical reactions were insensitive to changes in environment. Similar results were obtained by Matheson and his co-workers 21 in the polymerisation of pure methyl methacrylate. Here after the inception of the self-acceleration the value of the termination constant fell by a factor of 100 while the propagation constant again remained sensibly unchanged. If methyl methacrylate is polymerised in aqueous medium with a redox catalyst system the value of JCp/k:/z has been found to be 12-3 at 25°.46 An estimate of this ratio for pure monomer can be obtained from the data of Schulz and Harb~rth,~* and at 25" k p / l c ~ / 2 for pure monomer is found to be 5.2 x 10-2.If it is again assumed that the rate constant for propaga- tion is unchanged then this implies that the termination constant has been decreased by a factor of about 60,000. This very great change is not actually unreasonable since the use of water as a solvent is a rather extreme case. J. H. Baxendale and M. G. Evans 47 have attempted to deal with the o4 Faruduy SOC. Discwsions 1947 2 285. C6 G. M. Burnett and H. W. MeIvilIe Proc. Roy. Soc. 1947 A 189 494. dB J. H. Baxendale M. G. Evans and J. H. Kilham Trans. Faraday Soc. 1946 42 668. o7 Ibid. 1947 43 210. BURNETT RADICAL POLYMERISATION REACTIONS 309 problem of reactions in which the polymer is precipitated in terms of the coagulation of colloidal particles.The velocity constant for such conditions should then be described in the form where rl and r2 are the radii of the particles and if polymer chains of n and n2 units can be assumed to be spheres because of severe coiling the above equation becomes By using assumptions of this type it is possible to deduce the effect of the addition of emulsifiers since these will alter the charge on the colloid particles and hence affect the rate of coagulation. These views being taken into account it is evident that the viscosity and also the activation energy of viscous flow will enter into the termination constant of the reaction. The only possible weakness in the above discussion appears to be in the fact that coagulation does not appear necessarily to occur simultaneously with termination as the above postulates seem to indicate.A further phenomenon which has also been found in the case of methyl methacrylate but has so far not been investigated elsewhere may have some bearing on the subject under discussion. In their investigation of the catalysed polymerisation of this monomer Schulz and Harborth 34 measured the average degree of polymerisation a t different monomer conversions. As has been stated an increase in molecular weight was encountered in agreement with the theory of viscosity action in methyl methacrylate. If the " instantaneous " degree of polymerisation is defined by = drn/dn where n is the number of molecules of polymer and if the average degree of polperisation is = m/n then it can be shown that p and p are related by the equation - - P p LT ~ _ _ _ _ _ _ ~ _ _ 1 - (m/F)dF/dm Investigation shows that the value of the instantaneous degree of polymerisa- tion rises to very high values being as high as 47,000 under certain con- ditions.so that in theory the maximum possible degree of polymerisation ought to be 12,000. Now the question which has to be answered is whether in some way the value of the transfer constant has altered during the viscous phase of the reaction. Since the transfer reaction involves a monomer molecule this appears to be unlikely because the constancy of the propagation rate constant implies that the mobility of the monomer is unaltered. So far no concrete proposals have been given to account for this type of reaction but it may be that transfer to polymer to some extent might account for this increase in molecular weight above the limiting value.The resurgence of growth of dead polymer has been shown to be possible in the case of the copolymer- isation of styrene and butadiene (G.R.-S.).49 Ibid. 1948 70 3695. - At the same temperature the value of k~/lc,is 8.5 x 48 F. T. Wall J. Amey. Chem. Soc. 1947 69 1761. Y 310 QUARTERLY REVIEWS Emulsion Polymerisation of Styrene.-In the original discussion of the rate constants for various monomers two investigations were omitted since it was felt that they fitted more logically into a discussion of anomalous results. The polymerisation of styrene as an emulsion has been the subject of an investigation by W. V. Smith 50 based on a theory of emulsion polymerisation due to W.V. Smith and R. H. E w ~ r t . 5 ~ ~ The value of IC determined by this method is given as 3.5 x 10IOexp(- 11700/RT). This is very much greater than the value obtained in polymerisation of the pure monomer at the same temperature and the energy of activation is very much greater than that generally accepted. Smith also finds molecular weights very much in excess of the limiting value set by kt,/kp and contends therefore that the published values of the rate constants for transfer reactions must be too high. This seems to be rather unlikely in view of the consider- able accuracy with which the determinations can be made. The mechanism of emulsion polymerisation is not as well defined as that of the pure monomer and it may be that a situation similar to that obtaining in high conversion polymers may be operating.Vinylidene Chloride.-This monomer which has considerable commercial interest has two peculiarities in that the polymer is insoluble in the monomer and is also crystalline. An investigation of the reaction was carried out by J. D. Burnett and E. W. Melville 51 in order to ascertain the effect of precipitation on the values of the kinetic constants. Rates of polymerisation and overall energies of activation were found to be quite normal for this monomer despite the unusual properties of the polymer. The velocity constants evaluated by means of the sector technique and using direct photoinitiation gave the following results Rate constants for vinylidene chloride polymerisation Temp. 25 . . 15" * 35 .. (kcals./mole). l i i d ( I ' p I (1. mde-%ec.- l). i: ~ y:; i j 1016 1 1030 I 25 1 40 36.8 18.0 x 1 0 5 - BURNETT RADICAL POLYMERISATION REACTIONS 31 1 III. Polymerisation in Solution When a polymerisation reaction takes place in solution there is always the possibility that the growing radicals will react with the solvent to give dead polymer and a further radical by the reaction This type of reaction was first investigated by F. R. May0,5~ who showed that in the case of the thermal polymerisation of styrene the degree of polymerisation of the polymer i.e. the osmotically measured chain length was related to the molar ratio of solvent to monomer by the equation where P and Po are the degrees of polymerisation in solution and in pure monomer and ( X S / M ) is the molar ratio of solvent to monomer.It will be seen that the above equation will give a linear relationship between reciprocal degree of polymerisation and the solvent-monomer ratio. It should also be noted that the relationship holds only for fairly long chains. The consequences of shortening the chains will be considered later. The work was extended considerably and the technique of measurement much rehed by R. A. Gregg and F. R. M a ~ o ~ ~ who obtained the results shown in Table 111. TABLE I11 T h e m 1 polymerisation of styrene in hydrocarbon solvents@ R-+XS + R X + S - k; P-1 = Frl + (k>/kp}(.X&/iU) Solvent. Benzene . . . tert. -Butylbenzene Toluene . . . Ethylbenzene . isoProp ylbenzene Diphenylmethane Triphenylmethane Fluorene . . . Pentaphenylethane cycEoHexane . . n-Hexanea .. Decalin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . * . . . . ki x lo* (1. mole- %ec.- l). 60". 1-06 3-52 7-35 39-6 48.5 135 206 4420 118 x 104 1.42 24.8 23.6 100". 31 92.5 109 2 74 336 705 1340 20800 - 26-8 160 - E;' kcale./mole. 21.3 20.2 16.6 12.0 12.0 10-2 11.6 9.6 19.9 11.5 - - A;. 8.9 x 10% 6.0 x 109 5.7 x 107 6.4 x 104 2.8 x 106 3-4 x 106 1-3 x 108 1.2 x 106 - 1-4 x 109 8.9 x 104 - Values of k; Bi A; calculated from data of Bamford and Dewar.4 (a) Non-solvent for polymers. C. H. Bamford and M. J. S. D e ~ a r ~ ~ using a similar technique have obtained the results shown in Table IV. It will be seen from the figures of Tables I11 and IV that the values of IC; vary very considerably and in fact this measurement may be used as a method of determining the activity of free radicals towards other molecules.This is of particular interest in 62 J . Amer. Chem. SOL 1943 65 2324. 69 Paraday Soc. DiBcu~siom 1947 2 328. 64 Ibid. p. 314. Y* 312 QUARTERLY REVIEWS organic chemistry and comparison with many organic reactions comes readily. In the series of aromatic compounds investigated it will be seen TABLE IV Thermal polymerisation of styrene in solution Solvent. Carbon tetrachloride. Carbon tetrabromide 8.-Dichloroethane . 8.-Dibromoethane . Tolueneb . . . . Ethylbenzeneb . . Benzene6 . . . . 9 9 k; x 104 (1. mole-%ec.-*). 60". 1 80°. I 5300 8040 19400 19 ! 116 - 30 - 1 1.6 159 x 104 282 x 104 I 100". 30400 45400 - - 111.2 17 41 6.7 E; kcals./mole. 11.3 10.7 6.7 21.2 16.2 18.2 18.2 19-1 A;. 1-3 x 107a 7.6 x 107 3.92 x lo6 1-26 x 1Ol1 2.99 x lo8 6.68 x 108 8.79 x lo8 1-60 x 109 (a) Gregg and Mayo ; 63 other results by Bamford and D e ~ a r .~ ~ (b). Calculated from Mayo's data,6a using K = 4.57 x lo-* u = 0.65 in the viscosity equation. that benzene is less reactive than toluene ethylbenzene or isopropyl- benzene. This presumably must be ascribed to the presence in the latter of a-hydrogen atoms. This is further substantiated by the fact that as the lability of the a-hydrogen atom increases the efficiency of the transfer reaction increases also and if all the hydrogen atoms of the methyl group of toluene are blocked by methyl groups as in tert.-butylbenzene the activity is decreased. These results indicate that radical attack occurs preferentially at the a-hydrogen atoms. The other series which can be readily investigated is that of di- and tri-phenylmethane and fluorene whose activity is in that order this being bound up with the decreasing strength of the methane carbon-hydrogen bond.The acidities of these hydrocarbons 55 show a very close parallel with the value of Mayo's transfer constant i.e. k;/kp. The identity of these orders must be due to the saime factors increasing resonance stabilisation of the radical formed by the loss of a hydrogen atom or of the negative ion by the loss of a proton. Similar conclusions cannot however be reached in many other cases. For halogenated compounds the efficiency as transfer agents is very much increased. Thus the rate of the transfer reaction in the series CH,Cl*CH,Cl CHCl,*CHCI, CC1 increases in that order which one would expect from the fact that increase in chlorine constant enhances the stability of the radical.Bromine derivatives are more active than the corresponding chlorine compounds probably by virtue of the lower bond energy of the carbon-bromine bond. The very high value of k$ for the carbon tetra- bromide makes it possible to prepare very low-molecular-weight compounds and in fact the monomeric adduct Ph*CHBr*CH,*CBr, has been isolated.56 66 {a) J. B. Conant and G. W. Wheland J . Amer. Cltem. Xoc. 1932 54 1212 ; (b) W. K. McEwen ibid. 1936 58 1124. 6* M. S. Kharasch E. V. Jensen and W. H. Urry ibid. 1947 69 1100. BURNETT RADICAL POLYMERISATION REACTIONS 31 3 This incidentally shows that the point of radical attack must be on the IS-carbon atom of the styrene side chain.Of all the transfer agents the most potent class are the thiols all of which appear to have transfer-reaction velocity coefficients greater than the normal propagation rate constant. This feature appears to be independent of the nature of the hydrocarbon attached to the thiol group. Results for the ratio IC;lkp are shown below for some common thiols. Polymerisation of styrene in the presence of thiols 60'. 3.7 14.8 21 58 1 100". 2-3 5.8 13.7 3.9 - - - - tert.-Butanethiol . . . . . Dodecanethiol . . . . . 3-Ethoxypropanethiol . . . Ethyl thioglycollate . . . Monomer. kP* k;/kp. 1 k;. I - ____- Styrene . . . . . . . 207" 22 4550 Methyl methacrylate . . . 367b 0.67 246 Vinyl acetate . . . . 3700 48 178000 Methyl acrylate . . . . - 1.69 - I A;. An interesting method of determining the ratio of IC;/kp is the use of the radioactive isotope of sulphur 35S which is incorporated in the thiol.This method was devised by C. Walling.57 The determinations of the value of kj were made by using active n-butanethiol with the monomers styrene vinyl acetate methy methacrylate and methyl acrylate. The results are shown below Transfer constants for n-butanethiol 57 J . Amer. Chem. Soc. 1948 'SO 2561. 68 G. M. Burnett and H. W. Melville Faraclay SOC. D&cussions 1947 2 322. 59 Ibid. p. 337. 314 QUARTERLY REVIEWS with the stability of the radical formed during polymerisation. He showed that this order was Reference to the last table shows that the order of activity seems to have been altered slightly since the activity of the apparently heavily stabilised styryl radical is greater than that of either methyl acrylate or methacrylate both of whose substituent groups give only slight stabilisation.On the whole however the order given by Nozaki is fairly rigidly followed. Using exceptionally high concentrations of carbon tetrachloride for the thermal polperisation of styrene up to a ratio of solvent to monomer of 400 to 1 F. R. Mayo 6o has investigated the effect of chain length on the ratio k;/kp. With the very short chains involved in this reaction it is necessary to modify to some extent the equations which have been used previously since it is evident that the consumption of monomer in side reactions will be Comparable to that taken up in the polymerisation reaction. The results recorded in this work show that at 76" the ratio of is 0-0006 (or less) for one styrene unit 0.0025 for two units 0.007 for three units and for four or more units the value is constant at 0.0115.Studies of the rate of reaction showed that on account of the very great difficulty in obtaining the lowest molecular weights corresponding to one unit of styrene it is the transfer-rate coeficient k; rather than kp which varies. The assumption of constancy of the rate coefficients therefore requires some qualification defining the chain length. It will be noticed that in this case there is some similarity between those results reported here and those obtained in investigations of the effect of molecular size on the propagation and termination reaction velocity constants 41 (Table 11). In the latter case the value of ic,/kt decreased but k;& increased with molecular size.The significance of this trend is rather obscure since in the first instance there is a comparison of a radical-molecule reaction and an inter-radical reaction whereas in the second case both reactions are of radical-molecule nature. It is evident that following a transfer reaction with solvent the radical produced from the solvent must itself be capable of carrying on the reaction chain i.e. it must be capable of adding on to the double bond of the monomer. It seems to be thus far an impossible task to evaluate the rate constant for this type of reaction. This is unfortunate since it means that a powerful tool is denied to the physical chemist for the investigation of radical attack on the monomer double bond. A measure of the reaction velocity constant for the attack of the solvent fragment on the monomer would give a measure of the activity of the monomer by using the same solvent or by keeping to a single monomer the activity of many radicals could be investigated.It will have been noticed that throughout the entire range of results which have been presented here there is the trend that with high energies of activation high frequency factors are also found. This was first pointed out by Gregg and Mayo 53 in connection with the frequency factors and C,H,* > CH,:CH* > *CN N *CO,MS > C1 > GH2R CT *OAC > *H 6o J . A m y . Chem. SOC. 1948 70 3689. BURNETT RADICAL POLYMERISATION REACTIONS 315 energies of activation for a number of hydrocarbon solvents in the polymerisa- tion of styrene. In this case there was found to be a linear relationship between the logarithm of the frequency factor and the energy of activation.This has recently been extended by C . H. Bamford and N. J. S. Dewar 61 to a number of other reactions including the autoxidation of tetralin.62 They show that if the energy of activation is much less than 20 kcals. per mole it is unlikely that the frequency factor will appreciably exceed 10'. This is in direct contradiction to the normally accepted value of 1011-1012. A similar relationship holds in the case of oxidation reactions as has been pointed out by Bolland,l and is also known to be true in the case of ionic reacti0ns,~3 As yet the source of this relationship is unknown. W. Copolymerisation * Monomer Reactivity Ratios.-In recent years the study of copolymer- isation has been prosecuted with increasing vigour on account of the unique information which the results afford on the reactivity of radical types.Not only can the familiar monomers undergo copolymerisation but many com- pounds which do not themselves form high polymers can copolymerise with other monomers. In some cases the peculiar situation arises in which two monomers neither of which polymerises alone copolymerise with each other e.g. maleic anhydride and stilbene. Most of the work carried out has been with systems of two monomers. If these monomers are denoted by A and B then there are two possible modes of initiation in the case of either radical or photochemical chain starting A -+ P rate = X . * (11) B + Qz rate = Xb . * (12) in which P and Q are the active radicals derived from monomers A and B.There will evidently be four propagation reactions P + A + P rate = kplw[P][A] Q + A + P rate = kp,aa[Q1[Al (13) In the rate constant l ~ ~ ~ the first subscript denotes the nature of the radical end in this case P derived from monomer A and the second denotes the monomer concerned in the reaction i.e. B. If as is most likely the chains are stopped by the mutual destruction of two active ends then the possible reactions will be P + B -+ Q rate = rC,,a&E"[Bl . Q + B + Q rate = rC,,tdQI[Bl } . rate = kt,,[P]a P + Q + non-radical products rate = kt,d[P][Q] Q + Q P+pl rate = kt,ab[Qla 61 Nature 1949 163 256. 62 Proc. Roy. SOC. 1949 A 198 252. 63 R. A. Fairclough and C. N. Hinshelwood J. 1937 538. * The terminology used throughout this section is that adopted by Melville's school.In this case the main difference from other workers is the use of u and p for monomer reactivity ratios which are currently denoted by r1 and r2 by most American workers. Other symbols are almost universally accepted. 326 QUARTERLY REVIEWS If the monomers can undergo purely thermal polymerisation then there are three possible initiation reactions wix. t} j radical products B + B So far no investigation has shown the " crossed " initiation term to be significant (see below). If the normal approximation is introduced that the reaction chains will be long then the consumption of monomer will be confined solely to the propagation reactions. Hence the consumption of the individual mono- mers is and Also since under the stationary state conditions it can be shown that - d[Al/dt = kp,m[PI[AI + kp,tdQlEAl - d[Bl/dt = kp,bb[Ql[B] f kp,ab[P][BI Jc~,adPlCBl =I Jcp,ba[QI[Al 9 3 .. 9 . . . I . . . 9 . . . Y ? * . . Methyl methacrylate , I f 3 I 9 3 1 ) ,f Vinyl acetate . . Maleic anhydride . ? 9 , . . * ? y . . where 0 = kp,m/kp,ab and p = kp,bb/kp,ba these being called the monomer reactivity ratios and defining in effect the activity of the monomers towards 1.16 0.74 0.64 0.56 0-20 0.67 1.2 0.29 0.50 0-10 0-17 0-011 0.03 20 TABLE V Monomer reactivity ratios66 Monomer A. Styrene . . 9 7 9 9 1 9 ,? 9 9 ) 9 9 7 9 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . U. 0.52 0.75 0.75 0.30 0.41 0-54 2-0 0.043 8-5 0.21 55 Monomer B. Methyl methacrylate Methyl acrylate . . Methacrylonitrile . Acrylonitrile . . . fi-Chloroethyl acrylate Vinylidene chloride .Vinyl acetate . . . Maleic anhydride . Methyl maleate . . Methyl fumarate . . p-Methoxystyrene . p-Chlorostyrene . . m-Chlorostyrene . . o-Chlorostyrene . . 2 5-Dichlorostyrene Methacrylonitrile . Acrylonitrile . . . Vinyl acetate . . . p-Methoxystyrene . cr-Methylstyrene . . Methyl acrylate . . Ethyl maleate . . Ethyl fumarate . . Stilbene . . - . 9 9 9 9 . . P. 0.46 0.20 0.1 8 0.16 0.04 0.10 0.1 4 0.0 1 0 0.03 0.25 0.82 1.025 1.09 1.64 0.80 0.65 0.15 0-015 0.32 0.14 9.0 0.043 0.444 0.03 UP. 0.24 0.15 0.13 0.048 0.016 0.054 0.28 0.55 0 0.026 0.053 0.94 0.76 0.70 0.92 0.16 0-43 0.18 0-30 0-093 0.07 0.90 0.007 0.005 0.009 Temp. 60" 70 60 60 60 60 60 60 80 60 60 60 60 60 60 70 60 60 60 60 60 60 60 60 60 _ _ ~ BURNETT RADICAL POLYMERISATION REACTIONS 317 the same radicals.This copolymer composition equation derived inde- pendently by E'. R. Mayo and F. M. Lewis g4 and by T. Alfrey and G. Gold- finger,s5 has proved to be of immeasurable importance in studying the reactivities of monomers. By means of sensitive analytical techniques the monomer reactivity ratios for a wide range of monomer pairs have been determined a selection of these being set out in Table V. Since the monomer reactivity ratios are ratios of propagation constants it would not be expected that the values of o and p would vary very much with temperature. Table VI shows results obtained from two widely differing temperatures. The data show quite conclusively that there is a slight but definite difference in the monomer reactivity ratios at the higher temperatures. The authors 77 showed further that the entropy changes differed in most cases only slightly from zero so that whether two monomers TABLE VI EBect of temperature on monomer reactivity ratios Type.Styrene . . . . Methyl methacrylate Styrene . . . . Methyl acrylate. . Styrene . . . . Ethyl maleate . . Styrene . . . . Ethyl fumarats . Styrene . . . . p-Chlorostyrene . Monomer reactivity ratios 60". 0.520 f0.026 0.460 f0.026 0.747 +0*028 0.182 -&0-016 6.52 f 0.05 0.01 0.301 f0.024 0.0697 &0-0041 0.742 f0-030 1.032 f0.030 131". 0.590 f 0.026 0-536 f0.026 0.825 f 0.005 0.238 +0*005 5.48-&0*56 - 0,400 f0-014 0-0905 f 0.0008 0-81 6 -& 0.01 5 1*042f0*015 AH - AH (cals./mole). AS1 - ASS (E.U./mole). 480 &250 580 f 280 380fl40 1020f340 - 660 f480 - 1070 f 320 990f290 360 f 170 35k120 0*12f0-68 0.19 +O-76 0.54f0.36 0*66+0*86 1.87 f 1.36 - 0.82 f0.82 - 2.35 f.0.73 0.48 k0.43 0.40 f 0 * 32 64 J . Amer. Chem. Soo. 1944 66 1594. e5 J . Chem. Physics 1944 12 205. 66 Many other ratios are to be found in a review on copolymerisation by R. Simha 67 F. M. Lewis F. R. Mayo W. Cummings E. R. Briggs and C. Walling J . Amer. 68 T. Alfrey E. Merz and H. Mark J . Polymer Sci. 1946 1 37. 69 F. M. Lewis C. Walling W. Cummings E. R. Briggs and W. J. Wenisch 70 F. M. Lewis F. R. Mayo and W. F. Hulse ibid. 1945 67 1701. 71 F. R. Mayo C. Walling F. M. Lewis and W. F. Hulse ibid. 1948 70 1523. 7 2 T. Alfrey and E. Lavin ibid. 1945 6'9 2044. 73 F. M. Lewis and F. R. Mayo ibid. 1948 70 1533. 74 C. Walling E. R. Briggs K. B. Wolfstirn and F. R. Mayo ibid. p. 1537. 75 C. Walling E. R.Briggs and K. B. Wolfstirn dbid. p. 1543. 76 T. Alfrey and T. G. Harrison ibid. 1946 68 299. 77 F. R. Mayo F. M. Lewis and C. Walling Faraduy SOC. Discussions 1947 2 285. and L. A. Wall J . Nat. Bur. Stand. 1948 41 521. Chem. SOC. 1948 'SO 1519. J . Amer. Chem. SOC. 1948 'SO 1527. 318 QUARTERLY REVIEWS will copolymerise or not seems to depend solely upon a difference with a maximum of about 3 kcals. in the free energies of activation for the two competing reactions of a free radical. The reactivities of monomers and the ease with which they enter co- polymerisation appear to be influenced by three main factors (1) General overall reactivities of monomers towards all types of radicals ; (2) tendency of monomers to alternate on entering the copolymer; (3) steric effects. The reactivities of the monomers can be found from the reciprocals of the ratios set out in Table V but until sufficient values of kp,aa have been deter- mined there is little point in comparing monomer reactivity towards different radicals.Attention has to be focussed rather on the reactivity of monomers toward the same radical. Certain trends are however found in most cases although these may not be always duplicated as the radical type changes. For the exception it is generally found that there is a high tendency to alternate on entering the copolymer. Since the value of the propagation constants of styrene methyl meth- acrylate and vinyl acetate are relatively well established it is possible to evaluate the rate constants for the crossed type of propagation reaction. The values of rate constants for the interaction of radical and monomer are set out in Table VII.T8 TABLE VII Propagation rate constants of the type kp,ab for some copolymer pairs at 60" Styrene .. . . . . . . . Methyl methacrylate . . . . . Acrylonitrile . . . . . . Methyl acrylate . . . . . . Vinylidene chloride . . . . . Vinyl chloride . . . . . . . Vinyl acetate . . . . . . . Ally1 acetate . . . . . . Ethylfurnarate . . . . . Ethyl maleate . . . . . . . trans-Dichloroethylene . . . . cis-Dichloroethylene . . . . Styrene." 59 114 147 79 29-5 3.5 1.1 0.66 9.1 1.8 197 Vinyl acetate? 2.6 x 105 1.8 x 105 4.3 x 104 2.6 x 104 2.6 x 105 1.1 x 104 2-63 x 103 4.4 x 103 2.4 x 105 1.5 x 104 2.7 x 103 4.2 x lo2 I 0.26 Methyl methacrylate." 386 177 131 70 14 - 8.9 7.7 8.9 - I - (a) Calculated from the data of Bamford and Dewar.'ts2 ( b ) Calculated from Dixon-Lewis's data. Here some points are immediately apparent. The reactivities of the three radicals those derived from styrene vinyl acetate and methyl meth- acrylate fall into their expected order of reactivity i.e. vinyl acetate > methyl methacrylate > styrene. This is in agreement with the effect of substituents in stabilising radicals already given by N o ~ a k i ~ ~ i.e. *Ph > *CO,Me 21 *Me > *OAc. It will also be noted that the values of the 78 C. H. Barnford Paraday SOC. Di8cusaionrr 1947 2 317. BURNETT RADICAL POLYMERISATION REACTIONS 319 rate constant for some of the reactions involving vinyl acetate radicals rise to very high orders (> lo5). This would suggest by analogy with other radical reactions (see above) that the energy of activation for these processes is very low.This is further substantiated by the fact that the orders of magnitude of these velocity constants are of the same order as those normally associated with radical-radical reactions. Hence the nature of the sub- stituent does appear to have a profound effect on the stability of the radical. It will be seen from Table VII that there is a general trend in the values of the propagation constants which tend to become less as one goes down the table. There are some notable exceptions which require further con- sideration. In copolymerisation reactions generally two distinct types of reactionthe ideal and the alternating-have been distinguished. In the case of ideal copolymerisation the nature of the radical end in no way affects the addition of the next monomer molecule to the chain although some slight preference may be shown.In the ideal case in which the copolymer is formed in a purely random manner (r =p-l so that ap will be unity. Alternation is easily detected since 1 1 copolymers will be formed over a wide range of monomer mixtures. This must imply that radicals with free ends derived from monomer A will preferentially react with m0nomer.B and vice versa. Thus the nature of the radical end deter- mines the addition of the next monomer unit. This must mean that the constant for both monomers must be very much less than the constant for the crossed propagation reaction so that both CI and p will be small and ap will approach zero. As with all wide generalisations it is difficult to fit individual cases into either class with certainty but the value of the product of the monomer reactivity ratios can be taken as a very good indication of the tendency of a monomer pair to alternate.Table V gives the values of ap for a number of monomer pairs and it will be seen that in some cases unity is closely approached while in others the values lie below 0.5 showing a tendency to alternate and some do in fact reach zero showing complete alternation. Three lines of attack have been developed with a view to elucidate the nature and underlying cause of alternation. The first of these has been elaborated by C. C. Price 79 and by T. Alfrey and C. C. Price.** These workers assume that the controlling factor is the activation energy which is lowered by an electrical charge effect between radical and monomer.The rate constants for the addition of radicals to monomer can then be expressed in the form hp,aa = &a exp(- A W R T ) kp,ab = Aub e ~ p ( - AP&/RT) = A ex??[- (Pu + Qu + &2)l = A& =€e- (Pu + Qr + &u&b)l and in which the A factors denote probability factors pa denotes a reactivity factor for the end group qG and qa similar factors for monomers A and B ‘I9 “Reactions at Carbon Carbon Double Bonds” 1940. 8o J . Polymer Sci. 1946 1 83. 320 QUARTERLY REVIEWS and E and E~ are electrical (charge) terms. If it is assumed that there is no steric hindrance (Le. A, = A,& and that the same charge is conferred on both radical and monomer by the same substituent then kp,aa = PaQa exp( - &ae) k p a ~ = Pa&& exp( - Eaeb) where Pa is characteristic of the radical end and Qa and Qb are general monomer reactivities.Hence the nQonomer reactivity ratios can be written in the form (Qa/&b) e-E- Ea(&a - 411 and (QblQa) exp[ - d ~ b - &a)] By this method the monomer can be classified in terms of a general reactivity (Q) and a polar factor (E). As a result of a thorough study of the copolymer- isation of styrene with various substituted styrenes it was concluded that these ideas although giving a sound enough qualitative picture of the reactions could not be made sufficiently quantitative.s1 M. G. Evans 82 has shown that in the case of the interaction of radical A with monomer B to give a radical F the heat of reaction can be split into several terms as follows I;T = ( - RA - RM + RF) - -&:a + &-a where R are resonance energies and the D terms are dissociation energies.Changes in the activation energies will be given by AE = aAH = ~ ( R A + RH - RF) For values of propagation constants in monomer reactions therefore kp,a/kp,b = exPC(- Ea -k Eb)/RTI = ~XPC- ~(%?,a - &2,a)/R271 When this is checked against reasonable values for the resonance energy of methyl methacrylate styrene and vinyl acetate it gives values of a com- pletely at variance with those computed from the energies of activation for the propagation steps and the heats of reaction. This disagreement may however be due to the fact that in the above treatment all entropy changes have been neglected. Mayo Lewis and Walling *l investigated the copolymerisation of a number of substituted styrenes with styrene methyl methacrylate and maleic anhydride which show an increasing tendency to alternate with the styrene group of monomers.They deduce that the alternating effect is not wholly a polar one. This conclusion was reached as a result of compar- ing the reactivities obtained with Hammett’s o constant,83 defined by log(E/k,) = op where k and k are the velocity constants for substituted and unsubstituted benzene o is characteristic of the substituent and p of the reaction. The 0 constant has been interpreted in terms of the ability of the substituent to donate electrons to or withdraw them from the locus 81 F. R. Mayo F. M. Lewis and C. Walling J . Amer. Chem. Xoc. 1948 70 1529. aa Paraday SOC. DLwi.mions 1947 2 321. s3 ‘‘ Physical Organic Chemistry ” 1940. BURNETT RADICAL POLYMERISATION REACTIONS 321 of reaction.For styrene and substituted styrenes in which the tendency to alternate is negligible the correlation between reactivity and the Hammett constant is very good. Certain discrepancies begin to appear when meth- acrylate is the monomer and with maleic anhydride all correlation between the two quantities disappears. It is therefore concluded that the phenomenon is best explained in terms of the electron-accepting and -donating capacities of carbonyl conjugated and aromatic systems with a subsequent increase in the stability of the active complex due to resonance. In the case of styrene and maleic anhydride electron donation would result in structures of the tSlpes (I) and (11). The styrene portion would be stabilised by resonance between 26 possible structures while the carbonyl AE.- 0.6 - 0.3 - 0.1 0 (0) 0.35 portion becomes the relatively stable enolate ion. For the attack of the styrene radical on a conjugated double bond similar resonance stabilisation can occur. Not only does this explain the cause of alternation in a qualita- tive manner but it also indicates why maleic anhydride has a greater tendency to alternate than methyl methacrylate since the former has two carbonyl groups over which the odd electron and the charge can be distributed. This theory is very similar to that adopted by J. Weiss 84 to explain the formation of coloured complexes between these compounds although this type of complex must be carefully distinguished from the active com- plexes which the above theory requires. At the same time C. C.Price,85 using the values of CT and p obtained by a variety of workers has pointed out that the correlation between the alterna- tionin charge due to substitution and the Hammett constant is very good as the following table shows. Price therefore concludes that his theory is Correlation between Hammett’s constant and E values for c o ~ o l y m e r ~ s ~ t ~ o n of substituted styrenes d 88. - 0.44 - 0.27 - 0.17 - 0.07 (223 Substituent. p -Dimethylamino - p-Methyl- . . . m-Methyl- . . . (Hydrogen). . . p-Chloro- . . p-Methoxy- . . Substituent. p-Iodo- . . . p-Bromo- . . . m-Chloro- . . m-Bromo-. . . p-Cyano- . . . p-Nitro- . . . AE. 0-35 0.45 0.45 0-55 1.1 1.2 0.28 0.23 0.37 0.38 1.0 1.27 a (a) The values are for phenol derivatives. quite adequate to deal with the problem and it has at any rate the definite advantage that it enables the worker to put figures to the phenomena rather than deal in purely qualitative principles.E4 J. 1942 255. 86 J. Polymer Sci. 1948 3 772. 322 QUARTERLY REVIEWS That steric effects do play an important part in copolymerisation processes can be seen from Tables V and VII in which it will be noted for instance that ethyl maleate is much less reactive than the corresponding fumaric ester. In the former the whole molecule with the exception of the ethyl groups will be in the same plane whereas in the latter only one carbethoxy-group can be coplanar with the double bond. Hence the resonance contribution of the maleate to a radical adduct will be much less than that of the fumarate so it can be predicted that for this reason alone the fumarate will be the more active.In addition to this resonance effect there will be the purely geometrical steric hindrance of the bulky carbethoxy-groups which would tend to impede the approach of the attacking free radical. Complications such as these appear to be incapable of explanation on the basis of Price's theory. Termination in Copolymerisation.-As was shown previously three possible termination reactions can be envisaged as occurring in copolymerisa- tion reactions. To obtain information regarding these constants it is necessary to find the rate of copolymerisation. The analysis of the rates of reaction for such systems has been carried out by H. W. Melville B. Noble and W. F. Watson.86 If the termination reaction scheme is Pr + Ps + Mr + Ms kt,rs,aa Pr + Qs -j. Mr + Ms kt,rs,ab the reaction velocity coefficients require to be split in some way in order to cope with the mathematical analysis.As a first approximation it is taken that Qr + Qs + Mr + Ms kt,rs,bb 1 ' - (17) kt,rs,aa = kt,r,a + kt,s,a kt,m,ab I= kt,r,a f kt,s,b b,rs,bb = kt,r,b + kt,s,b which implies that the terminating capacity of a radical is the same for like and unlike radicals. This is however somewhat crude so that a more general form of (17) has to be applied i.e. k;t,rs,d = kt,r,ab - kt,u,ab (18) By using a system of kinetic analysis similar to that of Gee and Melville,5 the following expressions have been derived. Using the assumptions of equation (17) we have and using equation (18) we have (20) If # = 1 then it is evident that equation (20) will reduce to equation (19).All of the constants in equation (20) are known from monomer experiments and from copolymer composition except 4 so that a study of the rate will - d([A1 $. [ B ] ) - - {dAI2 -k 2[Al[Bl -k P [ B I 2 ) @ a -k Xb)'/' dt ( c ~ & ~ [ A ] ~ + 2+SaSbG&A][B] + ,U2fib2[B]2 )'/' * in which sa"(kt,r,m)1'2/kp,au ; (sb= (kz,r,bb)1'2/kp,bb ; #= (kt,r,ab * kt,u,ab)/(kt,m kt,sb). Be J . Polymer Sci. 1947 2 229. BURNETT RADICAL POLYMERISATION REACTIONS 323 immediately determine whether # is unity or not. If # > 1 then it must immediately follow that the termination reaction between unlike radicals is preferred to that between like radicals. The problem of determining the rate of copolymerisation is naturally a somewhat complex one but this has now been accomplished with considerable success.87 88 Melville and his collaborators have investigated the systems styrene-methyl methacrylate and styrene-butyl acrylate and Walling has studied the copolymerisation of styrene with methyl methacrylate methyl acrylate and vinyl acetate.In the case of styrene-methacrylate it was found that the addition of small quantities of styrene brought about a marked reduction in the rate of reaction while further additions produced no comparable effect as is shown in the following table. Rate of copolymerisation of styrene and methyl methacr~late I I I I I I I i Mole fraction of styrene 0 1 0-052 0.195 0.418 0-804 1-000 Rate moles yo per hour 1 4.65 1 1.88 j 1.13 1 0.82 I 0.80 1-10 1 On the assumption that the rate of initiation will vary linearly with monomer composition the value of # for this system will be approximately 45 indicating preferential " crossed " termination.When the rates of initiation are determined by osmotic pressure molecular-weight measure- ments it is found that the above assumption is invalid and by making the necessary calculations it was found that the value of # = 14 gave a very good fit with the experimental rates. Since the termination constants for the individual monomers are known it is possible to evaluate the velocity constant for the reaction between styrene radicals and methacrylate radicals. This was found to be 2.06 x 10s 1. mole-lsec.-l compared with 8 x lo6 for the interaction of styrene radicals and 2.7 x lo7 for methacrylate radicals. Thus it will be clearly seen that there is a definite preference for the termina- tion of the chains to take place by the interaction of unlike radicals.It has been suggested that a possible explanation of the non-linearity of the measured rates of initiation as a function of monomer composition may be that unlike radicals for some reason probably due to polarity combine rather than disproportionate. This postulate does give a closer agreement to the supposed linear variation of initiation with monomer composition and gives an upper limit of 30 to the value of #. The position with the butyl acrylate-styrene is not so clear as for the methacrylate-styrene systems. In this case no single value of # appears to be capable of covering the whole range of monomer compositions the values of # vary considerably but are again very much greater than unity having a most probable value of the order of 150.The results obtained by Walling for the methacrylate-styrene copolymer- isation give a value of # very close to that already given (# = 13). Some E. J. Arlman H. W. Melville and L. Valentine International Colloquium on Macromolecules Amsterdam 1949. 88 C. Walling J . Amer. Chem. Soc. 1949 71 1930. 324 QUARTERLY REVIEWS discrepancies in the actual working out of this value may give a slight alteration but will not seriously disturb the general agreement between the two sets of results. In the investigation of the methyl acrylate-styrene system the reaction was carried out at 60° a-azobisisobutyronitrile being used as catalyst. The rates of polperisation show the same trend as reported by Melville et al. i.e. the addition of small amounts of styrene causes a large reduction in the rate of reaction whereas further addition has little effect.The value of # stated to fit the rate curve is given as 40 but the values of 6 and 6 b are suspect and closer study reveals an arithmetical error which has the effect of raising the value of 4 to about 300.* In any case it is evident that the crossed termination mechanism is again very much favoured. Purely thermal initiation being used in the styrene- methacrylate copolymerisation the rate of initiation will be where the ki terms are rate constants for thermal initiation. Walling makes the assumption that the rate constant for the thermal polymerisation of methyl methacrylate is very much less than that of styrene and on this basis calculates that ki,& will be 6 x 1. mole-lsec.-l as compared with 2.15 x 10-12 for pure styrene.The work of Bamford and Dewar 32 makes it unlikely that the assumption used here is correct so little can be said as to the value of the initiation constant for ‘‘ crossed ” initiation or whether indeed it occurs at all. In the foregoing discussion it is evident that in all cases the value of 4 will be greater than unity so it is desirable to have an explanation of the under- lying reason why termination between two unlike radicals should be pre- ferred to other types of termination. Melville Arlman and Valentine suggest that there is a possible electronic explanation of this phenomenon derived along the lines of the Alfrey-Price theory mentioned in connection with monomer reacti~ity.7~ Modifications which are introduced allow the termination constants to be written in the form kt>t, = A&a2 exp(- €a2) k,bb = A b b p b 2 exp( - &b2) fit& = A a b P a P b @xp(- &a&b) where the A’s are probability factors Pa and p b are general reactivities of radicals and 8 and&b are electrical charge terms.Substitution in the definition of # and squaring gives since from equations (15) and (16) op = exp(- + 2&,&b + &b2). Reference to the data of Table V shows that the value of op for meth- acrylate-styrene copolymerisation is 0.24. Thus this alone will not be capable of raising the value of C# to the order of 14. It is therefore evident that the probability factor for crossed termination must also be exceptionally high in order that the necessary value of $ can be approached. This may * The Reviewer’s thanks are due to Dr.L. Valentine for pointing out some of the arithmetical errors in this paper. BURNETT RADICAL POLYMERISATION REACTIONS 325 be due to steric effects which as usual can be purely geometrical or may also give rise to greater or less stability of the radicals on the grounds of resonance alone. Again the methyl acrylate-styrene system gives a value of ap which is only about 0-5 times that of the former system so it is evident that a further enhanced probability factor will be necessary to give a # value of 300. At the same time one might in the latter case expect an increased probability of mixed termination in view of the removal of a blocking methyl group between methacrylate and acrylate molecules. Preliminary experi- ments which have been carried out with p-methoxystyrene-styrene systems for which up N 1 tend to show that in this case the value of 41 is much lower than those quoted and may indeed approximate closely to unity,ss thus indicating that no longer is crossed termination preferred.The following table sets out the values of individual velocity coefficients which have been obtained in this work. Rate constants in copolymer reactions Radical.* Styrene Styrene Butyl acrylate Butyl acrylate Styrene Butyl acrylate { gZ;Exylate Styrene Methyl methacrylate Methyl methacrylate Methyl methacrylate Methyl methacrylate { Styrene Methyl acrylate Styrene Methyl acrylate Methyl acrylate {Styrene b Methyl rtcrylate Molecule. Styrene Butyl acrylate Butyl acrylate Styrene - - Methyl Styrene Methyl methacrylate methacrylate - Methyl acrylate Methyl acrylate Styrene molecule a - Rate constant (1.mofe-%ec.-') At 25'. 39.5 89.7 22.5 13.1 77.0 7.9 x 106 1.6 x lo* (mean value) At 30'. 95 5.7 x 107 545 230 2.7 x 107 2.1 x 108 At 60". 4692 79 5.36 x 106 2-3 x 104 1.49 x 109 Energy of activation (kcals. per mole). 5.5 4.4 5.0 1.0 70 1 Frequency factor. 8.9 x lo6 5.1 x lo6 - 4.4 x 104 1.6 x 104 - 2.2 x 108 8.5 x 106 8.5 x 105 9.5 x 106 1.5 x lo8 2 x 108 - 5-60 x 1O1O - L * Where B second species is not given reaction is between two identical radicals. Sg L. Valentine unpublished data. 326 QUARTERLY REVIEWS In the oxidation of hydrocarbons there are also three possible termination reactions and in this case the assumption made is equivalent to a value of C$ = 1 for crossed termination. In the light of copolymerisation experiments it may be necessary to revise this view although there seems to be some support for the original assumption in that the radicals are very similar and there is little variation in the values of termination constants as the hydrocarbon is varied. 2o The Reviewer would like to record his thanks to Prof. P. D. Bartlett and Dr. M. S. Matheson for their help in forwarding manuscripts of unpublished papers and for other data incorporated in this review. i c Wuve-lellgtlls (clIl.-'~) broiiiitle ernulsion LI dycd with cyaiiiiie B dyed with Iiiiinc.yiinol

ISSN:0009-2681    

DOI:10.1039/QR9500400292

出版商:RSC    

年代:1950

数据来源: RSC