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QUARTERLY REVIEWS MECHANISMS OF OXIDATION BY COMPOUNDS OF CHROMIUM AND MANGANESE By WILLIAM A. WATERS M.A. Sc.D. F.R.S. (DYSON PERRINS LABORATORY OXFORD UNIVERSITY) Introduction The Essential Problems.-Chromic acid and potassium permanganate have been regularly used as oxidising agents for well over a century both in volumetric analysis and in degradative organic chemistry yet even today knowledge of the mechanisms of their reactions is only fragmentary. In organic chemistry few of their reactions are specific or even quantitative. The overall valency changes which the respective anions undergo when they are reduced to stable products (eqns. 1-3) cannot occur except by (HCrOJ- A Cr3+ (3-electron change) . . . ( 1 ) (Mn0,)- Mn2+ '(5-electron change) . . . (2) (Mn047 alkaline '5 MnO (3-electron change) .. . (3) a series of consecutive changes because quantum restrictions prevent more than a pair of electrons associated with any one atom from ever behaving in an identical manner. Again the electron transfers involved in these complete reductions of chromate and permanganate are not potentiometric- ally reversible; the redox potentials listed for them are spurious figures calculated from free-energy changes [AE = ( R T/nF)AG]. have shown that electron transfer between ions of similar atomic structure is fast and reversible (e.g. eqns. 4 5),2* 3 whereas reactions clearly involving the rupture of covalencies (e.g. eqn. 6) * are generally slow for with them appreciable activation energy is needed to reach the transition state. Stron Many studies of isotopically labelled ions *Mn2+ 4- Mn3+ -.- Mn2+ + *Mn3+ .(4) fMn02- + (Mn04)2- ?MnOJ2- + (Mn041- . (5) (H"0)- 9 (Mn03-0H)- (H"O-Mn03- + (OH)- - (6) C. B. Amphlett Quart. Rev. 1953 8 219; W. F. Libby J . Phys. Chem. 1952 56 2 A. W. Adamson ibid. 1951 55 93; M. J. Polissar J . Anzer. Chem. Soc. 1936 W. F. Libby ibid. 1940 62 1930; J. C. Sheppard and A. C. Wahl ibid. 1953 863; D. R. Stranks and R. G. Wilkins Chern. Rev. 1957 57 743. 58 1372; J. A. Happe and D. S. Martin ibid. 1955 77 4212. 75 5133; 1957 79 1020. 4M. C. R. Symons J . 1954 3676. S 277 278 QUARTERLY REVIEWS Both electron transfer and covalency breaking processes are involved in the sequence of consecutive reactions involved in the stripping of oxygen from anions of type (M04)n- to give cations Mm+ and it is now clear that the order in which the successive valency changes occur may differ from reaction to reaction according to the nature of the substrate that is attacked.From mechanistic studies of these changes there is for the inorganic chemist much to be learned concerning the structure and stability of chromium and manganese ions of intermediate valency level. Here the organic chemist can give valuable help because today a considerable body of evidence concerning the ways in which organic compounds of different structural type tend to oxidise can be used to forecast probable reaction paths. Again only by the use of organic compounds can one readily select oxidations that are slow enough for kinetic study and the possible detection of transient intermediates. This Review will therefore deal mainly with oxidations of organic substances.Routes of Oxidation of Organic Compounds.-Below pyrolysis tempera- tures oxidations are all bimolecular or concerted reactions. These can be classed as (a) 1-electron transfers (e.g. 7; Fenton's reaction) or ( b ) 2- electron transfers of type S,2 (e.g. 8; epoxidation of an olefin). HO. + H-CH,-OH - HO-H 4- .CHFW . . (7) Ph.CO.O?-OHw=CHR- Ph-CO-0- 4- HO-C%-EHR . (8) Processes (a) necessarily generate free radicals and often lead to chain reactions with very distinctive kinetics whilst processes ( b ) involve transient ions and are rarely chain reactions. Thus kinetic studies can often be dis- criminatory but may be difficult to interpret when consecutive reactions are involved. Detection of a free radical by a characteristic reaction e.g. catalysis of vinyl polymerisation or direct combination with oxygen is diagnostic of (a) whilst recognition of reactions typical of carbonium ions e.g.1 2-molecular rearrangement can point to (b). Recognising that favoured reactions take the path requiring the least activation energy one can taking solvent conditions into account forecast the route of oxidation from structural considerations. 1 -Electron transfers become exothermic and require little activation energy only if the immedi- ately resulting radical is a resonance-stabilised system. Related studies with several 1-electron abstractors (Ce4+ (Fe(CN) )3- Cu2+ etc.) of which the complexed ions of MnIII are typical (p. 296) indicate the general sequence of oxidisability to be 1 4 or 1 2-dihydric phenols > monohydric phenols > aldehydes and ketones (as enols) 1 2-glycols > allylic systems ethers > monohydric alcohols olefins.Olefins other than allylic systems are not easily attacked except by very powerful oxidisers such as Co3fY5 especially if of catio-enoid type (C:C*C:O or C:C*CN) since radical-ions =C-C= are entities of high energy level. Olefins however are easily attacked by electrophilic reagents of type ( b ) e.g. peracids or halogens particularly in solvents such as water C. E. H. Bawn and J. A. Sharp J . 1957 1866. + * WATERS OXIDATION BY CHROMIUM AND MANGANESE CORWOUNDS 279 which can co-ordinate and then react with carbonium ions. Again removal of hydrogen from H-C is greatly facilitated by a base-catalysed concerted 2-electron movement (9 cf. p. 284) whereas homolytic hydrogen abstraction (e.g.7) needs free radicals of very high energy such as hydroxyl or atomic chlorine unless the resulting organic radical has a high degree of resonance stabilisation as in Aryl-C-. By inductive reasoning along these lines tentative oxidation mechanisms can now be inferred with a sufficient degree of success to aid considerably the interpretation of experimental results. However much of the pub- lished evidence for propounding the foregoing generalisations comes from studies of oxidation by means of chromium or manganese compounds that are described more explicitly in the following paragraphs. Valency states of chromium and manganese Since oxidations by chromic acid and permanganate involve the eventual stripping of oxygen from anions (Cr0,)2- (Mn0,)- to give cations Cr3+ Mn2+ it is important to know when and how this removal of oxygen occurs To this end it is helpful to consider structurally similar ions of the whole transition group Ti V Cr Mn Fe Co.With manganese and vanadium each valency between that of a stable (XO,) anion and a cation X2+ is known and hence suggestions can be made concerning possible structures and properties of chromium ions of the intermediate valencies Crv and CrIV it being recognised that the stability and structure of any ion in solution depends on the acidity or alkalinity of its environment. The Ions of Manganese.-The purple permanganate ion (Mn0,)- is sym- metrically tetrahedral and the anion of a strong acid. In solution it does not exchange oxygen with water,4 but in strong alkali i t slowly decomposes to manganate and oxygen.4(Mn04)- + 4(0H)- - 4(Mn04)*-+ 2H20 + O2 . . . .(lo) (Mn0,)- + (OH)- + (MnOJ2- + .OH . . . . .(II) (Mn0,)- + (OH)- + 60)- (Mn04)2-+ (H0,)- etc. . (13) Symon~,~ using H2180 has shown that in equation (10) all the oxygen comes from the water and therefore favours a reaction sequence com- mencing with the electron transfer (1 1) first postulated by StammJ6 and continuing with further electron-transfer oxidation of dissociated hydroxyl (*O)- through H02* and (02*)- to oxygen (eqns. 11-13). Pol- lowing Stamm he has suggested that oxidations brought about by strongly alkaline permanganate are rea,lly effected by hydroxyl radicals or their *OH + (OH)’ GO)- 4- H,O . . . . . (12) H. Stamm “ Newer Methods of Volumetric Analysis ” trans. by Oesper Van Nostrarid & Co. Inc. New York 1938.280 QUARTERLY REVIEWS more active anions (*O)-. However this simple theory cannot suffice for those permanganate oxidations that can be effected rapidly in weak alkali.' The green manganate ion (Mn04)2- is stable only in alkali for solutions less than N in hydroxyl ion slowly disproportionate (eqn. 14). The sky-blue 3(Mn04)2- + 4H' - 2(Mn04)- 4- MnO + 2H,O . (14) (MnO4I2- + (OH)-= (MnOJ3- f .OM . . . . . (15) hypomanganate ion (MnO,) 3- obtainable as a sparingly-soluble salt K,MnO, by fusing together at red heat potassium permanganate and sodium hydroxide,* is even less stable and disproportionates similarly in solutions less than SN in hydroxyl ion. Reaction (15) can explain its route of formation. Whereas dilute aqueous (alkaline) solutions of hypomanganate can be prepared manganites X,MnO (with MnIV) and permanganites XMnO (with Mnv) are known only as solid products of alkali fusion of manganese dioxide.However there is now strong evidence (see p. 292) for the separa- tion of transient (MnO,) anions in several oxidations. Charge transfer between manganate and permanganate (eqn. 5) is fast and permanganate appears to convert hypomanganate very rapidly into rnang~~nate.~ Disproportionation of metastable solutions of (Mn04)2- and (Mno,)3- to stable (Mn04)- and hydrated manganese dioxide can be explained as occurring by electron transfer in this way for like Ti(OH), a compound Mn(OH) would be far too weak an acid to exist in any solution as (MIIO,)~- and would immediately be dehydrated to an insoluble product. The dissociation constants of H,MnO and H$hO4 are probably of the same order as those of H,CrO and H3VO4 and it can well be therefore that the disproportionations of compounds of Mnvl and Mnv involve the ions (HMn0,)- and (HMn0,)2-.10 These ions undoubtedly undergo displace- ment reactions involving hydroxyl for Symons has found that alkaline manganate in isotopic water H2180 slowly exchanges oxgyen with the water. This has been visualised as an X,2 substitution a t the manganese centre (eqn. 6). By using this equilibration and subsequently the dispro- portionation of manganate (eqn. 14) in more acid solution K. Wiberg has been able to prepare l80-labelled permanganate and therewith trace the route of the oxygen atoms in several of the oxidation reactions of perman- ganate solutions. Electronic structures absorption spectra and thermodynamic properties of ions (Mn04)n- have been studied by Symons and his colleagues.lo9 l1 The oxidising powers of the MnO anions decrease markedly in the order (Mn04)- > (Mn04)2- > (Mr~o,)~-.This is t o be expected for oxidation A. Y. Drummond and W. A. Waters J. 1953 435. H. Lux 2. Naturforschung 1946 1 281; R. Scholder D. Fischer and H. Water- J. S. F. Pode and W. A. Waters J. 1956 3373. stradt 2. anorg. Chem. 1954 277 234. lo A. Carrington and M. C. R. Symons J. 1956 3373. l1 A. Carrington D. J. E. Ingram D. Schonland and M. C. R. Symons J. 1956 4710; A. Carrington D. Schonland and M. C. R. Symons J. 1957 659. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 281 involves electron gain which has to be effected by electron movement contrary to the charge on the anion.With the metals from vanadium to iron the quadrivalent state marks the transition between stability as oxyanions and as cations; this can be associated with the symmetry of electrically neutral MIV(OK), and the tendency of such substances to be dehydrated to insoluble materials. Although manganese dioxide is a well-known oxidising agent long used in the presence of sulphuric or other mineral acids yet no homogeneous oxidations involving MnIV are known ; the fairly specific heterogeneous oxidations that can be effected by activated manganese dioxide in aprotic solvents l2 cannot yet be related to reactions of dissolved manganese compounds. Tervalent manganese gives a cherry-red cation Mn3+ in concentrated (> 9 ~ ) sulphuric acid.13 At lower acidities this cation rapidly dispropor- 2Mn3+ + Mn2+ + Mn4+ .. . . . . . . . (160) Mn4+ + 2H20 - MnO + 4H+ . . . . . . (166) tionates (eqns. 16a 16b) but MnIII can be stabilisecl in solutions of much lower acidity by chelating agents as for example pyrophosphate. Such solutions of MnIII are useful volumetric reagents.14 Their colours are not sufficiently intense to provide direct end-points but for this starch-iodide solutions can be used. The complexed ions e.g. {Mn(H2P20,),)3- are structurally similar to the octahedral co-ordination compounds of CrIII FeIII and ColI1 and have easily-displaced ligands. l5 The reactions of MnIII complexes are therefore similar to those of free Mn3+ but the redox potential of MnIII-MnII solutions naturally depends on the nature and concentration of the complexing agent.For the simple Mn3+-Mn2+ system this has the very high value of + 1.15 v. In neutral or weakly acid solutions Mn2+ is oxidised by permanganate to manganese dioxide. This Guyard reaction usually written as (17) is 2(Mn04)' + 3Mn2+ + 4(0H)' 5Mn02 + 2H20 . .(17) (Mn04)- + Mn2+ e (Mn0,)2- + Mn3+ . . .(18) not truly a reversible equilibrium for dissolution of hydrated manganese dioxide requires very much stronger acid than that which can completely prevent oxidation of manganous salts by permanganate. Again isotope exchange does not seem to occur between permanganate ions and man- ganous ions in the absence of a precipitate or colloid of manganese di- oxide.2 Several suggestions as to the mechanism of the Guyard reaction have been propounded all of which depend on postulates concerning the structures of transient manganese ions of intermediate valency.The first 12 See Ann. Reports 1952 49 142; 1953 50 170. l3 A. R. J. P. Ubbelohde J. 1935 1605. 1 4 I. M. Kolthoff and J. I. Watters Ind. Eng. Chem. Anal. 1943 15 8. l5 H. Taube Chem. Rev. 1952 50 47. 282 QUARTERLY REVIEWS stage is probably the reversible electron transfer (18). Reference to the published redox potentials (Mn0,)-(Mn0,)2- = + 0.6 v,l0 Mn3+-Mn2+ = + 1.51 v shows that oxidation of Mn2+ can proceed significantly only (i) if the acidity is low enough for the subsequent disproportionation of Mn3+ (eqn. 16) to follow with removal of Mn4+ as hydrated manganeEe dioxide or (ii) if a selective complexing agent has been added to lower the concen- tration of free Mn3+ to a requisite extent. In the presence of pyrophosphate for example manganous salts can be quantitatively titrated (electrometric- ally) at pH 6 to the MnIII stage with aqueous permanganate.14 Somehow the rapid reduction of (MnO,)- to Mn3+ in an acid solution must be possible by a route that does not involve formation of any significant amount of Mn(OH),.This is conceivable if compounds of Mnv and MnIV have am- photeric character and can form cations analogous to V02+ and V02+. Such ions have been postulated by A. W. AdamsonY16 F. C. Tompkins and E. Abel.17 The Ions of Chromium.-Chromic acid is a fairly strong acid ( K = 0.18; K = 3.2 x l8 andin dilute aqueous solution largely exists as (HCr0,)-. In more concentrated solutions the dehydration ( 19) occurs extensively. 2(HCr04)- (Cr20,)2- f HzO . . . . . .. . . .(I91 This vitiates any chance of using l 8 0 in studies of the mechanism of chromic acid oxidations. Alkaline chromate i.e. (Cr04)2- is devoid of oxidising power and it has been established that (Cr207)2- is a much weaker oxidiser than (HCrO,)-.lS Though the anhydride CrO, and the acid chloride CrQ2C12 are both powerful oxidisers particularly valuable since they can be used in orga.nic solvents the chlorochromate anion (Cr0,Cl)- is much less active than (Cr03-OH)-.20 Since strong acids enhance the oxidising powers of CrV1 it has been suggested that cations such as (HCrO,)+ can exist (cf. HSO,+). In this way too one can explain the solubility of chromium trioxide in acetic acid. In 1957 Bailey and Symons 21 prepared green K,CrQ4 by alkali fusion following the procedure for making K,MnO,.Its solutions in very strong alkali rapidly absorb oxygen and immediately disproportionate on dilution with water. Earlier Weinland and Fiederer 22 had prepared a series of salts such as Rb2CrOC15 and C,H,N,HCrOCl (C5H,N = pyridine) that appear to contain the anion (CrvOCl,)-. They will oxidise neutral solutions of iodide whereas neutral K2Cr2Q7 does not do so and are a t once decom- posed by water behaving stoicheiometrically as if they contained CrV. Quadrivalent chromium is known in CrF and CrCl,. The dioxide CrO, 16 A. W. Adamson J . Phys. Chem. 1951 55 293; F. C. Tompkins Truns. Faraday l7 E. Abel Mo?zutsh. 1949 80 455 and later notes. 18 J. D. Neuss and W. Riemann J . Amer. Chem. SOC. 1934 56 2235. 19 F. H. Westheimer and A. Novick J. Chem. Phys. 1943 11 500. 2o M.Cohen and F. H. Westheimer J . Amer. Chem. SOC. 1952 '74 4387. 21 N. Bailey and M. C. R,. Symons J. 1957 202. 2 2 R. F. Weinland and M. Fiederer Ber. 1906 39 4042. SOC. 1942 38 131. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 283 is thought to result from many chromic acid oxidations and has been isolated as a brown powder that immediately decomposes in ~ a t e r . 2 ~ By analogy with vanadium it can be suggested that CrO should be basic rather than acidic. Westheimer 24 has inferred that CrIV must be a stronger oxidising agent than MnTI1 (this has had some recent confirmation 25) and yet a powerful reducing agent. He considers that CrTV can oxidise iodide to iodine this being consistent with a cation (Cr4+ + I- + Cr3+ + 10) whilst Crv oxidises iodide to hypoiodite as would be expected for an oxyanion.How- ever the main evidence for transitory Crv and CrIV ions comes from the study of chromic acid oxidations of organic compounds. Analogies between chromium and the two adjacent elements vanadium and manganese are helpful in formulating hypotheses concerning mechanisms of reduction of chromate to C F . The strong reducer Cr2+ will not be considered in this Review. Chromic acid oxidations - inorganic reactions The mechanism of chromic acid oxidation was reviewed in 1949 by F. H. Westheimer 24 who had previously shown19 that the only active oxidising species in acidic aqueous solution is the acid chromate ion (HCr04)-. He pointed out that transient ions having intermediate valency states must be postulated to explain the occurrence of induced oxidations such as (20) and (21) under conditions in which direct oxidation of the second oxidisable component (I- Mn2+) did not occur.(HCrO,)- + Fe2++ 2 I- + 7H'- Cr3+ + Fe3+ + I + 4H20 (at pH 3) (20) 2(HCr04)- t 2H3As03 f Mn2++ 6H+ - 2Cr3' 4- 2H3As04 + Mn02 + 4H20 (21) . . . (23 I n rea,ctions such as (20) two equivalents of the second component can in the limit be oxidised per mole of chromic acid; this is indicative of oxidation through quinquevalent chromium. Kinetic study of the oxidation of ferrous ion by chromic acid indicates that the initial step is rapid and reversible a later reaction being rate-controlling. Thus the redox potential for the CrV1-Crv equilibrium must be similar to that of the FeIII-FeII system and by analogy with the chemistry of manganese (eqn. 18) re- action (22) can be postulated as a rapid equilibrium the ion of Crv resembling that of hypomanganate or of vanadate as recent work has verified.2l It may be noted that the inorganic reactions proceeding through Crv are those in which the primary reducing agent is an ion that can provide one eZectron e.g.Fe2+ Ti3+ Ce3+ V2+ VO+ VO,+. In reactions such as (21) only one equivalent of the second component is oxidised per chromate ion. This indicates that the reaction proceeds through quadrivalent chromium (reaction 23). Oxidations of alcohols 2 3 A. Leo and F. H. Westheimer J. Arner. Chem. Soc. 1952 74 4383. 24 F. H. Westheimer Chem. Rev. 1949 45 419. 28 A. E. Ogard and H. Taube J. Phys. Chern. 1958 62 6. 284 QUARTERLY REVIEWS (HCr04)- + (l-$Asq)- - Cr" 4- (H,As04)- . . . . (23) Crv' + CrIV - 2 ~ r V .. . . . . . . . . . . . (24 CrV + AS" - crm + AS^ . . . . . . . . . (29 hydroxy-acids and probably many other types of organic compound such as aromatic aldehydes are of this character and it may be noted that all these reactions involve t'he rupture of a covalency i.e. of an electron-pair bond. Kinetic analysis of oxidations of type (21) indicates however that both the valency levels of CrV1 and CrV are probably involved in t,he absence of the second oxidisable component and Westheimer has therefore postulated as a fast reaction the re-oxidation of CrIV and the 2-electron reduction of Crv (reactions 24 and 25) reaction (25) presumably being similar in type to reaction (23) as would be expected of an ion (H,Cr04)2-. Chromic acid oxidations - organic reactions (i) Oxidation of Alcohols.-This reaction has been studied in great detail.In 1943 Westheimer and Novick l9 established that the oxidation of iso- propyl alcohol resembles reaction (21) ; in the presence of manganous salts one equivalent of manganese dioxide can be obtained for every two equiva- lents of isopropyl alcohol oxidised. I n 1949 Westheimer and Nicolaides by comparing the rates of oxidation of H-CMe,*OH and D-CMe,*OH estab- lished that the H(D)-C bond of the alcohol is broken in the rate-determining stage of the oxidation.26 Westheimer 24 therefore concluded that a rapid reversible acid-catalysed esterification of the alcohol preceded the slower oxidation which he wrote as a concerted base-catalysed process giving quadrimlent chromium ((HCr0,)- or CrO,] that would rapidly oxidise Mn2+ but was usually destroyed by reaction (24).His reaction mechanism (eqns. 26 27) has been so widely applied as a model for other oxidations that it merits further consideration. (HCr04)- + 2H' + HO-CMe2-H H-CM5O.CrO2-OH:+ H20 . . (26) H,O - + H-CMe n- O-Cr02-OHZ H30++ CM%=O t H,CrO . (27) Dependence of the oxidation rate on [HCrO,-] and on [H+I2 (in moder- ately dilute acids) is consistent with an acid-catalysed esterification involving un-ionised H,Cr04. Chromate esters are now well known tert.-butyl chromate having been introduced by Oppenauer and Oberrauch 27 as a specific oxidiser for alcohols. It has been shown 23 that this reagent acts by rapid trans-esterification with primary and secondary alcohols this esterification and its reverse 28 usually involve electron-pair displacements at the chromium atom i.e.acyl-oxygen bond fission. Diisopropyl chromate which can be extracted by benzene from 26 F. H. Westheimer and N. Nicolaides J . Amer. Ghenz. Xoc. 1949 71 25. 2 7 R. V. Oppenauer and H. Oberrauch Anal. Asoc. Quim. argentina 1949 37 246. 28 M. Anbar I. Dostrovsky D. Samuel and A. D. Yoffe J. 1954 3603; H. H. Zeiss and C. N. Matthews J . Amer. Chem. Xoc. 1956 78 1694. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 285 chromic acid-isopropyl alcohol mixtures decomposes under anhydrous con- ditions in the latter solvent by a first-order reaction depositing a brown powder containing CrV1 and CrlI1 in the ratio 42 58 ( L e . slightly more Crvl than that required for CrIV). 23 This decomposition is catalysed by pyridine as would be expected for a base-catalysed fission of a C-H bond as shown in equation (27).However one can question whether neutral chromate esters R,CrO, are exact models for all reactions of chromic acid since solvents have a very great effect on the reaction velocity. In 86.5% acetic acid the rate of oxidation of isopropyl alcohol is 2500 times as fast as in water,29 a difference that can be ascribed to solvent displacement of the ionisation of H,CrO or to the formation of a highly reactive acetyl-chromic anhydride CH,*CO*O*CrO,*OH whilst chloride markedly retards reaction by formation of the more stable chlorochromate ion (Cl*CrO3)-.2O In glacial acetic acid chromic acid oxidations exhibit autoretardation owing to the formation of brown undissociated chromic-chromate com- plexes such as CrlI1(HCrO,)( O*CO*CH,),,30 that could easily be mistaken for chromium compounds of valency intermediate between CrV1 and C P .These however may be broken up by the addition of an acid stronger than H,CrO, e.g. sulphuric acid which markedly accelerates oxidations water and acetates being retarders that favour this complex formation The effect of phosphoric acid is similar,,l but more complex on account of its stepwise dissociation. For oxidations in strongly acid aqueous solutions Westheimer has sug- gested that proton addition to the acid chromate ester giving the cation shown in equation (27) favours the electron switch from the water molecule that acts as base catalyst. RoEek and Kr~picka,~2 by showing that the logarithm of the oxidation rate is linearly dependent on Hammett's acidity function H, argue that a molecule of water need not be required as catalyst and suggest that the oxidation proceeds by a bimolecular cyclic electron switch as in reaction (28) involving molecular H,CrO a t low acidities and a protonated species (H,CrO,)+ or (HCrO,)+ a t higher acidities.However Graham and Westheimer question the above argument ,29 since stoicheiometrically a reaction of rate dependent on the product [Ester of CrV1][H20][H+] is equivalent to the measured rate which depends on the product [H,CrO,][Alcohol][H+] and no extra water molecule. They concede however that in very strong acid a cyclic process such as (28) may operate. RoEek and Krupicka consider that the cyclic mechanism explains more satisfactorily than the catalysed mechanism the relative rates of oxidation 29 G.E. T. Graham and F. H. Westheimer J. Amer. Chena. Soc. 1958 80 3030. 30 R. Slack and W. A. Waters J. 1948 1666; 1949 599. 31 J. RoEek Coll. Czech. Chem. Comm. 1955 20 1320. 32 J. RoEek and J. Krupicka Chem. and Ind. 1957 1668; Chem. Listy 1058 52 1735. 286 QUARTERLY REVIEWS of para-substituted l-phenylethanols found by Kwart and Francis 33 to followthe order MeO- > Me&- > Me- > H- > C1- > NO,- electron-donating substituents favouring oxidation. Barton,34 accepting the validity of Westheimer's mechanism has sug- gested that for cyclic (e g. steroid) secondary alcohols molecules with acces- sible (equatorial) hydrogen atoms should be oxidised more readily than their hindered epimers (with axial H). This hypothesis assumes that the esterification equilibrium (26) is not markedly affected by steric conditions.Though qualitatively true and valuable for structural diagnosis Barton's conclusion is not quantitatively correct 35 and the cyclic mechanism (28) is in much better accord with observations. Oxidation of tertiary alcohols is more difficult than that of primary or secondary alcohols but can be effected in the presence of sulphuric acid. Though of first order with respect to the alcohol it is of zero order with respect to chromic acid and seems to involve the slow formation of an olefin which is then rapidly oxidised mainly to a ket0ne.~6 Esters of tertiary alcohols appear to be oxidised in a similar way. Chromic acid will also effect glycol fission and from the reaction between pinacol and chromyl chloride Slack and Waters 37 isolated an intermediate containing two atoms of chromium per pinacol molecule.This they supposed to decompose to give CrV however in view of later evidence of acid and base catalysts of glycol fission by lead tetra-acetate a concerted electron-pair mechanism may be more plausible. Compounds of CrV cannot however entirely be neglected in considering steps in alcohol oxidation. W. Mosher and his collaborators 3* noted that some C-C bond cleavage occurs during chromic acid oxidations of alcohols of type R,C*CHR'*OH. This phenomenon has been investigated more closely by Hampton Leo and Westheimer 39 for a-tert.-butylbenzyl alcohol Me,C-CHPh*OH which yields benzaldehyde and tert.- butyl alcohol as well as pivalophenone Me,C*COPh. Up to 67% of C-C cleavage may occur but this is sharply reduced by adding Mn2+ or Ce3+ ions to remove any CrlV + Ph-$H-CMe - Crm 4- Ph-$H-CMe3 .I . . .(29) OH 0. Ph-FH-CMe - Ph-C#=O t -CMe3 CrV' 0. + *CM% + HzO - CrV + HOOCMe . . . . .(30) + H+ . (31) 3 3 H. Kwart and P. S. Francis J . Amer. Chem. Soc. 3 4 D. H. R. Barton Experientia 1950 6 316. 1955 77 4907. 35 J. Schreiber and A. Eschenmosor Belv. Chim. Acta 1955 38 1529. 36 M7. F. Sagor J . Artier. Chem. Soc. 1956 78 4970; J. RoEek Coll. Czech. Chem. 37 R. Slack and W. A. Waters J. 1949 594. 38 W. Mosher and F. Whitmore J . Amer. Chenz. SOC. 1948 70 2544; W. Mosher and 39 J. Harnpton A. Leo and F. H. Westheimer ibid. 1956 '78 306. Cornna. 1958 23 833. E. Langerak ibid. 1949 71 286; 1951 73 1302. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 287 CrIV intermediate as soon as it is formed.It has been suggested therefore that the C-C cleavage is an oxidation involving Crv [formed from CrlV by reaction (24)] but a sequence of l-electron changes such as (29)-(31) can- not altogether be excluded if it is granted that reaction (24) may be rever- sible. Deuterium labelling by use of Me,C*CDPh*OH indicates that chromic acid itself does not effect C-C fission. (ii) Oxidation of Aromatic Aldehydes.-This is now well understood.29~ 40 The oxidation of benzaldehyde is of first order with respect t.0 both [PhCHO] and [HCr04-] and in the acid range 0.018-0*3~ the logarithm of the rate constant varies linearly with the Hammett function H,. Comparison with PhCDO shows that the rate-determining stage involves H-C cleavage (E,/lc = 4.3) as required by the mechanism Ph-CHO i- M+ i Ph&-OH .. . . . . . . . . . . . . (32) Ph-F' YH i- hCr04)- Ph-C+&-Cr02-!H PH A Ph-C=O PH + H+ + (Cr 02) + OH- (33) ti Fast H Electron-attracting groups (e.g. NO,) accelerate the oxidation as would be expected for the shift of the equilibrium of reaction (32) in formation of cations (Ar*CH*OH)+ and effects of substituents have magnitudes accord- ing with Hammett's a function. With benzaldehyde as with isopropyl alcohol oxidation it has been shown that the addition of nianganous or cerous ions reduces the oxidation rate by 30-40% by eliminating secondary reactions due to Cr1v.29 Oxygen greatly increases the oxidation rate.40 Uptake of oxygen is quite common in oxidations of organic compounds by chromic acid and naturally has been regarded as indicative of free-radical formation.41 However it is now thought that the oxygen absorption may be caused either by direct reactions of CrIV or CrV compounds with or by traces of organic free radicals formed by reactions between CrIV and organic molecules.Oxidations of aliphatic aldehydes have not received detailed study but may well occur by a different mechanism because oxidations of aliphatic ketones are thought to occur by way of their enols (cf. p. 297) to give mix- tures of a-hydroxy-ketones and a@-unsaturated ketones.42 In cyclic systems axial attack of the chromic acid is thought to occur. It is significant that aliphatic aldehydes are oxidised less easily than the corresponding primary alc0hols.~3 (iii) Oxidation of 0lefins.-Chromic acid oxidation of olefins eventually leads to carbon-carbon fission a t the olefinic link with formation of ketones and carboxylic acids but a very complex mixture of products can result from controlled reactions with limited amounts of oxidant e.g.40 K. B. Wiberg and T. Mill J . Arner. Chem. Xoc. 1958 80 3022. 41W. A. Waters (a) J. 1946 1151; ( b ) Trans. Faraday SOC. 1946 42 184. 42E. Wenkert and B. G. Jackson J. Amer. Chem. SOC. 1958 80 211. 43 W. Mosher and D. M. Preiss ibid. 1953 75 5605. 288 QUARTERLY REVIEWS The systematic studies of products by W. J. Hickinbottom and his collaborators both with chromic acid in aqueous sulphuric acid and with chromium trioxide in acetic anhydride have led to the conclusion that aliphatic olefins give mixtures that " can easily be explained by the initial formation of an epoxide 1 2-diol or of some polar product easily converted into one of the latter ",44~ 45 As the above example shows pinacolinic rearrangement products are often formed; this in fact was one of the main bugbears of early interpretations of oxidative degradations of ter- penes.With alicyclic compounds allylic oxidation is a frequent side re- action ; thus 1 -phenylcycZohexene gives up to 25% of 3-phenylcycZohex-2- e n ~ n e * ~ and cyclohexene 37 yo of ~ycZohex-2-enone.~~ In natural-product research this has been used for diagnosis of the -C=C-CH,- system and like oxygen uptake during the oxidation has been regarded as an indication of some homolytic attack on the methylene group. Even if this hypothesis is correct the reaction may be due to CrIV rather than Crvl and in any case it does not represent the main oxidation route.From Hickinbottom's studies the nature of the main initial reaction is now evident; it is electron-pair donation by the olefin to O=Cr giving a product that with water promptly yields the conjugate acid of an epoxide (I) 4 5 ~ 48 (reaction 34). 1 2-Diol formation is certainly a secondary reaction for epoxides can be isolated in high yields from olefins by the use of chromium n f i R,C=CHR' + O=$r=O -..r R2E-CHRiO-%r-O- -!% R,E-CHR.'-Oti + (HCr'"03)- (34) 0 0 (1) trioxide in acetic anhydride diluted with carbon di~ulphide.~~ Moreover pinacolinic products cannot be formed from 1 2-diols nearly as rapidly as the latter are oxidised under comparable conditions of acidity to C-C bond fission products. Even the epoxides cannot be the initial products for they can be oxidised by chromic-sulphuric acid mixtures more rapidly than they can be hydrolysed or isomerised by sulphuric acid of the same concentrati~n.~~ For aqueous chromic acid the oxidation is of first order with respect t o both the olefin and chromic acid.49 Representation as (35) giving a carbonium ion and not necessarily a cyclic intermediate such as (111) satis- 4 4 W.J. Hickinbottom I). R. Hogg I). Peters and D. G. M. Wood J . 1954 4400. 4 5 W. J. Hickinbottom D. Peters and D. G. M. Wood J. 1955 1360 and earlier papers of the series. 46 D. Ginsberg and R. Pappo J. 1951 516; L. F. Fieser and J. Szmuszkovicz J . Amer. Chem. SOC. 1948 70 3352. 4 7 F. C. Whitmore and G. W. Pedlow ibid. 1941 63 758. 48 M. A. Davis and W. 5. Hickinbottom J. 1958 2205. J9 H. H. Zeiss and F.R. Zwanzig J . Amer. Chem. Xoc. 1957 79 1733. WATERS OXIDATION BY CHROMIUM AND MANGANESR COMPOUNDS 289 factorily explains the difference between chromic acid oxidation and the attack on olefins of osmium tetroxide or potassium permanganate (p. 293); all subsequent stages can be adequately represented as passing via (I),48 RCH-CH O=Cr-CL (1 v) Mechanism (35) can be extended to oxidations of conjugated dienes which easily yield unsaturated 1 4-diketones or ketolsy50 and also to oxida- tion with double-bond shift of ,@unsaturated ket0nes,~1 reactions often encountered with terpenes and steroids possessing structures that could not yield cyclic C P intermediates $ 1 1 1 I I l l CH=C-C=CH + H,Crq - (HCrOJ- -t 'CH-C=C-CH-OH I I I I I I l l I I l l c (36) O=C-C=C- C=O F- HO-CH-C=C-C=O - HO-CH-C=C-CH-OH I I l l I I l l CH=C-C-C=O $H-C=C-C=O HY) k __t OH .. . . (37) HO-Cr 0 (HCrOJ- H+ The initial lead for the formulation of mechanisms (34) and (35) for chromic acid oxidations of olefins came from a study by Cristol and Eiler of the addition of chromyl chloride to 0lefins.~2 In carbon tetrachloride solution cyclohexene reacts rapidly to give C6H,,,Cr0,C1 and then much more slowly to form a brown solid C6Hlo(Cr02C12)2. Hydrolysis gives 30-40% of trans-2-chlorocycZohexano1 whilst olefins R*CH=CH, by similar treatment yield chlorohydrins R*CHCl*CH,*OH. This indicates that chromyl chloride acts as an electrophilic reagent adding by an oxygen atom to the n-electrons of the double bond giving a carbonium cation (R*CH*CH,-0-CrO-C1) + (cf. 11) for which various cyclic mesomeric or non-classical structures such as (IV) must be written so as to explain the eventual trans-combination of the chloride ion.(iv) Oxidation of Saturated Hydrocarbons.-This requires vigorous condi- tions and preferentially occurs at tertiary C-H groups or similar sites of re- activity such as a-CH groups of aromatic side-chains. This of course is the site of homolytic oxidation and possibilities of 1 -electron transfer have been considered by Slack and Waters.30 Oxidations of hydrocarbons a,re retarded by bases and catalysed by acids the effect of the latter being proportional 50 J. Elks R. M. Evans A. G. Long and G. H. Thomas J. 1954 451. 5 1 D. H. R. Barton N. J. Holness K. H. Overton and W. J. Rosenfelder J. 1952 5 2 S. J. Cristol and K. R. Eiler J . Arner. Chem.Soc. 1950 72 4353. 3751. 290 QUARTERLY REVIEWS to H,.53 This indicates attack by a cation e.g. (HCrO,)+. By comparing the rates of oxidation of Et,C-H and Et,C-D Sager and Bradley54 have shown that the rate-determining stage ruptures the C-H(D) bond. They find that the alcohol Et,C-OH is an early product and conclude that it is not formed from the free cation Et,C+ since this would promptly yield the much too easily oxidised olefin Et,C=CH*Me. A concerted electrophilic substitution followed by cleavage of an O-Cr link ass in hydrolysis of chromate esters thus seems to be indicated For such a mechanism steric factors should be important in H-C bridge- head molecules such as camphane which yields e~icamphor,~~ oxidative attack is transferred to a CH group. Undoubtedly the Btard reaction55 of chromyl chloride has a similar mechanism and it is significant that the Rtard complexes which generally contain two molecules of chromyl chloride per molecule of hydrocarbon are formed by direct addition and not by elimination of hydrogen chloride.It is suggested that they should be formulated as salts.56 Chromic acid oxidations of CH groups give high yields of ketones oxidations in acetic anhydride usually give their diacetyl derivatiGes >C( OAc), which usefully afford protection against subsequent oxidation. The oxidation may occur in stages but under the conditions that have to be used the isolation of easily-oxidised primary or secondary alcohols or their esters would clearly be impracticable. (v) Oxidations of Aromatic Hydrocarbons.-Though benzene is resistant to oxidation polycyclic aromatic and heterocyclic hydrocarbons such as anthracene and phenanthrene are easily oxidised to quinones.Qualitatively the order of reactivity of these compounds is that of their electron avail- ability and this is wholly consistent with attack by a cation such as (HCrO,) +. Again chromic acid easily destroys ring systems containing electron-donating substituents such as OH or NH,. Permanganate oxidations General Features.-The literature of oxidation by permanganate was reviewed in 1958 by Ladbury and C~llis.~V Consequently the following pages aim a t giving a broad picture of reaction mechanisms and show the very limited extent to which similarities can be traced between the chemistry of chromium and manganese. Enough has been said about the Guyard reaction (p.281) to make i t clear that it is only when the selection of experimental conditions allows of oxidation of Mn2+ and of the equilibration (16) that permanganate be- 53 J. RoEek Coll. Czech. Chem. Comm. 1957 22 1509 1519. 5 4 W. F. Sager and A. Bradley J . Amer. Chem. SOC. 1956 78 1187. 5 5 M. &tard Ann. Chim. Phys. 1881 22 218. 5 6 C. C. Hobbs and B. Houston J . Amer. Chem. Soc. 1954 76 1354. 57 J. W. Ladbury and C. F. Cullis Chem. Rev. 1958 58 403. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 291 comes a 5-electron acceptor. In minera'l acid solution where this oxidation of Mi2+ to Mi3+ can occur easily the manganic ions have a higher redox potential than (MnO,)- anions so many substrates are then oxidised by manganic cations and not directly by the permanganate.This is the case with those permanganate oxidations which exhibit autocatalysis the best known examples of which are the oxidations of hydrogen peroxide and of oxalic and malonic acids. By adding fluoride or pyrophosphate to remove free M n 2 + and Mn3+ these oxidations can be inhibited almost completely.58 Side-chain oxidation of aromatic hydrocarbons seems to be largely though not exclusively of this type.59 Consequently the mechanisms of oxidations effected by manganic salts considered separately below (p. 296) are of particular relevance to the understanding of the modes of action of solutions of permanganate in mineral acid. For oxidations in which the (Mn0,)- ion is directly involved there are two distinct paths (a) mere electron transfer e.g. (MnO,)- + e -+ (MIIO,)~- and ( b ) direct oxygen transfer; both of these can be investigated experi- mentally.Modes of Electron Transfer to (MnO,)"- ions.-Oxidations by electron transfer can be studied only in alkaline solutions in which the anions (Mn04)2- and (Mn04)3- are reasonably stable. Following Stamm,60 who introduced alkaline permanganate as a volumetric reagent and used baryta to prevent bulk reduction beyond the manganate stage (BaMnO is very insoluble) Drummond and Waters 61 verified that organic compounds of many types were rapidly and extensively attacked by cold alkaline perman- ganate often giving oxalate as the end-product. A significant feature of these oxidations was the repeated stepwise degradation of hydrocarbon chains occurring by oxidations of aldehydes and ketones through their enol anions.Of aliphatic compounds only ethers tertiary alcohols (excluding 1 2-diols) and the anions of saturated acids resisted attack. However the quantitative conversion of (Mn0,)- into BaMnO under these conditions does not indicate invariable 1 -electron abstraction since the immediate electron transfer (39) destroys any evidence for the transient existence of a Mnv valency leveLg Later work by Pode and Waters,g who studied oxidations effected by sodium manganate in 10N-potassium hydroxide in which the blue anion (&b~o,)~- has a reasonably long life showed that whilst (M~IO,)~- like (Mn04)- was not a specific oxidiser for particular organic groups yet its reactions could be divided into two categories according to whether they yielded dissolved blue (Mn0,)3- as a visible product or immediately gave brown hydrated manganese dioxide (MnIV).mond and W. A. Waters J. 1954 2456. These are tabulated. H. F. Lamer and D. M. Y o s t J. Amer. Chem. SOC. 1934 56 2571; A. Y. Drum- 59 J. W. Ladbury and C. F. Cullis J . 1955 555 1407 2850 4186. 6o H. Stamm 2. angew. Chem. 1934 47 579 791. 61 A. Y. Drummond and W. A. Waters J. 1953 435. 292 QUARTERLY REVIEWS Classification of oxidations effected by alkaline manganate Substances oxidised by l-electron transfer (MnvI + Mnv). Group A . Hydrogen peroxide sulphite,* thiosulphate.8 a-Hydroxy-acids formic acid.8 Ketones ketonic acids phenols. Group B. Substances oxidised by direct conversion of (Mn0,)2- into MnIv. Arsenite. Alcohols olefins unsaturated acids. The scope of this survey was naturally limited by considerations of solubility and stability in strong alkali ; thus aldehydes could not be tested.It is significant however that whilst manganate can attack though much more slowly all the types of molecule that are attacked by alkaline per- manganate only the compounds of Group A which yield (MnOJ3- from (Mn04)2- are easily oxidised by undoubted 1 -electron-abstracting ions such as Mn3f or [Fe(CN)6]3-. Other analogies support the general conclusion that substances of Group A but not those of Group B are oxidised by both (MnO,)- and (Mn0,)2- by mere I-electron abstraction. Thus the stepwise l-electron transfer is the invariable course of oxidation of hydrogen per- oxide to oxygen as established for example with Ce3+.62 In acid perman- ganate (free H202) it needs trace catalysis by Mn3+ but in alkali both (H0,:)- and (O,*)- lose electrons directly with (Mn04)- (Mn04)2- and (&tno,)3-.Sulphites it may be recalled undergo autoxidation that can be catalysed by cupric salts (eqn. 40) and radical dimers of sulphur com- Cu2+ + (HS03)- e Cu+ + HSO,. . . . . . .(40) pounds e.g. R,S, Na,S,06 Na2S,06 a're well known. In contrast oxidises only to (AsO*)~- and is not directly attacked by M n 3 + . 6 3 Phos- phites and hypophosphites which are rapidly attacked by manganate may however react by l-electron transfer. Other noteworthy experimental features are the stability of phenoxide anions to (MnO,)3- probably a matter of relative redox potentials and the rapid oxidations of 1 2-diols. Though it is possible to formulate oxidations of Group B as 2-electron transfers the evidence given below shows that most of them involve oxygen transfer as the prime reason for the valency change of the manganese.There may of course be oxidations in which the mechanistic course depends on the pH of the environment and it does seem as if even amongst organic reactions the direct electron transfer process occurs only in reactions be- tween two ions (cf. aldehyde oxidation p. 297). Oxidations known to involve Oxygen Transfer.-(i) Permanganate oxida- tion of oZeJins. It is well known that dilute aqueous permanganate very rapidly oxidises olefins to cis-1 2-diols. More recently alkaline manganate has been shown to exhibit the same stereospecificity of attack.9 64 How- G 2 S. Baer and G. Stein J. 1953 3176. c 3 H. Land and W. A. Waters J. 1957 4312. 6 4 W. Rigby J. 1956 2452.WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 293 ever a careful selection of experimental conditions is imperative if good yields of diol are required. By rapid addition of permanganate to a neutral solution of an olefin an acyloin R*CH( 0H)CO.R' is at once formed 65 whereas in decidedly alkaline solution (pH 12 or over) even in the presence of an excess of permanganate the diol predominates and when once formed is further oxidised only slowly.66 Organic co-solvents (acetone or alcohol) favour diol formation.67 Again glycol fission and Wagner rearrangement can both occur as side reactions. To explain cis-hydroxylation the formation of a primary cyclic adduct (V) containing MnV suggested by Wagner 68 and then Boeseken,Gg has long been favoured particularly since from spatially similar osmium tetroxide the analogous cyclic intermediates have been isolated.70 Only recently has clear support for this hypothesis come from the work of Wiberg and Saege- barth 67 who by using l*O-labelled permanganate under favourable con- ditions for diol formation have been able t o establish that in the oxidation of oleate up to 1-5 (i.e.virtually 2) atoms of oxygen can be transferred from perrnanganate anions to each olefin molecule. Thus the intermediate (V) must have two C-0-Mn bonds both of which hydrolyse between the manganese and the oxygen atoms. To explain the formation of either the 1 2-diol or the acyloin Wiberg and Saegebarth suggest that the cyclic intermediate rapidly hydrolyses to (VI) and then by bimolecular attack of hydroxyl anion on the manganese f-y "d" '0- cis-Diol (VII) 0 + I HC-YOH HC-%H I * 9~ - I * + (HM&J- +(H@ B ~ H - C - ~ ~ ~ O ?=O 0 (Vll) Acyloin.centre to hypomanganate and the cis-diol (41). Acyloin production is thought to involve oxidation of the Mnv ester (VI) by permanganate anion to (VII) containing MnV1 which then by a concerted base-catalysed process similar t o that operating in chromic acid oxidation of secondary alcohols (p. 284) gives the acyloin and MnIV. An alternative attack of a base on G 5 G. King J. 1936 1788. 66A. Lapworth and E. N. Mottram J. 1925 127 1628. 67 I<. B. Wiberg and K. A. Saegebarth J . Arner. Chena. Xoc. 1957 '79 2822. c8 G. Wagner J . Russ. Phys. Chern. SOC. 1895 27 219. O9 J. Boeseken Rec. Traw. chirn. 1921 40 553; 1928 48 683. 'O R. Criegee Annalen 1936 522 75.T 294 QUARTERLY REVIEWS the hydroxyl hydrogen of (VII) can lead to glycol fission. The electron transfer stage of oxidation of (VI) to (VII) is introduced to avoid postulating reduction of manganese below the valency level of &Iv. An alternative route of diol formation is of course hydrolysis of ester (VII) by 0-Mn bond fission but this like the hydrolysis of (VI) presumably needs free hydroxyl anions. This has also been proved to involve oxygen transfer from manganese to carbon in neutral or acid media.71 It is a bimolecular reaction showing general acid catalysis and has been assigned the following mechanism (42 43) because subst,itution of PhCDO for Ph*CHO decreases the reaction velocity 7-fold. A Hammett plot can be (ii) Oxidation of aromatic aldehydes. Ar-CHO t (H30)+ =i? Ar-ZH-OH + H,O .. . . . . . . . (42) Fast H H - 1 I. * OH Ar-5' + (*O-Mnq)- Ar-$.%O-,MnO Ht + Ar-$20 + (Mnvo3)-. (43) OH OH Fast drawn for relative oxidation rates of substituted aromatic aldehydes showing that electron-attracting substituents by preventing the formation of the organic catior in reaction (42) decrease the rate of oxidation. A similar mechanism to (43) was earlier suggested by Merz Stafford and for oxidation of alcohols by permanganate; they thought however that in this case a valency change from &Iv to MnII was concerned. The scheme (42,43) is exactly similar to that operating in oxidation of aromatic aldehydes by chromic acid (eqns. 32 and 33). In very alkaline solution when manganate results a more complex mechanism is involved since (i) the deuterium effect decreases and (ii) the overall rate rises only about %fold per unit change of pH indicating a rough reaction order of [Ar*CHOJ[MnO,-][OH-]h.For this a free-radical chain mechanism involving the anion Ar*CH( 0H)-O- and possibly hydroxyl radicals has been tentatively proposed. Formic acid is more easily oxidised by strongly alkaline than by acid permanganate and is scarcely attacked by manganic cations. Early work of Holluta in 1922,73 showed that both (MnO,)- and (Mn04)2- oxidised formate anions and gave the first indication of the transient existence of blue (&tr~O,)~-. In neutral or weakly alkaline solution the reaction is approximately bimolecular shows a primary salt effect and is not dependent upon PH,'~ 74 75 so that the initial reaction is one between two similarly charged ions (iii) Formic acid oxidation.This is more complicated. (HCq) - + (MnO,)-c CO + (MnOJ- + (OH)' . . . . (44) 71 K. B. Wiberg and R. Stewart J . Amer. Chem. Xoc. 1955 7'9 1786. 72 J. Merz G. Stafford and W. A. Waters J. 1951 638. 73 J. Holluta 2. phys. Chem. 1922 101 34 489; 102 32 276. 74 F. C. Tompkins Trans. Furaday SOC. 1941 37 201. 75 K. B. Wiberg and R. Stewart J . Amer. Chem. SOC. 1956 78 1214. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 295 A deuterium effect ( k H / k = 7.4) has been noted by Wiberg and Stewart who also showed by using labelled permanganate that there is a definite (18-300/0) transfer of lSO to the resultant carbon dioxide.75 This could be explained by a reversible addition to a carbonyl bond as in eqn. (43) followed by a slow concerted rearrangement involving both C-H and O-Mn links as in sequence (45) but they regard t'his as improbable since the 0 1 Slaw 7- -O-C..-ji Q- (I X ) -8 MnO -0-C=O*t Ht + (Mn03)- .. . (45) (MnO,)- anion would be expected to add to un-ionised formic acid (with a fairly definite C=O bond) more rapidly than to formate anion whereas formic acid is oxidised less easily than formate.73 To avoid this argument they suggest the partly-bonded transition state (IX) from which hydrogen- bonding to the whole (MnO,) can be thought to promote the chemical change. A structure similar to (IX) has been proposed by Stewart to explain the mechanism of permanganate oxidation of diphenylmethanol76 in dilute alkali. This is a base-catalysed second-order reaction in which there is a clear deuterium effect (k,/k = 6.6) but no oxygen transfer.A concerted -&3h,-H n + O=Mnq- - O=Cph + (H-O-Mn03)-. . . (46) u hydride-ion removal (46) would explain these observations though it does not suffice in the case of formic acid. (iv) Oxidations of tertiary C-H in aliphatic acids R2CH*[CH2],*C02H. Though most saturated carboxylic acids asre inert to cold alkaline perman- ganate or manganate solutions Kenyon and Symons,77 found that acids of the general structure given above can be converted into hydroxy-acids R2C( OH)*[CH2],*C0,H in preparative yields. With permanganate in con- centrated (> 5 ~ ) alkali the yields can reach 70-90% and optically active carboxylic acids give racemic products whereas with manganate in dilute (ca. 0 . 5 ~ ) alkali equally effective oxidation occurs but with complete reten- tion of optical activity.The latter reaction must therefore be a hydride removal involving an Mnvl to MnIV valency change and the transient formation of an optically inverted y-lactone. Kenyon and Symons suggest that the strongly alkaline permanganate may remove a hydrogen atom via hydroxyl radicals [formed as in eqn. (1 1 )I but this does not explain the selectiveness of attack on a y-C-H group. The sequence (47) can be compared with mechanism (38) suggested for chromic acid oxidation of saturated hydrocarbons. 7 6 R. Stewart J . Amer. Chem. SOC. 1957 79 3057. 7 7 J. Kenyon and M. C. R. Symons J . 1953 2129 3580. 296 QUARTERLY REVIEWS H20 + (HMnOJ3- - MnO + 3(OH)- A point of general interest to be noted from inspection of these oxygen transfers is that they show that both manganite (Mn03)2- and perman- ganite (MnO,) - anions must be entities capable of transient existence though they must be far less stable than anions of form (MnO,)%-.Oxidations involving manganic ions On account of the disproportionation (16) free manganic cations cannot exist in significant concentration in aqueous solution though the sulphate probably in the form of a complex ion is stable in sulphuric acid of over 60% ~oncentration.7~ Since most chelating agents form much more stable complexes with tervalent than with bivalent cations many complex man- ganics salts e.g. oxalates malonates,79 and pyrophosphates have been prepared and studied. Most of these have rapidly displaceable ligands.15 The various manganic pyrophosphates ranging from Mn(H,P,O,) to (Mn(H,P20,)3}3- according to the degree of ionisation of the pyrophosphate groups yield stable solutions at acidities down to pH 6 and are clearly the most suitable of these complexes for use in oxidation studies.The redox- potential measurements of Watters and Kolthoff 8O give values dependent on pH in the range + 1.1 to 1.2 v showing that manganic pyrophosphate is decidedly a more potent oxidiser than ferric salts but inferior to ceric salts. Towards organic compounds manganic pyrophosphate is quite a selective oxidant it attacks aliphatic aldehydes and ketones 1 2-diols cc-hydroxy- acids malonic and oxalic acids and phenols often at rates suitable for kinetic study.6l In general monohydric a.lcohols and olefins are not attacked though ally1 and crotyl alcohols can be oxidised very slowly.81 On account of the resistance towards oxidation of vinyl cyanide and methyl methacrylate the induced polymerisation of these monomers can be used to establish the presence and something of the nature of the organic free radicals generated by the initial stages of MnlI1 oxidations.82 The distinctive features of several of these ozidations which have been examined by Waters and his collaborators are noted below.78 L. Domange Bull. SOC. chim. France 1939 6 594. 7g G. H. Cartledge and P. M. Nichols J. Amer. Chein. Soc. 1940 62 3057. 8o J. I. Wetters and I. M. Kolthoff J . Amer. Chem. SOC. 1948 70 2455. 81 H. Land and W. A. Waters J. 1958 2129. 8 2 A. Y. Drummond and W. A. Waters J. 1953 2836. WATERS OXIDATION BY CHROMIUM AND MANGANESE COMPOUNDS 297 Oxidation of Aldehydes and Ketones,-These regularly seem t o occur through the enols or enolate ions.Thus t.he oxidation rates of propion- aldehyde and butyraldehyde are of zero order with respect to MnIII and of first order with respect to both aldehyde and hydrogen i0n.~3 With ketones the same features emerge only at high MnII concentrations for the oxidation step (eqn. 50) becomes measurable at low concentration^.^^ R-Cl-$COR'+ (H30)+ RCH&R'(OH)+ H20 (Immediate) . . . (48) 6 + RCHiER'(0H) ;t (BH)' + FtCH=CF(OH)(M~~sureaable). . . (49) R.CH=CR'(W) + Mn3+ - R-CH=CRiO. + Mn2+ + H+ . . - (50) a-Hydroxy-aldehydes or ketones are the first detectable molecular oxidation products and these are easily oxidised further. It has been suggested therefore that the true oxidation step is the removal from an enolate anion of an electron since this would leave a mesomeric resonance- stabilised radical that can then lose a second electron from the a-carbon atom or as in the case of cyclohexanone oxidation,g* disproportionate rapidly.It is noticeable that chloral hydrate formaldehyde and formic acid which cannot yield enols are not easily oxidised 83 (reactions 51-53). 6 + R.CH=CR!OH t (BH)' + RCH=CRLO' . . . . . . (51) Mn3' + R.w=CR<O- -c Mn2+ + R.CH=C&O* - R - i k i - C R k . (52) R.&-CR&O + Mn3+ - R.&-CR'=O - R.CH(OH)-CRLO . . (53) H p + R.CH=CR?CH=o 2 HO.CHR-CR/=CH-OH . . . (54) However ncraldehyde a-methylacraldehyde and crotonaldehyde which cannot form normal enols also oxidise at rates independent of the MnIII concentration; in these cases oxidation through a 1 4-conjugated enol hydrate (54) is ~uggested.~~ This can be hydroxylated in the a-position as in equations (52) and (53) and further oxidation can then occur as for a typical 1 2-diol.Pyruvic acid however is oxidised quantitatively to acetic acid by a pro- cess related to that of a-hydroxy-acid oxidation and not by enolisati~n.~~ Specific Oxidation of 1 2=Diols.-Leading t o quantitative carbon-carbon bond fission which can be brought about by several 1 -electron-abstracting agents these have several points of interest. With pinacol the initial oxidat'ion rate is of first order with respect to total MnIII but of less than first order with respect to pinacol the actual relationship being 85 of the form - d[MnIII]/dt cc a[Pinacol]/(b + [Pinacol]) This has been explained by suggesting that the pinacol must first (reversibly) displace a chelated pyrophosphate group from attachment to the manganese atom before the electron switch (55) can occur.Some a-hydroxy-acids seem to oxidise similarly with liberation of carbon dioxide,86 as also does 83 A. Y. Drummond and W. A. Waters J. 1953 440. 8 4 Idem J. 1955 497. 86 (Miss) P. Levesley and W. A. Waters J. 1955 217. 8 5 Idem J . 1953 3119. 298 QUARTERLY REVIEWS pyruvic acid (56). When isomeric cyclic cis- and trans-glycols are examined the relative rates of attack are not the same as for their oxidations by lead tetra-acetate or periodic acid and the reaction order with respect to Evidently the stereochemistry of glycol fission by 1 -electron abstraction needs further study. Malonic acid affords another case in which oxidation by MnlI1 occurs through a chelate complex.88 This oxidation like many others is pro- foundly affected by the presence of oxygen to which the initial organic free radicals are sensitive.Moreover it is specifically affected by man- ganous cations the initial-rate measurements showing that the critical first oxidation step [simplified in eqns. (57) and (58) by neglecting the fact that only a manganic-malonate complex reacts] is reversible. Though the radical *CH(CO,H) can also reduce MnIII it is as equation (57) implies is not the same in all cases.87 Mn3+ + H2C(qH)2 = Mn2++ H t -4- sCH(CO~H)~ . . . . (57) Mn3+ + .CH(Co,H) - Mn2' + +CH(CO,H) . . . . (5 8) HO*CH2-H + *CH(C02H)* -t HOCHi + H2C(C02H) . . . (59) an oxidiser for it can effect the oxidations of methyl and ethyl alcohols ethers and probably many other organic compounds (reaction 59).Here one has a novel form of induced oxidation for manganic salts themselves which unlike organic free radicals are merely electron abstractors cannot directly oxidise monohydric alcohols or ethers. This induced oxidation which is not observed with ethyl- and benzyl-malonic acids is thought to be due to the electronic structure of the radical *CH(CO,H) in which the two carboxyl groups are so powerfully electron-attracting that there is a marked tendency to further electron gain a t the tervalent carbon centre. Few organic free radicals as yet have been shown to exhibit this oxidising power for in fact the great majority of carbon radicals are reducing agents. For instance the radical *CMe,OH liberated during the oxidation of pinacol (eqn.55):82 or in other W ~ Y S ~ ~ can reduce mercuric ions to mercurous ions even in the presence of manganic salts or other oxidising agents. An interesting example of this induced reduction by a transient mdical is afforded by oxalic acid (60 61) for the oxidation of oxalate by manganic 8 7 (Miss) P. Levesley W. A. Waters and A. N. Wright J . 1956 840. 89 J. Merz and W. A. Waters J. 1949 S 15. Y. Drummond and W. A. Waters J. 1954 2456. WATERS OXIDATION BY CHXOMIUM AND MANGANESE COMPOUNDS 299 pyrophosphate can promote the reduction of mercuric chloride by oxalate this being a radical-catalysed chain rea~tion.~O (C20$’-+ Mn3+ - C02+ Mn2+ + (.Cq)- . . . . . . (60) (C02)- + Hg2+ - CO,+ Hg+ ’ . . . . . . . . . . (61) cCC2)- t (Mn0,)- - C02 + (Mn04)*- .. . . . . . - (62) Oxygen uptake by radicals is of course another instance of induced reduction here of the 0 molecule. Induced reductions and oxidations of this type are undoubtedly involved in quite a number of the oxidations that can be brought about by means of potassium permanganate in mineral acid solution. The important per- manganate-oxalic acid reaction for instance has been investigated and discussed so frequently57 that it needs but brief mention here except that it should be pointed out that the initial attack on oxalic acid is effected by manganic ions complexing of these with oxalate groups being important,Sl and that the autocatalytic nature of the overall oxidation is largely due to the rapidity with which the (GO,)- radical-ion can attack permanganate itself (62).A similar catalysis involving the radical *CH(CO,H) operates in the oxidation of malonic acid by acid permanganate.ss An important aspect of oxidations involving manganic ions is the influ- ence of oxygen which often changes the whole course of the reaction sub- sequent t o the initial production of an organic free radical. Thus whereas under nitrogen the oxidation of malonic acid takes the course ~ ( c c p ) + .CH(CC~,H)~- HO-CH(CO~H)~ 3% CO + *CH(OH)CO,H -c CO + H*CO,H with loss of carbon dioxide at the stage of tartronic acid and eventual pro- duction of a molecule of formic acid the reaction in the presence of oxygen seems to have the course proceeding through oxalic acid to complete oxidation to carbon dioxide. Detailed treatment of effects of oxygen would require discussion of mechan- isms of reactioiis between organic hydroperoxides and metallic ions ; this cannot be summarised here but it has been reviewed frequently in view of its great technical importance in hydrocarbon chemistry.It is sufficient to note that reactions of the types (63)-(66) (where X is a metal of variable R-0-0. f X’’ - 7 (R-0-0:)- + X3+ . . . . . (63) R-0-0. + X3+ - R+ 4- 0 + x2+ . . . . . (64) R-O-O-H + x2+ - R-0. + (:OH)- + X 3 + . . . . (65) R-0. + x2+ - (R-0:)- + X3+ . . . . . . (66) .CH(CO,H) + *O-O.CH(CO,H) + CO + (COZH) + 3C02 90 J. Weiss Discuss. Paraday Soc. 1947 2 188; G. H. Cartledge J. Anzer. Chem. 9lR. P. Bell and 0. M. Lidwell J. 1935 1303. SOC. 1941 63 906. 300 QUARTERLY REVIEWS valency) can all occur the favoured processes depending upon the redox potentials of the ions concerned in the particular environment of the reacting system.Of the above reactions (63) and (64) are involved in the oxidation of hydrogen peroxide by manganic salts and also in the permanganate- hydrogen peroxide reaction. Oxygen undoubtedly affects the course of many oxidations in which acid or alkaline permanganate is the reagent,41b but its presence has rarely been taken into account or searched for experi- mentally. Indeed all kinetic work on oxidations involving ions of the transition elements in which cognisance has not been taken of the possible intervention of oxygen effects should be viewed with some scepticism. Conclusions Though this Review presents a wide range of reaction mechanisms some substantiated in good detail but many still remaining as tentative hypotheses it can be seen that the systematic understanding of the re- actions of both chromic acid and permanganate is beginning to emerge.The major gap in experimental knowledge now concerns the inorganic chemistry rather than the organic chemistry of oxidation processes for whilst from other studies it is possible to adduce relevant evidence con- cerning the natures of transient organic ions and free radicals one can only make tentative suggestions concerning the structures and properties of the unstable ions of CrV Crlv Mnvl Mnv and MnIV in acid media. However taking note of the present interest of quantum chemists in struc- tures of compounds of the heavier elements one can hope for theoretical help in the elucidation of many of these problems.

ISSN:0009-2681    

DOI:10.1039/QR9581200277

出版商:RSC    

年代:1958

数据来源: RSC

摘要:

CHEMXSTRY OF p-XYLYLENE ITS ANALOGUES AND POLYMERS By L. A. ERREDE and M. SZWARC (MINNESOTA MINING AND MANUFACTURING COMPANY MINNESOTA AND COLLEGE OF FORESTRY SYRACUSE UNIVERSITY SYRACUSE N.Y .) THE development of quantum chemistry created considerable interest in hypothetical compounds and numerous calculations were made to predict the stabilities and properties of such molecules. p-Xylylene (I) a hydro- carbon closely resembling the well-known p-benzoquinone was one species that attracted the attention of many theoreticians. This compound 'was discussed as early as 1945 by Namiott Dyatkina and Syrkin and then again by Diatkina and Syrlrin 2 in 1946. p-Xylylene is the prototype of a class of hydrocarbons 1- sown as Chichibabin hydrocarbons. These compounds can be represented either by a quinonoid structure as exemplified by (I) or by a benzenoid structure possessing two uncoupled electrons e.g.as (11). Compounds corresponding to the latter should behave as diradicals and exhibit paramagnetic pro- perties whereas the quinonoid structure implies that the compound is diamagnetic. The magnetic properties of the known Chichibabin hydro- carbons e.g. (111) and (IV) were investiga.ted by Muller and Muller-Rodloff 3 CI CI Ph,.C#CPh - P h C m C P \ - - P h 2 e w t P h 2 - - (V) CI CI (m 0 v) who proved that these hydrocarbons are diamagnetic and therefore probably exist in the quinonoid form. Paramagnetic properties have been observed only in those Chichibabin compounds where the planarity of the molecule is rendered impossible by steric hindran~e,~ e.g. (V) and such compounds are best described as diradicals.The known Chichibabin hydrocarbons (111)-(V) are stabilised by the phenyl groups attached to the terminal carbon atoms. Furthermore these phenyl groups sterically shield the reactive carbon atoms a.nd thus prevent 1 A. Namiott M. Diatkina and J. Syrkin Cornpt. rend. Acad. Sci. U.R.S.S. 1945 2 M. Diatkina and J. Syrkin Acta Physicochim. [J.R.S.S. 1946 21 23. 3 Muller and Muller-Rodloff Annalen 1935 517 134. 48 285. Muller and Tietz Ber. 1941 74 807. 301 302 QUARTERLY REVIEWS dimerisation or polymerisation. On the other hand one would expect that molecules which belong to this class of compound but possess terminal unsubstituted methylene groups would be very reactive and should be easily dimerised or polyrnerised. Early theoretical consideration of the electronic structure of these com- pounds indicated that their triplet states are close to their singlet levels.l Later work by Coulson et aZ.,5 by both the valence method and the molecular- orbital method fully confirmed the calculations of Syrlrin and Diatkina.Moreover Coulson et u Z . ~ Daudel,6 and Pullman et aL7 calculated the bond order and free valence of p-xylylene both in t,he singlet and in the triplet state. The very high free valence computed for the terminal methylene carbons of the molecule in its ground state implies that this species should indeed be highly reactive (see Fig. 1). FIG. 1 Molecular diagrams of p-xylylene ‘0.92 ‘1.13 Ground state Excited state singlet triplet It is understandable therefore that the preparation and isolation of p-xylylene by the classic methods of synthetic chemistry eluded the experi- mentalist.The first evidence for its existence was obtained in an entirely different type of experiment namely pyrolysis of p-xylene,8 and subsequent investigations of the pyrolysis of related compounds proved that many analogues of p-xylylene can be produced by the same technique. Formation and Reactions of p-Xyly1ene.-The pyrolysis of toluene lo and of some related compounds 11 showed that the rupture of the PhCH,-H bond is one of the primary decomposition steps. Benzyl radicals or their derivatives are formed by this process and these eventually dimerise to give dibenzyl or the appropriate derivatives. The pyrolysis of p-xylene,s-lo however yields instead of a dimer a polymer which consists as shown later of linearly arrayed units (VI).In view of this observation it was Coulson Craig Maccoll and Pullman Discuss. Faraday Soc. 1947 2 36. Daudel ibid. p. 69. M. Szwarc Discuss. Faraday SOC. 1947 2 46. Idem J . Polymer Sci. 1951 6 319. ’ Pullman Berthier and Pullman Bull. SOC. chim. France 1948 15 450. l01dem J . Chem. Phys. 1948 16 128. M. Szwarc and J. S. Roberts ibid. p. 609. ERREDE AND SZWARC CHEMISTRY OF p-XYLYLENE 303 that the p-xylyl radical (VII) initially formed in the pyrolysis suggested is converted into p-xylylene which eventually polymerises. It has been found that monomeric p-xylylene is remarkably inert in the gaseous phase and that polymerisation occurs only on condensation. Thus no cloudiness or smoke .was ever perceptible in the gases flowing from the furnace and when a 3-m.tube heated t o about SO" separated the furnace from the cooled trap the polymer was formed readily in the trap but not in the tube. Since the gas required about 0.5 see. to flow through this long tube clearly no appreciable reaction of p-xylylene vapour takes place within this period of time. Still more convincing evidence for the inert nature of p-xylylene vapour was obtained in a flow system con- taining two traps in series the first being cooled by ice while the second was immersed in acetone-solid carbon dioxide. Polymer was formed in both traps although the tube connecting the traps which was maintained a t room temperature remained completely clear.8 To establish the identity of p-xylylene iodine vapour was introduced into the ice-cooled trap in which the polymer was forming suitable pre- cautions being taken to prevent the iodine vapour from diffusing into the furnace.The only iodine-containing material identified at the end of the experiment was p-xylylene di-iodide thus confirming the existence of p-xylylene in the gas stream.8 Auspos et aE. l2 investigated the reactions between p-xylylene and various substrates using the technique described above. Their observations con- firmed that the addition of iodine to p-xylylene yields p-xylylene di-iodide. Furthermore they showed that elementary chlorine and bromine react similarly yielding the corresponding dichloride and dibromide. They also obtained products of unknown nature from the pyrolysed vapour with NN-diethyl-p-nitrosoaniline which behaves as a stable free radical. On the other hand attempts to isolate the dinitroso- or dinitro-compounds from p-xylylene and nitric oxide or nitrogen dioxide respectively failed to give positive results.12s l3 However there are indicat,ions that the respective nitrogen-containing groups are retained as the end groups of the polymers formed in those experiments.Further evidence for the existence of p-xylylene was obtained from (VI I I> l2 Auspos Hall Hubbard Kirk Schaefgen and Speck J . Polymer Sci. 1955 15 9. l3 AUSPOS Burnum Hall Hubbard Kirk Schaefgen and Speck ibid. p. 19. 304 QUARTERLY REVIEWS careful studies of various by-products of the pyrolysis of p-xylene. Thus Brown and Farthing14-lG extracted from the polymer a cyclic dimer of p-xylylene i.e. (VIII) and showed that the benzene rings in this molecule lie parallel one above the other.p-Xylylene is the most probable precursor of this compound which is of interest because of the highly strained structure arising from the interaction of the closely located benzene rings and to the shortness of the CH,-CH bridges 14? l6 (see Fig. 2). At the time of its isolation the compound was new; however a few years later it was syn- thesised by Cram l7 who allowed a highly diluted solution of p-xylylene dibromide to react with sodium. 1.548 FIG. 2 Two views of the molecule of di-p-xylylene at right angles showing dimensions and distortion from planarity (Reproduced with permission from Brown J. 1953 3205) The very low yield of the cyclic dimer in the pyrolysis of p-xylene is attributed to steric strain. On the other hand the strainless cyclic trimer is formed more readily in the reaction.12 18 This compound is identical with the trimer prepared by Baker and his colleagues l9 by means of the Wurtz reaction.Smaller quantities of a cyclic tetramer were also isolated from the products of the pyrolysis. 2o The formation of these cyclic polymers provides further proof of the existence of monomeric p-xylylene in the pyrolysed gases.* The inconvenience of dealing with a short-lived labile species which was present only in the vapour introduced many limitations and difficulties in the early studies of the chemistry of p-xylylene. It was a great step forward therefore to show 2l that monomeric p-xylylene in admixture with p-xylene can be kept indefinitely in the solid state at liquid nitrogen temperature. I n addition a method has been developed 21 to produce reasonably stable solutions of p-xylylene in various solvents a t - 80".This was accomplished by carrying the pyrolysed gases containing p-xylylene over a surface of a suitable and well-stirred solvent cooled to -80". l4 C. J. Brown and A. C. Farthing Nature 1949 164 915. l5 A. C. Farthing J. 1953 3261. I'D. J. Cram and H. Steinberg J . Amer. Chem. SOC. 1951 73 5691. 18 L. Errede unpublished results. 20 L. Errede unpublished results. 21 L. Errede and Landrum J . Amer. Chem. SOC. 1957 79 4952. * It is not established whether these cyclic polymers are formed in the high-tem- The latter is more probable. 16 C. J. Brown J. 1953 3265. Baker McOmie and Norman J . 1950 1142. perature zone or during the cooling of the exit gases. ERREDE AND SZWARC CHEMISTRY OF $I-XYLYLENE 305 The concentration of p-xylylene in such solutions can be determined by withdrawing aliquot portions (using a pre-cooled pipette) mixing them rapidly with standard iodine solution and titrating the excess of iodine.p-Xylylene in solution reacts readily with iodine giving p-xylylene di-iodide quantitatively. While both di-iodide and polymer were formed in Szwarc's original experiments where the reactants were mixed in the gaseous phase no polymer is formed in the reaction in solution in presence of an excess of iodine. The stability of p-xylylene solutions is not increased by the addition of conventional inhibitors. This is not surprising in view of its very high reactivity and of its great tendency to polymerise. Indeed on heating these solutions polymerisation or copolymerisation takes place spontaneously and these reactions will be discussed fully in a later section of this review.Solutions of p-xylylene react also with bromine and chlorine 209 22 giving the corresponding dihalides and these reactions proceed more quantitatively than the corresponding reactions involving gaseous p-xylylene. The re- actions with oxygen 23 yields a copolymer of p-xylylene and oxygen. If an excess of oxygen is bubbled rapidly through a solution of p-xylylene an alternating copolymer results. This polymeric peroxide is precipitated as a powder insoluble in organic solvents and in water. It is stable a t room temperature but when heated carefully e.g. by suspending it in hot water it decomposes into hydrogen terephthalaldehyde and to a lesser extent into the products of its disproportionation.Hydrogen evolution was used to measure the rate of the decomposition. The reaction is of first order with a first-order rate constant E = 4.1015exp( -34,00O/R T ) . These observations are consistent with a mechanism involving the random rupture of the 0-0 bonds as the rate-determining step. This is followed either by the reaction (i) or by the disproportionation (ii). The reactions was found to be t erep ht halalde hyde increases difference in activation energies of these two approximately 13 kcal./mole. The yield of a t higher temperatures approaching a maxi- These reactions provide a basis for a novel Peroxide decompositions in which hydrogen is formed have been reported mum of 90% a t about 125". and simple synthesis of terephthalaldehyde from p-xylene.2 2 L. Errede unpublished results. 23 L. Errede and Hopwood J . Anzer. Chem. Xoc. 1957 79 6507. 306 QUARTERLY REVIEWS previously. The earliest example is attributed to Blank and Finkelbeiner 24 who studied the decomposition of the peroxide (HO*CH,*O*O*CH,*OH) obtained from formaldehyde and hydrogen peroxide. Similar conclusions on the mechanism of this reaction have been advanced by Wieland and Wingler z5 and by Errede and H0pwood.2~ Recently Mosher and his col- leagues 26 studied the decomposition of butyl hydroperoxide which also yields hydrogen and they proved eventually that the reaction proceeds via peroxide R*CH,*O*O*CHR-OH which decomposes into hydrogen molecules without forming hydrogen atoms.27 Physical Properties of p-Xyly1ene.-The labile nature of p-xylylene adds considerable technical difficulty to the determination of its physical pro- perties.The spectra of gaseous p-xylylene were studied by Tanaka 28 and by S~haefgen.,~ Absorption maxima were observed a t 2520 2560 2680 and 2770 8 suggesting the quinonoid (singlet) structure of the molecule. Spectroscopic studies in solution have not been carried out although this is now feasible. Some qualitative data are available 2* on the solubility of p-xylylene in solvents a t -80". Mechanism of Formation of p-Xyly1ene.-p-Xylylene formed in the pyrolysis of p-xylene is undoubtedly produced from p-xylyl radicals. The presence of the latter in the reaction was proved by Farthing 15 and by Schaefgen 29 who isolated from the products of the pyrolysis the linear dimer Me*CGH,-CH2*CH,*C6H,*Me the analogue of dibenzyl which is formed in the pyrolysis of toluene by the dimerisation of benzyl radicals.There are two possible reactions which might convert p-xylyl radicals into p-xylylene namely disproportionation (iii) and decomposition (iv) . When the reaction was first described Szwarc suggested disproportion- ation as the mode of formation of p-xylylene basing his argument on kinetic observations which in the light of more recent findings cannot be regarded as an unambiguous proof for this mechanism. The decomposition of p-xylyl radicals with the accompanying formation of hydrogen atoms should lead to a chain decomposition of p-xylene. On the other hand no chain reaction is expected if the radicals disproportionate. The kinetics of the pyrolyses of p-xylene m-xylene and toluene are all alike and the last two reactions are definitely not chain decompositions.This similarity in the kinetics was advanced therefore as an a'rgument in favour of disproportionation. 2 4 0. Blank and H. Finkelbeiner Ber. 1898 31 2979. 26 H. Wieland and A. Wingler Annalen 1929 431 301. 26 H. S. Mosher and C . F. Wurster J . Amer. Chem. SOC. 1955 77 5451. 2 7 Wurster Durham and Mosher ibid. 1958 80 327 332. 28Tanaka J . Chem. Xoc. Japan 1955 75 218. 29 Schaofgen J. Polymer Sci. 1955 15 230. ER;REDE AND SZWARC CHEMISTRY OF $I-XYLYLENE 307 Since this kinetic evidence is not entirely conclusive another approach was sought. It has been shown that the pyrolysis of benzyl bromide 30 and of its analogues 31 in a stream of toluene leads to the formation of benzyl radicals through the following sequence of reactions PhC3Br - Ph-CHd + Br ; Ph.CH + Br -c Ph-CH; + HBr This pyrolysis proceeds rapidly a t temperatures considerably lower than those required for the pyrolysis of p-xylene.It appeared therefore that the pyrolysis of p-xylyl bromide in a stream of p-xylene should lead to the formation of two p-xylyl radicals for every molecule of hydrogen bromide produced in the reaction. If the radicals disproportionate then for every molecule of hydrogen bromide produced one p-xylylene unit should be formed. However if these radicals decompose a chain reaction should ensue and molecular hydrogen and p-xylylene should be formed in stoi- cheiometric proportions. Hence the unequivocal discrimination between these two modes of reaction appeared to be feasible. This idea was pur- sued by Levy Szwarc and T h r ~ s s e l l ~ ~ who found that at about 500" the pyrolysis of p-xylyl bromide proceeds rapidly ; however the products were hydrogen bromide and the linear dimer of p-xylyl radicals i.e.Me*C6H4*CH2.CH,.C6H4*Me and neither hydrogen nor polymer was formed. This means that under the conditions of their experiments p-xylyl radicals neither disproportionate' nor decompose but dimerise instead. In a system containing radicals dimerisation and disproportionation proceed simultaneously. Dimerisation requires a very low activation energy if any. It might require however a third body to remove the excess of energy particularly if the reaction proceeds a t high temperature. Further- more a t sufficiently high temperature and low pressure the reaction is reversed owing to the shift in the equilibrium position.On the other hand disproportionation is essentially an irreversible reaction which in this particular case may require a not negligible activation energy. It should also be pointed out that the latter reaction does not require the presence of a third body. Hence higher temperature and lower partial pressure and total pressure favour disproportionation while dimerisation should be the predominant reaction a t lower temperature and higher pressure. * This was further substantiated by studying the ratio of the linear dimer to the polymer formed in the pyrolysis. Various attempts were made to produce p-xylylene at lower temperatures under homogeneous conditions. All however were unsuccessful. Roper and Szwarc 33 tried to produce p-xylyl radicals in the low-temperature pyrolysis The explanation of these results was provided by S~haefgen.2~ Numerous observations seem to confirm this basic picture.3O Szwarc Sehon and Ghosh J . Chem. Phys. 1950 18 1142. 31 Leigh Sehon and Szwarc Proc. Roy. Xoc. 1951 A 209 97. 32 Levy Szwarc and Throssell J. Chem. Phys. 1954 22 1621. 3 3 Roper and Szwarc unpublished results. * See in this connection Corley et al. J . Polymer Sci. 1954 13 137 who showed that the conversion of p-xylene into the polymer is affected by the temperature of pyrolysis pressure of the hydrocarbon and time of contact. 308 QUARTERLY REVIEWS of p-xylene by introducing into the furnace small amounts of bromine or tert.- butyl peroxide. Although radicals are formed under these conditions no formation of the polymer of p-xylylene was observed.Schaefgen 29 tried similar experiments using ethylene oxide as a source of radicals and again the results were negative. This shows clearly that in a homogeneous gas reaction a high temperature is imperative for the conversion of p-xylyl radicals into p-xylylene. At the same time it was found that an increase in total pressure decreases t,he yield of the polymer of p-xylylene. Szwarc 34 obtained a negligible yield of the polymer from the pyrolysis of p-xylene (4 mm. of mercury) in a stream of nitrogen (400 mm. of mercury). Similarly the yield of the polymer was decreased when the pyrolysis was carried out in stream of steam at atmospheric pressure although better results were obtained with steam a t lower pressure.33 On the other hand the polymer yield could be improved if small amounts of chlorine or bromine were added to p-xylene and the pyrolysis was carried out a t sufficiently high temperature and low pressure.29 The final proof for the disproportionation of p-xylyl radicals was pro- vided by Schaefgen's studies 29 of the pyrolysis of the linear dimer Me*C,H,*CH,*CH2*C6H4*Me.He obtained both p-xylene and poly-p- xylylene; the formation of the former is a conclusive evidence for the disproportionation mechanism. Although it appears to be well established that p-xylylene is formed through disproportionation of p-xylyl radicals a t conventional temperatures of tJhe pyrolysis (i.e. 800-1000") a t very high temperatures and very low pressures (10-5 mm. or less) the decomposition of p-xylyl radicals into hydrogen atoms and p-xylylene can be observed.Such observations were reported by Lossing and his colleagues 35 who studied the pyrolysis of the xylenes at very high temperatures mass-spectrographically. While CsHs was formed from p - or o-xylene only C,H was formed from m-xylene. Many attempts have been made to find a suitable catalyst for the dehydrogenation of p-xylene to p-xylylene but conventional dehydro- genation catalysts have proved unsuitable. The decomposition of p-xylene vapour on hot metal wires was also investigated and in many cases the formation of p-xylylene was observed. However this method did not show any advantage over flow pyrolysis in a hot tube.12 p-Xylylene is probably formed as an intermediate in a few reactions other than pyrolysis for example the decomposition of the azide N,*CH,*C6H4*CH,*N3 and the Hofmann degradation 36 of the base Me*C6€14=CH,.NMe,oOH.The latter takes place a t 60-200" in a vacuum and proceeds according to the equation n [CI-$SyiCI-&N(CH& OH-} .- [CH;CbH4-CHJn + n NCHJ + n H20 34 Szwarc unpublished results. 35 Farmer Marsden and Lossing J . Chem. Phys. 1955 23 403. 36 Fawcett U.S.P. 2,757,146/1956. ERREDE AND SZWARC CHEMISTRY OF P-XYLYLENE 309 Formation of Analogues of p-Xyly1ene.-Pyrolysis has also proved most useful for the production of other compounds belonging to the same class as p-xylylene. Pyrolysis of p-xylene derivatives like pseudocurnene," durene isodurene hexamethylbenzene 2 -phenyl-p-xylene 2 - chloro-p -xylene 2 5-dichloro-p-xylene 2-fluoro-p-xylene and 2 5-difluoro-p-xylene leads to the formation of the corresponding substituted p-xylylenes and eventu- ally to the formation of the respective p~lyrners.~~ lo 37 The identity of the monomeric species was proved by analysing the structure of the polymers formed and in a few instances by trapping the monomeric species with iodine and isolating the corresponding di-i~dides.~ 9 21 It was found also that some other compounds resembling p-xylene yield on pyrolysis analogues of p-~ylylene.~ Thus the respective polymers were formed from 1 4-dimethylnaphthalene 2 5-dimethylpyrazine and 5 8- dimethylquinoline.The polymer formed from 2 5-dimethylpyrazine was particularly interesting in that although it was insoluble in organic solvents a t moderate temperatures it was easily dissolved in warm dilute aqueous hydrochloric acid.Some unsuccessful attempts were made to produce other compounds of the p-xylylene class. Pyrolysis of 2 6-dimethylnaphthalene led to the linear dimer Pyrolysis of 4 4'- dimethyldiphenyl and 9 10-dimethylanthracene gave unidentified products,g and further study is needed in order to interpret the results so far obtained. Attempts to obtain species like (X) or (XI) from the pyrolyses of pi"-cresol and p-toluidine were also unsuccessful. instead of the expected compound (IX). The conventional pyrolysis of o-xylene which was expected to yield o-xylylene has been extensively investigated. Although none of the isolated products indicated the existence of o-xylylene there is reason to believe that it might be formed in the reaction. Indeed a t very high temperature and at extremely low pressure the respective monomer CsH8 was observed by Lossing et An important side reaction in the pyrolysis of o-xylene involves apparently a rupture of carbon-carbon bonds since in many experi- ments anthracene and its methyl homologues were isolated.Bailey 38 succeeded in producing poly-o-xylylene by pyrolysing the 37 Roper U.S.P. 2,798,052/1957. 38 See e.g. J. Rosenberg Ph.D. Thesis Wayne University 1951. * Szwarc reported that no polymer was formed from pseudocumene. Later it was found that the pseudocumene used was contaminated with a considerable amount of other isomers (having the same b.p.) and when experiments were repeated with the pure substance the results were positive. U 310 QUARTERLY REVIEWS diacetate (XII). The reaction probably proceeds in the usual way yield- ing o-xylylene and acetic acid and the former species then polymerises rapidly to (XIII).The polymer produced resembles poly-p-xylylene to some extent. A similar product was obtained by action of magnesium on the chloride (XIV). This reaction was investigated by Mann and Stewart 39 who suggested the following sequence of steps The same workers found that the para-isomer reacted similarly producing a p-xylylene polymer of low molecular weight. Interesting work was carried out recently by Korschak ; 39a he treated p-diisopropylbenzene with tert.- butyl hydroperoxide. The primary radicals abstract the tertiary hydrogen atoms of the hydrocarbon and the recom- bination of the secondary radicals leads to poly-p-xylylene methylated in the side chain. Because of the low temperature there are no complicating side reactions and one obtains linear soluble high-melting products which can be moulded cast or spun.* Mechanism of Polymerisation of p-Xyly1ene.-Condensation of gaseous p-xylylene leads to a rapid polymerisation and eventually a tough coherent and continuous film is deposited on the walls of the condenser.This film is transparent in thin but opaque or white in thick layers. Apparently the reaction is started by the p-xylylene units’ combining into large diradicals e.g.7 -CH; C 6 q CHiC3 C6Hi C%C% CeH4*C~CHiC6H~CH~ which continue their growth a t both ends. The initiation seems to be spontaneous although it is remarkable that it does not occur in the gas but only in the condensed phase. The first step leading to the formation of the polymer is probably the dimerisation This lack of polymerisation in the gas merits further discussion.The central CH,-CH bond of such a dimer is extremely weak since con- siderable binding energy is lost when two C=C double bonds are opened and form a C-C single bond leaving two electrons uncoupled. Hence such a diradical should readily revert to the more stable xylylene units. On the 39 Mann and Stewart J. 1954 2826. 39a Personel communication from Professor H. Mark. * The consumption of radicals in this reaction is exceedingly high since it is not a chain process. ERREDE AND SZWARC CHEMISTRY O F $I)-XYLYLENE 311 other hand if four units are combined into a linear tetrameric diradical (see the formula above) then a comparatively stable species is produced and the dissociation energy of its central CH,-CH bond is probably ‘‘ normal ”.Rupture of this central CH,-CH bond in the tetramer pro- duces two “ true ” diradicals from one diradical and consequently no addi- tional driving force is available to facilitate the process.* Hence the critical stage in the polymerisation is overcome when three or four units are com- bined. Since a simultaneous interaction of three or four units is highly improbable in the gas but quite probable in the condensed phase the nuclei for the polymerisation are expected to be formed readily in the condensed phase only. There is still another factor which makes polymerisation in the con- densed phase more probable than reaction in the gas. Most probably the polymerisation of p-xylylene involves diradicals. It was pointed out by Haward 40 and by Zimm 41 that cyclisation competes efficiently with pro- pagation in a diradical polymerisation particularly in the early stages of the growth when the chain is only 3 or 4 units long.I n the gas the con- centration of monomer is low and therefore cyclisation is much more prob- able than propagation. On the other hand the situation is probably reversed in the condensed phase and moreover crystallisation of the polymer hinders the cyclisation considerably by imposing restrictions on the motion of the segments. The creation of polymerisation nuclei is obviously a slower process than the propagation. Consequently the polymer grows very rapidly from the initiating centres a process favouring the formation of dendrites often observed in the reaction. However if the nuclei are formed uniformly a homogeneous film is produced.For example if an object to be coated with p-xylylene polymer is rotated in the stream of gas flowing out of the pyrolysis zone a very uniform homogeneous film is formed.? Such films act as excellent protective materials and adhere well to the various surfaces. Investigation conducted by the Polaroid Corporation led to the interest- ing observation that a laminated film is produced if the pyrolysis is inter- rupted and then continued again. Apparently the growing active centres are terminated during the interruption and then on resumption of the process a new film begins to form on the surface of the old one. Most probably the termination of the growing molecules breaks the interlocking process which is responsible for the formation of a cohesive and homogeneous film.The temperature of condensation of p-xylylene vapour is obviously a factor which determines the physical characteristic of the resulting polymeric film. This point was investigated by carrying out the reaction a t different 40 Haward Trans. Faraday Soc. 1950 46 204. 41 Zimm and Bragg J . Polymer Sci. 1952 9 476. * I n any n-meric diradical the weakest bond involves the terminal unit. I n a trimeric diradical this bond would be stronger than the bond in the dimer although weaker than the central bond of tetramer. f These experiments were performed in the M. W. Kellogg Co. laboratories. 312 QUARTERLY REVIEWS condensation temperatures (0" 20° 40" and SO"). The results suggest that the molecular weight of the polymer formed or its degree of cross-linking increases with decreasing temperature of polymerisation.33 On the other hand the nature of the surface on which the condensation takes place appears not to be critical. To quote an extreme case films of poly-p- xylylene were produced by carrying the vapour of the monomer over the surface of an organic liquid. The polymer formed a crust on the liquid and its properties and structure seemed to be identical with those of films formed on solid surfaces.33 The nature of the termination reaction in p-xylylene polymerisation is not known. If the growing species are diradicals then termination cannot occur by coupling which would merely double the molecular weight of the active diradical species. Furthermore disproportionation is impossible in polymerisation of p-xylylene since no terminal double bond can be formed.Hence cyclisation seems to be the only possibility for termination involving the growing chains. Obviously this reaction is responsible for the formation of the cyclic trimers and tetramers but it is highly improbable for production of mat.sria1 of high molecular weight. Nevertheless the formation of lamin- ated films is an obvious indication that the growing ends are somehow terminated or '' buried " in the polymeric material.* The possibility of termination by impurities which act as monoradicals also should be taken into consideration although experiments designed to check this hypothesis have not been conclusive. Development of the technique of producing solutions of p-xylylene has given impetus to the study of the kinetics of p-xylylene polymerisation.Experiments by Errede z2 indicated that the reaction is of first order with respect to monomer and its rate is proportional to the number of existing " nuclei ". The latter are formed during the preparation of the solution and their number seems to remain constant when the solution is left un- disturbed. However if a '' hot " object is immersed in the solution e.g. a pipette kept a t room temperature (the polymerisation was studied a t temperatures ranging from -80" to -40") the number of nuclei apparently increases and the polymerisation is suddenly enhanced (see Fig. 3) although its new rate again obeys first-order kinetics with respect to monomer con- centration. It appears therefore that under these conditions the poly- merisation does not involve termination each growing chain continuing its growth indefinitely.No experimental data exist to show whether any chain-transfer process takes place in the polymerisation of p-xylylene. Probably the chain- transfer rate constants of poly-p-xylyl radicals are not significantly different from that of polystyryl radicals. However since the propagation rate constant is obviously much greater in the p-xylylene system as compared with the styrene system the corresponding chain transfer constants ( kt,/kp) should be significantly smaller in p-xylylene polymerisation. * It was reported by Corley et u Z . ~ ~ that the concentration of free radicals if they were indeed present in the solid polymer is less than 10-lo mole/c.c. i.e. less than the minimum amount detectable by electron spin resonance. ERREDE AND SZWARC CHEMISTRY OF $I-XYLYLENE 313 0.06 0.05 A \ d w 0.04 0 -.4 5 c .- 4 0.03 0 4 c U c 0 0 0.02 0.0 I 1 I I I . 2 4 6 8 10 12 Time (hr.) FIG. 3 Unimolecular plot representing the kinetics of polymerisatton of p-xylylene. The arrows denote the time when a warm object was introduced into solution causing a sudden increase in the number of growing centres. There are some indications discussed in the last part of this review that the p-xylylene polymers are crosslinked or branched. These linkages might arise from a radical-addition reaction followed by a hydrogen abstraction In these equations R itself is a polymeric radical. It is to be expected that the sequence of the two reactions would be favoured by lower temperature of the polymerisation and indeed the decreasing solubility of the polymer formed at lower temperature of polymerisation i.e.a t lower temperature of condensation can be construed as evidence for an increasing number of crosslinks or bran~hes.~ Actually a polymer isolated from traps cooled by liquid nitrogen appears to be completely insoluble,l2 and this might indicate a high proportion of crosslinks formed a t this temperature. The suggested scheme of addition of radicals to the benzene nucleus might be criticised on the ground that similar reactions can be envisaged in polymerisation of styrene although no crosslinking or branching of this 314 QUARTERLY REVIEWS nature has been discovered in polystyrene. However the styryl radical is more shielded than the radical responsible for the polymerisation of p-xylylene and this may account for the difference in their behaviour.An dternative explanation of the formation of the crosslinlis or branches involves an assumption that in the propagation reaction p-xylylene adds in two ways the normal addition to the methylene group and the very (xv) infrequent addition to the benzene nucleus. The latter leads to the forma- tion of the unit (XV) in the polymer chain which in turn participates in the branching or crosslinking process. Copolymerisation.-Numerous attempts to copolymerise p-xylylene with various conventional monomers were mostly unsuccessful. Thus Roper and Szwarc 33 tried to condense p-xylylene with styrene or butadiene or to bubble p-xylylene vapour through a solution containing these monomers. I n all these experiments poly-p-xylylene was the only polymeric material formed in the reaction.Similar techniques had been used by Kaufman et aZ.,43 Corley et u Z . ~ ~ and Auspos et aZ.,13 and only the last workers reported any success. They were able to copolymerise maleic anhydride and chloroprene with p-xylylene. On the other hand p-xylylene copolymerises with other monomers of the same class e.g. with monomers derived from pseudocumene chloro- p-xylene and 2 5-dimethylpyrazine. 33 The last example is particularly interesting since the polymer of 2 5-dimethylpyrazine is soluble in aqueous hydrochloric acid and thus extraction of the polymeric product with hydro- chloric acid should distinguish between the true copolymer and a mixture of two homopolymers. That no polymer could be so extracted indicates that the investigated material was indeed a copolymer.* Further evidence that copolymerisation occurred is provided by studies of the X-ray diffraction patterns of the 33 In one a mixture of two precursors say p-xylene and pseudocumene was pyrolysed and the product condensed; in the other each precursor was pyrolysed in a different furnace and the gaseous products mixed and condensed together.The advantages of the second method are obvious although technical difficulties encountered in this type of process are much greater. Recently these copolymers were prepared by mixing the desired proportion of the respective monomers kept a t low temperature in solution and then initiating the polymerisation by raising the temperature. 21 Structure of Poly-p-xylylene Polymers.-Poly-p-xylylene seems to be a linear polymer of units (VI).Its oxidation by chromic acid was reported * One may argue however that the investigated material contained two homo- polymers entangled to such an extent that their separation was impossible. We believe that such entanglement could slow down the extraction process or make it less efficient but would not prevent it entirely. -cHkK- CH* Two techniques were used for the preparation of copolymers. FIG. 4 X- Ray diffraction patterns of lin-poly-p-xylylene A Pyrolysis polymer By Wurtz polymer C a-form of polymer D a + small amount of /3 E a + p approx. equal amounts P p-form of polymer (Beproduccd with permission from Brown and Farthing J. 1953 3270) FIG. 5 X - R a y fibre diagram of poly-p-xylylene (Figs. 5 and 6 are reproduced with permission from Kaufman Mark and Mesrobian J.Polymer Sci. 1954 13 3.) ERREDE AND SZWARC CHEMISTRY OF (P-XYLYLENE 315 by Brown and Farthing 4 2 who isolated terephthalic acid as a major product and detected only a trace (about 0-10/,) of isophthalic acid. The ultra- violet and the infrared spectra reveal only this grouping which is consistent with the simple linear structure. Furthermore the X-ray diffraction pat- terns 9 13 42 43 indicate a highly crystalline structure which excludes the possibility of many irregular branches or frequent cross-links in the polymeric composition. The crystalline structure of poly-p-xylylene was thoroughly investigated by Brown and Farthing.42 They were the first to point out the existence of two modifications classified by them as the cc- and the /3-form. The X-ray pattern of the cc-form shows two strong and sharp diffraction rings corresponding to the spacings of 4.0 and 5.2 A (see Table l) while the ,&modification is characterised by a ring corresponding to the spacing of 4-4A.These workers suggested also that the configuration of the cc-form is similar to that of the molecule of 4 4'-dimethyldibenzyl-the rings lie parallel to each other in the same molecule however they are not coplanar but arranged stepwise. On the other hand in the p-form the benzene rings are both parallel and coplanar as in the molecule of diphenyl. Brown and Farthing also observed that the cc-form was converted into the ,&form by heat preferentially above the melting point. This process seems to be irreversible and it was suggested by these workers that it might involve a chemical change namely dehydrogenation of poly-p-xylylene into a linear polystilbene in which each benzene ring would possess a quinonoid structure.Such structures were suggested by Goldfinger for polyphenyl- ene. I n our opinion the chemical reaction suggested by Brown and Farthing does not take place under mild conditions although it'might occur during pyrolysis of the polymer. Hence we consider the ,!3-modification to be polymorphic with the a-form. Studies by Brown and his colleagues also showed a similarity between the crystal structures of linear poly-p-xylylene obtained by pyrolysis and of the polymers obtained by the Wurtz reaction from p-xylylene dichloride. The proportions of 01- and P-modification in the polymer depend on the method of preparation and are affected by the subsequent treatment of the sample.Fig. 4 reproduced from the paper by Brown and Farthing illustrates the observed changes in the X-ray diffraction patterns of polymers obtained under different conditions. X-Ray studies of polymers obtained from other monomers of the p - xylylene class were reported by Szwarc and by Kaufman Mark and Me~robian.~~ Their results are also included in Table 1. Moreover Kauf- man Mark and Mesrobian studied the structure of fibres obtained by stretching samples of poly-p-xylylene polypseudocumene and polydurene. The high melting points of these polymers made it necessary to stretch the fibres a t temperatures above 150". The phenomenon of " necking down " 4 2 Brown and Farthing J. 1953 3270. 43 Kaufman Mark and Mesrobian J. Polymer Xci. 1954 13 3. 44 Corley Haas Kanc and Livingstone ibid.p. 137. 4 5 Goldfinger ibid. 1949 4 93. 316 QUARTERLY REVIEWS frequently associated with the cold drawing of crystalline polymers was not.ed during the elongation of poly-p-xylylene films. This phenomenon TABLE 1. X- Ray diffraction patterm of various polyxylylenes (XVI) (XXI I) (XXl I I) (XXIV) * Po I y m e r 2-0 Plane spacings (i) 3.0 4.0 5.0 6-0 7-0 8.0 ......... ......... ...... ......... I ......... I (XVl),oc-form I . . . . . . . . I J ,..I I (xvi) fi-form ,. .... . . . . I . ........ ,.... I . . . . . I ......... I . . . . . . . . . I ......... (XVI) Dc+p I I I...! ,..I I"'"" I""""' I ......... ......... ..... ...... (XVl I) (XVIII) ,.". ... I.,. ........ 1 .........,.. .I ...... ,..I ......,... I ""'I I. ..... . . ' I . . .......I ......... ....... I... .. . " . I . .. 1.. .. . (x I x) ,. ',.. I.. ,.. I (xx) I ....... .I I .........,... I .....,... I I I (XXI I) I ' """' I * * - * * I. I T I I ...... .... ......... ...... ......... I"" - I ......... ..... ......... ......... ................ ......... ..... ......... i ......... !'"" I. ....I..' I (XXI I I) ......... ..... ...... I"' ...... I""'"" I -. - * - * *. I (xx 1) I I 1...,.1. ........ ....... ......... ......... .... (XXIV) I I I. I I " ' " " " 1 2.0 3.0 4.0 5.0 6.0 7.0 8.0 [=sharp E d i f f use is typical of polymers of high molecular weight and is therefore further evidence that poly-p-xylylene obtained by pyrolysis has a high molecular weight. ERREDE AND SZWARC CHEMISTRY OF p-XYLYLENE 317 Evidence for the formation of a well-oriented fibre is apparent from the X-ray fibre diagrams shown in Fig.5 . The repeat distance calculated from the diagram is 6.55 8 corresponding to an ordered sequence of p-xylylene units (see Fig. 6). The calculated value for such a configuration is 6.52 A Ordered Sequence Disordered Sequence FIG. 6 Schematic representation of the arrangement of p-xylylene chains under stretch. while a repeat distance of 6.37 was calculated for the disordered configura- tion shown also in Fig. 6. The former repeat distance was found also for the oriented polypseudocumene and p~lydurene.~~ Determination of the molecular weight of poly-p-xylylene presents technical difficulties because of its insolubility and high melting point. I n work carried out a t the Du Pont laboratories over 250 cheniicals were tried as solvents for these polymers; 1 3 the best were the chlorinated diphenyls benzyl benzoate a-methylbenzylphenols and the terphenyls.The polymer does not dissolve however below 250" and precipitates again if the temperature of the solution falls below 200°.91 1 3 5 43 Some degradation probably takes place during dissolution this degradation apparently con- tinuing if the solution is kept a t these high temperatures.13 Some observations by Kaufman Mark and Mesrobian 43 should now be mentioned. These workers claim that poly-p-xylylene dissolves only if oxygen is present during the heating and when this was rigorously excluded a sample of the polymer did not dissolve when heated for two days with a suitable solvent a t 250". They concluded therefore that some cross-links exist in the polymer and that oxygen accelerates their rupture.318 QUARTERLY REVIEWS The viscosity of diluted solutions of poly-p-xylylene was measured by Auspos et aZ.13 who developed a viscosometer suitable for work a t high temperatures (around 300"). They found that all " soluble " polymers i.e. materials which could be dissolved at 302" in less than 6 minutes had low inherent viscosity. The viscosity increased however as the rate of solution of the polymers decreased. The relevant results are collected in Table 2 and indicate that the molecular weight of some samples is a t least 20,000. Similar results were obtained by Kaufman et aZ.43 who determined the molecular weight of a soluble fraction of polypseudocumene by measuring the osmotic pressure of its solution in chloroform (Bn -24,000).Further evidence for the high molecular weight of poly-p-xylylene is provided by Kaufman Mark and Mesrobian,43 namely its ability to be stretched to give highly oriented fibres the swelling in water of sulphonated poly-p-xylylene and the nondiffusibility of the polymer solution through TABLE 2. Viscosities of poly-p-xylylene Preparation Pyrolysis Pyrolysis Pyrolysis (I Wurtz reaction Wurtz reaction c Pyrolysis Pyrolysis Time t o soh. min. 40 25 6 10 11 5 30 Concn. g./100 C.C. 0.768 0.673 0.504 0.838 0.865 0.261 0.618 0.27 0.325 0.153 0-095 0-15 0.35 0.73 a Polymer is drawable at 150" before dissolving. Reaction a t -30". C Reaction a t -60". Polymer melts at 390-400" when heated mpidly; degradation quite slow at 259". Viscosity a t 259" (more soluble polymer). Swollen gel present after 5 min.At 259". Dissolution after 30 min. the solution becoming amber. a sintered-glass membrane which allegedly prevents diffusion of polymers of molecular weight above 20,000. In connection with the ability of poly- p-xylylene to yield fibres it is generally recognised among fibre chemists that crystalline polymers must have a minimum molecular weight in order to exhibit spinnability and drawing capacity; for polyesters this is above 8000. There are several indications that the network chains in (unstretched) poly-p-xylylene are deposited on the walls of the condenser in some unique manner. First the transparent polymer film exhibits negative birefringence when viewed edge-on,a39 44 and secondly the sulphonated polymers swell anis~tropically.~~ Since the sulphonated polymer exhibits pronounced swelling in length and width but not in thickness it might be assumed that the network chains are lying approximately parallel to the flat face of the polymer film.Attempts to define this arrangement more exactly by X-ray diffraction were unsuccessful; however it has been reported that ERREDE AND SZWARC CHEMISTRY O F p-XYLYLENE 310 in the detection of chain orientation X-ray measurements are less sensitive than optical birefringence measurements or anisotropic swelling behaviour.46 A very pronounced birefringence develops in films of poly-p-xylylena during stretching. The stretching birefringence constant is positive and very namely + 0.2 while considerably lower values characterise other polymeric films [e.g. poly (vinylacetate) + 0.035 "Ethocel" + 0.016 cellulose acetate + 0.005 poly(methy1 methacrylate) - 0.001 and poly- styrene - 0.024 to - 0.0311.At the same time films of poly-p-xylylene exhibit a positiveand very high Brewster constant44 of + 75. 10-13 cm.2/dyne (the corresponding value for polystyrene is + 10 in the same units). The high and positive birefringence stretching constant and Brewster constant make poly-p-xylylene a very good photoelastic material. Intractability of poly-p-xylylene polymers is frequently attributed to the combination of crystallinity and cross-linking. It is difficult to separate the two effects for no amorphous polymer has ever been obtained-even the polymer produced a t - 190" shows ~rystal1inity.l~ Presence of ortho- substituents in the repeating units only slightly lessens the crystallinity.The polymer does not dissolve until the temperature of the solution approaches that of the crystalline melting point.13 This observation led many to believe that the intractable nature of the polymer is due solely to its extreme crystallinity. However this property alone cannot explain the following observations (1 ) Although ortho-substituents somewhat lower the crystallinity of the polymer they do not lower the flow point; for example the melting point of crystallites of poly-p-xylylene and polypseudo- cumene are 400" and 250° respectively while their flow points are at about 4OO0.l3 (2) Although the swellability is increased by this structural modifica- tion the solubility is hardly affected.13 I n fact the poly- (2-phenyl-p- xylylene) the least crystalline of this class of polymer is even less soluble than poly-p-xylylene.l3 One is forced therefore to assume that the polymer is cross-linked in addition to being crystalline although neither infrared spectra nor X-ray examination indicates the presence of cross-links. How- ever it was pointed out by Powers and Austin47 that these analytical devices are not sensitive enough to detect the presence of cross-links in amountr that still could impart marked effects on the physical properties. Poly-p-xylylene undergoes many of the reactions typical of p-xylene itself although the rate is markedly retarded because of the polymer's insolubility. The polymer is not attacked appreciably by concentrated sulphuric acid at 150". Sulphonation occurs readily however in the presence of traces of silver ion,13 439 44 and the product contains one sulphonic acid group per benzene ~5ng.4~ The sulphonated product swells about 100-fold in water and about 400-fold in dilute aqueous base without dis~olving.~~ The swelling is anisotropic that is the sulphonated film expends in length and width but not in thickness.4 6 K. H. Meyer " Natural and Synthetic High Polymers " 2nd edn. Interscience 4 7 Powers and Austin J . Polymer Sci. 1951 6 775. Publ. New York 1950 p. 89. 320 QUARTERLY REVIEWS The polymer can be chlorinated by treatment with sulphuryl chloride in pyridine in the presence of ultraviolet light.44 Chlorination occurs pre- dominantly a t the ethylene groups although some aromatic substitution and chlorinolysis also take place. The final product again displays aniso- tropic swelling and it contains 2-4 chlorine atoms per p-xylylene unit.The chlorinated product is fairly soluble in low-boiling solvents such as toluene and xylene because of chain scission. Clear pale yellow polymeric films are deposited when the solutions are evaporated to dryness. These films are reported 44 to be tough and flexible and can be cold-drawn. The product melts with darkening a t about 200" The once-solubilised chlorin- ated polymer is amorphous and is readily soluble in low-boiling solvent such as dioxan or benzene. Nitration occurs 13 when the polymer is immersed in concentrated nitric acid at 50" for a prolonged period giving poly(dinitr0-p-xylylene). This material is it high explosive with the sensitivity of pentaerythritol tetra- nitrate and with the power of trinitrotoluene.The nitrated product is soluble in nitrobenzene and in cyclohexanone but films cast from these solutions are of very poor quality.13 Poly-p-xylylene is unaffected by prolonged treatment with boiling 5 ~ - nitric aci6.4z About one half of the available polymer is oxidised however by 24 hours' treatment with excess of chromic oxide in boiling acetic acid,42 yielding terephthalic acid as the main product. The polymer is slowly oxidised a t high temperature by atmospheric oxygen and the material burns fiercely when ignited. 13 Degradation of the polymer takes place on melting a t about 400". It was expected that the polymer would decompose into monomeric units i.e. p-xylylene molecules which would re-polymerise on condensation. Hence a pseudosublimation of the polymer was attempted by heating it in one end of an evacuated tube cooled at the other end by solid carbon dioxide.The experiment failed however and closer examination of the results showed that dehydrogenation occurred readily. 34 Madorsky and Strauss 48 have studied the decomposition of poly-p-xylylene in a vacuum and observed random chain scission a t 415". Benzene toluene xylene p-methylstyrene and p-ethyltoluene were identified as volatile products of the decomposition. The activation energy was reported to be 76 kcal./mole. These workers compared the thermal stability of numerous polymers and found poly-p-xylylene to be second to "Teflon " in this respect. We thank the National Science Foundation for their support. We also express our appreciation to the late Dr. F. Kind Dr. H. Steiner Dr. W. E. Hanford and Dr. J. W. Copenhaver for encouragement and interest. 48 Madorsky and Strauss J . Res. Nut. Bur. Xtund. 1955 55 223.

ISSN:0009-2681    

DOI:10.1039/QR9581200301

出版商:RSC    

年代:1958

数据来源: RSC

摘要:

STRUCTURE AND PROPERTIES OF C-NITROSO-COMPOUNDS By B. G. GOWENLOCK M.Sc. PH.D. (DEPARTMENT OF CHEMISTRY THE UNIVERSITY BIRMINGHAM 15) and W. LUTTKE DR.RER.NAT. (INSTITUT FUR PHYSIKALISCHE CHEMIE THE UNIVERSITY FREIBURG I. BR.) C-NITROSO-COMPOUNDS have been known for over eighty years the first examples being NN-dimethyl-p-nitrosoaniline,l nitrosobenzene which was prepared by Baeyer from reaction of nitrosyl chloride with diphenyl- mercury and the pseudo-nitroles obtained by Meyer 3 from the reaction of nitrous acid with secondary nitro-alkanes. These and other similar aromatic and substituted aliphatic nitroso-compounds gave blue or green solutions but also formed in most cases colourless crystals. Piloty Bamberger and others showed by molecular-weight measurements that these colour changes corresponded to the existence of two different molecular forms i.e.a blue or green unimolecular compound and a colourless bimolecular compound. This feature distinguished C-nitroso- from N-nitroso-compounds.* A fur- ther generalisation established by about 1905 was that primary and second- ary nitroso-compounds were not capable of more than transient existence owing to rapid isomerisation to the oxime R1R2CH*N0 --+ R1R2C:N*OH. This generalisation which sufficed for the compounds then known was given a wide currency by Sidgwick and was repeated by Walker in the most up-to-date review. Exceptions to this " rule " were recognised in m-nitrosotoluene (C,H,*CH,*NO) first prepared by Behrend and Konig, and in the substituted secondary nitroso-compounds (R*CHX*NO), where X = C1 Br or C0,Et.In some cases the name " isonitroso-compound " was given to the oxime derived from the primary or secondary nitroso- compound. In addition it was recognised that many of' the bimolecular tertiary nitroso-compounds dissociated into the monomer in solution or when molten and that some nitroso-compounds existed as monomers only. It was also shown that all dimeric nitroso-compounds could be dissociated to the monomer in suitable conditions though such dissociation might only be very limited in its extent. Before 1914 German chemists (particularly Piloty Bamberger Wieland Staudinger and Schmidt) made major con- tributions to our understanding of these compounds; in the inter-war period Baeyer and Caro Ber. 1874 7 809. Meyer Annalen 1875 175 88; 1876 180 133. Sidgwick " The Organic Chemistry of Nitrogen " Clarendon Press Oxford 1937 Walker " The Chemistry of Carbon Compounds " Vol.lA Elsevier Amsterdam Behrend and Konig Annalen 1891 263 212. 2 Baeyer ibid. p. 1638. p. 204. 1951 p. 370. * We shall henceforth refer to C-nitroso-compounds as nitroso-compounds except where it is necessary to distinguish between C- N - 0- and halogen-nitroso-compounds. 321 322 QUARTERLY REVIEWS Ingold Aston and Hammick extended that knowledge and within the last ten years a variety of contributions pursued independently in many different countries have so extended knowledge of the preparations and properties of nit,roso-compounds as to make previous reviews 47 inadequate. In particular the structure of the dimeric nitroso-compounds which Sidgwick classed as ‘‘ an incompletely solved problem ” has been solved.Preparation The variety of available preparative methods may be classified as follows (i) Oxidation methods. Until recently the only oxidative procedures were those developed about fifty years ago neutralised permonosulphuric acid (Caro’s acid) being the best means of oxidising aromatic amines 7 or tert.- alkylamines to the corresponding nitroso-compound. The yields are often poor (e.g. 4% for ButNO). Oxidation of the N-substituted hydroxylamine by aqueous chromic acid,6 ferric chloride,g or chlorine 10 has been employed. The use of mercuric oxide l1 with N-cyclohexylhydroxylamine has resulted in synthesis in good yield of nitrosocycbohexane a secondary nitroso-com- pound. This nitroso-compound can also be prepared by oxidation of cyclo- hexylamine by Caro’s acid 12a (21% yield) or potassium permanganate 12b in aqueous formaldehyde (8O%) weakly acid conditions being necessary in the latter case it is possible that the reaction mechanism is similar to the Emmons synthesis.A new preparative method for primary secondary and tertiary nitroso- compounds has been established by Emm0ns.1~~ Neutralised peracetic acid in methylene dichloride is employed to oxidise either the amine or the diethyl ketimine to the nitroso-compound. This method which proceeds by oxidation to the oxaziran and further oxidation to the nitroso-compound offers the possibility of production of many nitroso-alkanes in large quantity. It is to be noted that tertiary nitroso- alkanes are prepared in better yield by oxidation of the corresponding hydroxylamines than by oxidation of the imine.Modifications of the Caro’s acid oxidation have recently been pursued by Krimm 13b who prepared nitroso-alkanes -aralkanes and -cycloalkanes in good yield by oxidation of either secondary amines by peracetic acid in ethereal solution by use of compounds of molybdenum or tungsten as catalysts or primary amines (attached to primary or secondary carbon atoms) by a variety of -peracids in benzene solution. No preparative method for nitroso-alkanes by reduction of a nitro-compound has been reported although reduction of some nitro-compounds by zinc dust yields the nitroso-compound as an The results are listed in Table 1. (ii) Reduction methods. ’ Bamberger Ber. 1899 32 1675; 1903 36 3803. Bamberger and Seligman Ber. 1903 36 685. Bamberger Ber. 1895 28 245.lo Piloty Ber. 1898 31 1879; 1901 34 1864; 1902 35 3093. l1 Flam Swiss Patent No. 324,434 Class 360 1957. l 2 Okamura and Sakurai (a) Chem. Abs. 1953 47 2992; ( b ) 1954 48 4225. l3 (a) Emmons J. Amer. Chem. SOC. 1957 79 6522; (b) IZrimm G.P. 948,417 1956; Krimm and Hamann G.P. 956,069 1956. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS TABLE 1. P" Pri PhCH n-cl zH25 n-C18H35 cyclo-C,H, 9% - C,H ,.CHMe Ph-CH ,*CH But Me,C*CH,*CMe Yield (%) 33 37 37 60 {:; 83 71 86 87 Preparation of (RNO) Starting materials Me,CH.N:CMe PhCH ,*NH ,,E t ,CO n-C,,H,,-NH,,Et ,CO n-C,,H,5*NH,,Et,CO c~cZO-C,H,~*NH,,E~ ZCO J cyclo-C,H ,.NH 'I C,H,,*CH.NHMe,Et ,CO Ph*CH,.CH ,-NH ,E t ,CO ButNHeOH Me,C.CH,*CMe,*NH.OH Osidising agent MeCO ,H Me-CO ,H Me*CO,H Me-CO,H MeC0,H MeC0,H Me.CO,H NaOH,Br NaOH,Br 323 intermediate product ; some aromatic nitroso-compounds can be prepared in poor yield by reduction of the nitro-group.Reduction of 2 2'-dinitro- diphenyl by either zinc dust l4 or sodium sulphide l5 gives the internal cis-dimer (I). (iii) Nitrous acid reactions. A nitroso-group can be introduced into the benzene ring in the para-position by reaction between a tertimy aromatic amine or a phenol with nitrous acid. Alternatively Meyer's test affords a method of production of a pseudo-nitrole. (iv) AZkyZ nitrite reactions. Coe and Doumani 16 first produced nitroso- methane on photolysis of gaseous tert.-butyl nitrite at room temperature Me,C*ONO -+ Me,CO + MeNO. It has been further shown l7 l8 that the reaction is a general one for alkyl nitrites when filtered radiation is used (A < 3300 A).The same nitrites will also yield nitroso-alkanes on pyrolysis at about 320" c when the correct conditions of low pressure of reactant and short reaction time are employed.18 The pyrolysis probably involves free-radical reaction whereas the photolysis is predominantly intramolecular. The well-known inhibiting action of nitric oxide on free-radical chain reactions suggested the possibility of production of nitroso-compounds by this means. Although nitric oxide was first em- ployed as an inhibitor in 1937 i t was only in 1953 that preparation of nitroso-compounds was reported by this means by several workers. The early qualitative results have been summarised l9 and it is noteworthy that dimeric nitrosomethane was probably prepared 20 (unknown to the authors l4 Tauber Ber.1891 24 3081. l5 Ross Kahan and Leach J . Arner. Chem. SOC. 1952 74 4122. l6 Coe and Doumani ibid. 1948 70 1516. l7 Tarte Bull. SOC. roy. LiBge 1953 26. l 8 Gowenlock and Trotman J. 1955 4190; 1956 1670. 19Chilton and Gowenlock J. 1953 3232; 1954 3174. 2o Thompson and Linnett Trans. Paraday SOC. 1937 33 874. (v) Radical + nitric oxide. 324 QUARTERLY REVIEWS concerned) in 1937. The gaseous radicals are produced thermally,lg 21 photolytically,z1 22 z3 or by a metathe~is.~~ A new preparative method for nitrosomethane which may well be of this type is due to Kharasch and his co-workers,25 acetyl peroxide being decomposed in either boiling sec.-butyl or sec.-pentyl (l-ethylpropyl) nitrite. Alternatively this reaction may be of a complicated character. (vi) Halogen-oxime reactions.Substituted chloro- and bromo-nitroso- compounds are prepared by this means,4 10 ketoximes reacting in a one-stage process R'KLC:N.OH + X - .R'R~CX.NO + HX and aldoximes in a two-stage process consequent upon isomerisation of the secondary nitroso-compound formed (which can also be isolated) R.CH:N.OH 4- X -.- RCH X * NO - R*CX:N*OH R.CX:N-OH + X --c RCHX-NO + HX R.CXiNO f HX N-Bromosuccinimide can serve as the source of bromine for production of bromonitroso-compounds. 26 (vii) Nitrosyl chloride and relccted methods. Aromatic nitroso-compounds can be prepared by reaction of nitrosyl chloride with a diarylmercury 2 or arylmercuric 27 or arylmagnesium halides. 28 Chlorine-containing nitroso- compounds result from addition of nitrosyl chloride to ethylenic systems,29 which has had considerable importance in determining the structure of terpenes 30 and is paralleled by the similar addition of dinitrogen trioxide (" nitrous fumes ").Both reactions occur according to the scheme I R'R2C:CHR' + N0.X - R'R2XC-CH(NO)R3 It has not been unambiguously established that the usual method of pro- duction of nitrosyl chloride (pentyl nitrite and hydrochloric acid) in the presence of the unsaturated compound does not also produce dinitrogen trioxide which aJso reacts. Therefore detailed evidence is necessary to confirm the identity of the product or products. Other preparations of a similar kind are the reactions of an alkyl rritrite with certain ketones containing the group >CH*CO in presence of hydro- chloric acid whereby compounds containing > C*NO*CO are formed,31 and the reaction of nitrous fumes upon a-acyl esters R*CHR1*C02Et where R1 21 Pratt U.S.P.2,683,078 1954. 22 Haszeldine J. 1953 2075. 23 Banus J. 1953 3755. 24 Muller and Metzger Chem. Ber. 1953 88 165. 25 Kharasch Meltzer and Nudenberg J. Org. Ghenz. 1957 22 37. 26 Iffland and Criner J . Amer. Chem. SOC. 1953 75 4047. 2 7 Smith and Taylor ibid. 1935 57 2460. 29Tilden and Sudborough J. 1893 63 479. 30 TVallach " Terpeno und Campher " Enke Leipzig 1914 p. 69. 31 Aston Menard and Mayberry J . Amer. Chem. Soc. 1932 54 1530; 1935 57 28 Oddo Gaxxetta 1909 39 659. 1888. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 325 is an acyl group such as acetyl and R is an alkyl group the cc-nitroso-esters R-CH (NO) CO,E t being formed. A survey 33 of preparative methods for nitroso-compounds has recently appeared.In general it can be stated that whereas our knowledge of preparative methods for aromatic nitroso-compounds has remained virtually unaltered for over fifty years the major advance has been directed to the production of the hitherto unknown nitroso-alkanes. It will be seen that only one general method 18 (pyrolysis of alkyl nitrites) is available for the production of the cis-nitroso-alkane dimers. It is possible that the production of the cis-dimer from the products of pyrolysis of an alkyl nitrite is due to the fact that the monomeric nitroso-compound is orientated in a regular manner (a) on condensation from the vapour phase on the wall of the liquid-oxygen- cooled vessel. ( 0 ) (b ) On being warmed to about - 90" to - 70° the solid first melts and then rapidly forms the white solid cis-dimer.It may be supposed that in the viscous liquid formed on melting the orientated monomers react rapidly to give the cis-dimer (b) even though this is less stable thermally than the trans-dimer. As most C-nitroso-compounds exist in one monomeric and two dimeric forms which possess different physical and chemical properties we shall consider the detailed properties under separate sections of this Review. All other methods yield the trans-dimers. Structure and properties of C-nitroso-monomers The nitroso-compounds discussed in this section are those that exist solely as the monomer or as dimers that dissociate completely to the monomer on dissolution in organic solvents or when heated (gas or liquid phase) except as noted. Physical Properties.-( i) Electronic absorption spectra.The most obvious property of a monomeric nitroso-compound is its blue (aliphatic) or green (aromatic) colour. The electronic absorption spectra of a variety of nitroso- monomers have been inve~tigated,~41 35 and three characteristic absorption bands located namely 6300-7900 ( I - SO) and < 2200 A ( E - 5000). The first of these is given by all nitroso- monomers whereas the other two bands are characteristic of aliphatic nitroso- compounds only being submerged in the strong phenyl absorption ( E - 1-60) 2700-2900 32 Schmidt Annalen 1910 317 30. 33 Metzger in Houben-Weyl-Muller " Methoden der organischen Chemie " Vol. 34 Pesteiner and Bruck in Landolt-Bornstein " Tabellen Zahlenwerte and Funk- 35 Hershenson " Ultraviolet and Visible Absorption Spectra " Academic Press 10-1 Georg Thieme Stuttgart 1958 in the press.tionen " I Band 3 Teil (Molekeln 11) Springer-Verlag Berlin 1951. New York 1956. X 326 QUARTERLY REVIEWS in the aromatic series. The position of the long-wavelength absorption maximum depends on the nature of the substituents in the alkyl or aryl group. In the cases of some aliphatic and all ortho-substituted aromatic nitroso-compounds the extinction coefficient is a function of both concen- tration and temperature and thus depends on the degree of dimeri~ation.~~ 37 By this means accurate values of the energy of dimerisation have been measured (see p. 339). With those compounds which are present only as the monomer in solution cmax is altered by structural changes. Havinga and other workers 38 have shown that the " normal value " of E,, is -45-60.The low values of cmsx for p-nitrosophenols are partly due to the tautomeric equilibrium annexed. The high value of E,, for dimethyl- p-nitrosoaniline is ascribed to the contribution of the quinonoid structure (11) to the resonance hybrid. Confirmation of this contribution is also provided by dipole moments polarography and kinetics (see pp. 328 and 329). On the basis of the relatively low intensity of the visible absorption Lewis and Kasha 39 suggested that it was due to a singlet-triplet transition. A theoretical treatment of the spectra of the molecules O=O O=NH O=N*R has been given by Orge1,40 the visible absorption being ascribed to a singlet- singlet n-n*(N) transition one electron from the lone pair of the nitrogen atom is promoted to an antibonding n*-orbital.Confirmation of this assign- ment is provided by the following observations. On dimerisation or on oxidation to the nitro-compound when these electrons participate in bond formation the visible absorption disappears.41 Also the absorption band is displaced when the solvent is changed from a non-polar to a polar medium (e.g. hexane to ethanol to water) which is in full agreement with the results for other n-n* transition^.^^ The investigations of Fenimore 43 and of Nakamoto and Suzul~i44 on the dichroism of the absorption showed that the visible absorption exhibited a polarisation peypendicubar to the plane of the molecule this also is in accord with prediction for n-n* transitions. Finally analysis of the complex diffuse vibrational structure of this visible absorption 457 46 showed that the N=O stretching frequency in the 36 Ingold and Piggott J.1924 125 168. 37 Keussler and Luttke 8. Elektrochem. 1958 in the press; Luttke Angew. Chem. 38 Schors Kraaijevel and Havinga Rec. Trav. chim. 1955 74 1243. 39 Lewis and Kasha J . Anzer. Chem. SOC. 1945 67 994. 40 Orgel J. 1953 1276. 41 Luttke Habilitationsschrift Freiburg. i. Br. 1956. 42McConnell J. Chem. Phys. 1952 20 700. 43 Fenimore J . Amer. Chem. SOC. 1950 '72 3114 3226. 44Nakamoto and Suzuki J . Chem. Phys. 1952 20 1971. 4 5 Tarte Bull. SOC. chim. belges 1954 63 525. 46Mason (Banus) J. 1957 3904. 1958 70 442. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 327 excited state is about 200-300 cm.-l less than in the ground state such a drop in this frequency and a related increase in the N-0 bond length is such as to be expected when the n*-orbital is filled by only one electron.It has been shown by Hammick and his co-workers 47 that the visible absorption is associated with photodecomposition for some aliphatic nitroso- compounds the reaction C.NO-C$ - ,C=C + HNO \ \ / / occurring. The lower-wavelength bands for aliphatic nitroso-compounds have been assigned 41 46 to the n-n*(O) transition (2700-2900 A) and the n-n* transition of the N=O group. Although assignments of the N-0 stretching frequency for a few nitroso-compounds have been made by Glusker and Thompson,48 Muller and Metzger,49 Jander and Haszeldine,50 and Goubeau and Fromme 51 (Raman) yet complete confirmation awaited the detailed assignment of the infrared spectra of monomeric nitroso- compounds made by Luttke,52 Tarte,45 and Mason and Dunderdale 53 who demonstrated that the N=O stretching frequency was in the region 1539- 1621 cm-l (aliphatic and halogenated) and 1488-1513 cm.-l (aromatic).The C-N frequency couples with characteristic vibrations of the remainder of the molecule generally resulting in two bands at about 1100 and between 760 and 860 cm.-l. The characteristic frequencies were assigned 52 on the basis of detailed comparison of the absorptions in a series of related molecules (for tertiary aliphatic nitroso-compounds) by a study of the infrared ab- sorption spectra of the gas undergoing the isomerisation )CH*NO -+ )C=NOH a t 170" (for CH,*NO and C6Hll*NO) and by an infrared study of the liquaid-phase disproportionation (first observed by Bamberger 54) 3RNO -+ RN(-+O)*NR + RNO for nitrosobenzene.This detailed assign- ment implies the rejection of the assignment 55 of the - 1500 cm.-l band to conjugation of C=C stretching with the nitroso-group. The N-0 stretch- ing frequency plotted against the N-0 bond length gives a smooth curve and inspection of the graph implies that the N-0 bond length in the nitroso- monomer will be about 1.27 which is the value for a " normal " double bond.52 It may also be noted that in the aliphatic nitroso-compounds substitution of hydrogen by an acetyl group lowers the N-0 frequency whereas substitution by one C1 CN or NO group raises the NO frequency di- or tri-substitution leads to an even larger value for this frequency. (ii) Infrared and Raman spectra. 4 7 Anderson Crumpler and Hammick J. 1935 1679; Haminick and Lister J.48 Glusker and Thompson Spectrochirn. Acta 1954 6 434. 49 Muller and Metzger Chem. Eer. 1954 87 1282. 50 Jander and Haszeldine J. 1954 912. 51 Goubeau and Fromme Z. anorg. Chew. 1949 258 18. 52Luttke J . Phys. Radium 1954 15 633; 2. Elelctrochem. 1957 81 302. 63Mason (Banus) and Dunderdala J . 1956 754. 53 Bamberger Ber. 1900 33 1939; 1902 35 1606. 55 Nakamoto and Rundle J . Amer. Chem. SOC. 1956 78 1113. 1937 489. 328 QUARTERLY REVIEWS In aromatic nitroso-compounds substituent effects are small but can be correlated for para-substituents with their electron-donating or -attracting properties. It has been established 5 3 that perfluoro- nitroso-compounds have normal Trouton constants (21.9 23-8) and it may be presumed that other nitroso-monomers exhibit similar behaviour.There are no direct determinations of heats of formation although approximate equivalence of D(R-NO) and D(R-NO,) gives 56 AHf(MeNO) = - 4 3 4 kcal.mole-1 and AH,(EtNO) = 0 Only one X-ray crystal- lographic examination of a monomeric nitroso-compound has been reported ; 57 p-iodonitrosobenzene is a planar molecule with a C-N-0 angle of 125" C-N distance 1-28 8 N-0 distance 1-24 8. This implies considerable double-bond character in the C-N bond. Strong dipole-dipole interaction between molecules is suggested by the short iodine-oxygen contact distance. The remaining physical data are dipole moments and diamagnetic suscepti- bilities both of which give information about the structure of the monomers. The diDole-moment data for substituted nitrosobenzenes are summarised 58 (iii) Other physical properties.4 lical.mole-~. J. in Table 2. TABLE 2. Dipole moment ( D) for p-X-nitrosobenxene and C6H5*X. X H NO 2 c1 Br I Me NMe 3.2 0.84 1.84 1-92 2.16 3.79 6.90 0 3-97 1.59 1.57 1.42 0.37 1.61 Vector sum of moments 3.2 0.77 1.61 1.63 1.78 3.57 4.81 The effect of the p-NMe group is particularly striking and is additional confirmation of the contribution of the quinonoid form (11) to the resonance hybrid which the evidence from electronic spectra also suggests in this case. Dipole moments for a few aliphatic nitroso-monomers have also been mea- sured and are slightly lower than that for nitrosobenzene. It has already been noted that nitroso-compounds are diamagnetic a variety of measurements confirming this fact. The most detailed survey 59 shows that the discrepancy between the observed molar susceptibilities and the values calculated from Pascal's constants is between 10 and 18 units.It is therefore concluded that the magnetic contribution of the monomeric nitroso-group is paramagnetic by this amount. Such a value is very much larger than for other groups ( e . g . ketone C 0 + 6.3 units) but is very much smaller than for compounds possessing permanent magnetic moments. 1957 75. 5 6 Gowenlock Trotman and Batt Chemical Society Special Publication No. 10 6 7 Webster J. 1956 2841. 69 Matsunaga Bull. Chern. SOC. Japan 1956 29 969. 58 Ref. 34 p. 463 et seg. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 329 Chemical Properties.-We may treat the reactions of monomeric nitroso- compounds under seven separate headings. Such rea,ctions for aromatic nitroso-compounds have been surveyed by Hickinbottom 6o and detailed references for many of the reactions listed will be found in that and other reviews.4 Many oxidising agents e.g.nitric acid hydrogen per- oxide and permanganate produce nitro-compounds ; oxidation accounts for the low yields obtained in the oxidation of amines by Caro’s acid. Little attention has been paid to the gas-phase oxidation of nitroso-alkanes though it has been reported61 that nitric oxide will oxidise nitrosomethane to nitromethane. The consequences of this for the inhibition of free-radical chain reactions by nitric oxide have yet to be explored. For this class of reaction as for most reactions of nitroso-compounds detailed kinetic studies are singularly lacking. The polarographic reduction of nitroso-compounds has been investigated by a number of workers (for references see Holleck and Schindler 62).(i) Oxidation. (ii) Reduction. The general reversible reaction scheme R.NO + 2H+ + 2e 1-) R*NH.OH operates and half-wave potentials for a number of para-substituted nitroso- benzenes over the pH range 1-10 are in Table 3. A linear relation is shown to exist between the half-wave potential at pH 7 and the frequency of the visible absorption band when the para-substituent is CHO C1 Br H I CH, or OCH,. This relationship very clearly shows the interrelated nature of the reduction process and the light -absorption mechanism (sum- marised in the Figure). Antibonding r*- . state + -- I I I n - . Bonding r- * . state- . . .{= . . . . . . . . 0 . . . . . . . 0 . Electron transition Ground Electron addition in absorption of state through reduction light It is noteworthy that the anomalous behaviour of the p-OH and p-NMe substituents is reflected in their different bonding (see later).The only kinetic study of reduction of nitroso-compounds 63 gives results that confirm the above pattern the rate-determining step being the electron-transfer Ph*N:O + SO:- -c (Ph*~.O-;SO~) 6o Hickinbottom “ The Chemistry of Carbon Compounds ” Vol. IIA Elsevier 6 1 Levy I n d . Eng. Chem. 1956 48 762. 6 2 Holleck and Schindler 2. Elektrochem. 1956 60 1138 1142. 63 Kresze and Manthey Chem. Ber. 1956 89 1412. Amsterdam 1954 p. 148. 330 c1 H CH NMe 2 QUARTERLY REVIEWS t - 469 13,369 8.5 0.5 -479 13,423 ' 8.7 10.5 - 525 13,605 ' 8.4 10.9 -615 1 15,000 4.5 6.3 TABLE 3 p-Subst.E (mv)62 Y (cm.-1)6a . log Ati3 E (k~nl./inole)~~ The relevant data are in Table 3. Again it is evident that p-NMe substituents produce a fundamental alteration in the properties of the nitroso-group . Reduction of aromatic nitroso-compounds with a variety of reducing agents produces the amines or the azo- or azoxy-compound according to the conditions; in some cases substitution in the ring also occurs. The internal oxidation-reduction (disproportionation) reaction of molten nitroso- benzene has already been mentioned; the mechanism of this reaction is unknown but presumably PhN does not participate as azobenzene is not a product. Reduction of the gem. -chloronitroso-compounds 64 by catalytic hydro- genation lithium aluminium hydride or sodium borohydride yields the corresponding oxime.I n these reactions the nitroso-group behaves in similar fashion to the carbonyl group (as in the isoelectronic aldehyde). Typical reactions are (iii) Condensation reuctions. R.NO + H,N.R~ - R.N:N-R' + H,O RNO t HO~NHR' - R.N:N.OR' + H,O R*NO + H2N.0H --c R.N:N*OH 4- H2O R-NO + CH2R'R2 -C R*N:CR'R2 -I- H20 Kinetic investigations have been made in very few cases although the reactions lend themselves to spectrophotometry. The reactions between aniline and substituted nitrosobenzenes 65a yield the results listed in Table 4. An almost linear relation exists between E and log A and thus substituent effects must be related to both the energy and the entropy of activation. In a more detailed in~estigation,~~~ Ogata and Takagi confirmed that a linear relation does exist between energy and entropy of activation when various substituents are present in both nitrosobenzene and aniline but they also showed that Ueno and Akiyoshi's work was incorrect because buffered solutions were not used.A kinetic investigation 66 of the formation of azoxybenzene shows that in the bimolecular reaction the free hydroxylamine-free nitrosobenzene step pre- dominates in neutral solution whereas in acid solution the rate-determining 6 4 Muller Metzger and Fries ibid. 1954 87 1449; 1955 88 1891. 8 5 (a) Ueno and Akiyoshi J. Amer. Chem. SOC. 1954,76 3670; (b) Ogata and Takagi 66 Ogata Tsuchida and Takagi ibid. 1957 79 3397. ibid. 1958 80 3591. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 331 I X I ' E (kcal.mole-l) 1 1ogA I TABLE 4. Reaction between Ph*NH and X*C,H,-NO H o-NO m-N02 p-NO 5-85 10.6 11.8 11.8 5.44 7.58 8.74 8.74 step is predominantly between the free hydroxylamine and protonated nitrosobenzene.It was also shown that rapid equilibrium is set up when para-substituted compounds are employed thus Consequently the formation of pp'-disubstituted azoxybenzenes is to be expected. Addition to the N=O group is extremely facile and will take place with saturated molecules e.g. p-X + C,H,*NO + C,H,.NH-OH p*X*C,H,*NH*OH + C,H,*NO (iv) Addition reactions. Ph*NO t Ph.NH*NH-Ph -C Ph*NH*OH + Ph*N:N.Ph It is however of greater interest to focus attention on the ready reaction of nitroso-compounds with unsaturated and conjugated compounds. Addi- tion reactions of nitrosobenzene with ketens,,' substituted ethylenes,68a and Schiff bases 68b have been observed and four-membered rings result Ph,C.-$O Ph,C=CO + Ph*NO - O-N-Ph Ph,F-CH Ph*N-& Ph,C=CH + Ph*NO Ph*y-y H,C-N-R R*N=CH + PhNO - A parallel reaction 69 is observed between perfluoronitroso-compounds and perfluoro-olefins the reaction taking place in the dark at 45"; a 1 1 co- polymer also is formed the repeating unit being -N( RF)-O-CF,-CF,.Nitroso-compounds will also participate in the Diels-Alder reaction the major studies having been made by A ~ ~ u z o v ~ ~ Wichterle,7l and their co- workers. Oxazine-ring compounds result from this reaction and addition compounds have been prepared from nitrosobenzene nitrosotoluene 2-cyano- 2 -nitrosopropane 2 - chloro-2 -nitrosopropane 1 - chloro - 1 -nitrosocyclohexane and l-cyano-l-nitrosocycZohexane. A variety of conjugated dienes have been employed and the predominant addition reactions classified.Direct addition across the -N=O group results on reaction of nitroso-compounds 6 7 Staudinger and Ielagin Ber. 1911 44 365. 68 ( a ) Ingold and Weaver J. 1924 125 1146; ( b ) Ingold ibid. p. 93. 69 Barr and Haszeldine J. 1955 1881; 1956 3416. 70 Arbuzov and Pisha DokZady Akad. NaukS.S.S.R. 1957,116,71 (includes references 71 Wichterle and Gregor Chem. Listy 1957 51 605 (includes references to previous to previous papers). papers). 332 QUARTERLY REVIEWS with Grignard reagents and zinc alk~ls.7~ The nitroso-group adds very readily. to free radicals producing a trisubstituted hydroxylamine 73 RNO + 2R1 -+ RRINOR1. This reaction has not been postulated in gas-phase reactions of nitroso-compounds but is a logica,l possibility.Additional confirmation of the high reactivity of the -N=O group to free radicals is provided by the observation 74 that the methyl affinity of nitroso- benzene is 105 whereas other monosubstituted benzenes have low methyl affinities (e.g. PhCN 12.2 PhCl 4.2). Radical addition of perfluoroalkyl to perfluoronitroso-compounds has been postulated 69 as the basic step in the formation of (R&NO*NO but there are cogent grounds 46 for assuming that the reaction occurs by addition of an excited form of the perfluoro- nitroso-alkane across the N-0 bond. Nitric oxide also adds to nitroso- monomers to give the diazonium nitrate 75 RNO + 2N0 -+ RN,NO, a possible mechanism being This reaction also has never been considered in relation to inhibition of chain reactions by nitric oxide.(v) Substitution in the ring. The orientation of substitution in nitroso- benzene has been the subject of investigation by a number of authors and is of interest in that it provided the one apparent exception to the Hammick- Illingworth rule. It has been suggested that the presence of a small quantity of dimer is responsible for the anomalies though this cannot now be accept- able on the basis of accurate molecular-weight determinations of nitroso- benzene.,7 52 A kinetic investigation of bromination in carbon tetra- chloride shows that variable rates and induction periods occur in absence of added hydrogen bromide. It is suggested that prior rate-determining addition of hydrogen bromide to the NO group occurs and that subsequent fast reactions account for the substitution reactions.It may therefore be possible that the early discussions (see ref. 4 for details) are not in fact dealing with the directing effect of the NO group but with reaction con- sequent upon addition to the NO group. 7 2 Aston and Menard J . Amer. Chem. Soc. 1935 57 1920. 73 Gingras and Waters J. 1954 1920 3508; Gregor Chem. Listy 1957 51 2304. 7 4 Heilman Rembaum and Szwarc J. 1957 1127. 7 5 Bamberger Ber. 1897 30 506. 7 6 Robertson Hitchings and Will J. 1950 808. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 333 (vi) Dimeriaation and isomerisation. The self-addition to give a dimer and the isomerisation of primary and secondary nitroso-compounds to give an oxime form two related and little understood reactions. It is possible for the reactions to compete thus 2RR'CH-NO (RR'CH-NO) ; RR'CH.NO RR'C=N.OH the relative rates of dimerisation and isomerisation being given by the factor E,[RR'CH*NOJ/lc,.It might be expected that when the monomer is produced in large concentration (cf. refs. 13 and 25) dimer formation will predominate. When the monomer is present in small concentration there is evidence 18 l9 to suggest that oxime formation is preferred. It is noteworthy that dimer formation predominates in non-hydroxylic solvents and this suggests that the mechanism of the oximation may vary with the solvent and involve acid-base catalysis (cf. keto-enol reactions). It is however quite certain that in the gas phase the reaction is not instan- taneous 41 56 and that the activation energy is of the order of 35 kcal.mole-l. In the case of monomeric nitroso-compounds condensed as solids at about - loo" melting which takes place in the region of - 80" leads to pro- duction of the dimer and not the oxime.Our knowledge of the mechanism of both the reactions is scanty but suggests a solvent effect upon a t least the isomerisation step. Until detailed kinetic evidence is forthcoming upon the mechanisms of both these steps and of the influence of substituent groups generalisations are premature. As it is evident that the early in- correct generalisation concerning instantaneous isomerisation of primary and secondary nitroso-alkanes was based upon studies conducted in aqueous solutions and low concentrations it suggests that other early generalisations such as the instantaneous isomerisation for Me.CO.CHi NO - Me COCH N-OH may also be incorrect for similar reasons.under any of the previous groups. and examples of these reactions are given below (vii) Other reactions. It is difficult to classify the formation of nitrones Full details are given by Hickinbottom 60 Ph.CH,Cl + Ph-NO --c PhCH:N(O)Ph + HCl Ph-CiC-Ph + 2PhaNO -C Ph*$-$-Ph O y y o PhPh These compounds are also classifled as 0-substituted oximes (=NOR) and it is evident that a further investigation of these reactions and their products is needed. A violet complex ion [Fe(CN)5*PhN0]3- is formed on reaction 334 QUARTERLY REVIEWS of nitrosobenzene with ferrocyanides.77 As a nitroso-monomer has a pair of non-bonding electrons on the nitrogen atom it might be expected t o behave as a Lewis base Certainly CF,=NO is not protonated by concen- trated sulphuric acid but the decomposition of 2 5-dimethyl-2-nitrosohexane by hydrochloric acid to give a variety of products 7* may proceed by this mechanism.Isomerisation in solution to the oxime may also depend upon Lewis-base action by the nitroso-monomer. Structure.-The structure of the monomeric nitroso-compounds has been systematically treated by Luttke 41 79 on the basis of the infrared absorption spectra and other physical properties. The nitrogen atom is trigonally hybridised and bears a c-electron lone pair and one n-electron. This n-electron couples with the n-electron of the oxygen atom to form a n-bond. Thus the CNO group can be described as K K K c c ~ ~ c ~ o ~ c y ~ o o ~ When an aromatic nitroso-compound is formed a delocalisation of n-electrons over the whole molecule results. As a result the N-0 bond should be of slightly lower order than in an aliphatic nitroso-monomer and this is reflected in the lower stretching frequencies for aromatic nitroso-compounds.Natur- ally the n-electron delocalisation will be affected by the nature of the sub- stituents in the benzene ring. This explanation of the structure is in accord with the observations on electronic spectra and polarographic reduction to which we have already referred. The frequency for the visible absorption maximum for nitrosobenzene is lower than for nitroso-alkanes because the energy level of the n* antibonding orbital (to which one of the lone-pair a-electrons is raised) will be lower for the aromatic compound owing to the conjugation with the n-electrons of the aromatic ring. It is also possible that the alkyl group exerts a different effect from the aryl group on the energy level of the lone-pair electrons.Orgel 4O has shown that the dia- magnetism and the spectra of the nitroso-compounds can be explained on the basis of the relationship of the parent molecule HNO to the isoelectronic oxygen molecule. The effect of displacing the hydrogen atom from the N-0 axis is to lower the level of the lAg state of the molecule below that of the ,Zg- state (which is the ground state of oxygen) so that the ground state of the nitroso-compounds is a singlet. Nitroso-compounds exhibiting Special Features.-Nitrosoanilines and nitrosophenols have been mentioned as possessing distinct properties ; the former is a resonance hybrid and the latter exhibits tautomeric equilibria with quinone monoximes.Infrared 80 and electronic spectroscopy provide valuable evidence for such descriptions and for o-and p-nitrosophenols it has been shown that a molecular-orbital treatment 81 predicts that the quinone rnonoxime will be more stable than the isomeric nitrosophenol. It has also been shown4 that o-dinitrosobenzene does not exist as a true nitroso-compound but is benzofurazan oxide (111). 7 7 Baudisch Ber. 1021 54 413. 79Luttko Angew. Chenz. 1956 68 417; 1957 69 99. so Hadii J. 1956 2725. 81 Jaffd J . Amer. Chern. SOC. 1956 77 4448. Aston and Ailman J . A.mer. Chem. SOC. 1938 60 1930. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 335 Other polynitrosobenzenes,82 e.g. '' hexanitrosobenzene " or benzotri- furoxan (IV) exhibit simila'r behaviour and form crystalline complexes with ar omat ic hydro car boils.0 cm> 8 O' rx & 10 =N t 0 Nitrosoethylene and its derivatives would be expected to polymerise only one such compound is known 83 (2-methyl-3-nitrosobut-Z-ene). As this polymer possesses infrared frequencies characteristic of C=N and N-0 stretch- ing it is probable that the monomer polymerises as a diene and not as a vinyl compound. Structure and properties of C-nitroso-dimers m7e have already stated that dimeric nitroso-compounds can exhibit cis-trans- isomerism [(V)-(VI)] and a brief account of previous formule for (RNO) may be instructive. The first structural formulz suggested (VII and VIII) were soon regarded as inadequate by organic chemists as chemical evidence Physical Properties and Structure.-( i) MoZecuZar formula. suggested direct N-N bonding [thus disproving (VII)] and absence of per- oxidic character [thus disproving (VIII)].These four-membered ring struc- tures invoke only single bonds and molecular models indicate strained non-planar structures. The existence of geometrical isomerisni was accepted as a logical possibility but was discounted both on the grounds of being unlikely (ref. 4 p. 216) and because another formulation the resonance hybrid (IX) was better able to explain some experimental observations namely (a) aromatic substitution ( b ) dipole moment of the dimer (c) N-N linkage and (d) ease of dissociation to the monomer. The last two are also explained by structures (V) and (VI) so we shall consider the nature of the other experimental evidence. Electrophilic substitution of nitroso- benzene leads to para-isomers' being obtained and it was suggested that the absence of the meta-isomer was due to fast reaction with the ortho- para-directing dimer of structure (IX) and not with the meta-directing monomer.This argument is invalid as no dimer is present in solution and also it is very probable that reaction occurs between the electrophilic reactant and the monomeric -N=O group substitution in the ring being due t o subsequent reaction. The dipole-moment evidence was based upon 8 2 Bailey and Case Proc. Chem. Soc. 1957 176 211. 8 3 Brown J. Amer. Chem. SOC. 1955 77 6341. 336 QUARTERLY REVIEWS change of the dielectric constant with time of a solution containing a dis- sociating dimeric nitroso-compound. Not only is it difficult to determine the dipole moment of a compound in a solution containing other polar molecules especially when the concentrations of both species are changing but in order to interpret their results Hammick New and Williams 84 had to make assumptions that have now been recognised by Smith 85 as being a further source of error.Smith’s criticism of the resonance formula (IX) was consequent upon the preparation of nitroso-alkane dirners in both the forms (V) and (VI) which together with the demonstration of geometrical evidence from infrared spectroscopy affords a complete refutation of Harnmick New and Williams’s resonance formula and suggests a family relationship between azo- azoxy- and dimeric nitroso-compounds in both the cis- and the trans-isomer. To this evidence we shall now turn. Dimeric nitroso-compounds are colourless solids and the observation that dimeric nitrosomethane prepared on pyrolysis of tert.-butyl nitrite possessed a different ultraviolet absorption maximum in aqueous solution from that prepared photolytically led l8 to the realisation that two different dimers could be prepared which exhibited the familiar interconversion patterns of cis-trans-isomers.The absorption is of high intensity (E,, - 10,000) and is due to a n-n* transition. The primary and secondary alkyl trans-dimers which are stable in a variety of solvents at room temperatures show a regular variation in A,, for each compound with variation of solvent and for each solvent with variation in size of the alkyl group. Thus for trans-dimeric nitrosomethane A varies from 276 mp (H,O) and 282.5 mp (EtOH) to 291 mp (CC1,). Increase of size of the alkyl group results in a variation of Amax (H,O) from 276 mp (Me) to 287 mp (sec.-C,H,,) and A,, (CC1,) from 291 mp (Me) to 300 mp (sec.-C,H,,).The cis-dimers are stable only in aqueous solution organic solvents resulting in conversion into the trans-dimers; values of A are always lower than for the corresponding trans-dimers and again a regular variation of Amax with size of the alkyl group is observed. Thus Amax (H,O) varies from 265 mp (Me) to 271 mp (sec.-C5H,,) a smaller variation than for the trans- dimers. The ultraviolet spectra of other dimeric nitroso-alkanes and cyclo- alkanes 13a 2 4 j 419 ,9 64 86 confirm the position of the absorption region and indicate that these investigators have produced only the trans-dimers. Simultaneously with the discovery of cis-trans-isomerism in nitrosoalkanes Luttke 79 87 found that there were two different patterns exhibited in the infrared spectra of dimeric nitroso- compounds and that on the ground of‘ symmetry these could only be ascribed to cis-trans-isomerisation.The combined results of Luttke and other workers 45 839 88 are presented in Table 5. Comparison with the spectra of the two compounds (X) and (XI) whose cis-configuration is obvious assists in assigning the frequencies. This (ii) Electronic spectra. (iii) Infrared and Raman spectra. $4 Hammick New and Williams J. 1934 29. s 6 Schindler Luttke and Holleck Chem. Ber. 1957 90 157. Smith J . 1957 1124. Luttke 2. Elektrochem. 1957 61 976. Gowenlock Spedding Trotman and Whiffen J. 1957 3927. GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS Characteristic NO frequencies in (RNO) TABLE 5.R ~~ Aliphatic Aromatic trans-Dimer Single band between 1176 and 1290 cm.-l (29 compounds) Single band between 1253 and 1299 cm.-l ( 18 compounds) cis-Dimer ~~ - _ _ Double band between 1323 and 1344 cm.-l and 1330 and 1420 cm.-1 (8 compounds) Double band between 1389 and 1397 cm.-l and 1409 cm. -l ( 2 compounds) 337 evidence is reinforced by the Raman spectra of dimeric nitrosocyclohexane (trans) and dimeric nitrosobenzene (cis).41 It is noteworthy that nitroso- benzene and l-nitrosonaphthalene form only the cis-dimer and that it is presumably impossible to prepare the trans-dimers; also that the NO bond in the dimers is of lower order than that in the monomers. This would correspond to a longer bond and calculations show *7 that the difference of NO bond order on passing from monomer to dimer is of the order Heats of formation of the dirners are lacking with the exception of one (probably unreliable) measurement for nitros~benzene.~S X-Ray crystallography has been carried out for dimeric 2 4 6-tribromonitrosobenzene 43 and p-bromonitrosobenzene and for the internal cis-dimer (X) 1 4-dichloro- 1 4-dinitrosocycZohexane.91 The first two compounds are shown to be trans-dimers and the former has N-N ca.1.4A the second gives N-N 1-31 C-N 1-40 and N-0 1.35rf. The cis-compound (X) gives N-N 1.34 N-0 1.32 8. Thus we see that the N-N and N-0 bonds are intermediate between single and double bonds. The bond angles indicate that the nitrogen atom is trigonally hybridised. The dipole moments 84 for dimeric nitrosomesitylene and 2 5-dimethyl- 2-nitrosohexane implied a finite dipole moment (1-1.5 D) for the bisnitroso- group and led (together with the information on orientation of substitution in the benzene ring) to the formulation of the resonance structures (IX) for dimeric nitroso-compounds.It was suggested that the dipole moment arose owing to rotation about the central N-N bond out of the trans-position. Not only is this formulation contradicted by the detailed evidence for geo- metrical isomerism already cited but it has been shown *5 that the method 0.3-0.4. (iv) Other physical properties. 89 Drucker and Flade 2. wiss. Phot. 1930 29 29. 91 Hodgkin personal communication. Darwin and Hodgkin Nature 1950 166 827. 338 QUARTERLY REVIEWS of calculating the dipole moments involves unjustifiable assumptions.The molecular polarisation and molecular refraction of trans-dimeric nitroso- methane differ by only 7.5 c.c. a difference interpreted in terms of a fairly high atom polarisation the major contribution to which arises from bending of the polar N-0 bonds. The resonance structures (IX) are frequently quoted in text books as the accepted structures for nitroso-dimers. It is therefore necessary to emphasise that the supposed supporting evidence of dipole moments and aromatic substitution is in fact illusory. It may also be noted that when Hammick suggested these resonance structures the existence of geometrical isomerism for azo- and azoxy-compounds was not known and thus the repeated pattern of geometrical isomerism in the compounds was not suspected. Diamagnetic susceptibilities for crystalline dimeric nitroso-compounds have been measured by Matsunaga.59 The values listed for the molar susceptibilities (both observed and calculated) are useless as they have been obtained on the basis of the molecular weight of the monomer.Recal- culation on the basis of the correct molecular weight leads to the conclusion that the group contribution of N202 to the molar susceptibility in dimeric nitroso-compounds is almost zero. In the dimer each nitrogen atom is trigonally hybridised and forms three a-bonds (to C N and 0 severally) ; in addition each nitrogen atom contributes two n-electrons and each oxygen atom one n-electron to form the six n-electron system of the ON*NO dimer system. On the basis of this structure 419 56 certain generalisations 567 79 concerning the stability of the dimers can be made.I n dimeric nitrosobenzene conjugation takes place between the n-electrons of the ON-NO system and those of the phenyl rings thus weakening the N-N bond and lowering the stability of the dimer. As trans-dimeric nitroso- benzene is unknown presumably the N-N bond is so weakened by this conjugation that the N-N bond in the hypothetical compound (XII) cannot (v) Electronic structure and stability of dinzer. be formed. In the cis-dimer (XIII) the phenyl groups will be twisted out of the plane of the ON*NO system because of steric restriction and thus conjugation will be sufficiently reduced to permit formation of a dimer. This twisting of the phenyl groups is paralleled in cis-stilbene. In the aliphatic dimers the remainder of the molecule cannot participate in conjugation and will therefore possess a more stable structure whereas phenylnitrosomethane dimer will be similar in stability to nitrosomethane GOWENLOCK AND LUTTKE C-NITROSO-COMPOUNDS 339 as the interposed CH groups will prevent the n-electron conjugation be- tween the phenyl groups and ON-NO system.Similarly ortho-substitution in aromatic nitroso-dimers (trans) will enhance stability as the phenyl group will be twisted from coplanarity with the ON-NO system in order to accom- modate both the ortho-group and the oxygen atoms and thus conjugation is reduced. In general substitution in the ring will also have an effect upon this conjugation; thus an iodine atom in the para-position acts as an electron donor and completely prevents dimer formation whereas in the ortho-position the large steric hindrance between the iodine and oxygen atoms destroys coplanarity and outweighs this donor effect and a dimer is formed.The recent measurements by Keussler and Luttke 37 have given quantitative proof of these assertions (see Table 6). Chemical Properties.-The major studies of reactions of nitroso-dimers relate to reduction and production of monomers; the knowledge of other reactions is extremely scanty. Polarographic reduction 86 of dimeric nitrosocyclohexane takes place by an irreversible 6-electron step to form 1 2-dicyclohexyl- hydrazine. In this reduction as in chemical reductions it is necessary to ascertain that the dimer is reduced. The work of Aston and his school 9 2 shows that according to the reactants and conditions trans-(Me,C*NO*COMe) can be reduced either to the azoxy-compound or to hydrazine whereas trans-( Ph*CH,*NO) yields the amine and 1 2-dibenzylhydrazine on reduc- tion with aluminium amalgam.93 In neither case does monomer production make a significant contribution.Dimeric trans-nitrosomethane has been utilised 2 l for the production of hydrazine on reduction. (ii) Monomer production. Production of monomer from the dimer on dissolution or on heating forms an essential preliminary to most reactions of dimeric nitroso-compounds. Kinetic investigations are few,56 9% 95 but results available show that the unimolecular decomposition exhibits many interesting features and that generalisations on monomer production are fraught with difficulty. It is particularly noteworthy that entropies of activation may vary considerably and that the solvent effects show marked differences for different compounds.Heats of dimerisation were lacking until recently with the exception of one older measurement for nitroso- mesitylene. 36 Recent photometric measurements 37 of the intensity of the n-n*(N) transition as a function of concentration and temperature give the results summarised in Table 6. These refer to the equilibrium (RNO) 2 ZRNO whereas the kinetic data treat only the forward reaction. Although tentative explanations for the dissociation process have been advanced on the basis of the electron structures of both monomer and dimer it is evident that further generalisation awaits a better foundation on further experimental data. (i) Reduction. O 2 Aston Menard and Mayberry J .Anaer. Chem. SOC. 1932 54 1530; Aston and O3 Gundlach Dissertation Munich 1905. O 4 Anderson and Hammick J. 1935 30. O 5 Schwartz J. Amer. Chem. SOC. 1957 79 4353. Parker ibid. 1934 56 1387. 340 QUARTERLY REVIEWS TABLE 6. Heats of dimerisation for nitroso-compounds ~ ___________ 1L Ph p-Br*C6H p-NMe,-C,H Ph-CH Me,C*COMe 2 4 6-Me3C,H C’lJClO-C6H, AK (kc;tl.mole-l) ( 1 < 1 < 1 20.5 f 0.2 20.6 f 0.2 25.0 & 0-2 12-1 * 0.2 AS (c.u.) - - - -42 -35 -65 36.2 (iii) Other reactions. Mechanisms for the cis-trans-interconversions would be purely speculative; the data are summarised for R = alkyl in the annexed scheme by saying that heat or the presence of a solvent of low dielectric constant favours the movement towards the right and a quantum of radiation movement towards the left.Interaction of different dimers to give RN202R’ has been observed in one case only 96 though it may be possible to prepare other mixed dimers. trans-Dimeric nitrosomethane and dry hydrogen chloride in ether solution form a white solid 25 of empirical formula Me*NO,HCl whose structure is unknown. The qualitative analysis of nitroso-compounds has been summarised by Hulle.97 Technical uses of nitroso-cornpounds A commercial method for the production of nitrosomethane dimer and its subsequent reduction to hydrazine has been patented; 21 the use of pseudo-nitroles as anti-knock agents in Diesel engine fuels has also been patented. NN-Dimethyl-p- nitrosoaniline has a powerful germicidal action. Aromatic nitroso-com- pounds are also used in the vulcanisation of some synthetic rubbers as antioxidants in lubricating oils and in stabilisation of halogenated dielectric materials.Some silicon-containing nitroso-alkanes have found use in pharmaceuticals elastomers and resins. The major details of the structures of monomeric and dimeric nitroso-compounds ha.ving been solved it appears to the Reviewers that apart from further confirmatory evidence for the struc- tures the major problems to be solved are the related thermochemical and kinetic investigations our knowledge of which is scanty. Also it would be useful to know more of the mechanism of the addition reactions to the N=O group and the formation of nitrones. A few technical uses have been reported. Future developments. 9 6 Hammick Edwards Illingworth and Snell J. 1933 671. 9 7 Hulle in Houben-Weyl-Miiller “ Methoden der organischen Chemie ” Vol. 2 Georg Thieme Stuttgart 1954 p. 615.

ISSN:0009-2681    

DOI:10.1039/QR9581200321

出版商:RSC    

年代:1958

数据来源: RSC

摘要:

COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS By P. C. CROFTS B.Sc. Ph.D. (MANCHESTER COLLEGE OF SCIENCE AND TECHNOLOGY MANCHESTER 1) PHOSPHORUS- CONTAINING organic compounds present a variety of features of chemical interest. Many complex esters and amides of phosphoric and condensed phosphoric acids play important parts in metabolic processes but compounds whose molecules contain carbon atoms linked directly to phos- phorus do not occur naturally. They are discussed together with other organic compounds of phosphorus in Kosolapoff’s book which summarises the literature up to the beginning of 1950 and is a mine of information for all workers in this field. It is usefully supplemented by a recent review 2 deal- ing with the preparation and properties of phosphonic acids the most important class of compounds containing C-P bonds Although phosphorus follows nitrogen in Group V of the Periodic Table there is very little resemblance between the organic chemistry of these two elements.Phosphorus analogues of important classes of organic nitrogen compounds such as nitro-compounds aromatic nitrogen-heterocycles and azo-compounds are mostly unknown and where formal similarities do exist as between primary phosphines and primary amines or between phosphine oxides and amine oxides there are considerable differences in reactions. These dissimilarities arise because of the lower electronegativity of phos - phorus which leads to its forming stronger bonds with oxygen and with halogens and because of the greater reactivity of the unshared electrons on tervalent phosphorus which results in a strong tendency to quinquevalency.Nomenclature of Organic Compounds of Phosphorus.-The problem of providing suitable unambiguous names for organic phosphorus compounds is one which has caused considerable difficulty. The majority of organic phosphorus compounds can be considered as derived from various acids of phosphorus; the existence of two valency states and of both mono- and di-basic acids together with the problem of whether negative substituents such as chlorine or amino-groups should be considered as replacing hydrogen or hydroxyl in the parent structure led to several different systems of nomenclature. The resulting confusion was dispelled as far as British and American publications are concerned by the adoption in 1952 by the Chemical Society and the American Chemical Society of a new system of nomenclature for compounds containing one phosphorus atom.3 This system uses as parent structures a number of phosphorus hydrides and acids some of which exist only hypothetically.In naming compounds groups which are attached by C-P bonds are considered as replacing hydrogen 1 Kosolapoff “ Organophosphorus Compounds ” John Wiley and Sons New York 1950. Freedman and Doak Chenx. Rev. 1957 57 470. J . 1952 5122. Y 34 z 342 QUARTERLY REVIEWS in the appropriate parent structure and are prefixed to its name whereas negative groups are considered as replacing hydroxyl or oxygen (either doubly bonded to phosphorus or in a hydroxyl group) and are indicated either as a separate word (e.g. chloride or amide) replacing the word acid or if the compound is itself an acid or an ester by a suitable affix (e.g.-chlorid- or -amid-) immediately preceding the valency suffix (-ic acid -ate -ous acid or -ite). Ester groups precede the name as a separate word or words as in normal usage and in cases of ambiguity (as in acids in which some but not all of the oxygen atoms of the parent structure have been replaced by sulphur atoms or in substituted amides) the symbols 0- S- N- P- etc. may be used. The parent structures most frequently met are H,P phosphine P(OH) phosphorous acid H-P(OH) phosphonous acid H2P( OH) phos- phinoua acid H,P( 0 ) phosphine oxide P( 0)( OH) phosphoric acid H-P(O)( OH) phosphonic acid and H,P(O)(OH) phosphinic acid. Examples of this system of nomenclature are Me,P trirnethylphos- phine EtMePhP( 0) ethylmethylphenylphosphine oxide P( OEt) triethyl phosphite MeP( OEt) diethyl methylphosphonite Me,P(NEt,) NN-diethyl- dimethylphosphinous amide MeP( O)Cl methylphosphonic dichloride MeP( 0) (0Et)Cl ethyl methylphosphonochloridate and Me,P( S)( OEt) 0- ethyl di methylphosphinot hioate.Starting Materials €or the Synthesis of Compounds containing Carbon- Phosphorus Bonds.-Many reactions leading to the formation of C-P bonds start with inorganic phosphorus compounds such as phosphine phosphorus trichloride and phosphorus oxychloride but for the preparation of com- pounds in which an aliphat,ic carbon atom is linked to quinquevalent phos- phorus the dialkyl and trialkyl esters of phosphorous acid are of particular value. Phosphorus trichloride reacts readily with primary and secondary alcohols in the presence of a tertiary base (preferably diethylaniline because its hydrochloride is non-hygroscopic and readily filtered off) to give trialkyl phosphites PCl t 3ROH + 38 - P(OR& + 3B,HCl These esters are reactive because they contain a phosphorus atom the unshared electrons of which make it strongly nucleophilic together with readily displaced alkyl groups.Partial dealkylation of trialkyl phosphites to the dialkyl esters occurs very rapidly under the influence of hydrogen chloride. Because of this reaction of phosphorus trichloride with primary and secondary alcohols in the absence of a base results in the formation of dialkyl phosphites PCL + 3ROH - P(0R);OH + 2HCl + RCI Although they are almost always referred to as such these compounds are not true phosphites but are phosphonates HOP( 0) (OR), the quinquevalent phosphorus atom being the cause of their relative stability towards further dealkylation.They provide the best-known example of the instability of a hydroxyl group attached to a tervalent phosphorus atom; in practically CROFTS COMPOUNDS CONTAINING CARBON-PIIOSPIIORUS BONDS 343 all cases when a compound having this structural feature might be expected the isomer in which the phosphorus atom is quinquevalent is obtained instead. It may be that a tautomeric equilibrium exists between the two forms with the equilibrium well over on the phosphonate side. This idea is supported by the fact that whereas dialkyl phosphites themselves possess little nucleophilic reactivity they react readily with sodium evolving hydrogen and giving derivatives which are powerful nucleophilic reagents.The anions of these sodium derivatives'appear to be mesomeric with the charge divided between phosphorus and oxygen since in different reactions groups may become attached to either of these atoms. A similar reactivity Na L is shown by dialkyl phosphites on addition of sodium alkoxide solutions or of tertiary bases. Formation of a mesomeric anion which can be alkyl- ated (and which can also add to activated double bonds) is reminiscent of compounds that exhibit keto-enol tautomerism and there is in this respect a general resemblance between dialkyl phosphites and diethyl malonate resulting in several analogies with familiar organic reactions. Tspes of Compound containing Carbon-Phosphorus Bonds.-Compounds of Tervalent Phosphorus.-These are considered for nomenclature purposes to be derived from one of the three parent structures phosphine PH, phosphonous acid HP( OH), or phosphinous acid H,P(OH) by replacement by organic radicals of some or all of the hydrogen atoms attached to phos- phorus and by replacement of the hydroxyl groups by negative substituents.They include primary secondary and tertiary phosphines R*PH, R,PH and R,P phosphonous dichlorides R-PCl and phosphinous chlorides R,PCl. From previous remarks on the instability of hydroxyl groups attached to tervalent phosphorus atoms it will be evident that phosphonous and phos- phinous acids are not stable compounds although their esters R*P(OR') and R,P*OR' are like trialkyl phosphites capable of existence. This is particularly true of those in which the phosphorus atom has attached to it alkyl groups which since they are electron-repelling increase the already high electron density on the phosphorus atom.Compounds with bonds between aliphatic carbon and tervalent phosphorus atoms tend therefore to be rather troublesome to prepare and to work with. This together with the nauseating smells tendency to spontaneous inflammability and toxicity of the lower members has led to their being relatively little investi- gated. In particular few preparations of primary and secondary aliphatic phosphines have been reported since the pioneering work of Hofmann who alkylated phosphine by heating phosphonium iodide with alkyl iodides and All tervalent phosphorus compounds are susceptible to oxidation. 4Hofmann Ber. 1871 4 430 605; 1873 6 292. 344 QUARTERLY REVIEWS zinc oxide.This leads to mono- di- and tri-substitution; a variant achieving better control of the extent of alkylation is the reaction of alkyl halides with sodium or potassium phosphides in liquid amrn~nia.~ Another route which has been used more particularly for the preparation of primary and secondary arylphosphines is hydrolysis of phosphonous and phosphinous chlorides the monosubstituted phosphinic acids and disubstituted phosphine oxides which are formed initially undergoing oxidative disproportionation when heated 6 3R-PC12 - 3R.PH (O)*OH 2R,PC1 -2R2PH (0) -C R-PH t 2RP(0)(OH)2 -c R2PH + RP(O).OH Phenylphosphine has been prepared by reduction with lithium aluminium hydride of phenylphosphonous dichloride,' phenylphosphonic dichlorideYs and phenylphosphinic acid.9 Similar reductions appear to provide rela- tively easy routes to other primary and secondary phosphines.Tertiary phosphines containing three identical groups are readily pre- pared from phosphorus trichloride and Grignard reagents. By using less reactive organometallic compounds substitution may be limited to inter- mediate stages and phosphonous and phosphinous chlorides thus obtained Esters of phosphonous and phosphinous acids are obtained by reaction of the chlorides with alcohols or phenols in presence of a tertiary base or less satisfactorily with sodium alkoxides. Compounds of Quinquevalent Phosphorus.-This group includes more chemical types and far more known compounds than those which contain tervalent phosphorus. Almost all quinquevalent phosphorus compounds having C-P bonds contain four atoms covalently bound to phosphorus and the majority of these are related to one of the three parent structures phos- phonic acid H*P( O)(OH), phosphinic acid H,P(O)*OH and phosphine oxide H,P(O).They thus contain three single bonds (one two or three of which may be C-P bonds) to phosphorus and one double (or semipolar) bond between phosphorus and oxygen the place of which may be taken by other bivalent atoms or groups such as =S =NR or =CR,. Other quinquevalent phosphorus compounds containing C-P bonds are quaternary phosphonium salts R,P+X- and the small but very interesting group of penta-aryl- phosphoranes Ar5P which contain five covalent bonds to phosphorus. These compounds are often referred to as phosphorus penta-aryls ; phosphorane is the systematic name for the hypothetical parent compound H5P.The most numerous class of compounds containing C-P bonds a're the alkyl- and aryl-phosphonic acids and their simple derivatives. Phosphonic acids or their chlorides or esters from which the acids themselves are easily obtained may be prepared by most of the general methods for forming 5 Watt and Thompson J . Arne?. Chem. SOC. 1948 70 2295; Wagner and Burg ibid. 1953 75 3869. 6 Michaclis Annden 1896 293 193; 294 1; 1901 315 43; Michaelis and Gleich- mami Ber. 1882 15 801. 7 Horvat and Furst J. Amel.. Chem. SOC. 1952 74 562. 9 TVeil Prijs and Erlenmeyer HeZv. Chi7n. Actu 1953 36 142. Freedman and Doak ibid. p. 3414. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 345 C-P bonds which are discussed in the next section. The acids are generally crystalline the lower members being extremely soluble in water.ReactioL s and interconversions of these acids and their derivatives are dealt with later. Fewer phosphinic acids R,P(O)*OH are known but the reactions by which they and their derivatives are prepared and which they undergo are similar to those of phosphonic acids. Trialkyl- and triaryl-phosphine oxides R3P( 0) may be obtained by oxidation of the corresponding tertiary phosphines either directly with air or oxygen or by wet methods or by conversion into phosphine dihalides R,PHal (which resemble phosphorus pentahalides and probably have similar structures) and hydrolysis of these. Tertiary phosphine oxides may also more conveniently be prepared by reaction of phosphorus oxychloride or of phosphonic or phosphinic chlorides with Grignard reagents.Quaternary phosphonium salts which contain a t least one alkyl radical may be prepared by reaction of an alkyl halide preferably the iodide with the appropriate tertiary phosphine. Tetra-arylphosphonium salts cannot be prepared in this way but may be by reaction of triarylphosphines with arylmagnesium halides and oxygen l o with aryl halides and aluminium chloride at high temperatures l1 with aryldiazonium acetates l2 or with an aryl halide together with a Grignard reagent and cobalt ch10ride.l~ The group which is introduced in the last reaction comes from the aryl halide rather than from the Grignard reagent. That the bond between phosphorus and oxygen (or other elements) which is formally written as double does in fact have a considerable amount of double-bond character is shown by the fact that its dipole moment is much lower than that of co-ordinate links involving phosphorus (as in com- plexes of tervalent phosphorus compounds with metallic salts),14 by its short length and by its resemblance to the carbonyl group in producing cc-methyl- enic reactivity.If this double-bond character were complete it would require an outer shell of ten electrons and these bonds are therefore probably best represented as hybrids of P=O and P+-0-. Compounds with five covalent bonds to phosphorus also require a ten- electron outer shell and only a few such compounds are known. Treatment of tetraphenylphosphonium iodide with phenyl-lithium gives pentaphenyl- phosphorane the covalent character of which is shown by its insolubility in water solubility in organic solvents and comparatively low melting point (124").15 The equivalence of the five C-P bonds in phosphoranes is shown by the identity of tetraphenyl-p-tolylphosphorane prepared by reaction of p-tolyl-lithium and tetraphenylphosphonium iodide with that prepared from phenyl-lithium and triphenyl-p- tolylphosphonium iodide.l6 lo Dodonow and Medox Ber. 1928 61 907; Willard Perkins and Blicke J. Amer. l1 Lyon and Mann J. 1942 666. 12 Horner and Hoffmann Chem. Ber. 1958 91 46. l4 Phillips Hunter and Sutton J. 1945 146. l5 Wittig and Rieber Annalen 1949 562 187. l6 Wittig and Geissler ibid. 1953 580 44. Chem. SOC. 1948 70 737. l3 Idem ibid. p . 50. 346 QUARTERLY REVIEWS The preparation of tetraphenyl(triphenylmethy1)phosphorane from tri- phenyl( triphenylmethy1)phosphonium iodide and phenyl-lithium (but not from tetraphenylphosphonium iodide and triphenylmethylsodium) shows that it is not essential that all the C-P bonds in phosphoranes should involve aromatic carbon atoms.16 Attempts to prepare phosphoranes of the form R4P*CHR' have however been unsuccessful and have instead given com- pounds which may be written with a formal double bond between carbon and phosphorus.17 9 l 8 I n fact this bond appears to resemble the P=O double bond in having a considerable amount of dipolar character so that these compounds are best represented as resonance hybrids as shown above.They are often referred to as phosphinemethylenes ; although not mentioned in the new nomenclature scheme they mould appear to be more systematic- ally called me6hylenephosphoranes.Methylenephosphoranes are usually very reactive being rapidly decom- posed by air or moisture. This high reactivity is associated with the car- banionic centre in the dipolar structure and is decreased when the charge is dispersed by the attachment of phenyl or other electron-attracting groups to the methylenic carbon atom. Very stable methylenephosphoranes are known in which the methylenic carbon atom forms part of a cyclopentadiene ring as in 9-fluorenylidenetriphenylphosphorane l9 (I) and triphenylcyczo- pentadienylidenephospliorane 20 (11). I n these the instability normally associated with a carbanion is more than offset by the stabilisation arising from the extra aromatic ring. The dipole moment (7.0 D) of compound (11) indicates roughly equal contributions from the double-bonded and the dipolar structure.2o Stabilisation of methylenephosphoranes may also occur by mesomeric displacement of the negative charge to an oxygen atom as in the compounds ~ - C H = C R / (5- '(In) Ha 1- l7 Coffman and Marvel J . Amer. Chem. SOC. 1929 51 3496. l9 Pinck and Hilbert J . Anwr. Ghevz. SOC. 1947 69 723. 2o Ramirez and Levy ibid. 1957 79 67. Wittig and Sehollkopf Chew&. Ber. 1954 87 1318. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 347 (111) obtained by treatment with alkali of phenacyltriphenylphosphonium bromide and acetonyltriphenylphosphonium chloride.18 2 1 These coin- pounds may be regarded as vinylogues of phosphiiie oxides. Methylenephosphoranes which are not stabilised in any of the above ways react with carbonyl groups with the formation of carbon-carbon double bonds.18 This often referred to as the Wittig rea,ction has been found synthetically useful both on a laboratory and an industrial scale and has been reviewed.22 R,CO + R;P=CR; - R~C=CR; + R ~ P O General Reactions €or the Formation of Carbon-Phosphorus Bonds.- I n this section some reactions which are of particularly wide application or which have been more thoroughly investigated than those already men- tioned are discussed with regard to their general scope and to the reaction mechanisms involved.All of these general methods can be used either directly or indirectly for the preparation of phosphonic acids and their derivatives; that so many examples are of the synthesis of these compounds is an illustration of the pre-eminent position which they occupy amongst- compounds containing carbon-phosphorus bonds.(a) Reactions in which the Phosphorus Atom acts as a Nucleophilic Centre. -In these a phosphorus atom which is tervalent or potentially so because of tautomerism (as in dialkyl phosphites) becomes quinquevalent with the establishment of a C-P bond. Since phosphorus cannot readily be reduced from the quinquevalent to the tervalent state these reactions cannot in general be used for forming several C-P bonds in one molecule. Com- pounds whose molecules contain more than one C-P bond (e.g. phosphinic acids and their derivatives) may however be prepared by reactions of this type by starting with tervalent phosphorus compounds which already con- tain one or two C-P bonds. This reaction is the most completely investi- gated and one of the most widely used methods of forming C-P bonds.23-26 Its simplest form is the reaction of an alkyl halide with a trialkyl phosphite to give a dialkyl alkylphosphonate e.g.the reaction of n-butyl bromide with triethyl phosphite to give diethyl n-butylphosphonate BuBr + P(OEt) - Bu.P(O)(OEt) -I- EtBr I n general the alkyl group of the ha,lide becomes attached to the phosphorus atom and one of the alkyl groups from the trialkyl phosphite appears as alkyl halide. This by-product alkyl halide may compete with that used as starting material so that in the above example some diethyl ethylphos- 21 Michaelis and Kohler Bcr. 1899 32 1566; Ramirez and Dershowitz J. Org. Chern. 1957 22 41. 2 2 Levisalles Bull. Soc. chim. France 1968 1021. 23 A. E. Arbusov J . Russ. Pkys.Chenz. SOC. 1906 38 687. 2 4 Kosolapoff J. Anaer. Chem. Xoc. 1945 67 1180. 25 Ford-Moore and Williams J. 1947 1465. 2o Ford-Moore and Perry Org. Syrzth. 1951 31 33. (i) The Arbusov reaction. 348 QUARTERLY REVIEWS phonate would also be formed and would contaminate the product. Separa- tion is usually easily effected by distillation and unless the original alkyl halide is particular unreactive the amount of contaminant is not unduly large. There is abundant evidence that the reaction which is carried out by heating the reactants together a t 120-160" for several hours occurs in two stages 27-29 the formation of an ionic intermediate and its subsequent decomposition vix. The intermediate formation of these quasi-phosphonium compounds has been observed by changes in the refractive index and density of reaction mixtures 28 and by their isolation in favourable cases in crystalline form.29 30 The Arbusov reaction may be written more generally as A and B may be primary alkoxy secondary allioxy aryloxy alkyl aryl or dialkylamino-groups.Since the reaction involves nucleophilic attack on RX by the unshared electrons of the phosphorus atom it is assisted if A and B are electron-repelling and hindered if they are electron-attracting groups ease of reaction thus increasing in the order A B = aryloxy < alkoxy < aryl < alkyl. For the reaction to proceed normally R should be aliphatic; if it is an aryl group the second stage which involves nucleophilic attack by the ion X- on the O-R bond requires very strong heating result- ing in widespread decomposition.The quasi-phosphonium compounds obtained by reaction of alkyl halides with triaryl phosphites 30 may how- ever be broken down by treatment with an alcohol. This causes rapid replacement of an aryloxy-group by an alkoxy-group which is then attacked by halide ion in the normal way (ArO&P -t ArO,+ OAr ArO,+ OR ArO +O + R'Hal Ar OH' R' Ha l- -@+ [ArO":R' Hal-] - Ar0"'R' + RHal This sequence of reactions provides a method for the preparation of some alkyl halides (e.g. neopentyl iodide) which are not otherwise readily a~cessible.~1 Arbusov reactions on dialkyl aryl- and alkyl-phosphonites yield alkyl 27 Kosolapoff J . Amer. Chem. SOC. 1944 66 109; Gerrard and Green J. 1951 2550; Pudovik Doklady Akad. Nauk S.S.S.R. 1952 84 519; B. A. Arbusov and Fuzhenkova ibid. 1957 114 89. 28 abrarnov and Bol'shakova Zhur.obshchei Khim. 1957 27 441. 29 Razumov and Bankovskaya Doklady Akad. Nauk S.S.S.R. 1957 116 241. 30 Michaelis and Kaehne Ber. 1898 31 1048. 81 Landauer and Rydon J. 1953 2224. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 349 alkylarylphosphinates RArP( O)*OR' 32 and alkyl dialkylphosphinates RR'P( O)*OR" 33 which have two different groups attached to the phos- phorus atom and cannot therefore be obtained hy reactions in which both C-P bonds are formed at the same time. Similarly Arbusov reactions on alkyl dialkylphosphinites provide routes to tertiary phosphine oxides in which the organic radicals linked to the phosphorus atom are not all identi- cal.29 34 Although reactions of these types proceed readily they have been little used because of the difficult accessibility of the starting materials.Variations in the structure of R"X are from a preparative point of view of much greater importance than those in the tervalent phosphorus com- pound which is usually triethyl phosphite. The reaction is by no means confined to halides since dialkyl ~ u l p h a t e s ~ ~ alkyl toluene-p-s~lphonates,~~ and alkyl fluoroborates react similarly whilst the reaction of trialkyl phos- phites with lactones 36 is probably closely related In practice however the majority of Arbusov reactions involve halogen compounds and here it appears that virtually any halide which is capable of reacting with nucleophilic reagents by an X,2 mechanism and does not contain potentially interfering groups (such as carbonyl or nitro) is suitable. Amongst alkyl halides the reaction only proceeds satisfactorily if R' is a primary alkyl group but contrary to an earlier report, long-chain primary alkyl halides react as well as those of lower molecular weight.3' Aryl and vinyl halides are of course insufficiently reactive but ally1 halides react normally 38 although isomerisation may cause difficulties in some cases.3p Benzyl diphenylmethyl and triphenylmethyl halides a'll give the expected phosph~nates,~~ as do halogenomethyl derivatives of condensed aromatic hydrocarbons 41 and of heterocyclic compounds.42 3 2 A. E. Arbusov and Razumov Izvest. Akad. Nauk S.S.S.R. Otdel. khim. N a u k 1945 167; Kamai Doklady Akad. Nauk S.S.S.R. 1947 55 219; 1949 66 389. 33 B. A. Arbusov and Rizpolozhenskii Izvest. Akad. Nauk S.S.S.R. Otdel. khim. N a u k 1952 854.34 A. E. Arbusov and Nikoronov Zhur. obshchei Khim. 1948 18 2008. 35 Myers Preis and Jensen J . Amer. Chem. Xoc. 1954 76 4172. 3 6 McConnell and Coover ibid. 1956 78 4453; Kreutzkamp Naturwiss. 1956 37 Kosolapoff J . Amer. Chem. SOC. 1954 76 615. 38 A. E. Arbusov and Razumov Izvest. Akad. N a u k S.S.S.R. Otdel. khim. N a u k 39 Pudovik and B. A. Arbusov ibid. 1949 522. 4O A. E. Arbusov and B. A. Arbusov J . Russ. Phys. Chem. Soc. 1929 61 217; Lugovkin and B. A. Arbusov Doklady Akad. Nauk S.S.S.R. 1948 59 1301. 4 1 B. A. Arbusov and Lugovkin Zhur. obshchei Khim. 1950 20 1249. 4 2 Idem ibid. 1951 21 1869; 1952 22 1193; B. A. Arbusov and Zoroastrova 43 81. 1951 714. Ilxuest. Akad. Nauk S.S.S.R. Otdel. khirn. N a u k 1954 806. 350 QUARTERLY REVIEWS Arbusov reactions of halogeno-ethers proceed normally as do those of esters of 2-bromoethanol with aliphatic carboxylic acids.43 The free phos- phonic acids could not be obtained in the latter case because hydrolysis occurred more readily a t the carboxylic ester linkage than a t the phosphonic ester group.Acyl halides 44 and acid anhydrides45 react readily with trialkyl phosphites to give dialkyl acylphosphonates. Because of the weak- ness of the C-P bond in acylphosphonic acids they cannot be obtained from these esters by ordinary methods of hydrolysis although they may be pre- pared by dealkylation with dry hydrogen halides.46 Esters of a-mono- chloro- and x-monobromo-monocarboxylic acids undergo normal Arbusov reactions with trialkyl phosphites giving esters which can be hydrolysed to the free a-carboxyalkylphosphonic acids,4' but halogeno-esters such as ethyl trichloroacetate and diethyl bromo- and dibromo-malonate which have more than one halogen atom and one carboxyl group linked to the same carbon atom undergo anomalous reactions 48 like those of a-halogeno-alde- hydes and -ketones described below.Trialkyl phosphites react readily with x-halogeno-aldehydes (exotherm- ally without heating with chloral and bromal). The products do not contain C-P bonds but are unsaturated esters of phosphoric a~id.4~7 49 The reaction appears to involve nucleophilic attack of the trialkyl phosphite on the carbonyl group elimination of chloride ion and dealkylation analogous to the second stage of the Arbusov reaction. Thus triethyl phosphite and chloroacetaldehyde give diethyl vinyl phosphate Cl-CH,-CH=O 4- P(OEt) - C a C H c EtCl + c1- +y(oEt)3 CH,=CH -0.P (0) (OEt) - CH,=CH -0 Formation of a solid intermediate was observed in the reaction of ethyl ethylene phosphite with chloral.48 a-Halogeno-ketones with trialkyl phosphites give both products of normal Arbusov reactions and anomalous products similar to those from o(- halogeno-aldehydes.Unsaturated phosphates are the principal products from a-chloro-ketones whilst a-iodo-ketones give mainly phosphonates.50 9 51 The formation of phosphonates is also favoured by carrying out the reaction a t high temperatures so that whereas bromoacetone and triethyl phosphite 4 3 Ackerman Jordan and Swern J . Amer. Chem. Soc. 1956 78 6025. 4 4 Kabachnik and Rossiskaya Izvest. Akad. Nauk S.S.S. R. Otdel. khim. Nauk 1945 364; Ackerman Jordan Eddy and Swern J .Amer. Chem. SOC. 1956 78 4444. 4 5 Kamai and Kukhtin Zhur. obshchei Khim. 1957 27 949. 4 6 Cooke Gerrard and Green Chem. and Ind. 1953 351. 4 7 Ackerman Chladek and Swern J . Amer. Chem. SOC. 1957 '79 6524. 48 Allen and Johnson ibid. 1955 77 2871. 49 Perkow Krokow and Rnoevenagel Chem. Ber. 1955 88 662. 50 Pudovik Doklady Akad. Nauk S.S.S.R. 1955 105 735. 51 Jacobson Griffin Preis and Jensen J . Amer. Chem. SOC. 1967 79 2608. CROFTS COMPOUNDS CONTAINING CARRON-PHOSPHORUS BONDS 351 give largely diethyl isopropenyl phosphate in ether at a low temperature a t 150-160" diethyl acetonylphosphonate is the main pr~duct.~O Methylene and other dihalides react normally but less readily than alkyl halides in the Arbusov reaction one or both halogen atoms being replaced according to the ratio of the rea~tants.~5 Chloroform does not react with triethyl phosphite even under drastic conditions,52 but carbon tetrachloride does so a t lower temperatures than are required for Arbusov reactions of alkyl halides and gives excellent yields of diethyl trichloromethylphos- phonate.53 This surprisingly ready reaction has been shown to proceed by a free-radical chain mechanism being inhibited by quinol and accelerated by dibenzoyl peroxide at 80" and occurring a t room temperature on exposure t o ultraviolet radiation.54 (ii) Formation of alkylphosphonic dichlorides from alkyl chlorides phos- phorus trichloride and aluminium trichloride.The nucleophilic character of the phosphorus atom in phosphorus trichloride being decreased by the influence on the unshared electrons of the inductive effect of the chlorine atoms is insufficient to enable it to attack alkyl chlorides and form a,lkyl- phosphonic tetrachlorides R-PCl, by a bimolecular mechanism (as occurs in the formation of the quasi-phosphonium intermediates in the Arbusov reaction).If however alkyl chlorides are added with cooling and stirring to mixtures of phosphorus trichloride and aluminium trichloride crystalline complexes which are probably alkyltrichlorophosphonium tetrachloroaluminates are rapidly formed. On careful hydrolysis these complexes yield alkylphosphonic dichlorides 55 7 5~3 whilst treatment with alcohols gives dialkyl alkylphosphonates 57 RCI i- AICL - R+ AICb- R * P (0) C 1 R- P(OX0d) Evidence that carbonium ions are involved in these reactions is pro- vided by the isomerisation and degradation which has been observed with some alkyl groups.Thus n-propyl n-butyl and isobutyl chloride gave isopropyl- sec.-butyl- and tert.-butyl-phosphonic dichloride 56 whereas tert.-amyl chloride yielded a mixture of tert.-butyl- and tert.-amyl-phosphonic dichlorides.58 With these limitations this reaction provides a good route to many phosphonic dichlorides derived from alkyl cycloalkyl and aralkyl chlorides and polychloroalkanes. The polychloroalkanes always yield chlorides of monophosphonic acids which is to be expected since reaction of two chlorine atoms would require the formation of doubly charged carbonium ions. 5 2 Crofts and Kosolapoff J . Amer. Chem. SOC. 1953 '75 5738. 5 3 Kamai and Egorovs Zhur. obshchei Khim. 1946 16 1521 ; Kosolapoff J.Amer. 54 Griffin Chem. and I n d . 1958 415. 5 6 Kinnear and Perron J. 1952 3437. 57 Hoffmann Simmons and Glunz J . Anaer. Chem. Xoc. 1957 79 3570. 5B Crofts and Kosolapoff ibid. 1953 75 3379. Chem. SOC. 1947 69 1002. 5 5 Clay J . Org. Chem. 1951 16 892. 352 QUARTERLY REVIEWS Alkylphenylphosphinic acids RPhP( O)*OH have been prepared by reaction of alkyl chlorides with phenylphosphonous dichloride Ph-PCl, and aluminium trichloride followed by hydrolysis.59 (iii) Reaction of diaxonium salts with halides of tervalent phosphorus. Reaction of aryldiazonium fluoroborates with phosphorus trichloride in a dry solvent (ethyl acetate or dioxan) in presence of cuprous chloride or bromide followed by addition of water and steam-distillation to hydrolyse the products and remove the solvent has proved a valuable general method for the synthesis of arylphosphonic acids.60 Aryldiazonium fluorosilicates may be advant,ageously used instead of fluoroborates in some cases.61 Although yields are variable (10-70%) the reaction is useful because of its generality (few failures having been reported) the ready availability of a large number of aromatic amines and the known orientation of the products.The reaction mechanism does not appear to have been investi- gated but presumably involves formation of an aryltrichlorophosphonium fluoroborate. This may occur by nucleophilic attack of phosphorus tri- chloride on aryl cations formed by decomposition of the diazonium salt. Alternatively the occurrence of an induction period followed by a rapid reaction suggests a chain-reaction involving aryl radicals Ar- + P C b t Ar.N - ArPCl t N2+ At.In many cases the diarylphosphinic acid is also formed sometimes in quite considerable amounts. This presumably arises by an excha,nge reaction resulting in the formation of arylphosphonous dichloride and reaction of this with more diazonium fluoroborate Ar.PCl,f BF; + PCL - Ar.PC12 + FC1 BF; IA~N BF,- 9 Ar2PC1 BF; t N Ar,P(O)*GH By using alkyl- or aryl-phosphonous dichlorides instead of phosphorus (iv) The Michaelis reaction. Alkali-metal derivatives of diallryl phos- trichloride arylalkylphosphinic acids may be prepared.62 phites react with alkyl halides to give dialkyl alkylphosphonates 6 3 Reactions of this type may be carried out with diethyl phosphite dry ether being used as solvent but nowadays dibutyl phosphite is more usually 59 Biddle Kennedy and Willans Chem.and I n d . 1957 1481. 6o Doak and Freedman J . A m e r . Chem. SOC. 1951 73 5658; 1952 74 753; Ashby and Kosolnpoff ibid. 1953 75 4903; Freedman and Doak ibid. 1955 77 173. 61 I d e m ibid. 1953 75 4905. 6 2 I d e m ibid. 1952 74 2884; J. Org. Ghem. 1955 23 769. 63 Michaelis and Becker Ber. 1897 30 1003. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 353 employed because of the high solubility of its sodium derivative in light petroleum and aromatic hydrocarbons which are more satisfactory as reaction media. 24 After completion of the reaction under reflux for several hours the sodium halide is removed by filtration or by washing with water and the solution is distilled to give the phosphonate. Complete removal of sodium halide is important as otherwise undistillable sodium salts are formed on heating.The Michaelis reaction has been used almost as widely as the Arbusov reaction for the preparation of esters of phosphonic acids. Its mechanism has not been investigated to the same extent but clearly involves a bi- molecular nucleophilic attack on the alkyl halide by the mesoineric anion and resembles alkylation of diethyl malonate and ethyl acetoacetate. Examples of the Michaelis reaction are not confined to those involving dialkyl phosphites but also include reactions of the sodium derivatives of monoalkyl aryl- and alkyl-phosphinates with alkyl halides to give alkyl alkylarylphosphinates RArP( 0)-OR' 64 and alkyl dialkylphosphinates RR'P( O)*OR" 65 respectively. These routes to unsymmetrical phosphinic acids are like analogous Arbusov reactions hampered by the difficulties of preparing the phosphonitks required as starting materials.As with the Arbusov reaction alkyl toluene-p-sulphonates 35 and dialkyl sulphates will react as well as halides although in practice the latter are almost invariably used. The structural requirements for these are similar to but generally somewhat more rigorous than those in the Arbusov reaction because of the intervention of other reactions of the strongly basic and nucleophilic anion. Primary alkyl halides 24 and benzyl halides 66 (but not diphenylmethyl or triphenylmethyl halides) give good yields of phos- phonates. Reactions of sodium dialkyl phosphites with ally1 halides are complicated by the formation of diphosphonates by addition of dialkyl phosphite to the double bond,39* 67 but by carrying out the reaction in a large volume of benzene or with free dialkyl phosphite present dialkyl allylphos- phonates may be obtained as the sole products.68 Michaelis reactions have been used for the preparation of o-amino- alkylphosphonic acids by reaction of sodium dibutyl phosphite with N-cu- bromoalkylphthalimides and hydrolysis of the undistilled products and also (preferably because it yields esters which may be purified by distillation) by reaction of o-bromoalkylamine hydrobromides with two niols.of sodium dibutyl p h ~ s p h i t e . ~ ~ Reactions of sodium diallryl phosphites with a-halogeno-ketones do not give dialkyl2-oxoalkylphosphonates which would be expected from normal Michaelis reactions and are one of the products of interaction of trialkyl 6 4 Kosolapoff J .Amer. Chem. Soc. 1959 '72 4292. 6 5 B. A. Arbusov and Rizpolozhensku Txvest. Akad. Nauk S.S.S.R. Otdel. khim. 6 G Kosolapoff J . Amer. Chem. Soc. 1945 67 2259. Nut& 1955 253. Rueggeberg. Chernak and Rose. ibid. 1950,72 5336; Schwarzenbach Ruckstuhl and Zure Helv. Chim. Acta 1951 34 455. 68 Pudovik and Frolova Zhur. obshchei Khim. 1952 22 2052. 69 Chavune Con@. rend. 1947 224 406. 354 QUARTERLY REVIEWS phosphites and a-halogeno-ketones. Sodium diethyl phosphite and chloro- acetone have been reported 7O to yield diethyl isopropenyl phosphate (the abnormal product from the reaction of triethyl phosphite and chloroacetone) but B. A. Arbusov and his co-workers 71 have found that the products of these and similar reactions are dialkyl 1 2-epoxyalkylphosphonates (IV) which they suggest are formed by attack of the dialkyl phosphite anion on the parbonyl group followed by formation of the epoxide ring with expulsion of chloride ion The first stage is similar to that proposed (see p.350) for the formation of anomalous products in reactions of trialkyl phosphites with cc-halogeno- aldehydes and -ketones. That the negatively charged oxygen atom forms a bond to phosphorus in the a,nomalous Arbusov reaction and to the cc-carbon atom in the anomalous Micha.elis reaction is presumably due to the positive charge which is present on the phosphorus atom in the former but not in the latter case. (v) Reactions of aldehydes and ketones with chlorides of tervalent phosphorus. Whenaldehydes (1 mol.) are heated with phosphorus trichloride (1-1.5 mols.) a t about 200 O for several hours a-chloro-phosphonic dichlorides are f~rrned.~z 7 3 The reaction proceeds well with most aromatic aldehydes and with formaldehyde but gives poor results with other aliphatic aldehydes because of ready elimination of hydrogen chloride from the products.Similar reactions occur with other tervalent phosphorus chlorides including diary1 phosphorochloridites alkyl- and aryl-phosphonous dichlorides and diarylphosphinous chlorides.' 2 74 I n addition to these high-temperature reactions a-hydroxyalkylphos- phonic acids may be prepared (at room temperature or slightly above) from phosphorus trichloride by reaction either with three mols. of an aldehyde followed by water,75 or with one mol. of an aldehyde or ketone followed by addition of glacial acetic acid and subsequent hydrolysis :76 (vi) Addition of compounds containing P-H bonds to carbonyl and imino- groups.Aldehydes react with phosphine in presence of hydrogen chloride to give tetra- (a-hydroxyalky1)-phosphonium chlorides :77 PH + 4R-CHO + HCl --C [R-CH(OH)]4PtCL- 70 Krentzkamp and Kayser Chem. Ber. 1956 89 1614. 71 B. A. Arbusov Vinogradova and Polezhayeva Doklady Akad. Nauk S.S.S. R. 7 Kabachnik and Shepeleva Doklady A kad. Nauk S.S.S. R. 1950 75 2 19. 73 Idem Izvest. Akad. Xauk S.S.S.R. Otdel. khim. Nauk 1950 39; 1951 185. 7 4 Idem ibid. 1953 862. 7 5 Possek Monatsh. 1884 5 121; 1886 7 20; Page J. 1912 101 423. 7 6 Conant MacDonald and Kinney J. Amer. Chem. SOC. 1921 43 1928. 7 7 de Girard Ann. Chim. Phys. 1884 2 1; Reeves Flynn and Guthrie J .Amer. 1956 111 107; B. A. Arbusov Chem. SOC. Special Publ. No. 8 1957 p. 47. Chem. SOC.. 1955 7'7 3923. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 355 Similarly reaction of aldehydes and ketones occurs with phosphorous acid giving a-hydroxyalkylphosphonic acids and more readily wit,h hypo- phosphorous acid giving a-hydroxylalkyl- and di-(a-hydroxyalky1)-phos- phinic acids.'g In reactions with hypophosphorous acid it is possible to isolate the product of the first addition and by reaction with a different aldehyde or ketone to prepare an unsymmetrical di- (a-hydroxyalky1)- phosphinic acid H o - R R'C pH# RR'CO + H,P(O)-OH - RR'C(OH).PH (O).OH R':cHo_ HO-R"HC/ 'OH All these reactions require several days for completion and involve rather tedious separations of the water-soluble acids.I n contrast to these slow reactions dialkyl phosphites react rapidly and exothermally with aldehydes and ketones on addition of a few drops of sodium methoxide in methanol to give dialkyl cc- hydroxyall~ylphosphonates. 79 The reactions of phosphorous (and hypophosphorous) acid and of dialkyl phosphites with carbonyl groups are similar to acid-catalysed and base-catalysed aldol reactions and like these and other additions to carbonyl groups a.re reversible. The acids are more stable than the esters being Me OH R ' R ' ~ (OH) -P(o) (OR& - + Meo- unaffected by alkali and reasonably RRT-O- P (0) (OR) stable towards acids so that they may be obtained by acid-hydrolysis of the esters; they are however hydrolysed to the carbonyl compound and phosphorous acid by prolonged boiling with strong acids.The esters on the other hand are readily decomposed by dilute sodium hydroxide 80 or sometimes (with bulky carbonyl compounds) on attempted distillation.*l Addition reactions of compounds with P-H bonds similar to both the above acid-catalysed and base-catalysed types take place with carbon- nitrogen double bonds in a d s . Thus hypophosphorous acid reacts with anils (or with primary amines together with an aldehyde or ketone) to give substituted cc-aminoalkylphosphinic acids g2 R'"'C=NR + H,P(O).OH - R'R"C(NHR) .PH(o)-oH 78 Ville Ann. Chim. Phys. 1891 23 289; Marie ibid. 1904 3 35. Abramov Zhur. obshchei Khirn. 1952 22 647 and numerous other papers by Abramov Semenova and Semenova Doklady Akad. Nauk S.S.S.R. 1952,84,281. Abramov et al.81 Abramov and Semenova Sbornilc Statey PO obshchei Khirn. 1953 1 393. 82 Schmidt Chem. Ber. 1948 81 477. 356 QUARTERLY REVIEWS whilst base-catalysed additions of dialliyl phosphites to anils yield N-mono- substituted dialkyl or-aminoalkylphosphonates.83~ 84 These and NN-disub- stituted or unsubstituted esters may also be prepared 83 85 by reaction of a dialkyl phosphite with an aldehyde or ketone and a primary or secondary amine or ammonia in Mannich-type reactions 86 which are unusual in that the carbonyl compound need not be formaldehyde. Examples using less common starting materials of the reactions con- sidered in this section are those of dialkylphosphine oxides with aldehydes and ketones giving dialkyl-or-hydroxyalkylphosphine oxides 87 and those of alkyl alkyl- and aryl-phosphinates with anils 88 and with ammonia and aldehydes or ketones.89 (vii) Nucleophilic addition of compounds containing P-H bonds to activated double bonds.These reactions are 3 4-additions corresponding to some of the 1 2-additions described in the previous section. The discovery that dialkyl phosphites are able to add to carbon-carbon double bonds activated by electron-withdrawing groups was made as a result of attempted Michaelis reactions of ally1 halides which yielded esters of diphosphonic acids instead of dialkyl allylphosphonates. 39 67 These additions are complicated by allylic shifts resulting in the formation of both 1 2- and 1 3-diphosphon- ates ; 68 more straightforward and synthetically more useful reactions are the additions of dialkyl phosphites to or@-unsaturated ketones esters and nitriles.These have been extensively investigated by Pudovik and his co- workers They are carried out by addition to the reactants of a few drops of an alcoholic sodium allioxide solution (having the same alkyl group as in the dialkyl phosphite in order to avoid interchange) followed by distillation. Yields vary but are often around 70%. The phosphonic acid group enters the @-position 92 as would be expected of attack by a dialkyl phosphite anion 91 and are analogous to Michael reactions. Substituents particularly if attached to the @-carbon atom make re- action more difficult 9 3 and may lead to addition a t the carbonyl group rather than a t the olefinic double bond.g1 This also occurs with a@-un- saturated aldehydes which yield or@-unsaturated or-hydroxyalkylphosphon- ates instead of ~-oxoalkylphosphonates.g~~ 94 83 Fi-lds J .Amer. Chem. SOC. 1952 '74 1528. 8 4 Pudovik Doklady Akad. N a u k S.S.S.R. 1952 83 865. 8 5 Chalmers and Kosolapoff J . Amer. Chem. SOC. 1953 75 5278. 8 6 Blicke Organic Reactions 1942 1 303. 8 7 Miller Miller Rogers and Hamilton J . Amer. Chem. SOC. 1957 '79 424. 88 Pudovik Doklady Akad. N a u k S.S.S.R. 1953 92 773. 89 Kabachnik and Medved Izvest. Akad. N a u k S.S.S.R. Otdel. khim. N a u k 1954 90 Pudovik and B. A. Arbusov Doklady Akad. N a u k S.S.S.R. 1950 '73 327. 91 Pudovik ibid. p. 499. 9 2 Pudovik and B. A. Arbusov Zhur. obshchei Khim. 1951 21 382. 9 3 Pudovik ibid. 1952 22 1371. 1024. 9 4 Pudovik and ICitaev ibid. p. 467 CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 357 Other compounds with P-H bonds take part in similar reactions.Thus alkyl alkyl- and aryl-phosphinates 95 and dialkylphosphine oxides 96 add to unsaturated ketones esters and nitriles. The only reported reactions of this type involving tervalent phosphorus compounds are those of phenyl- phosphine and diphenylphosphine with acrylonitrile when the reactants are heated without a catalyst at 130" for 6-7 hours 97 Ph-PH + XH,=CH.CN -c Ph*P(CH;;CH&N) (b) Reactions in which the Phosphorus Atom acts as an Electrophilic Centre. These reactions in which atoms and groups attached to phosphorus are replaced by organic groups by reactions with organometallic compounds aromatic hydrocarbons or olefins mostly involve phosphorus halides. They provide methods for the formation of C-P bonds in many types of com- pounds and include the majority of reactions for the formation of bonds between carbon and tervalent phosphorus atoms.Interaction of phos- phorus halides and of some other compounds with metal alkyls and aryls provides routes to many classes of compounds containing C-P bonds. Grig- nard reagents have been most frequently used. When they (or organo- lithium compounds) are present in excess their reactions with phosphorus trichloride and oxychloride result in replacement of all three chlorine atoms and the formation of trialkyl (or triary1)-phosphines and -phosphine oxides. Unsymmetrical compounds of these types can be prepared by reactions of phosphorus chlorides which already contain C-P bonds with a Grignard reagent containing a different groupm98 The products are isolated after (i) Reactions involving organometallic compounds.R.PC1 t 2Rf.MgCl - RRf2P f 2MgCL decomposition in the usual way care being taken t o avoid oxidation when working with phosphines. Yields are usually good if aryl or primary alkyl groups are being introduced but reactions of secondary alkylmagnesium halides with phosphorus trichloride give only poor yields of tri-see. - alkylp hosphines .gg Reactions of organometallic compounds resulting in incomplete replace- ment of halogen atoms attached to phosphorus and the formation of com- pounds containing only one or two C-P bonds may be achieved in several ways. With bulky groups the extent of substitution appears to be limited by steric hindrance and dialkylphosphinic acids have been prepared by reactions of tert.-butyl- and isopropyl-magnesium chloride with tert.-butyl- phosphonic chloride followed by hydrolysis.b*l 100 Attempts to limit the Qs Pudovik et al.Ixvest. Akad. Naulc S.S.S.R. Otdel. khim. Nauk 1952 902; 1954 g 6 Miller Bradley and Hamilton J . Amer. Chem. SOC. 1956 78 5299. 9 7 Mann and Millar J. 1952 4453. g8 Davies and Waltors J. 1935 1786; Davies and Mann J. 1944 276; Morrison 99 Davies J. 1933 1043. loo Crofts and Fox J. 1958 2995. G3G; Zhur. obshchei Khim. 1954 24 1026; 1955 25 778. J . Amer. Chern. SOC. 1950 72 4820. z 358 QUARTERLY REVIEWS number of chlorine atoms replaced by employing ‘‘ reversed addition ” i.e. by adding the Grignard reagent to the phosphorus halide have given only poor yields of alkylphosphonic and dialkylphosphinic chlorides from phosphorus oxychloride and methyl- or ethyl-magnesium halides 101 but have been more successful in reactions involving aromatic Grignard reagents.102J03 For the preparation of phosphonous dichlorides R-PCI, the usual method has been to employ less reactive organometallic compounds.Organomercury compounds were the first to be used 104 but difficulties in obtaining completely mercury-free products have been reported and organic derivatives of other metals are now preferred. Excellent yields of ethyl PbEt + 3PC1 - 3Et*K12/ PbCL,+ EtC1 phosphonous &chloride may be obtained by using tetraethyl-lead.105 This and other phosphonous dichlorides can also be prepared in moderate yields by reaction of phosphorus trichloride with organo-cadmium lo6 and -zinc c~mpounds,~O~ which are readily available from Grignard reagents and cadmium or zinc chloride.Diarylphosphinous chlorides Ar,PCI have been prepared by the use of aryl-mercury lo8 and -zinc compounds,1o9 but most preparations of these and of dialkylphosphinous chlorides have been not by this route but by thermal decomposition of tertiary phosphine dichlorides (see p. 363).11° Limitation of the extent of substitution in reactions of organometallic compounds can also be achieved by the use of less reactive phosphorus halides. Phosphorus-fluorine bonds are less easily attacked by nucleophilic reagents than are phosphorus-chlorine bonds so that reaction of an alkyl- phosphonic difluoride with Grignard reagents may be used for the prepara- tion of unsymmetrical dialkylphosphinic acids 111 R-P(0)F2 t R!MgHal --c RRfP(0)F - RR’P(O)*OH Reactions of thiophosphoryl chloride with alkylmagnesium halides result largely in disubstitution.l12 With methylmagnesium halides tetramethyl- diphosphine disulphide a peculiar insoluble compound is f0rrned.11~ Oxida- lol Jean Bull.SOC. chim. France 1956 569. loa Kosolapoff J. Amer. Chem. SOC. 1942 64 2982. loa Burger and Dawson J. Org. Chem. 1951 16 1250. lo4 Michaelis Ber. 1880 13 2174; Guichard Ber. 1899 32 1572. loS Kharasch Jensen and Weinhouse J. Org. Chenz. 1949 14 429. lo6 Fox J . Amer. Chem. SOC. 1950 72 4147. lo’ Weil Prijs and Erlenmeyer Helv. Chim. Acta 1952 35 1412. loS Michaelis Annalen 1901 315 43. loQ Weil Helv. Chim. Acta 1954 37 654. 110 Collie and Reynolds J. 1915,107 367; Plets Dissertation Kazan 1938 quoted ll1 Dawson and Kennard J . Org. Chem. 1957 22 1671. 112 Strecker and Grossman Ber.1916 49 63. llS Kabachnik and Shepeleva Izvest. Akad. Nauk S.S.S.R. Otdel. khim. Nauk in ref. 1 p. 57. 1949 66. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 359 tion of this or of the products from similar reactions yields dialkylphos- phinic acids 1 1 3 7 114 Another general route for the preparation of symmetrical dialkylphos- phinic acids is reaction of dialkyl phosphites with Grignard reagents or organolithiurn compounds and oxidation (with hydrogen peroxide or by reaction with phosphorus pentachloride followed by hydrolysis) of the result- ant dialkyl- or diaryl-phosphine oxides. 115-11' Many of these intermediat'es have been isolated and characterised.ll6 A general method for restricting the extent of substitution in reactions of Grignard reagents with phosphorus chlorides is the blocking of one or two of the positions by replacement of the chlorine atoms by unreactive groups whj ch can subsequently be removed.Alkoxy- or aryloxy-groups are not gelierally satisfactory as they are replaced almost as readily as chlorine atoms,l17 7 118 unless the Grignard reagent is sterically hindered,l03 but dialkylamino-groups are very effective for this purpose. 118 3 119 Sym- metrical phosphinic acids may be prepared by blocking one of the positions in phosphorus oxychloride POCL -c P(0)61R!&I2 - I?.ffZ))-NR\ - VO).OH and unsymmetrical dialkylphosphinic acids have been obtained by similar reactions starting with alkylphosphonic dichlorides.lo0 Blocking of one position in phosphorus trichloride has been used for the preparation of dimethylphosphinous chloride 120 the dimethylamino- group being replaced in this case by reaction with dry hydrogen chloride at a low temperature PCL - PCliNMe -c Me,P.NMe Ic Me2pCL (ii) Aromatic substitution by phosphorus halides oxides and sulphides.Phenylphosphonous dichloride can be conveniently prepared by heating the mixed vapours of benzene and phosphorus trichloride 21 but similar reac- tions with other aromatic hydrocarbons do not give useful results. Friedel- Crafts reactions of aromatic compounds with phosphorus trichloride and 11* Kosolapoff and Watson J . Amer. Chem. Xoc. 1951 73 5466; Fox Thesis 115 Kosolapoff and Watson J . Amer. Chem.. SOC. 1951 73 4101. 116 Williams and Hamilton ibid. 1952 74 5418; 1955 '77 3411. 117 Willans Chem. and Ind. 1957 235. l18 Michaelis and Wegner Ber.1915 48 316. 119 Kosolapoff J . Amer. Chem. SOC. 1949 71 369. lZo Burg and Slota ibid. 1958 80 1107. 121 Michaelis Ber. 1873 6 601; Bowles and James J . Amer. Chem. Soc. 1929 Man Chester 1957. 51 1406. z* 360 QUARTERLY REVIEWS aluminium trichloride provide however a method of preparing arylphos- phonous dichlorides which is generally applicable to aromatic hydrocarbons halides ethers and tertiary amines but not to compounds which contain meta-directing The yields of products isolated were originally very low because of the formation of complexes with aluminium chloride but have been increased to about 70% after the discovery that these com- plexes may be broken down on completion of the reaction (2-8 hours under reflux with stirring) by the addition of phosphorus oxychloride which forms a stronger complex with aluminium chloride and thus liberates the aryl- phosphonous dichloride.123 The main products from monosubstituted ArH -I- PCI Ar.PCl,,AlCL Ar.FCI,t POC$ALCl benzenes are the para- compounds although some ortho- and meta-substitution has also been rep0rted.1~~ An alternative to the above isolation procedure useful when arylphosphonic acids or their esters are ultimately required is chlorination followed by reaction with ethanol and distillation of the result- ing diethyl arylphosphonate 125 .Ar-PCs AICI %+ Ar.PC14,AlCI ArP(O)(OEt) Prolonged heating in Priedel-Crafts reactions of phosphorus trichloride leads to the formation of diarylphosphinous chlorides. These have not been isolated a.s such but have been converted into ethyl ciiarylphosphinites by the above procedure.125 Pormation of C-P bonds also occurs on reaction of phosphorus penta- sulphide and phosphorus pentoxide with aromatic compounds.Phosphorus pentasulphide and excess of benzene react in presence of aluminium chloride a t the boiling point to give an 80% yield of diphenylphosphinodithioic acid 126 8PhH t p4Sl0- 4PhJ'(S).SH + 2H2S Benzene o- xy lene anisole phenetole naphthalene and 2 -isopropyl- naphthalene also react with phosphorus pentasulphide in the absence of aluminium trichloride,l27 although considerably higher temperatures (150- 225") are required and only one aryl group becomes attached to each phos- phorus atom. It is suggested that the products which have the empirical formula Ar*PS2 and are found to be dimeric by cryoscopic measurements have structures of the type (V).Reaction with chlorine yields first aryl- phosphonothioic dichlorides and then arylphosphonic tetrachlorides ; pro- longed hydrolysis gives arylphosphonic acids. lZ2 Michaelis Annul& 1896 293 193; 294 1; 1901 315 43. l z 3 Dye J . Amer. Chem. SOC. 1948 70 2595; Buchner and Lockhart ibid. 1951 lZ4 Kosolapoff J . Amer. Chem. SOC. 1952 74 4119. lz5 Iiosolapoff and Huber ibid. 1947 69 2020. lZ6 Higgins Vogel and Craig ibid. 1955 77 1864. l z 7 Lecher Greenwood Whitehouse and Chao ibid. 1956 78 5018. 73 755; Org. Synth. 1951 31 88. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 361 4ArH + F,S, - 2 + 2H2S Ar *P(OxOH)2 y CI Ar-PCL Ar.P(S)Cl Reaction of aromatic compounds (benzene o-xylene chlorobenzene and naphthalene) with phosphorus pentoxide a t 250-325" yields pitch-like materials which give arylphosphonic acids on hydrolysis.128 The reaction is of great interest but is not at present a useful preparative method. Phos- phorus pentachloride adds to olefins which have a terminal methylene group giving 2-chloroalkylphosphonic tetrachlorides. These readily lose hydrogen chloride arid yield on hydrolysis @-unsaturated alkylphosphonic acids. 1 2 9 The reaction is usually carried out by adding an olefin to a cooled suspension of phosphorus pentachloride in benzene and keeping the mixture overnight. Hydrolysis of the adduct then yields an alkenylphosphonic acid ; treatment with sulphur dioxide or with hydrogen sulphide gives an alkenylphosphonic dichloride 130 or an alkenylphosphonothioic dichloride 131 respectively.(iii) Addition of phosphorus pentachloride to oleJins and acetylenes. RCH=CH + PCL - RCHCl*CH,-PCl R-CH=CH.P(S)CL R-CH=CH .Po) (OH2 4 + R-CH=CH.P(O)CL Terminal acetylenic compounds with phosphorus pentachloride form similar adducts which are hydrolysed without elimination of hydrogen chloride 132 R.C&H -t PCL - R-CCl=CH.PCI a RCc1=CH*P(0)(OH)2 (c) Free-radical Reactions.-Although many oxidation reactions of terval- ent phosphorus compounds undoubtedly involve free radicals reactions of this type are of relatively minor importance for the formation of C-P bonds. Some examples have already been noted e.g. some reactions for the forma- tion of tetra-arylphosphonium salts l o r 12 l3 Arbusov reactions involving carbon tetrachloride,54 and possibly the formation of arylphosphonic acids via aryldiazonium fluoroborates (see p.352); only two more reaction types oxidative phosphonation and free-radical additions to unsaturated com- pounds justify further discussion. (i> Reactions of organic compounds with oxygen and chlorides of tervalent phosphorus. Alkylphosphonic dichlorides are formed by reactions which may be represented by the oversimplified equation RH + PCL t &02 - R.POC12+ HCl 128 Lecher Chao Whitehouse and Greenwood J . Amer. Chem. SOC 1954 '76 1045. 129 Bergmann and Bondi Ber. 1931 64 1455; Kosolapoff and Huber J . Arner. 130 Anisimov and Nesmeyanov Izvest. Akad. Nauk S.S.X.R. Otdel. khirn. Nazck 131 Anisimov Kolabova and Nesmeyanov ibid. p. 796. 132 Bergmann and Bondi Ber. 1933 66 278. Chem. Soc. 1946 68 2540. 1954 610. 362 QUARTERLY REVIEWS when oxygen is passed through mixtures of phosphorus trichloride and saturated hydrocarbons.133 134 Because part of the phosphorus trichloride is simply oxidised to phosphorus oxychloride it is usual to employ an excess of this reactant but even so yields calculated on the hydrocarbon rarely exceed 30%.Usually oxygen is passed into the phosphorus trichloride- hydrocarbon mixture until the initial temperature rise subsides. Single products are only obtained from hydrocarbons such as ethane,135 neopen- tane 136 cycZohexane,133 and cyclopentane 137 in which all the hydrogen atoms are identically situated; in other cases a mixture of all possible isomeric products is obtained. As in other radical reactions tertiary carbon atoms are most easily attacked and primary carbon atoms are least reactive.13* It has been suggested that the initial process is formation of a peroxide-like diradical which can then react in either of two ways CL3P t 0 - cr,b-o-o* CL3b-0-O* + PCL - 2pOCl C.L,b-O-O* + RH + PCL -c POC1,t R-P(0)CL2+ HCL the last equation representing several consecutive stages.135 3 138 Benzene does not react but toluene and ethylbenzene are attacked in the side-chain.136 Reaction occurs with alkyl chlorides (n- butyl chloride giving all four possible chlorobutylphosphonic dichlorides) and with ethers although in the latter case cleavage a t the oxygen atom also occurs.138 By using methyl- ethyl- and phenyl-phosphonous dichloride instead of phosphorus trichloride phos- phinic chlorides RR’P( 0)Cl have been ~repared.13~ Olefins and acetylenes also react when oxygen is passed through mixtures of these hydrocarbons with phosphorus trichloride the overall reactions in these cases corresponding to addition of phosphorus oxychloride across the double or triple bond.Free-radical mechanisms may also be involved here as is suggested by the formation of both 1 1- and 1 2-dichloroethylphos- phonic dichloride from vinyl chloride ; 140 with acetylenes however the phosphonic dichlorides obtained are single compounds identical with those obtained from reactions of the same acetylenes with phosphorus penta- chloride. 141 (ii) Free-radical addition reactions of phosphorus compounds to olgfinic bonds. Addition of phosphorus compounds to carbon-carbon double bonds may occur by free-radical mechanisms as well as by those in which the phosphorus atom acts as a nucleophilic or electrophilic centre.Most of the 133 Clayton and Jensen J . Amer. Chem. SOC. 1948 70 3880. 134 Soborovskii Zinov’ev and Englin Doklady Akad. Nauk S.S.S.R. 1949 67 293. 135 Graf Chem.. Ber. 1952 85 9. 136 Jonsen and Noller J . Amer. Chem. SOC. 1949 71 2384. 13’ Isbell and Wadsworth ibid. 1956 78 6042. 138 Soborovskii Zinov’ev and Englin Doklady Akad. N a u k S.S.S.R. 1950 73 333. 139 Soborovskii and Zinov’ev Zhur. obshchei Khim. 1954 24 516; Zinov’ev and 140 Soborovskii Zinov’ev and Muler Doklady Akad. Nauk S.S.S.R. 1956 109 98. 141 Zinov’ev Muler and Soborovskii Zhur. obshchei Khim. 1954 24 380. Soborovskii ibid. 1956 26 3030. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 363 reactions initiated by peroxides or by ultraviolet irradiation have involved compounds with P-H bonds but phosphorus trichloride has also been found to undergo peroxide-induced addition to olefins the product from oct-1 -ene being 2-chloro-octylphosphonous dichloride.14* The same direction of addition phosphorus becoming attached to the terminal carbon atom has been found for additions of phosphine 143 and hypophosphorous acid 144 to olefins and of diethyl phosphite to isopropenyl acetate 145 and to vinyl acetate.146 I n the last case addition occurs in the opposite direction to the base-catalysed reaction of the same compounds 9 CH3CO-OCH2CH2*P (O)(OEt) %$p CH,CO.O$H.CH CH,-CO.OCH=CH + HP(O)(OEt) P(O)(OE<j Reactions of Compounds containing Carbon-Phosphorus Bonds.- These reactions are of three types; those which result in the breaking of C-P bonds those involving other atoms or groups directly attached to phosphorus and those which occur a t points remote from the phosphorus atom but are affected by it.Cleavage of Carbon-Phosphorus Bonds.-Reactions of this type are rela- tively uncommon. Carbon-phosphorus bonds are comparable in strength to carbon-carbon bonds and are only broken under drastic conditions or when particularly weakened by their environment. Because of this power- ful reagents may generally be used for bringing about interconversions of compounds which contain C-P bonds without risk of these being broken. Thus for example prolonged heating under reflux with constant- boiling hydrochloric acid may be used for hydrolysis of esters of most phosphonic and phosphinic acids and triarylphosphine oxides may be nitrated with mixtures of concentrated nitric and sulphuric acids.Pyrolyses of phosphonium halides and of analogous compounds of the type R,PHal,5-,) (n = 1-4) result in cleavage of one C-P bond and con- version of the phosphorus atom from the quinquevalent to the tervalent state e.g. R,PCl - R2PCl t RCI I n view of the apparent similarity between these thermal decompositions and those of quaternary ammonium hydroxides it is of particular interest that pyrolysis of quaternary phosphonium hydroxides does not proceed in this way but yields tertiary phosphine oxides ,R4P+OH- - R,P(O) f RH 142 Kharasch Jensen and Urry J . Amer. Chem. Soc. 1945 67 1864. 143 Stiles Rust and Vaughan ibid. 1952 74 3282. 144 Williams and Hamilton ibid. 1955 '77 3411. 145 Preis Myers and Jensen ibid.p. 6225. 146 McConnell and Coover ibid. 1957 79 1961. 364 QUARTERLY REVIEWS A similar reaction is the formation of phosphinic acids from phosphine oxides by fusion with alkali.14' The structural requirements for cleavage of C-P bonds in reactions occurring a t lower temperatures differ according to whether an alkyl or an aryl group is being detached. I n general C-P bonds are stable towards hydrolysis but when instability does occur cleavage of C-alkyl bonds tends to take place under alkaline conditions and is assisted by electron-attracting substituents in the alkyl group. Thus trichloromethyl148 and trifluoro- methyl 149 groups attached to phosphorus are removed in alkaline solution and dialkyl 2-chloro- 1 -hydroxyalkylphosphonates 150 and dialkyl 1 - cyano- 1 -hydroxyalkylphosphonates both undergo rearrangement involving fission of the C-P bond under alkaline conditions.Cleavage of C-aryl bonds occurs in quite different circumstances being brought about by hydrogen ions or other electrophilic reagents when the aryl group has electron-repelling groups in the ortho- or para-position. Examples of this are the hydrolysis of p-hydroxyphenylphosphonic acid by hot dilute hydrochloric acid and the formation of tribromophenol by the action of bromine water on this phosphonic acid a t room temperature.152 Other Reactions at the Phosphorus Atom.-Reactions involving the forma- tion and breaking of other bonds to phosphorus are usually not greatly affected by the presence or absence of C-P bonds. The predominant reactions of tervalent phosphorus compounds involve addition of reagents and cause the phosphorus atom to become quinque- valent.Thus all compounds of this type add oxygen sulphur or halogen more or less readily whilst the more reactive ones (phosphines and phos- phites) also combine with hydrogen halides alkyl halides and salts of transition metals. 3P-CuCl 96-R Hal' T U 2 R Y ~ Other reactions such as elimination of alkyl halide may follow but these do not involve a reversion of the phosphorus atom to the tervalent state. Hydrolysis of chlorides of tervalent phosphorus although a substitu- tion also cause the phosphorus atom to become quinquevalent >PCl t H,O - Most reactions of quinquevalent phosphorus compounds are substitutions Thus chlorine atoms in phosphonic and are quite readily brought about. 147 Homer HoEfmann and Wippel Clzem.Ber. 1958 91 64. 148 Bengelsdorf J. Anzer. Chem. SOC. 1955 77 6611. 149 Emelkus Haszeldine and Paul J. 1955 563. 150 Barthel Alexander Giang and Hall J . Amer. Chem. SOC. 1955 77 2424. 151 Hall Stephens and Drysdale ibid. 1957 79 1768. CROFTS COMPOUNDS CONTAINING CARBON-PHOSPHORUS BONDS 365 and phosphinic chlorides may be replaced by hydroxy- alkoxy- aryloxy- or substituted amino-groups by reactions with water alcohols (the liberated hydrogen chloride being removed by a base or reduced pressiire) phenols or amines respectively. Esters and ainides of phosphonic and phosphinic acids are generally readily hydrolysed by heating them under reflux with constant-boiling hydrochloric acid and the acids may be converted into the chlorides by phosphorus pentachloride or thionyl chloride.1 ROH (+ base or in vac.) or ArOH. 2 HCl reflux. 3 H,O. 4 PC1 or SOCl,. 5 Excess of R-NH or R,NH. 6 HC1 reflux. The effects of substituents on the dissociation constants of phosphonic and phosphinic acids are generally as would be expected the dissociation constants being decreased by electron-repelling groups whilst electron- attracting groups strengthen the acids.52 58 > 153 Alkylphosphonic and di- alkylphosphinic acids a’re weaker than phosphoric acid and become more so with increase in the size and degree of branching of the alkyl groups. Reactions not involving Bonds to Phosphorus.-Because of the ease with which tervalent phosphorus compounds undergo reactions a t the phosphorus atom most of the information on reactions occurring a t other points refers to compounds in which the phosphorus atom is quinquevalent and particu- larly to those such as phosphonic acids phosphinic acids and tertiary phosphine oxides which contain a phosphoryl (P=O) group.This group being dipolar is electron-attracting and many of its effects resemble those of carbonyl and nitro-groups. Thus when attached to a benzene ring it is deactivating and meta-directing for electrophilic aromatic substitutions such as nitration,154 but facilitates nucleophilic replacement of halogen at’oms in the ortho- or para-position by amines and phenols.155 In aliphatic compounds the phosphoryl group behaves like a carbonyl or ethoxycarbonyl group in activating hydrogen atoms attached to the adjacent carbon atom so that triethyl phosphoacetate (EtO),P( O)*CH,*CO,Et and diethyl acetonylphosphonate which contain a methylene group flanked by a (EtO),P( 0)- group a’nd an ethoxycarbonyl or carbonyl group respectively form potassium derivatives which may be alkylated by alkyl halides.lS6 Tetraethyl methylenediphosphonate in which the central carbon atom is flanked by two (EtO),P(O)- groups undergoes similar reactions 157 152Bell and Kosolapoff ibid.1953 75 4901. 153 Jaffb Freedman and Doak ibid. p. 2209; 1954 76 1548. 154 Kosolapoff ibid. 1949 71 4021. 156 Kosolapoff and Powell ibid. 1950 72 4198. 15’ Kosolapoff ibid. 1953 75 1500. Bauer ibid. 1941 63 2137. 366 The phosphoryl group is also able to activate an adjacent double bond for addition of nucleophilic reagents. Thus amines diethyl malonate ethyl cyanoacetate dialkyl phosphites and conjugated dienes will add to esters of vinylphosphonic acid l58 CH,:CH*P(O)(OR) + CH 2 ( q E t ) 2 (Et0,C)2~H-CHiCH2.P(0)~OE~* 15* Pudovik Doklady Akad. Nauk S.S.S.R. 1951 $0 65; Pudovik and Imaev Izvest. Akad. Nauk S.S.S.R. Otdel. khim. Nauk 1952 916.

ISSN:0009-2681    

DOI:10.1039/QR9581200341

出版商:RSC    

年代:1958

数据来源: RSC