摘要:

QUARTERLY REVIEWS THE CHEMICAL SOCIETY PATRON HER MAJESTY THE QUEEN President J. M. ROBERTSON C.B.E. M.A. D.Sc. F.R.S. Vice-presidents who have filled the office of President H. J. EMEL~~US C.B.E. M.A. D.Sc. SIR CHRISTOPHER INGOLD D.Sc. SIR CYRIL HINSHELWOOD 0. M. M. A. SIR ERIC RIDEAL M.B.E. M.A. D.Sc. E. L. HIRST C.B.E. D.Sc. LL.D. LORD TODD M.A. D.Sc. F.R.S. F.R.S. F.R.I.C. F.R.S. Sc.D. F.R.S. F.R.S. F.R.S. Vice-presidents D. H. R. BARTON D.Sc. Ph.D. E. R. H. JONES D.Sc. F.R.I.C. E. J. BOWEN M.A. D.Sc. F.R.S. J. CHATT M.A. Sc.D. F.R.S. F.R.S. F.R.S. B. LYTHGOE M.A. Ph.D. F.R.S. M. STACEY Ph.D. D.Sc. F.R.S. Honorary Treasurer J. W. BARRETT Ph.D. A.R.C.S. F.R.I.C. Honorary Secretaries J. W. LINNETT M.A. D.Phil. F.R.S. A. W. JOHNSON Sc.D. Ph.D. A.R.C.S. K. W. SYKES M.A. D.Phi1.Ordinary Members of Council G. 0. ASPINALL D.Sc. Ph.D. J. BADDILEY D.Sc. Ph.D. F.R.S. L. J. BELLAMY B.Sc. Ph.D. W. COCKER M.A. D.Sc. F.R.I.C. D. P. CRAIG M.Sc. D.Sc. L. CROMBIE D.Sc. F.R.I.C. F. S. DAINTON M.A. Sc.D. F.R.S. D. H. EVERETT M.B.E. M.A. D.Phi1. G. W. A. FOWLES B.Sc. Ph.D. W. GERRARD D.Sc. Ph.D. F.R.I.C. R. N. HASZELDINE M.A. Sc.D. F.R.I.C. M.R.I.A. F.R.I.C. A. K. HOLLIDAY Ph.D, D.Sc. J. HONEYMAN Ph.D. D.Sc. A. R. KATRITZKY M.A. D.Phil. T. J. KING B.Sc. M.A. D.Phi1. E. A. MOELWYN-HUGHES D.Phil. W. J. ORVILLE-THOMAS Ph.D. D.Sc. H. M. POWELL B.Sc. M.A. F.R.S. R. A. RAPHAEL D.Sc. F.R.I.C. J. C. ROBB D.Sc. Ph.D. A. I. VOGEL M.Sc. D.Sc. F.R.I.C. F.R.I.C. Ph.D. D.Sc. Sc.D. F.R.S. Ex Oficio J. W. COOK D.Sc. F.R.I.C. F.R.S. (Chairman of the Chemical Council) D. H. HEY D.Sc. F.R.I.C.F.R.S. (Chairman of the Publication Committee) E. D. HUGHES D.Sc. F.R.I.C. F.R.S. (Chairman of the Joint Library Committee) General Secretary J. R. RUCK KEENE M.B.E. T.D. M.A. Librarian R. G. GRIFFIN F.L.A. QUARTERLY REVIEWS VOL. XVI 1962 Publication Committee Chairman D. H. HEY D.Sc. F.R.I.C. F.R.S. G. 0. ASPINALL D.Sc. F.R.S.E. G. KOHNSTAM Ph.D. J. W. LINNETT M.A. D.Phil. F.R.S. WESONBAKER M.A. D.Sc. F.R.S. J. F. W. MCOMIE M.A. D.Phil. C. H. BAMFORD M.A. Sc.D. F.R.I.C. J. W. BARRETT Ph.D. A.R.C.S. R. S. Nyholm D.Sc. F.R.I.C. R. P. BELL M.A. F.R.S. L. E. ORGEL M.A. D.Phil. F.R.S. D. M. BROWN Ph.D. L. N. OWEN Ph.D. D.Sc. F.R.I.C. G. M. BURNETT Ph.D. D.Sc. R. A. RAPHAEL D.Sc. F.R.I.C. I. G. M. CAMPBELL B.Sc. Ph.D. G. E. COATES M.A. D.Sc. F.R.I.C. J.M.RoBERTsoN,C.B.E.,M.A.,D.SC.T. COTTRELL B.Sc. D.Sc. F.R.I.C. D. P. CRAIG Ph.D. D.Sc. F.R.I.C. A. G. SHARPE M.A. Ph.D. F.R.I.C. D. F. ELLIOTT Ph.D. A.R.C.S. K. W. SYKES M.A. D.Phi1. J. C. TATLOW B.Sc. Ph.D. F.R.I.C. V. GOLD D.Sc. Ph.D. H. J. V. TYRRELL B.Sc. M.A. R.H.HALL,P~.D.,A.R.C.S.,F.R.I.C. B. C. L. WEEDON D.Sc. A.R.C.S. C. H. HASSALL M.Sc. Ph.D. F.R.I.C. A. W. JOHNSON Sc.D. Ph.D. D. H. WHIFFEN M.A. D.Phi1. M. C. WHITING M.A. PhD. A.R.C.S. C. KEMBALL M.A. Ph.D. F.R.I.C. W. W1qPh.D. J. A. KITCHENER D.Sc. Ph.D. G. WILKINSON Ph.D. A.R.C.S. F.R.I.C. D.Sc. F.R.I.C. F.R.S. F.R.S. F.R.S. A.R.I.C. F.R.I.C. A.R.C.S. Editor R. S. CAHN M.A. D.Phil.Nat. F.R.I.C. Deputy Editor L. C. CROSS Ph.D. A.R.C.S. F.R.I.C. Assistant Editors A. D. MITCHELL D.Sc. F.R.I.C.; I. J. CANTLON Ph.D. L O N D O N T H E C H E M I C A L S O C I E T Y CONTENTS PAGE DIRECT STRUCTURAL EVIDENCE FOR WEAK CHARGE-TRANSFER BONDS IN SOLIDS CONTAIMNG CHEMICALLY SATURATED MOLECULES.By 0. Hassel INORGANIC REACTIONS IN LIQUID AMMONIA. By G. W. A. Fowles and and Chr. Rermming . - 1 D. Nicholls . . . . 19 FLUOROCARBON CHEMISTRY. PART 1. THE FLUORINATION OF ORGANIC COMPOUNDS. By R. Stephens and J. C. Tatlow . . 4 4 METAL OXIDATION. By M. W. Roberts . . . 71 D. W. Theobald . 101 RECENT ASPECTS OF SESQUITERPENOID CHEMISTRY. By T. G. Halsall and THE SYNTHESIS OF DI- AND TRI-TERPENES. By N. A. J. Rogers and J. A. THE CHEMISTRY OF THE PSYCHOTOMIMETIC SUBSTANCES. By D. F. Downing THE EFFECTS OF SOLVATION ON THE PROPERTIES OF ANIONS IN DIPOLAR CYANIDE COMPLEXES OF THE TRANSITION METALS. By W. P. Griffith Barltrop . . 117 133 A~ROTIC SOLVENTS.By A. J. Parker . . . . 163 . 188 By J. I. G. Cadogan . . 208 PROBLEMS. By M. A. D. Fluendy and E. B. Smith . . 241 ADDITION POLYMERISATION AT HIGH PRESSURES. By K. E. Weale . . 267 OXIDATION OF TERVALENT ORGANIC COMPOUNDS OF PHOSPHORUS. THE APPLICATION OF MONTE CARL0 METHODS TO PHYSICOCHEMICAL HALOGEN CATIONS. By J. Arotsky and M. C. R. Symons . . 282 AN OUTLINE OF TECHNETIUM CHEMISTRY. By R. D. Peacock and R. Colton . . 299 KINETICS AND MECHANISM OF REPLACEMENT REACTIONS OF CO-ORDINA- TOPOTACTIC REACTIONS IN INORGANIC OXY-COMPOUNDS. By L. S. Dent TION COMPOUNDS. By R. G. Wilkins . . 316 Glasser F. P. Glasser and H. F. W. Taylor . . . . 343 STEREOREGULAR ADDITION POLYMERISATION. By C. E. H. Bawn and A. Ledwith . . . . 361

ISSN:0009-2681    

DOI:10.1039/QR96216FP001

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

INORGANIC REACTIONS IN LIQUID AMMONIA By G. W. A. FOWLES and D. NICHOLLS (THE UNIVERSITY SOUTHAMPTON. THE UNIVERSITY LIVERPOOL) 1. Introduction LIQUID ammonia which is the best known non-aqueous ionising solvent is quite a good solvent for many inorganic compounds particularly halides and ammonium salts even though it has an appreciably lower dielectric constant than water. It has a modest degree of auto-ionisation 2NH + NH4+ + NH,- so that solutions of ammonium salts act as acids and those of amides as bases; normal acid-base reactions can be carried out. Since N-H bonds of ammonia are less easily broken than 0-H bonds of water compounds are less likely to be ammonolysed than hydrolysed and for this reason liquid ammonia may sometimes be used as a medium for the preparation of compounds which can be obtained only in low yields from aqueous solutions.Germane GeH, for instance which may be prepared by the action of acids on Mg,Ge is obtained only in 20% yields when aqueous acids are used but an 80 % yield is made possible by the use of solutions of ammonium bromide in liquid ammonia. A unique property of liquid ammonia is that of dissolving alkali metals without further reaction. In contrast to the violent reaction between alkali metals and water the liquid ammonia solutions are quite stable although decomposition to the amide and hydrogen take place in the presence of suitable catalysts such as plati- num black. As two recent reviews1$2 have appeared on the nature of the metal-ammonia solutions we need only very briefly summarise the situa- tion. The metals are considered to dissolve in ammonia giving metal cations and electrons M - t M + + e the electrons are associated with the solvent and in concentrated solutions at least the cation also appears to play some part in determining the environment of the electron.The solutions thus contain a very ready source of electrons and they accordingly constitute very powerful homo- geneous reducing agents. In 1950 Watt3 thoroughly reviewed the use of these solutions in the reduction of both inorganic and organic compounds and Birch4 has also summarised many organic reductions. Jolly “Progress in Inorganic Chemistry,” 1959 1 235. Symons Quart. Rev. 1959 13 99. Watt Chem. Rev. 1950 46 289. Birch Quart. Rev. 1950 4 69. 19 20 QUARTERLY REVIEWS Apart from a brief account by Watt5 in 1957 and a survey of non- aqueous solvents by GutmamQ there has been no recent general review of the various lines of liquid-ammonia research that have been developed since the war.In this present Review we shall first discuss the reactions of ammonia with metal halides with especial emphasis on ammonolysis. Fernelius and Bowman’ reviewed early work (prior to 1940) on the ammo- nolysis of both organic and inorganic compounds (including halides) ; in our account we shall restrict ourselves to those halides which either best illustrate the general principles or have been the most fully studied. The remainder of the Review is concerned with the synthetic procedures in both the inorganic and the organometallic field that have been evolved in the past decade or so. 2. Reactions of liquid ammonia with metal halides We can conveniently consider these reactions as falling into two categories one in which simple addition compounds are formed and the other in which one or more metal-halogen bonds are ammonolysed i.e.(i) (ii) MX + mNH -+ MXn-m,a(NH2)m,p + $NH,X MX + xNH +. MX,,xNH The ammonobasic metal halide formed by ammonolysis (reaction ii) may of course take up ammonia and form an addition compound. It is not possible to classify the halides rigidly under these two headings because the products of the reaction depend on experimental conditions and even those halides that are very easily ammonolysed do form addition compounds initially and these intermediates can on occasion be isolated under suitable conditions. In general though it is true to say that am- monolysis occurs only with the more covalent halides.(a) Ionic Halides of Non-transition Elements.-Thus the ionic halides of non-transition metals (e.g. alkali and alkaline-earth elements) form simple “ammoniates” in which the ammonia is but weakly held and can be removed in vacuo at relatively low temperatures leaving the un- changed halide. A great deal of early work in this field has been done by Biltz who in a summary,8 points out that (i) for a given metal halide the heat of formation of the ammoniate increases as the number of bonded ammonia molecules decreases and (ii) for a given Periodic Group ofmetals the heat of formation of similar ammoniates decreases with increasing atomic weight. The ammonia molecules are undoubtedly associated with the cation although the anion plays an indirect part in determining such details as lattice spacings.Watt J. Chem. Educ. 1957 34 538. Gutmann Quart. Rev. 1956,10,451. Fernelius and Bowman Chem. Rev. 1940,26 3. 13 Biltz 2. amrg. Chem. 1923 130 93. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 21 The bonding in these addition compounds is best regarded as ion- dipole in character resulting from interaction between the cation and the lone-pair of electrons of the nitrogen atom and since the degree to which the electron charge cloud is distorted depends on both the size and the charge of the cation small highly-charged cations form the strongest bonds e.g. Mgs>Ca2+> Sr2+>Ba2*. (b) Halides of Transition Metals in their Lower Valency States.-These halides are usually considered to be highly polymeric with fairly polar metal-halogen bonding.The chlorides of bivalent manganese iron cobalt and nickel for instance which come into this category form addition compounds with ammonia and these adducts do not differ markedly from those formed by the alkaline-earth halides since the ammonia is still lost quite easily in vacuo. However these transition elements of the first row have vacant 4d-orbitals energetically available and the bonding is to be considered more donor-acceptor than ion-dipole in character. The ammoniates formed by these halides of the transition metals in their lower-valency states are often true ammines and the bonding of the ammonia groups to the metal ion is best understood in terms of ligand- field or the molecular-orbital theory. However some halides (e.g. VCl, VBr, MoBr,) have been incorrectly described1* as forming hexa- ammines [M(NH,),]X, whereas they do in fact undergo ammonolysis.Thus both the chloride and the bromide of tervalent vanadium are ammonolysed in liquid ammonia,ll and the so-called ammine is really a mixture [V(NH,),]X E VX,(NH,),4NH3 + NH,X Similarly molybdenum(r1I) bromide is ammonolysed rapidly at room temperature with the formation of MoBr(NH2),,NH3. Titanium(rr1) chloride on the other hand is not appreciably ammonolysed evm at room temperature but much of the ammonia associated with the chloride can be removed by heat in vacuu showing that the ammonia is only weakly bonded. The dichlorides of bath titanium and vanadium form adducts when treated with liquid ammonia at room temperature but at -33.5” there is. virtually no reaction.(c) Covalent Halides of Non-transition Metals.-Themore covalent halides invariably undergo at least partial ammonolysis when treated with liquid ammonia and the extent to which the halogen atoms are replaced by amido-groups decreases as the metal-halogen bonds become more polar. For a given element in a particular valency state the chloride bromide and * Orgel “An Introduction to Transition Metal Chemistry Ligand-field Theory,” Methuen London 1960. lo Sidgwick “The Chemical Elements and Their Compounds,” Oxford Univ. Press 1950. l1 Fowles Lanigan and Nicholls Chem. and Znd. 1961 1167. 22 QUARTERLY REVIEWS iodide usually react similarly with liquid ammonia but the fluoride is always ammonolysed less readily and often only forms an ammoniate. We can illustrate these points by reference to the halides of the Group I11 and IV elements.Boron(m) fluoride acts as a typical Lewis acid towards ammonia and forms an adduct BF3,NH3 which does not undergo ammonolysis even at 50°.12 Although no analogous adducts have been isolated from the reactions of ammonia with the other three halides it is probable that they are formed initially and subsequently undergo ammonolysis N H4X Further co-ordination and elimination of hydrogen halide then leads to the formation of the amide which readily decomposes to the imide 2B(NHJ -+ B,(NH)S + 3NH The reluctance of boron(rrr) fluoride to undergo ammonolysis is a reflec- tion of the high B-F bond strength. In the presence of amide ions or alkali metals ammonolysis takes place e.g. + K + BH + KF This is to be expected since the amide ion is a much stronger nucleophilic reagent than ammonia while the alkali metals provide a source of elec- trons and help the formation of fluoride ions.It is interesting that when the ammonolytic mixture [BF,(NH& + KF] is passed down a cation-exchange the potassium ions are re- moved and the adduct (BF3,NH3) is re-formed. This is consistent with the initial formation of a complex boron anion which breaks down on removal of excess of ammonia N H (i) BFz(NH2) + KF - K[BF,(NHz)I N H3 (ii) K[BF,(NH,)] + NH,(Resin) -+ NH,[BF,(NH,)] + K(Resin) (ii i) N H4[BF,(N H2)] Removal of + BF,,NH + NH excess Of NH Thus when the excess of ammonia is removed the solid complex is unstable and transference of a proton from the ammonium ion to the amino-group is followed by the loss of a molecule of ammonia.We shall see that the formation of similar complex ions helps to account for the behaviour of a number of other halides that are only partially ammono- lysed. As we go from the boron halides to those of aluminium there is a marked change since none of the aluminium halides is appreciably l2 Jenkins J. Amer. Chem. SOC. 1956 78 5500. l3 McDowell and Keenan J. Amer. Chem. SOC. 1956 78 2065. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 23 ammonolysed in liquid ammonia. The ammoniates are ammonolysed however on treatment with solutions of the alkali metals in liquid ammonia.14 The halides of the Group IV elements carbon to tin all undergo ammonolysis on direct treatment with ammonia (cf. Table 1). TABLE 1. Ammonolysis of Group 1V halides. Halide CCI CBr Cr SiCl Si,CI GeCI GeI SnCI SnBr snI4 PbCI Reaction temp.140" 100-250 - 33.5 25-150 - 50 - 50 - 33-5 - 33.5 - 33.5 - 33.5 -33.5 20 Ammonolysed product HN C(NH& HN:C(NH& CI,,2NH3 HN C(NH,) Si(NH3 Ge( NH) Ge( NH) SnCl(NH,) SnBr(NH,) No ammonolysis [Si( NH)(NH,)I SnI(NH2)3 Ref. 15 16 17 16 18 19 20 21 22 23 23 7 Carbon tetrahalides are not ammonolysed unless the reactions are carried out above room temperature. This is understandable since carbon has no vacant orbitals of low enough energy to accept electrons from an ammonia molecule and no intermediate adduct can be formed. Hence the mechanism proposed for the ammonolysis of the boron halides is not applicable and the breaking of a C-X bond occurs only at higher tem- peratures. It should be mentioned that carbon tetraiodide forms a diam- moniate when treated with liquid ammonia but the ammonia is held only by weak van der Waals interactions and will not modify the C-X bonds sufficiently to facilitate ammonolysis.In contrast to the inertness of the carbon tetrahalides the halides (other than fluorides) of quadrivalent silicon and germanium are readily and completely ammonolysed to the amides M(NH2), at low temperatures ; these readily break down to the imides M(:NH),. Tin@) chloride l4 Watt and Braun J. Amer. Chem. SOC. 1956,78 5494; Taylor Griswold and Klein- berg ibid. 1955,77 294. l5 Stiihler Ber. 1914 47 909. l6 Watt and Hahn J. Amer. Chem. SOC. 1955,77 312. l7 Watt McBride and Sowards J. Amer. Chem. SOC. 1956,78 1562. l8 Vigoroux and Hugot Compt. rend. 1903 136 1670. l9 Schwarz and Sexauer Ber.1926,56,333; Billy Compt. rend. 1960,250,4163. 2o Schwarz Angew. Chem. 1935,48,221. *l Johnson and Sidwell J. Amer. Chem. SOC. 1933 55 1884. 22 Bannister and Fowles J. 1958 751. 23 Bannister and Fowles J. 1958,4374. 24 QUARTERLY REVIEWS bromide and iodide are not completely ammonolysed however and even after prolonged washing with liquid ammonia the last tin-halogen bond remains intact. The ammonobasic tin(1v) halides are almost insoluble in liquid ammonia so that they can be obtained free from ammonium halide by extracting the latter with liquid ammonia. Schwarz and Jeanmai~e,~~ who did earlier work with tin(1v) chloride found that even on prolonged washing with ammonia the insoluble product still had a Sn Cl ratio of 1 1.5 although this could be lowered to 1 1 if the product were heated to 100" between the washings.More recent work has however shown that SnCl(NHA can be obtained without this heating. The insoluble ammonobasic tin(1v) halides are almost certainly polymers resulting from the elimination. of hydrogen halide between neighbouring molecules or from condensation through chlorine or nitrogen bridges or both e.g. The composition SnCI(NH,) is probably only an average one the degree of ammonolysis being above average at the extremities of the polymer. The presence of some tin compounds in the ammonia-soluble portion of the products (solubility increasing in the order Cl<Br<I) suggests that the ammonium halide formed in the ammonolysis produces soluble complex anionic species such as [SnC13(NH,)3]2-. Solubility is not complete however because of the polymeric nature of the bulk of the product and the relatively low concentrations of ammonium halide.The tin(1v) halide-ammonia systems have also been examined in some detail by a tensimetric procedure in which a known excess of ammonia is condensed on to the halide (SnX, NH3 - 1 30) and the resulting mixture is allowed to come to equilibrium over a period of days. Under these experimental conditions a small amount of liquid ammonia is present in the reaction bulb; small quantities of ammonia are then removed pro- gressively and the vapour pressure of ammonia measured for each com- position. The annexed Figure shows tensimetric plots for the ammonium chloride-ammonia and the tin(1v) chloride-ammonia system. From Fig. (a) it is clear that ammonium chloride forms a triammoniate with a character- istic dissociation pressure of 91 mm.at -36" and the similar univariant portion in Fig. (b) shows that ammonium chloride is present in the product forrned by the reaction of tin(rv) chloride with ammonia. The length of this univariant step is a direct measure of the amount of ammonium chloride that is present. * Schwarz and Jeanmaire Ber. 1932 65 1443. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 25 1 I I I t3Oo b 1 I I I I I -200 1 I I I r -100 ,.+d 1,l g13 15 17 19 NH3- SnCL4 Tensimetric plots (-36") for the systems formed by ammonia with (aj ammonium chloride and (b) tin(Iv) chloride. TABLE 2. Tensimetric studies on tin(iv) halide. Halide SnCI Temp. -63" -45 and -36 - 36 - 36 -45 and -36 - 36 - 36 No. of mol. of NH,X forming ammoniates 2.6 1.0 2.8 1.2 3.0 5-0 2.0 At -63" the tin@) chloride-ammonia system shows the presence of almost three mol.of ammonium chloride in agreement with the reaction but at somewhat higher temperatures only one mol. of ammonium chloride can be detected. Now the degree of ammonolysis at the higher temperatures must be at least as great as that at -63" so that the "missing" two mol. of ammonium chloride must for some reason be incapable of forming the characteristic ammoniates. The explanation is that at higher temperatures the ammonium chloride is more soluble in the excess of ammonia and the more concentrated solution reacts with the ammono- basic tin(1v) chloride to give an anionic complex Three mol. of ammonium bromide are detected in the tin(1v) bromide- ammonia system at -45" and -36" however indicating that although SnCI + 6NH3 + SnCI(NH,) + 3NH,CI SnCI(NH,) + 2NH,CI -f (NHJ,[SnCI,(NH,)3] 26 QUARTERLY REVIEWS complex formation may occur (giving the soluble product in washing experiments) the complex breaks down when it is precipitated as a solid upon removal of the excess of liquid ammonia.This agrees with the decrease in stability of complex hexahalogenostannate ions [SnX,I2- in the order I > Br > C1. Only two mol. of ammonium iodide are however found in the tin(1v) iodide-ammonia system and this must be because ammonolysis of tin(~v) iodide is less complete than that of the bromide and chloride. (We have seen that complex formation is ruled out since complexes formed by the bromide are unstable under tensimetric conditions.) The third stage of the ammonolysis of the iodide Snl,(NH,) + 2NH3 -+ Snl(NH,) + NH,I is completed when ammonium iodide is removed by washing but the high concentration of ammonium iodide (saturated solutions) suppresses the ammonolysis in the tensimetric experiments.Lead(1v) chloride is reported to be resistant to ammonolysis although no detailed study appears to have been made. While the lack of ammono- lysis is not too surprising it is in marked contrast to the behaviour of diammonium hexachloroplumbate(Iv) (NHJaPbC1, which is extensively ammonolysed. The explosive nature of the product precludes more rigorous examination but the ratio Pb C1 N appears to be in the region of 1 1 1. The original workers obtained figures of 5 6 4 and proposed a cyclic constitution to account for the instability of the product but it seems more likely to be a polymeric mixture which owes its explosive properties to the presence of unstable Pb-N bonds.(d) Halides of Transition Metals in Their Higher Valency States.-We limit our review of the ammonolysis of these halides to the elements of the titanium vanadium and chromium subgroups as much more work has been done recently in this field. By and large the investigations fall into two categories those in which the extent of ammonolysis has been studied by tensimetric methods and those in which the halides have been allowed to react with liquid ammonia at various temperatures and the products then washed with ammonia to remove the ammonium halide; the latter technique is not always possible since in one or two cases [e.g. niobium@) chloride] the products are completely soluble in ammonia.Table 3 summarises the results obtained by the two methods for many halides and for a number of complex anionic halides of the elements. In their ammonolytic behaviour the titanium(1v) halides resemble the halides of quadrivalent tin rather than those of germanium in that only three titanium-halogen bonds can be ammonolysed even at room tempera- ture. Contrary to the view sometimes expressed the amide Ti(NH& is not prepared by ammonolysis of the titanium(1v) halides although it is formed when potassium amide is added. The ammonobasic titanium(rv) chloride, FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 27 TABLE 3. Ammonolysis of halides and complex halides of the Group IVA VA and VIA transition elements in their higher valency states.Halide Solubility Insoluble product (Reaction (% of metal) at -33-5" then washed with with excess of ammonia) TiCl (NH&TiCI Tar (NH,),TiBr TiI K2TiCI6 ZrC1 (NH&ZrC16 Rb,ZrC16 Cs,ZrCI ZrBr ThCl ThBr ThI4 VF5 VCl NbF NbCI TaCl MoCI wF6 wa * Reaction at 25". ammonia 5 90 70 70 100 100 10 70 20 5 - - - - 0 50 0 100 100 60 15 0 90 TiCl(NH& TiCl(NHd + TiCl 2(N H2) TiCl(NH2) +TiCl,(NH& +KCI TiBr(NH,) +TiBr2(NH2) - ZrCI,(NH,) ZrCl,(NH2) ZrCl,(NH& +RbCl ZrC13(NH,) + CsCl ZrBr,(NH,) + ZrBr2(NH2),* - Th14,7NH3 VCl(NH&,+ VC1,(NH2) NbF5,2NH VFd,NH,+NH,F - TAmmoniates formed. Mol. ratio Ref. NH,X/M detected tensimet- rically 2.7 4.0 2.0 2.7 4.0 2.5 1.0 3.0 1.0 1.0 ot ot - - - 2.0 2.0 2-0 2.0 - - - <2-7 25 26 26 27 26 27 28 29 29 29 30 28 31 32 33 34 33 35 36 37 38 39 - 25 Fowles and Pollard J.1953 2588. 26 Fowles and Nicholls J. 1961 95. 27 Fowles and Nicholls J. 1959 990. 28 Fowles and Pollard J. 1953 4128; Drake and Fowles J . Less-Common Metals 2 9 Drake and Fowles J. Less-Common Metals 1961 3 149. 30 Bowerman and Fernelius J . Amer. Chem. SOC. 1939 61 121. 31 Young J. Amer. Chem. SOC. 1935 57 997. 32 Watt Soward$ and Malhotra J . Amer. Chem. Soc. 1957 79,4908. 33 Cave11 and Clark J. Inorg. Nuclear Chem. 1961 17 257. 34 Fowles and Nicholls J. 1958 1687. 35 Fowles and Pollard J . 1952 4938. 36 Moureu and Hamblett J. Amer. Chem. SOC. 1937 59 33. 37 Edwards and Fowles J. Less-Common Metals 1961 3 181. 38 Clark and Ernel&us J. 1957 4778. 33 Fowles and Osborne J. 1959 2275. 1960,2,401. 28 QUARTERLY REVIEWS TiCl(NHh, is isolated from the reaction mixture by washing with ammonia although the bromide is still somewhat contaminated with TiBr,(NH,),.With all three halides the tensimetric studies detect rather less than three mol. of ammonium halide indicating that the third stage of the ammonolysis is somewhat suppressed by ammonium halide the effect being greatest with the iodide. The marked increase in solubility of the ammonolysis products along the series TiCl,<TiBr,<TiI may indicate a decreasing degree of polymerisation while the modest solubility of the ammonobasic titanium(1v) chloride in a concentrated solution of am- monium chloride shows that the polymers can be broken down to some extent. Experiments with anion-exchange resins prove that in the iodide solution at any rate all the titanium is present in anionic form so that similar complex anions are probably present in the solutions formed by the other halides.These complexes are stable only in solution and break down to the ammonolytic mixture on removal of the excess of ammonia. Hexachloro- and hexabromo-titanates with liquid ammonia give products that are much more soluble than those formed by the tetrahalides. It is tempting to consider these complexes as though they react as a mixture of the tetra- halide and ammonium halide. e.g. NHS (NH,),TiCI -+ 2NH4CI + TiCI -+ Ammonolytic mixture and then to explain the increased solubility and decreased ammonolysis as resulting from the presence of the extra ammonium halide; but if the potassium salt behaved like this the liberated potassium chloride would have no marked effect on the ammonolysis of the titanium(1v) chloride since it has only a slight solubility in ammonia.A more convincing explanation requires the direct ammonolysis of the anion followed by the precipitation of potassium chloride when the potassium salt reacts i.e. K,TiCI -+ K,[TiCI,(NH,),] + 2NH,CI -f (NH4)2[TiC14(NH,)21 + 2KCI I Removal of .J excess NH TiCI2(NH,) + 2NH4CI Thus virtually all the titanium is found in solution. The chloro-complexes are the least soluble in ammonia so that small amounts remain un- dissolved in the first washing; on removal of ammonia these complexes break down and the usual ammonobasic titanium(1v) chloride is formed on further treatment with ammonia. Zirconium(1v) halides are less extensively ammonolysed and zirco- nium(~~) chloride for instance forms ZrCl,(NH,) when treated with liquid ammonia at -33.5"; the product is again insoluble in ammonia in the absence of ammonium chloride and probably is polymeric in nature.FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 29 It has been observed that when the compound is heated to 100" in vacuo and treated again with ammonia further ammonolysis takes place such that the Zr:C1 ratio may be lowered to 1.0:2.5; this indicates that the heating tends to break down the polymer so that part of it may be further ammonolysed. The diammonium hexachlorozirconate(1v) behaves as might be expected three mol. of ammonium chloride being detected tensimetrically. The products formed from the rubidium and casium salts are not very soluble presumably because the original complexes themselves are almost insoluble.The ammonolysis of zirconium(1v) bromide was studied at 25" by Bowerman and Fernelius who obtained an insoluble product which they formulated as 3Zr(NH),,7NH4Br,5NH,; it seems more likely that this product is either a mixture of ZrBr,(NH,),xNH and ZrBr,(NH,),,yNH, or a polymer with an average composition corres- ponding to Zr Br = 1.0 2.3. At room temperature one might expect the second Zr-Br bond to be partly ammonolysed. None of the thorium(1v) halides shows any evidence of ammonolysis in liquid ammonia although all form well-defined ammoniates. 3. Reactions of ammono-bases In view of its relatively high solubility in ammonia potassium amide is the most extensively used ammono-base. Sodium amide is practically insoluble but can be used as a suspension or when only a low concentra- tion of amide ions is required.These amides are readily prepared in situ by catalytic decomposition of their alkali-metal solutions and their reactions in ammonia show striking resemblances to the reactions of alkali-metal hydroxides in aqueous systems. Thus simple neutralisation reactions occur KNH + NH,CI 3 KCI + 2NH and potassium permanganate (which forms a violet solution in ammonia) gives a green precipitate of the manganate K,Mn04 when treated with potassium amide ; this precipitate redissolves to a violet solution upon treatment with ammonium Ammonolytic reactions proceed much further in basic solutions than in ammonia alone since by removal of ammonium ions ammonolytic equilibria e.g. MX,(NH,) + 2NH3 + MX(NH,) + NH,X are disturbed and many compounds which show practically no ammono- lysis in ammonia alone are ammonolysed in basic solutions.Thus while carbon@) iodide is not ammonolysed in liquid ammonia at low temperatures the addition of potassium amide leads to the formation of iodoform and iodoamine :41 C14,2NH + NH 3 HCI + INH + 2NH 40 Inone Takamoto and Kurokawa Nippon Kaguku Zasshi 1957 78 274. 41 Watt McBride and Sowards J. Amer. Chem. SOC. 1956 78 1562. 30 QUARTERLY REVIEWS The reaction is complicated by the secondary reactions INH + KNHZ + N2H4 + KI 2CI + N,H4 + 2NH3 -+ N + 2NH.J + 2HCI,. The decomposition products of chloramine in liquid ammonia depend upon the basicity of the medium.42 In acidic solutions the formation of hydrazine by the ammonolysis is suppressed whereas in solutions of potassium amide potassium chloride is precipitated and hydrazine produced.Again nitrogen tri-iodide merely forms an ammoniate in liquid ammonia but ammonolysis occurs when potassium amide is added :43 NH,CI + 2NH + NH2-NH + NH4Cl N13 + 3KNH -f 3KNHI + NH (Fast) 3KNHI 3 3KI + N + NH (Slow) Franklin has given an account4* of the reactions of potassium amide with the salts of many non-transition metals which lead to the formation of amides imides or nitrides e.g. AgNO 3 AgNH Pbl + PbNH Hg12 * Hg3N2 Of late interest has centred around the polymeric transition-metal amides which have been prepared from ammonobasic compounds by further solvolysis or by metathetical reactions of the more ionic salts with potassium amide. (a) Reactions of Ammonobasic Compounds with Alkali-metal Ar~ides.~~- The extent to which metal halides are ammonolysed may be greatly in- creased by the addition of potassium amide and in some instances complete ammonolysis can thus be effected.The resulting amides are inclined to lose ammonia and form the corresponding imides [cf. B(NH&]. The halides that are completely ammonolysed are those of the transition metals in their higher valency (i.e. more acidic) states so that the amides formed are invariably amphoteric and react with either potassium or potassium amide to give potassium salts; consequently it is very difficult to isolate these amides in an uncombined state. Table 4 summarises the products obtained by the reaction of the halides of a number of transition metals. It can be seen that potassium salts are formed with the halides of quadri- valent titanium and zirconium and of quinquevalent tantalum molyb- denum and tungsten.42 Jander 2. anorg. Chem. 1955,280,264. 43 Jander and Schmid 2. anorg. Chern. 1957,292 178. 44 Franklin “The Nitrogen Systems of Compounds,” Reinhold Publ. Jnc. New York 45 Levine and Fernelius Chem. Rev. 1954,54,452. 1935. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 31 Since cerium(rI1) iodide and thorium(1v) bromide are considerably less acidic they are incompletely ammonolysed. The product formed by cerium(rr1) iodide was originally considered to be CeI,,Ce(NH,), 10NH3 but it is more likely to be an ammonobasic cerium(Ir1) iodide polymer [cf. SnCl(NH,),] in which the Ce I ratio averages 2 3. For simplicity this is represented in Table 4 as a mixture; in the same way most of the other products listed in this Table are likely to contain polymeric anions.TABLE 4. Reaction of trmsitional-metal halides with potassium arnide- liquid ammonia solutions. Halide CeI TiBr ZrBr ThBr TaBr MoCl wBr5 Product of reaction with potassium amide CeI(NH2),+CeI2(NH2) Ti( NH)( NK) Zr( NK) ThBr,(NH,) Ta(NH&(NHK),+ Ta(NH,),(NHK) W(NH2X NK) Mo(NH,)(:NK) Ref. 46 47 30 48 45 49 49 (b) Reactions of Ammono-bases with Transition-metal Salts.-Where the metal halides are not ammonolysed the metal amides are more conveniently prepared by metathetical reactions of the more soluble nitrates or thio- cyanates with potassium amide. Schmitz-Dumont and his associates have in this way succeeded in preparing a large number of transition-metal amides. The mechanism involved in the formation of these amides from metal salts is comparable to that encountered in the formation of metal hydrox- ides in water.Equilibria of the type [M(NH,),]”+ + NH + [M(NH3)5(NHz)](E-1)+ + NH,+ may exist and in the presence of amide ions the ammonium-ion concen- tration is reduced and ammonolysis is favoured. A process analogous to olation can then occur so that ultimately polymeric metal amide ammoniates are produced; usually these lose their ammonia quite readily in vacuo. Thus hexammine- chromium(II1) nitrate reacts50 with potassium amide to give the bright red amide [Cr(NH,),](NO,) + 3KNH -+ 3KN0 + 6NH + Cr(NH,) 46 Bergstrom J. Amer. Chem. SOC. 1937,59 1374. 4i Franklin and Hine J. Amer. Chem. Soc. 1912 34 1497. 48 Watt and Malhotra J . Inorg. Nuclear Chem.1959 11 255. 49 Bergstrom J. Amer. Chem. SOC. 1925,47 2317. Schmitz-Dumont Pilzecker and Pepenbrink 2. anorg. Chenz. 1941 248 175. 32 QUARTERLY REVIEWS With an excess of potassium amide some of the bridging bonds are broken and a potassium salt of a polymeric anion is obtained { Cr(NH2)3 I n + KNH2 -f (K[Cr(NH2)41>~ The salt produced from the corresponding cobalt(1rr) amide51 appears to be less polymeric and accordingly soluble in liquid ammonia H,N\ /NH,\ 7 2 2 [$0(NH2)J *+ 3 n KNH n K H N-CO-NH 43-NH2 [H2N ’ \NH,’ ‘NH When the solution is evaporated however de-ammoniation results in polymerisation and thermal decomposition of the product gives a nitrido- salt Bergsfroms2 used the anhydrous thiocyanates as starting materials for the preparation of the amides of bivalent manganese and cobalt and more recently complex thiocyanates have been used of metals that do not form simple thiocyanates.Thus the amides of tervalent titaniums3 and vana- d i ~ m ~ ~ can be prepared K,M(SCN) + 3KNH2 -+ M(NH2) + 6KSCN both amides react with an excess of potassium amide to give the imide salt K[M(:NH),]. The same vanadium compounds are formed by reaction between a solution of potassium in ammonia and vanadium(rr1) bromide.55 Because of their solubility in ammonia complex nitrates may also be useful starting materials for the preparation of amides. Thus the unstable amide Th(NH2)4 of quadrivalent thorium may be prepared56 by treating the anhydrous complex nitrate K,Th(NO,), with four equivalents of potassium amide. The amide decomposes spontaneously { K3[Co2(NH)3(NH2)31 I n + K3n[(Co2Ns)nI f 3nNH3 Th(NH,) -f Th(:NH)(NH,) + NH3 and forms a variety of complex imidoamide salts with an excess of potas- sium amide the composition depending on the amount of potassium amide used; e.g.with a 6-fold excess K[Th(:NH\(NH&,] is believed to be formed. The reaction of potassium amide with halides of the Group VIII metals has been studied extensively by Watt and his Cationic nickel(r1) 51 Schmitz-Dumont and Kron 2. anorg. Chem. 1955,280 180. 52 Bergstrom J. Amer. Chem. SOC. 1924,46 1552 2631. 53 Schmitz-Dumont Simons and Broja Z. anorg. Chem. 1949,258 308. 54 Schmitz-Dumont and Broja 2. anorg. Chem. 1948,255 299. 55 Nicholls J. Inorg. Nuclear Chem. in the press. 50 Schmitz-Dumont and Raabe 2. anorg. Chem. 1954 277 297. 57 Watt and Dawes J. Amer.Chem. Soc. 1959 81 8; Watt Walling and Mayfield ibid. 1953 75 6175; Watt and Davies ibid. 1948 70 3753; Watt Choppin and Hall J. Electrochem. SOC. 1954 101 235. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 33 salts react at -33" to give nickel(r1) amide diammoniate and this amide yields elemental nickel when thermally decomposed in vacuo at 585". 'Tetrammineplatinum(n) bromide yields platinum(I1) amide diammoniate This amide also gives the metal when thermally decomposed [Pt(NH,),]Br + ZKNH -+ Pt(NH2),,2NH -l- 2KBr + 2NH 3Pt(NH2),,2NH -+ 3Pt + lONH + N With bis(ethylenediamine)platinum(Ir) iodide potassium amide causes replacement of the iodine atoms according to the equations [Pt en,]l + KNH -+ [Pt( en-H) en11 + KI + NH [Pt en,] I + ZKNH + Pt( en-H) + 2KI + ZNH [where (en-H) represents the abstraction of a proton from a nitrogen atom bonded to the central platinum atom].With an excess of potassium amide the potassium salt K[Pt(en-H)(en-2H)] is formed; the addition of am- monium halides to these compounds regenerates the original complex Pt( en-H) + 2NH,X -f [Pt en,]X + 2NH The titration of iridium(r1I) bromide with potassium amide provides evidence for the stepwise replacement of bromine atoms by amide groups followed by conversion of the resultant yellow iridium(II1) amide into a red-brown solution of potassium hexa-amidoiridate(II1). The reactions (co-ordinated ammonia omitted) are IrBr + KNH + IrBr,NH + KBr IrBr,NH + KNH + IrBr(NH,) + KBr IrBr(NH,) + KNH -f Ir(NH& + KBr lr(NH2)3 + 3KNH2 -+ K,[lr(NH,),] Bromopentamminerhodium(Ir1) bromide reacts similarly with potassium amide but only as far as the formation of rhodium(1u) amide.(c) Reactions of Ammono-acids with Transition-metal hides.-We have seen that ammonobasic metal halides are almost invariably soluble in liquid ammonia solutions of ammonium halides and this solubility has been attributed to the formation of soluble complex anionic species of the metal e.g. TiI(NH,) + 2NHJ -f (NH4),[TiI,(NH,),] With the amides (or potassium derivatives) of those metals whose halides are ammonolysed in ammonia we should expect dissolution to occur upon treatment with ammonium halide solutions (provided that the amides are not resistant to attack through polymerisation) e.g. NH,CI NH,CI NH,CI Mo(NH,)(:NK) Mo(NHJ5 [Mo(NH,)~CI]- -6 [Mo(NH,),CI,]- KNH NHZ- N H,- Where the metal halide or salt is not ammonolysed in ammonia it is often 2 34 QUARTERLY REVIEWS possible to regenerate this compound by treatment of the amide with the corresponding ammonium salt e.g.Ni(NH,),,2NH3 + 2NH,I -+ Nil,,6NH3 Even with the polymeric diamidothorium(1v) imide { HN Th(NH,) In all the existing bridging bonds are broken upon treatment with ammonium iodide in ammonia and colourless crystals of thorium(rv) iodide octa- ammoniate ThI,,8NH3 are obtained. Low-temperature treatment of the polymeric chromium(rI1) and cobalt(rI1) amides with ammonium salts results in dissolution of the amides. Schmitz-Dumont suggests50 that the residues remaining after removal of ammonia from these solutions contain high polymers such as { [Cr(NHzi) 3(NH 2)BrIBr I n and { [Co(NH3) 3(NH 2) 2 IN03 >n - 4.Reactions of alkali-metal solutions (a) Reduction and Effect on Ammono1ysis.-Solutions of the alkali and alkaline-earth metals in liquid ammonia have attracted widespread interest not only in the physical nature of the solutions but also in the use of the solutions as reducing agents. Early work utilising alkali- metal solutions as reducing agents for inorganic substances is reviewed el~ewhere.~ Attention in recent years has been largely devoted to the production of unusual valency states of metals and to the synthetic uses of the solutions particularly in the field of organic and organometallic chemistry. With many metal salts reduction by an alkali metal in ammonia yields the elemental metal. Such reductions are usually complicated however since the metal formed is in a finely divided and highly reactive state.Thus many metals e.g. zinc cadmium mercury tin and lead form poly-ide salts with an excess of alkali metal and others catalyse the decomposition of the alkali-metal solution i.e.; K + NH3 + KNH + iH2 so that the final products of the reduction may contain the metal amide; e.g. with nickel salts58 there are the reactions NiBr + 2K -f Ni + 2KBr 2K + 2NH3 + ZKNH + H ZKNH + NiBr + 2NH3 -+ Ni(NH2),,2NH + 2KBr The catalytic breakdown of the alkali-metal solutions to the amides may however occur without permanent reduction of the transition metal and 58 Burgess and Eastes J. Amer. Chem. SOC. 1941 63. 2674; Watt and Davies ibid. 1948,70 3753. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 35 many halides give the same products.with alkali-metal solutions as they do with solutions of alkali-metal amides e.g. N H3 K-NHs ZrBr -+ ZrBr,(NH,) -+ Zr(:NK) NH3 K-N H VBr -+ VBr,(NH,) -2 V(:NH)(NHK) and hydrogen gas is liberated. We have already seen that neither boron(n1) fluoride nor the aluminium(rI1) halides are ammonolysed unless an alkali metal is present and it is interesting that while only one B-F bond is ammonolysed on addition of potassium both sodium and lithium effect greater ammonolysis namely K-NHS BF - -+ BF,(NH2) Na-NH Li-NHa 4 ( N H .J2 B e N H * B F( N H ,) -+ ( N H,),B* N H * B( N H) Sometimes a difference in reactivity of the various alkali-metal solutions may be attributed to the difference in the solubilities of the alkali-metal amides (LiNH and NaNH almost insoluble; KNH soluble).Thus tetramethyl-lead reacts with alkali metals in ammonia to give [Pb(CH,),]- ions and methyl radicals (which yield methane and ethane) :69 Pb(CH,) + e- -+ [Pb(CH,),]- + CH3. In the presence of potassium solvolysis of the [Pb(CH,),]- ions occurs giving lead imide as the final product [Pb(CH,),]- + NH3 -+ [Pb(CHJ,NH,]- + CH [Pb(CH,),NH,]- + NH3 -f [Pb(CH3)(NH,)z]- + CHI [Pb(CH,)(NH,),]- + NH3 + [Pb(NH&]- + CHI [Pb(NH,),]- + PbNH + NH + [NH,]- With lithium and sodium however the insolubility of the amides (at -78”) retards the ammonolysis through the reaction [Pb(CH,),NH,]- + Na+ -f Pb(CH,) + NaNH so that only two methyl groups are removed and the product is dimethyl- lead. With the tetra-alkylammonium salts R,NX the N-C bond is ruptured by potassium to give a trialkylamine and a hydrocarbon; e.g.for (CH,),NCl “(CH,),]+ + e -+ N(CH,) + CH,. [CH& + NH + CH + “41- CH,. + e -+ [CH,]- 69 Holliday and Pass J. 1958 3485. Hazlehurst Holliday and Pass J. 1956 4653. 36 QUARTERLY REVIEWS The amide ion produced yields an olefin if R contains a /3-carbon atom e.g. [N(C2H!5)41+ + lNH21- -f N(C2H5)3 f C2H4 f NH3 [(CH3*CHz*CH2)3NCH3]+ + [NHz]- -+ (CH,CH,.CH2),N*CH3 + C,H6 -t NH Evolution of hydrogen during an alkali-metal reduction was used by earlier workers to infer the presence of an ammonium ion. Thus the “diammoniate” of diborane was formulated61 as an ammonium salt (NH,) [H,B(NH,)BH,] on account of its reaction with a sodium solution NH (NH,)[H,B(NH,)BH,] + Na -+ Na[H,B(NH2)BH,l + 3H2 + NH More recent work by Parry and his co-workerss2 has shown the compound to be a borohydride [NH3(BH2)NH3]BH, whose reaction with sodium may be written [NH,(BH,)NH,]BH + Na -+ NaBH1 + BH2NH2 + 4H2 + NH N H* Trimethylboron gives hydrogen when treated with a solution of sodium or potassium and the salt M[B(CH,),NH,] is formed? the trimethylboron can be regenerated by treatment with a solution of ammonium bromide.Tetramethyldiborane is split symmetrically into BH(CH3),,NH3 and the salt Na,BH(CH,) when treated with an excess of sodium in ammonia at -78°,64 and an analogous calcium salt CaBH(CH,),,NH can be piepared. The [BH(CH,),]- ion in the sodium salt shows the high base strength expected of a boron atom with a lone pair of electrons in that it forms a stable compound Na [BH(CH,),],B(CH,), with trimethylboron; this compound appears to contain B-B links.Conductimetric studies of the reactions of alkali-metal solutions with a number of covalent h y d r i d e ~ ~ ~ have shown that disubstitution by alkali metals can occur in SnH, ASH, SbH, and GeH, but not in PH,. Thus disodiostannane SnH,Na, may be isolated after reaction of stannane with a ten-fold excess of sodium; in the presence of excess of the hydride however conversion into the monosubstituted derivative occurs SnH,Na -t SnH + 2SnH,Na The analogous germanium compound NaGeH, reacts with phenyl bromide to give germanium(1r) hydride NaGeH + C6H,Br -+ C6H6 + GeH + NaBr 61 Schlesinger and Burg J. Amer. Chem. SOC. 1938 60 290. 62 Schultz and Parry J . Amer. Chem. SOC. 1958 80 4. 64 Burg and Campbell J. Amer. Chem. SOC. 1952,74 3744; 1957,79,4023. 65 EmelCus and Kettle J.1958 2445; Emeleus and Mackay J. 1961,2676. Smith and Kraus J . Amer. Chem. SOC. 1951 73 2751; Holliday and Thompson J. 1960 2695. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 37 this hydride may be recrystallised from liquid ammonia and it reacts with one atom of sodium to form a soluble red salt which gives germane GeH, on treatment with ammonium bromide. Alkylgennanes with sodium give the monosubstituted salts,66 e.g. NaEtGeH,. (b) The Production of Unusual Valency States.-Whereas simple nickel@) salts are reduced to the metal with alkali metal-ammonia solutions the complex nickel(I1) cyanidess7 yield products in which the metal appears to exhibit a valency of less than two. Thus when an excess of potassium tetracyanonickelate(I1) is treated with sodium or potassium in ammonia a bright red precipitate is formed of composition K,[Ni(CN),] [Ni(CN),I2- + e -+ [Ni(CN),la- + CN- If an excess of alkali metal is used in the reduction a bulky yellow precipi- tate of the tetracyanonickelate(0) separates [Ni(CN),I2- + 2e -+ [Ni(CN),I4- The potassium salt can be isolated as a copper-coloured solid extremely unstable in air and blackening on exposure.It dissolves in water evolving hydrogen and giving a red solution with the characteristic properties of the tricyanonickelate(1) anion. The tetracyanonickelate(0) ion [Ni(CN414- is of course isoelectronic with the carbonyl Ni(CO),. The corresponding zero-valent palladium ion,sa [Pd(CN),I4- may be made by the reduction of [ Pd(CN),I2- salts. In liquid ammonia both of the zero-valent salts reduce azoben2ene to hydrazobenzene and silver and mercury(I1) salts to the metals.The analogous cobalt salt may be prepared69 by reduction of potassium hexacyanocobaltate(n1) K,[Co(CN),] + 3K+ + 3e + K,[Co(CN),] + 2KCN although potentiometric titrations the intermediate formation of a cobalt(I1 complex K3 [Co(CN),]. The reduction of the hexacyano- chromate(II1) yields a product containing chromium(I) while the hexacyanomanganate(Ir1) ion appears to give a product containing both the uni- and the zero-valent state of manganese 2K,[Mn(CN),] + 5K+ + 5e + K,[Mn(CN),],K,[Mn(CN),],2NH This formulation of the product is consistent with the amount of potassium used in the reduction its reducing action towards silver nitrate and chemical analysis. 66 Glarum and Kraus J . Amer. Chem. SOC.1950,72,5398. 67 Eastes and Burgess J . Amer. Chem. SOC. 1942,64,1187. Ga Burbage and Fernelius J. Amer. Chem. SOC. 1943 65 1484. 6 9 Hieber and Bartenstein Nuturwiss. 1952,39 300; 2. unorg. Chem. 1954,276 12. 70 Watt Hall Choppin and Gentile J. Amsr. Chem. SOC. 1954 76 373; Watt and 71 Davidson and Kleinberg J. Phys. Chem. 1953 57 571. Thompson J. Inorg. Nuclear Chem. 1959,9 31 1. 3s QUARTERLY REVIEWS Some unusual ammine complexes of platinum and iridium have been prepared by Watt and his co-~orkers.~~ When tetrammineplatinum(I1) bromide is reduced with potassium in ammonia at the boiling point exactly two gram-atoms of potassium are consumed per mole of the bromide and a yellowish-white solid is precipitated. Thermal decomposi- tion of this solid yields only platinum and ammonia and the solid appears to be tetrammineplatinum(0) [Pt(NH,),]Br + 2K+ + 2e -+ Pt(NH,) + 2KBr Diamagnetic pentammineiridium(0) is similarly ~btained’~ when bromo- pentammineiridium(rr1) bromide is reduced with potassium in boiling ammonia [Ir(NH,) BrIBr + 3K+ + 3e -+ lr(NH3)5 + 3KBr The stability of these zero-valent complex ammines is unusual in that the ligands are incapable of forming 7.r-bonds with the metal.A co-ordination compound with a bidentate nitrogen ligand that may contain platinum(0) has been as an unstable pink precipitate [identifiable as bisethylenediamineplatinum(0) only by its thermal de- composition products] when two equivalents of potassium react with bisethylenediamineplatinum(r1) iodide. At room temperature it de- composes by two paths Pt -/- 2en t Pt en -+ Pt( en-H) + H When only one equivalent of potassium is used in the reduction of the iodide an unstable compound [Pt en,]I is formed which disproportionates at room temperature 2 Pt en,l -+ Pt en + [Pt en,]& 5.Recent synthetical reactions So far we have discussed reactions involving ammonia reactions of alkali-metal amides and reductions with alkali-metal solutions. There remains the large field of reactions in which ammonia acts merely as a solvent medium. The advantages of liquid ammonia as a solvent medium for synthetic procedures are by now well known. The solubility relation- ships shown by ammonia especially those which show different trends from those found in water are often most valuable. The smaller tendency of ammonia than of water to solvolysis and the greater stability of products at the lower temperatures employed often permit the study and isolation of compounds which cannot be prepared or which can be obtained only with difficulty from other solvents.Many examples of metathetical reactions are already reviewed74 and we shall mention only those syntheses 7 2 Watt and Mayfield J. Amer. Chenz. SOC. 1953 75 6178. 73 Watt McCarley and Dawes J. Amer. Chem. SOC. 1957 79 5163; 1959 81 8. 74 Audrieth and Kleinberg “Non-aqueous Solvents,” J. Wiley & Sons Inc. New York 1953. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 39 which have been used in the last decade. These are conveniently divided into two sections (a) inorganic and (b) organo-metallic syntheses. (a) Inorganic Syntheses.-One interesting preparation is that of am- monium nitrite,75 which is obtained when sodium nitrite and ammonium chloride are mixed in ammonia at -33.5".Phosphine reacts rapidly with alkali-metal solutions in ammonia to give the salts MIPH, which are valuable for the synthesis of a variety of phosphine derivatives. Treatment of a solution of the sodium salt (NaPH,) with methyl chloride results in precipitation of sodium chloride and the formation of methylph~sphine~~ which can be obtained in good yield from the filtrate CH3CI + NaPH -+ NaCl + CH3.PH2 When trimethylene dichloride is used in place of methyl chloride tri- methylenediphosphine and polymeric alkylphosphines of formula PH,-CH(CH,)CH,.[PH.CH(CH3)CH,];PH2 are ~btained.~' Heavy- metal hydrogen phosphides have been prepared by metathesis of KPH and an ammonia-soluble metal salt .'* From hexamminecobalt(~rr) nitrate a dark brown pyrophoric phosphine Co(PH,), is obtained [Co(NH,),](NO,) + 3KPH2 + Co(PH2)S + 3KNO3 + 6NH3 This compound decomposes spontaneously at O" to leave a black residue of Co(PH), and from this a crystalline phosphide COP,., is formed at 560".With an excess of KPH and the hexammine nitrate the insoluble amorphous salt KCo2(PH2) is obtained. Nickel(1r) thiocyanate gives Ni(PH,), which is unstable at 0" and reacts with an excess of KPH to give a soluble complex which is probably Kz[Ni(PH,),]. Peroxides of magnesium cadmium and zinc have been isolated for the first time79 by treatment of the metal nitrates with sodium or potassium superoxides (which are sparingly soluble and insoluble respectively in ammonia). The unstable superoxide intermediate dissociates to give the peroxide Zn(NO,) + 2K0 -f Zn(02) + 2KN0 Zn(O2)2 -f ZnO2 + 0 2 Strontium nitrate gives strontium peroxide but reaction of barium nitrate with four equivalents of potassium superoxide gives a compound whose analysis suggests the stoicheiometry :*O Ba(NO,) + 4K0 -+ K,Ba(O,),O -+- 2KN0 + 0 This is the only case found of mixed superoxide-peroxide formation; it is i 5 Larbouillat-Linemann Compt.rend. 1954 238 902. i 6 Wagner and Burg J. Amer. Chem. SOC. 1953,75 3869. '' LefHer Groch and Teach Abs. Papers Amer. Chem. SOC. meeting Sept. 1959. '* Schmitz-Dumont Nagel and Schaal Angew. Chem. 1958,70,105. i Q Schechter and Kleinberg J. Amer. Chem. SOC. 1954,76 3297. * O Seyb and Kleinberg J. Amer. Chem. SOC. 1951,73 2308. 40 QUARTERLY REVIEWS noteworthy that in this compound the cations have almost identical ionic radii K+ 1-33 A; Ba2+ 1.35 A.The reactions of solutions of alkali-metal borohydrides in ammonia have been used to afford,s1 by metathesis borohydrides such as (b) Organo-metallic Syntheses.-(i) Coupling reactions. In early synthetic work on organometallic compounds Kraus made extensive use of liquid ammonia and alkali metal-ammonia solutions. The reactions of organometallic compounds with alkali-metal solutions were reviewed in 1950,82 so that coupling reactions of the type R,MNa + XR' -+ R,MR' + NaX (R R' = alkyl or aryl X = halogen) need not be considered here in detail; as general illustrations of this type of reaction two examples may be cited 83984 [Cr(NM,),I [BH413 and [Mg(NH3),1 EBH412. 2Ph,GeNa + Br.[CH,],-Br -+ Ph,Ge.[CH,],.GePh + 2NaBr Et,PbCI + 2Li + PhCH,CI -+ Et,PbCH,Ph + 2LiCI When the ainmoniate of trimethyl-lead borohydride is distilled at -5 O a most interesting reaction occurs,s5 in which trimethylplumbane is ob- tained with elimination of ammonia-borine N Ha Me,PbCI + KBH4 -+ [Me,Pb]BH,,xNH -33" -5" -+ Me,PbH + H,B,NH + (x-l)NH This hydride reacts rapidly with ammonia at -78" to form ammonium trimethylplumbate and addition of trimethyl-lead chloride then gives hexame thyldiplumbane Me,PbH + NH -+ NH,[PbMe,] NH,[PbMe,] + Me,PbCI -+ (Me,Pb) + NH,CI (ii) "Sandwich" compounds.Cyclopentadienyl and indenyl compounds of a number of transition metals have been prepareds6 in liquid ammonia by reactions of the type [Co(NH,),](SCN) + 2C5H5Li -+ [Co(NH,),](C,H,) + 2LiSCN The ammoniates are readily decomposed thermally in vacuo and the resulting cyclopentadienyl compounds may then be obtained by sublima- tion.The method has been used in particular for the preparation of the Parry Schultz and Girardot J. Amer. Chem. Soc. 1958 80 1. Watt Chem. Rev. 1950 46 317. Smith and Kraus J . Amer. Chem. SOC. 1952 74 1418. 84 Gilman and Leeper J. Org. Chem. 1951 16,466. 85 Duffy and Holliday J. 1961 1678. 86 Fischer and Fritz Adv. Inorg. Chem. and Radiochem. 1959 1 55. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 4 1 cyclopentadienyls of chromium(III) manganese(rr) iron(rr) cobalt(rr) and nickel(r~) and the indenyl of cobalt(r1). (iii) Acetylene derivatives. Acetylene derivatives of the main-group elements have been known for some time.When acetylene is bubbled through a solution of sodium in ammonia the monosodium derivative NaC,CH can be obtained and this is a useful starting material for liquid-ammonia preparations of metal-acetylene compounds. With the Group IV metals for example,87 those triaryl- or trialkyl-metal halides (R,MX) which are not ammonolysed in ammonia (Le. tin and lead compounds) react with sodium acetylide giving compounds of the type Ph,PbCi C-PbPh and Et,SnCi CaSnEt,. Monosubstituted products are not isolated because they readily disproportionate to the disubstituted product and acetylene R,SnCI + NaCiCH -+ R,SnCiCH + NaCl 4 R,SnCiC.SnR + C,H The R,SnCiCH intermediates do however react with an excess of the sodium salt and form complex compounds of the type R,Sn.[ Ci CH],.Na. An alternative route to the disubstituted derivatives is by the reaction 2Ph,SnNa + C21 -+ Ph,SnCiC.SnPh + 2Nal The phenyl compounds are stable to water but the ethyl compounds are more sensitive ; the ease of hydrolysis decreases with decreasing polarity of the carbon-metal bond i.e.Pb>Sn>Si. Transition-metal acetylides have recently been synthesised by Nast and his co-workers.88 Once again the soluble thiocyanates are used as starting materials [Ni(NH,),](SCN) + 4K.CiC.Ph + Vacuo K,[Ni(Ci C.Ph),],ZNH + 2KSCN + 4NH -+ K,[Ni(Ci C-Ph),] K,[Ni(Ci C.Ph),] + [Ni(NH,),](SCN) + Ni(Ci C*Ph),,4NH -4 (PhX i CeNiCi C.Ph)x Vacuo The ammonia-free complexes are rather unstable decomposing in several days at room temperature and derivatives of alkylacetylenes are prone to detonation. The diamagnetism of K,[Ni(C:C-Ph),] suggests that the nickel atom is in a square-planar environment.Like the analogous complex cyanide the acetylide K,[Ni(Ci CH),] can be reduced to the zero-valent compound K,[Ni(Ci CH),] by the action of a solution of potassium in ammonia. 87 Beerman and Hartmann Z. anorg. Chem. 1954,276 20. Nast Internat. Conference on Co-ordination Chemistry Chem. SOC. Special Publ. No. 13 1959 p. 103; Nast Z. Nuturforsch. 1953,8b 381; Nast and Pfab Chem. Ber. 1956,89,415; Nast and Vester,Z. anorg. Chem. 1955,279,146; Nast and Lewinsky ibid. 1955 282 210; Nast and Urban ibid. 1956 287 17; Nast and Pfab ibid. 1957 292 287. 42 QUARTERLY REVIEWS Analogues of hexacyanoferrate(rI1) and hexacyanoferrate(I1) complexes can be obtained from iron(r1) thiocyanate Fe(SCN),,4NH3 + 6MCiCR -f M,[Fe(CiCR),] + 2MSCN + 4NH 2K,[Fe(CiCH),] + 2NH + O2 -+ 2K,[Fe(CiCH),] + H20 + 2KNH2 The iron(II1) complex is unstable exploding above -30".The correspond- ing cobalt compounds Na,[Co(Ci CMe),] (paramagnetic) and Na [Co(C i CMe),] (diamagnetic) are obtained similarly. Reaction of copper(r) iodide with potassium acetylide in ammonia yields the orange cuprous acetylide which decomposes above -45" to black Cu,C and acetylene CUI + K'CiCH --+ Cu-CiCH -4 C U ~ C ~ + C2H2 -78" -45" Alkynylcuprates(1) of the types M,[Cu(CiCR),] and M [Cu(Ci CR),] can also be obtained e.g. when potassium methyl or phenyl acetylide is used NH Cul + 3KCiCMe -+ K,[Cu(CiCMe),],xNH + KI Vacuo - K,[Cu(Ci CMe),] (iv) Carbonyls. Reactions of alkali-metal-ammonia solutions with metal carbonyls have recently been studied in some detail by Behrens and his co-worker~.~~ With the mononuclear hexacarbonyls of Group VIA metals reduction occurs with sodium in ammonia yielding the com- pounds Na,M(CO) (M = Cr Mo or W).These can be recovered from the solution by evaporation of the solvent; they contain ammonia of crystal- lisation unless warmed to 70" in vacuo. A large excess of alkali metal decomposes them. Sodium reduction of the iron carbonyls Fe(CO), { Fe(CO) j3 Fe,(CO), and Fe(CO),I leads in each case to Na,[Fe(CO),]. At -75" almost no carbon monoxide is liberated when sodium reacts with {CO(CO),}, but at higher temperatures ammonia reacts with the carbonyl 3[CO(CO).J2 + 12NH3 -+ 2[CO(NH,),][CO(CO),]2 + 8CO this ammine is then reduced by sodium to cobalt and Na[Co(CO),] 2Na + [Co(NH,),][Co(CO),] -+ 2Na[Co(CO),] + Co + 6NH Nickel carbonyl also forms anions of the type [Ni,(C0),I2- and [Ni,(C0)J2- when reduced by solutions of alkali metals in liquid am- monia.Carbonylcyano-complexes of nickel have been isolatedgl from liquid 8g Behrens and Weber Z . anorg. Chem. 1955 281 190; 1957 291 122; Behrens Behrens and Lohofer Chem. Ber. 1961,94,1391,1497; Hieber Kroder and Zahn Z. Naturforsch. 1952,7b 321. Z . Naturforsch. 1960,15b 325. 91 Nast and Roos 2. anorg. Chem. 1953 272,242. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 43 ammonia as yellow crystals by passage of oxygen-free carbon monoxide through solutions of complex nickel cyanides at -40" K,[Ni,(CN),] + 2CO +- 2K2[Ni(CN),CO] K,[Ni(CN),] + 2CO + K,[Ni(CN),(CO),] + 2KCN The magnetic properties of K,[Ni(CN),CO] indicate that this compound is dimeric. Reaction with potassium acetylidesg2 yields yellow explosive. octaalkynyldinickelates K,[Ni,(CN),(CO),] + 8KCiCH -+ K,[Ni,(CiCH),] + 6KCN + 2CO Thanks are offered to Dr. M. Allbutt for valuable discussions. 92 Nast and Kasperl Chem. Ber. 1959 92 2135.

ISSN:0009-2681    

DOI:10.1039/QR9621600019

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

FLUOROCARBON CHEMISTRY. PART I. THE FLUORINATION OF ORGANIC! COMPOUNDS By R. STEPHENS and J. C. TATLOW (CHEMISTRY DEPARTMENT UNIVERSITY OF BIRMINGHAM) IT is now obvious that the carbon-fluorine system forms the basis of a new branch of organic chemistry paralleling the vast and better known field arising from carbon and hydrogen. The carbon-fluorine bond is a very stable one; fluorine is univalent and it has a fairly small atomic size which enables it to take the place of hydro- gen in organic chemistry. The bond of carbon with chlorine is rather less stable and the atomic size of chlorine is appreciably greater than that of fluorine so that chlorocarbon chemistry is not as comprehensive as fluorocarbon chemistry and in the latter but not the former almost complete duplication of the major sections of hydrocarbon chemistry is possible.Nearly all the most important organic functional groups can be linked to fluorocarbon chains or rings giving rise to stable series of fluorocarbon derivatives. The range of compounds capable of existence is thus enormous particularly since mixed fluorohydrocarbon species and their derivatives are usually stable and can give rise to vast numbers of isomers. The size of the fluorine atom (van der Waals radius 1.35 A; cf. hydrogen 1-2 A chlorine 1.8 A) is such that saturated fluorocarbon chains or rings e.g.9 CF,. [CF,] n.CF3 CF2-C F /cF2-cF2 \ cFz F2C 2 F2-2 F \CF,-CF can exist with little steric strain. This would not be so if the atomic size were much greater (there would then be insufficient room for all the necessary fluorine atoms on the carbon skeleton without some interactions occurring between them).The greater atomic size means however that a fluorocarbon chain is stiffer than a hydrocarbon chain and further that the fluorine atoms shield the backbone of carbon atoms against chemical attack much more effectively than hydrogen atoms are able to do. Great chemical stability is one of the chief characteristics of saturated fluoro- carbons and many of their derivatives and it is due not only to the in- trinsic stability of the carbon-fluorine bond but also to the difficulty of approach of chemical reagents to the carbon atoms. This Review and a later one will summarise the main features of fluoro- carbon chemistry treating the fluorocarbons and their simple derivatives in the same manner as that in which orthodox organic chemistry is dealt 44 STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY.PART I 45 with in general text-books. At present organic fluorine compounds are nearly always made from related hydrocarbon-type materials and in the present article the methods available for the introduction of fluorine into organic structures are described systematically. Methods for the Introduction of Fluorine into Organic Structures Only two organic fluoro-compounds have so far been found1y2 occurring naturally (potassium monofluoroacetate in the South African plant gifblaar and fluoro-oleic acid in ratsbane from West Africa). Perfluoro- propane and higher members of the series of saturated fluorocarbons were first recognised3 after synthesis from carbon and elementary fluorine. Similar reactions of carbon with halogen fluorides or with mixtures of fluorine and another halogen gave4p5 perfluorohalogenocarbons or under certain drastic conditions,'j tetrafluoroethylene (in this Review halogen means chlorine bromine or iodine).However these reactions have not so far been used widely; and most organic fluorides are made by the intro- duction ef fluorine into hydrocarbon or halogeno-hydrocarbon precursors of similar carbon skeletons. One of the reasons for the comparatively late development of fluorocarbon chemistry has been undoubtedly the relative difficulty of fluorination. Even now this is often troublesome to carry out and routes to desired compounds are usually sought via conversions of commercially available precursors containing fluorine. Though more stages may be involved such an overall process is often preferable to one involving introduction of fluorine.The methods available for forming a carbon-fluorine bond may be divided into two broad types. In Section A fluorine is introduced only at functional groups in the organic molecule by exchange or addition hydrogen normally being unattacked. The fluorinating agent used is hydrogen fluoride or an inorganic fluoride in which in general the other constituent if multivalent exerts a lower valency and can be made without the use of elementary fluorine. Section B consists of processes for exhaustive fluorination of organic compounds in which all substituents on a carbon chain functional groups and hydrogen can be replaced by fluorine and unsaturation can be removed. Though a saturated fluorocarbon is the ultimate end-product of such a reaction it is usually possible to arrange either for a functional group or for some hydrogen to be retained.The fluorinating agents are either elementary fluorine or some related species for example a halogen fluoride or a high-valency fluoride of a transition metal which can be made conveniently only with elementary fluorine. Marais Onderstepoort J. Vet. Sci. An. 1943,18,203; 1944,20,67. Peters Biochem. J. 1960 76 32 P. Simons and Block J. Amer. Chem. Soc. 1937,59 1407; 1939 61 2962. Collins Wadsworth and Leech B.P. 653,879/1951. Mantell Passino and Teeters U.S.P. 2,670,389/1954 2,684,987/1954 2,774,797/ Farlow and Muetterties U.S.P. 2,732,410/1956. 1956. 46 QUARTERLY REVIEWS Another closely related method is electrochemical fluorination i.e.electrolysis of anhydrous hydrogen fluoride containing the organic starting material; it resembles the process used for the generation of fluorine. This classification though useful is not completely rigid partly because reactions are not always carried out under comparable conditions. Thus for example under mild conditions a high-valency fluoride might give an exchange reaction typical of Section A. In fact one or two high-valency fluorides are useful exchange reagents of type A. SECTION A This section can be further sub-divided into five main types of process (i) exchange of fluorine for another halogen atom; (ii) exchange of fluorine for some other univalent functional group usually hydroxyl or a sulphon- ated hydroxyl group; (iii) exchange of fluorine for oxygen in carbonyl or carboxyl groups; (iv) additions to unsaturated linkages ; (v) exchange of fluorine for diazonium groups in aromatic and heterocyclic compounds.A(i) Exchange of Fluorine for Other Halogen Atoms.-This was one of the earliest fluorination methods alkyl fluorides being made’ from silver monofluoride and alkyl iodides in the 1880’s. It was exploited extensively by Swarts who used antimony trifluoride to make a variety of aliphatic fluorides in his pioneer researches in the field.8 It was developed further by Henneg and put into commercial operationlo for the production of refrigerants with hydrogen fluoride as the source of fluorine. Quite large quantities of organic fluorides are now rnade,l1 mostly still by this general method. Many fluorinating agents have been used but the most important are hydrogen fluoride antimony trifluoride and related systems potassium fluoride silver monofluoride mercurous and mercuric fluoride and antimony pentafluoride.Silver mercurous and potassium fluoride exchange isolated halogen atoms for fluorine hydrogen fluoride and antimony trifluoride systems both exchange halogen in polyhalogenated groups but not usually in monohalogenated groups whilst mercuric fluoride and antimony pentafluoride are the most reactive exchange agents and often effect both reactions. Brief accounts have appeared12J3 of the fluorinating activities of various inorganic fluorides. 7 Moissan Compt. rend. 1888 107 260; Moissan and Meslans Compt. rend. 1888 Q Henne “Organic Reactions,” 1944 Vol. 11 John Wiley New York p. 49. 10 Midgley Henne and McNary U.S.P.1,930,129/1933; Daudt and Youker U.S.P. 2,005,705-1 1/1935; Holt and Mattison U.S.P. 2,005,712 2,005,713/1935. 1 2 Lovelace Rausch and Postelnek “Aliphatic Fluorine Compounds,” Reinhold Publ. Corp. New York 1958 p. 1. 13 Park in “Fluorine Chemistry,” ed. Simons Academic Press New York Vol. I 1950 p. 523. 107 1155; Meslans Compt. rend. 1889 108 352. Swarts Bull. SOC. chim. belges 1930 39 444. Chem. Eng. News July 18th 1960 p. 92. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 47 Silver monofluoride. This converts alkyl monohalogeno-compounds into fluorides having been employed by Moissan and Meslans' to make the lower alkyl fluorides from the iodides C,H,,+,.X + 2AgF+ C,H,,+,*F + AgF AgX. Since an organic fluoride boils lower than analogous halides the tech- nique is straight-forward the product being distilled from a mixture of the organic halide and an excess of silver fluoride.The reaction modified where necessary has been used in the synthesis of w-fluorocarboxylic acid derivatives1* (e.g. Br.[CH2],1C0,Et-+ F-[CH2]11.C0,Et) and of fluoro-~teroids.~~ Though this salt has been applied mostly to monohalo- geno-compounds there are a few reports of its effecting conversions involv- ing polyhalogeno-groups e.g.,16 CF,COCBr -+ CF,-COCBr,F -+ CF,COCBrF,. Silver fluoride is rather difficult to prepare and it forms troublesome complexes with the silver halide produced during the ex- these also reduce the amount of fluorine available for reaction. For many purposes potassium fluoride is superior but silver fluoride still finds uses as in the recent synthesis1' of 2,4,6-trifluoropyrimidine by refluxing the chloro-analogue with the reagent.C I F Mercurousfluoride. This is similar in reactivity to silver fluoride but is claimed to be better.l* It gives alkyl fluorides fairly readily. Potassium fluoride. This easily available and convenient reagent has found extensive use. Reactive halogen atoms in aliphatic compounds are exchanged readily as for example in the preparation of acyl fluorideslg and of derivatives of a-fluoro-carboxylic acids20a21 from analogous chlor- ides. A mixture of the reactants is heated to about 200" and agitated often in an autoclave. Alkyl fluorides and esters of fluoro-alcohols can be made21 with potassium fluoride best in a solvent either ethylene glycol2 or aceta- mide,23 at 150-200". Use of a solvent avoids trouble due to coating of the potassium fluoride with potassium chloride and allows otherwise difficult exchanges in the groups -CH2X and >CHX to be effected in ordinary glass equipment.Further a small amount of moisture in one of the re- l4 Buckle Pattison and Saunders J. 1949 1471 ; Pattison and Saunders J. 1949 l5 Tannhauser Pratt and Jensen J . Amer. Chem. SOC. 1956,78 2658 l6 Shepard and Loiselle J . Org. Chem. 1958,23 2012. Schroeder J. Amer. Chem. Soc. 1960 82 4115. Swarts Bull. SOC. chim. belges 1924 35 1533; Henne and Renoll J. Amer. Chem. 2745. Soc. 1938 60 1060. l9 Nesmeyanov and Kahn Ber. 1934,67 370. 2o Saunders and Stacey J. 1948 1773. 21 Gryszkiewicz-Trochimowski Sporzynski and Wnuk Rec. Trav. chirn. 1947 66 22 Hoffmann J. Amer. Chem. SOC. 1948,70,2596; J .Org. Chem. 1949,14,105; 1950 23 Bergmann and Blank J. 1953 3786. 41 3 ; Gryszkiewicz-Trochimowski ibid. p. 427. 15 425. 48 QUARTERLY REVIEWS actants can apparently lead to dangerous pressure increases in a closed system. If the reaction mixture is irradiated with ultraviolet light mild conditions suffice for the exchange;24 irradiation under more drastic conditions effects exchange with chloroform and carbon tetra~hloride.~~ Toluenesulphonyloxy and similar groups can be exchanged with potassium fluoride (see p. 52). Fluoroformates (e.g. F.COZEt) can be made26 from chloroformates (e.g. C1CO 2Et) and on treatment with pyridine or boron trifluoride give alkyl fluorides (e.g. EtF). In some cases aryl fluorides can be made from aryl chlorides and potas- sium fluoride. This conversion was originally thought to be confined to compounds with a very reactive halogen e.g.1 -chloro-2,4-dinitro- benzene but other examples are now k n ~ w n . ~ ~ ~ ~ ~ One interesting casez9 is the conversion of tetrachloro- into tetrafluoro-p-benzoquinone. The successful exchange of the halogen in less activated halides with potassium fluoride has been achieved by the use of non-aqueous solvents such as dimethylformamide and dimethyl sulphoxide. An explana- tion of this effect has been based30 on the lower degree of solvation of the fluoride ion in dipolar aprotic solvents. A study28 of the relative efficiencies of the alkali-metal fluorides in this type of fluorination has revealed a much greater ease of exchange with czesium fluoride (CsF>RbF>KF>NaF> LiF). Thus whereas l-chloro- 2,4-dinitrobenzene did not react with lithium fluoride at 20O0 casium fluoride gave a quantitative yield of 1 -fluoro-2,4-dinitrobenzene.Systems based on antimony trijluoride and hydrogen fluoride. Antimony trifluoride alone is not a very vigorous exchange reagent but if antimony pentachloride,1° or chlorine32 is added a much more reactive quinquevalent species (e.g. SbF,Cl,) is formed. The reactions are carried simply by heating the organic halide with the fluorinating agent usually without a solvent. Glass apparatus can be used sometimes but metal (steel nickel or copper) is best. Anhydrous hydrogen fluoride is an exchange reagent though not a particularly vigorous one but if a small amount of an antimony salt is addedlO the reactivity is enhanced considerably.Many different systems have been suggested hydrogen fluoride-antimony pentachloride being used most widely; presumably a species such as antimony dichlorode trifluoride is continuously generated. The reactions can be conducted in the liquid phase in a pressure vessel with due precautions for handling 24 Olah and Pavlath Acta Chim. Acad. Sci. Hung. 1953 3 191 199. 25 Olah and Pavlath Acta Chim. Acad. Sci. Hung. 1954 4 119. as OlAh and Kuhn J. Org. Chem. 1956,21 1319. 2T Finger and Kruse J. Amer. Chetn. Soc. 1956 78 6034. 28 Vorozhtsov and Yakobson Khim. Nauk i Prom. 1958 3,403. 2Q Wallenfels and Draber Chem. Ber. 1957 90 2819. 30 Miller and Parker J. Amer. Chem. Soc. 1961,83,117 Tulloch and Coffman J. Org. 31 Swats Bull. Acad. roy. Belg. 1892 24 309 474. 32 Henne U.S.P. 1,978,840/1934 1,990,692/1935.Chem. 1960,25 2016. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 49 hydrogen fluoride. Alternatively a vapour-phase process can be used hydrogen fluoride and the organic halide being passed through a tube at fairly high temperatures usually over a c a t a l y ~ t . ~ ~ s ~ ~ ~ ~ * This catalyst is generally activated carbon (alone or impregnated with a metal halide) or a metal halide; many different systems have been claimed in the patent literature. A catalyst is however not always Antimony trifluoride systems and hydrogen fluoride systems have roughly equivalent reactivities hydrogen fluoride is used commercially and is best for large- scale work ; antimony trifluoride has advantages for isolated small-scale conversions. Hydrogen fluoride can of course add to olefins (see p.53). In general the reactivities of organic halides are as follows (i) Aryl (e.g. C,H,Cl) and alkyl halides (e.g. RCH,Cl) are unattacked; vinyl halides are not attacked directly but if hydrogen fluoride is used addition followed by exchange may occur (CH,=CHCl -+ CH,CHClF + CH,CHF,). (ii) CCl groups are most reactive; if attached to an aryl or heterocyclic ring or to an ethylenic group they are readily converted into CF groups (Ph-CC1 -+ PhCF,; CCl,=CCl-CCl -+ CCl,=CClCF,) ; if they are attached to a saturated aliphatic chain the reaction is slower giving CC1,F and thence CClF but only rarely CF groups. (iii) >CCl and -CHC12 groups behave similarly but are in general less reactive. (iv) The presence of fluorine decreases the ease of replacement of halogen on an adjacent carbon atom.(v) If they are in equivalent positions the order of reactivity is iodine> bromine>chlorine but chlorine in a more reactive position may be replaced before bromine in a less reactive one. Since polyhalogeno-compounds are fluorinated most easily the usual starting materials are chloro-derivatives. The commercially important convex sions are the following. CCI + CCI,F -+ CCI,F -+ CCIF (refs. 9 10 13 34) CHCI -+ CHCI,F + CHCIF (refs. 9 10 13) CCI,-CCI .+ CCI,CCI,F -+ CCI,F.CCI,F -+ CCI,FCCIF - CCIF,.CCIF (refs. 9 13) They are usually carried out with hydrogen fluoride and a catalyst. Amongst the many other reactions reported are CHBr,.CH,Br l+ CHBrFCH,Br -+ CHF,CH,Br (ref. 35) CHCI,-CCI,CCI ?- CHCI,*CCI,CCI,F -+ CHCI,-CCI,.CCIF -+ CHCI,CCIF-CCIF -+ CHCIFCCIF-CCIF (ref.36) CBrCI,.CH,Br !- CF,CH,Br (ref. 37) 33 Leicester B.P. 468447/1937; Woolff and Miller U.S.P. 2,673,139/1954. 34 McBee Hass Frost and Welch Ind. Eng. Chem. 1947 39 404. 35 Swarts Bull. Acad. roy. Belg. 1909 728. 36 Henne and Haeckl J. Amer. Chem. Soc. 1941 63 3476. 37 Dickey Towne Bloom Taylor Hill Corbitt McCall and Moore Ind. Eng. Chem. 1954,46 2213. 50 QUARTERLY REVIEWS CF3 3 cI,c~c\N,c~cc13 F,CC #C.CF3 (ref. 39) N @Cl2 - 4 Q / CHCL,; (ref. 38) Reagents 1 SbF + Br,. 2 SbF,. 3 HF-SbCI,. 4 HF. Mercuric fluoride. This reagent is similar in behaviour to those described in the previous sub-section though rather more r e a c t i ~ e . ~ ~ ~ It seems to fluorinate a polyhalogeno-compound to a higher degree more readily and will replace a lone halogen atom on carbon by fluorine if it is not adjacent to fluorine or to other halogens :40 CHBr,CHBr -+ CHBr,-CHBrF + CHBr,.CHF (C,H,),CCI -+ (C,H,),CF CHCI,*CCIF -> CHF,*CCIF The reagent may be prepared from mercuric chloride and elementary fluorine or made in situ by passing hydrogen fluoride into a mixture of mercuric oxide and the organic halide.40 Though the latter method is more convenient experimentally water is liberated and may be harmful e.g.in the synthesis of benzyl fluorides where the preformed difluoride was used.41 Very rapid exchanges such as occur with ethyl iodide or iodoform must be performed in a solvent e.g. chloroform or methylene chloride to moderate the reaction. An alcohol ester or ether must not be used as solvent since the exchange is then inhibited. Such easy exchanges may be carried out in glass vessels.With polyhalides requiring temperatw es in the range 80-160" a copper reaction vessel is suitable and from 160" to 240" nickel is unaffected. The fluorinated product is usually distilled from the reaction mixture. This is a very reactive compound which has some of the character of other high-valency fluorides (p. 61) but is a liquid (b.p. 150') and so cannot be employed analogously. It is included here since its main use has been as a drastic halogen-exchange agent as for example in routes 42 to C and C8 fluorocarbons and chlorofluoro- carbons requiring a minimum of elemental fluorine and involving ex- haustive chlorination followed by halogen exchange. Complete replace- ment of chlorine in a chlorofluoroalkane is seldom possible however.Addition as well as exchange can occur readily as for example,43 in the conversion of hexachlorobenzene into 1,2-dichloro-octafluorocyclohexene Antimony pentafluoride. 38 Fernandez-Bolaiios Overend Sykes Tatlow and Wiseman J. 1960 4003. 39 McBee Pierce and Bolt Ind. Eng. Chem. 1947 39 391. 40 Henne and Midgley J. Amer. Chem. SOC. 1936 58 884; Henne ibid. 1938 60 *l Bernstein Roth and Miller J. Amer. Chem. Soc. 1948,70,2310. 42 Couper Downing Lulek Perkins Stilmar and Struve Ind. Eng. Chem. 1947 39 43 McBee Wiseman and Bachman Ind. Eng. Chem. 1947 39 415; Leffler J. Org. 1569; Henne and Flanagan ibid. 1943,65 2362. 346. Chem. 1959,24 1132. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 51 and of hexachlorocyclopentadiene into 1,2-dichlorohexafluorocyclo- pentene. This reagent was also used in the first reported synthesis of hexa- fluorobenzene (p.66). Most reactions have involved compounds with little hydrogen; if much were present it would be attacked. Antimony pentafluoride can be made from hydrogen fluoride and antimony penta- chloride and this system though probably rather less reactive is often used under drastic conditions in preference to the pentafluoride alone. Sulphur tetrafluoride. It has been reported very recently44 that this reagent will exchange fluorine for chlorine or bromine in halogeno-alkanes -alkenes and -pyrimidines and for chlorine in cyanuric chloride. The replacement was usually incomplete even though elevated temperatures and pressures were employed e.g. carbon tetrachloride gave chloro- fluoromethanes. The halogenoalkanes higher than those of methane underwent exchange less readily even under extreme conditions.Thus hexachloroethane gave tetrachloro-1,2-difluoroethane and 2,2,3,3-tetrachlorohexafluorobutane gave 2,2,3-trichloroheptafluorobutane. Hexachlorobenzene reacted both by addition and exchange affording dichloro-octafluorocyclohexene and trichlorononafluorocyclohexane a reaction analogous to that with antimony pentafl~oride.~~ Other jluorides. Various other metallic and non-metallic fluorides have been mentioned as possible exchange agents but it does not appear that any has outstanding advantages. Thallous fldoride has been to make fluoroformates and caesium fluoride28 to make fluoro-aromatic compounds instead of potassium fluoiide. They appealed to be more reactive (see p. 48). The use of aprotic solvents in exchange methods of fluorination has recently been extended.30 Sodium fluoride in solution in tetrahydrothiophen 1,l-dioxide acetonitrile or dimethylformamide has been found to exchange halogen rapidly with carboxylic acid chlorides cc-chloro-ethers trichloromethanesulphenyl chloride thiocarbonyl chloride selenium oxychloride thiophosphoryl chlorides and phosphonitrilic chlorides.The conversion was usually accomplished by heating the reactants at tempera- tures up to 250" at atmospheric pressure. In general sodium fluoride seems less reactive than potassium fluoride as was demonstrated in the aromatic field. 28 If the readily available calcium fluoride could be used to fluorinate organic compounds directly some economies might result but no really competitive processes appear to have been found.A@) Exchange of Fluorine for Univalent Functional Groups other than halogen.-It was early that reactions of an alcohol with hydrogen 44 Tullock Carboni Harder Smith and Coffman J. Arner. Chem. Suc. 1960 82 45 Nakanishi Myers and Jensen J. Arner. Chem. SOC. 1955,77,3099,5033. 46 Moissan and Meslans Compt. rend. 1888 107 1155; Meslans Compt. rend. 1892 5107. 115 1080. 52 QUARTERLY REVIEWS fluoride or phosphorus fluorides gave very little alkyl fluoride too many side reactions interfering. Sulphur tetrafluoride can apparently effect this conversion; other alternatives are treatment of a sulphonate ester with potassium fluoride or the sequence:45 ROH -+ ROCOCl-+ ROCOF -f RF + CO,. The halogen exchange can be effected with thallous or potassium fluoride.26 The hydrogen in activated methylene groups can be replaced by use of perchloryl fluoride.Sulphur tetrafluoride. This new reagent4' apparently converts hydroxyl groups into fluoride. Though few examples have been reported as yet it appears that acidic hydroxyl groups are exchanged best e.g. H0.CH2-C02H -+ F*CH2CF With simple aliphatic alcohols there is some ether formation. Potassium fluoride with sulphonate esters. This typical nucleophilic substitution is one of the oldest routes48 to alkyl fluorides. The usual t e c h n i q ~ e ~ ~ ~ ~ is to reflux a toluenesulphonic ester with potassium fluoride in 2,2'-dihydroxydiethyl ether. Fairly vigorous conditions are required since fluoride ion is a weak nucleophile. Even more drastic conditions than usual are needed to replace49 the tightly held oxygen of the toluenesulphon- ate of a fluoro-alcohol such as trifluoroethanol.PerchZoryZJluoride. Though this reacts with aromatic compounds under Friedel-Crafts conditions to give aryl perchlorates it will replace the hydro- gen of activated methylene groups by fluorine. Thus from diethyl sodio- malonate diethyl difluoromalonate was prepared;51 and fluorine was introduced into certain keto-steroids in a related fashions2 A(iii) Exchange of Fluorine for Oxygen in Carbonyl and Carboxyl Groups.-Sulphur tetrafluoride. This remarkable reagent is a gas (b.p. -38") which can be made by reaction of (a) sulphur with fluorine or iodine pentafluoride (b) sulphur dichloride with sodium fluoride or (c) sulphur and chlorine with sodium fluoride. It can effect the following conversions directly:47 >C=O -+ >CF,; -C02H + -COF -+ -CF,.Reactions are carried out in pressure vessels at up to 30O0 and in the more difficult conversions a catalyst (HF BF, AsF, or TiF,) is added. Both alkyl and 47 Hasek Smith and Englehardt J. Amer. Chem. SOC. 1960,82,543. 49 Edge11 and Parts J. Amer. Chem. SOC. 1955,77,4899. 5 0 Bergmann and Shahak Chem. and Ind. 1958 157. 51 Inman Oesterling and Tyczkowski J. Amer. Chem. SOC. 1958,80,6533. 52 Kissman Small and Weiss J. Amer. Chem. SOC. 1960 82 2312. Dumas and Peligot Annafen 1835 15 59. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 53 aryl compounds react and ethylenic acetylenic and ether groupings are unattacked. Some representative conversions out of a considerable number described4' are as follows CH3CH2COzH -+ CH3*CH,*CF3 HCzCCO2H +- HC,C.CF CH2=CH.CO,H -+ CH2=CH*CF3 o - C ~ H ~ ( C O ~ H ) ~ -+ O-CgH4(CF3)2 C,H,*CHO + C,H,.CHF CH3CO*CH3 +- CH3CF2*CH3 This process undoubtedly is one of the most important recent advances in synthetic fluorine chemistry.Phenylsulphur trzjluovide. This is made from diphenyl disulphide and silver difluoride and appears53 to have similar reactivity to that of sulphur tetrafluoride though only the synthesis of benzylidene fluoride has been reported so far; being a liquid it should be easier to use. A(iv) Additions to Unsaturated Compounds.-Hydrogen fluoride. This can react analogously to the other hydrogen halides towards unsaturated compounds as was shown long ago54 in the synthesis of pentyl fluoride from pentene. Under fairly drastic conditions additions to simple 01efins~~ and acetylenes56 proceed normally and with the expected orientation of the products.There is an extensive patent literature on the process (e.g. for vinyl fluoride and fluoroprene). Boron trifluoride has been suggested as a catalyst ;57 in the patent literature58 a specially prepared aluminium fluoride has been recommended. Chloro-olefins have been used widely as starting materials in a combina- tion59y37 of addition and exchange processes (see p. 48). A typical conver- sion was CH3COC2H (+PCl,) + CH3.CCl=CHCH3 (+HF) -+ CH3.CF2C2H5; there are various other examples of syntheses of com- pounds containing >CF2 groups. Additions of hydrogen fluoride to fluoro-olefins have been carried out with potassium fluoride in formamide.60 This is a difficult reaction with hydrogen fluoride itself but in the solvent used owing presumably to the absence of ~olvation,~* the fluoride ion exhibits a considerable degree of nucleophilic activity.Hexafluoropropene gave 2H-heptafluoropropane- the expected orientation. Other reactions can occur notably rearrangement by elimination of fluoride ion from the intermediate carbanion (F- + Additions of hydrogen fluoride are not limited to carbon-carbon bonds being possible with > C =N- groups. Isocyanates gave N-fluoroformyl- CFB=CF*CF2R -+ CF,*-CF.CF,R -+ CF,*CF=CFR + F-). j3 Sheppard J. Amer. Chem. SOC. 1960 82 4751. 54 Young J . 1881 39 489. 55 Grosse and Linn J. Org. Chem. 1939 3 26. 56 Grosse and Linn J. Amer. Chem. SOC. 1942 64 2289; Henne and Plueddeman 57 Henne and Arnold J. Amer. Chem.SOC. 1948 70 758. 58 Miller and Smith U.S.P. 2,669,590/1954. 59 Henne and Haeckl J. Amer. Chem. SOC. 1941 63 2692; Renoll ibid. 1942 64 6o Fried and Miller J. Amer. Chem. SOC. 1959 81 2078; Miller Fried and Gold- ibid. 1943,65,587. 1115; Henne and Plueddeman ibid. 1943 65 1271. white ibid. 1960 82 3091. 54 QUARTERLY REVIEWS ated amines,61 whilst periluoro-azomethines afforded perfiuoro-secondary amhesg2 (e.g. C2F,.N=CF -f C2F,*NHCF3). The group >CF-NH- is stable only in the absence of water. Ethylene oxide rings are broken by hydrogen fluoride,63 to give 13-fluoro- hydroxy-groupings. An extension of this process has been to make fluoro-steroids. Other fluorides. Certain high-valency metallic fluorides or metal oxide-hydrogen fluoride systems have been used to add two atoms of fluorine across the double bonds of halogeno-olefins (see p.65). As has been mentioned (pp. 50,5 1) antimony pentafluoride and sulphur tetrafluo- ride can add to as well as exchange with unsaturated chloro-compounds. A(v) Exchange of Fluorine for Diazonium Groups in Aromatic or Heterocyclic Compounds.-This cannot be done by a Sandmeyer-type reaction but is possible by diazotisation in hydrogen fluoride itself. Alternatively the diazonium tetrafluoroborate or hexafluorosilicate may be made and decomposed to give the aryl fluoride. This reagent was originally used to decompose diazoamino-compounds or diazopiperidides. The exchange is now effected by diazotising the amine in anhydrous hydrogen fluoride with sodium nitrite65 or nitrosyl chloride,66 and warming the system to decompose the diazonium fluoride formed.Benzenes,65,66 naphthalene^,^^ and pyrid- inePv6* have been made but only with one fluorine atom. TetraJEuoroboric acid. This or its sodium salt is the usual fluorinating agent of the Balz-Schiemann reaction,69 being added to the solution obtained after normal diazotisation of an aromatic amine with consequent precipitation of the diazonium fluoroborate. Such a salt is usually almost insoluble in water and relatively stable. It may be dried and if heated decomposes at a definite temperature ArN,+BF,- -+ ArF + BF + N,. A review of the process has been published.73 The reaction has been used to prepare fluoro-benzenes -biphenyls -naphthalenes -fluorenes -phenan- threnes -pyridines -quinolines -isoquinolines -benzothiazoles and -carbazoles. It is possible to introduce fluorine stepwise by repetition of the sequence nitration reduction diazotisation introduction of fluorine.From benzene itself 1 ,2,4,5-71 and 1,2,3,5-tetrafluoroben~ene~~ were made 62 Pearlson and Hals U.S.P. 2,643,267/1954; Barr and Haszeldine J. 1955 2532. 63 Knunyants Compt. rend. Acad. Sci. U.S.S.R. 1947 55 223; Knunyants Kil’disheva and Petrov 2. obshchei Khim. 1949 19 95; Knunyants Kil’disheva and Bykhovskaya ibid. p. 101. 64 Fried and Sabo J. Amer. Chem. SOC. 1954,76 1455; Fried Hen Sabo Borman Singer and Numeroff ibid. 1955,77 1068; Herr Hogg and Levin ibid. 1956,78 500. 66 Shenk and Pellon U.S.P. 2,563,796/1952. 67 Beaty and Musgrave J. 1952 875. 6s Gruber Canad. J. Chem. 1953 31 1020. 6 9 Balz and Schiemann Ber. 1927 60 1186. 70 Roe “Organic Reactions,” 1949 Vol.V John Wiley New York p. 193. 71 Finger Reed Burness Fort and Blough J. Amer. Chem. SOC. 1951,73 145. 72 Finger Reed and Oesterling J . Amer. Chem. SOC. 1951,73 152. Hydrogen fluoride. Buckley Piggott and Welch J. 1945 864. Ferm and Van der Werf J. Amer. Chem. SOC. 1950 72,4809. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 55 thus but further nitration was not possible. A complication is that fluorine in a fluorobenzenediazonium fluoroborate is activated to nucleophilic displacement and may be replaced73 by chlorine from co-precipitated sodium chloride. Alternative conditions have been proposed74 for the decomposition of diazonium fluoroborates. Diazonium hexafluoroph~sphates~~ and hexafluor~silicates~~~~~ have been used in a similar manner but with no real advantage.Only in a very few cases do the methods of Section A give full fluorina- tion in one operation. Usually reactions stop when some hydrogen and/or halogen remains and sometimes very little fluorine is introduced. However the products can usually be subjected to preferential dehydrohalogenation or dehalogenation. These processes can be carried out either by pyrolysis or with chemical reagents. Dehydrohalogenation can be done with alcoholic alkali and dehalogenation with zinc dust in a medium such as ethanol or acetamide. Halogens other than fluorine are almost always eliminated preferentially and only rarely is fluorine itself removed as well. Such treatment can be followed if necessary by other fluorination stages. In this way fluorocarbon-type materials can often be synthesised without undue difficulty.Some examples are CHCL CHCLF CF2=CF + HCl (ref. 10.77) CCl ,CCl L CCL F2.CCL F 2 CF,=CCL F 4/ ( ref. 9 l3,78,79 ,) CF,-CCLF J3 CF2-CF CF,- CCl F CF2 - CF I 1 - 1 1 1 CF,-CO2H CCL*=CCl*CCL=CCI CF3.CCl=CCL*CF3 cF,.c PC-CF (ref .80) CBr - 6 CBr,F - 4 FCfF-$F + Br (rcf.81,82) Reagents 1 HF. 2 Pyrolysis. 3 Zn-EtOH. 4 Heat 5 KMnO,. 6 SbF,. CF=CF 73 Finger and Oesterling J. Amer. Chem. SOC. 1956 78 2593. 74 Bergmann and Bentov J. Org. Chem. 1954 19 1594; Bergmann Berkovic and Ikan J. Amer. Chem. SOC. 1956 78 6037; Brunton and Suschitzky J. 1955 1035; Barben and Suschitzky Chem. and Ind. 1957 1039. 75 Lang and Muller Ber. 1930 63 1058. 76 Cheek Wiley and Roe J. Amer. Chem. SOC. 1949 71 1863. 77 Park Benning Downing Laucius and McHarness Ind. Eng. Chem. 1947,39 354.78 Booth Burchfield Bixby and McElvey J. Amer. Chem. SOC. 1933 55 2231; Locke Brode and Henne ibid. 1934,56 1726. 7 9 Henne and Ruh J. Amer. Chem. SOC. 1947 69 279; Harmon U.S.P. 2,404,3741 1946 2,436,142/1948; Buxton Ingram Smith Stacey and Tatlow J. 1952 3830. Henne and Trott J. Amer. Chem. SOC. 1947 69 1820; Henne and Finnegan ibid. 1949 71 298. 81 DCsirant Bull. Classe Sci. Acad. roy. Belg. 1955 41 759; Bull. SOC. chim. belges 1958 67 676; Hellmann Peters Pummer and Wall J. Amer. Chem. SOC. 1957 79 5654. 82 Birchall and Haszeldine J 1959 13. 56 QUARTERLY REVIEWS The products of these sequences are important synthetic intermediates in fluorocarbon chemistry. Their general reactivities have been described in earlier reviews83 and will be dealt with again in the second part of this one.SECTION B This section deals with methods for exhaustive fluorination of which there are four all capable of producing fluorocarbon-type material directly. Reagents are (i) elementary fluorine ; (ii) transition-metal fluorides in which the metal has a high valency; or (iii) a halogen fluoride. In method (iv) (electrochemical fluorination) a current is passed through a solution of a carbon compound in anhydrous hydrogen fluoride. With all of these methods unsaturation and hydrogen in organic compounds are attacked and when the reactions are allowed to go to completion saturated perfluoro-compounds are formed. Functional groups are often attacked but are sometimes preserved. B(i) Direct Fluorination with Elementary Fluorine.-There is a very recent reviewa4 on this subject.In all early attempts at direct fluorination of organic compounds the violence of the reaction could not be controlled and little progress was made until the mid-1930’s. Then two techniques were evolved vapour-phase fluorination in a packed vessel and a simple liquid-phase reaction in which fluorine usually diluted with nitrogen was bubbled through an organic compound normally in a solution. Sub- sequently it was found practicable to carry out vapour-phase fluorina- tions in unpacked vessels. All reactions between elementary fluorine and hydrocarbon-type compounds are highly exothermic since heats of formation of 9C-F and H-F bonds are high (ca. 105 and 134 kcal./mole respectively) and the dissociation energy of the F-F bond is low (37 kcal./mole). Unless this heat is rapidly dissipated combustion and fragmentation of the carbon skeleton ensue.The techniques evolved have all achieved to some extent at least rapid removal of heat from the reaction site. This process usually called the “catalytic method,” followed from the observation that fluorine and an organic vapour reacted smoothly within the meshes of a finely divided metal packing particularly at elevated temperatures ; com- pounds with a high degree of fluorination were produced. It was developed by B i g e l o ~ ~ ~ ~ * ~ and was subsequently adapted as part of the original Atomic Energy Projects for the production of fluorocarbons from hydro- Direct vapour-phase fluorination over a metal packing. 83 Musgrave Quart. Rev. 1954 8 331 ; Haszeldine Ann. Reports 1954 51 279. ** Tedder in “Advances in Fluorine Chemistry,” ed.Stacey Tatlow and Sharpe 85 Bigelow Chem. Rev. 1947 40 51. 86 Bigelow in “Fluorine Chemistry,” ed. Simons Academic Press New York Vol. I Butterworths Scientific Publs. London 1961 Vol. 11 p. 104. 1950 p. 373. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 57 carbons a typical form of apparatus being illustrated in Fig. 1. Usually streams of the organic vapour and of fluorine both diluted with nitrogen were mixed in a metal tube filled with some form of finely divided metal such as gauze turnings or shot and heated to about 200-250”. FIG. 1. Apparatus for vapour-phase fluorination. Pyfex flow-meters. Mlxing chamber for fluorine and nitrogen. Copper spiral for preheating the fluorine-nitrogen mixture. Steel reaction vessel ( 3 4 x 3”) filled with gold-plated copper turnings.Thermometer pocket. Nickel baffle plate extending + length of R; lower half drilled with h‘‘ holes +” apart. Furnace for preheating hydrocarbon-nitrogen mixture. Copper-glass ground joint. Graduated 50-C.C. Pyrex vessel carrying a thermometer T2 Silica tube furnace for heating the hydrocarbon reservoir. Traps for the condensation of polymeric material. Copper tube dipping into “Cerechlor” or a fluorinated oil to act as a safety valve. Potassium fluoride scrubber (removed during preparation of high-boiling fluorocarbons). Two copper U tubes joined in series and cooled by a mixture of solid carbon dioxide and alcohol. Liquid-air trap to condense any volatile decomposition products. (Reprinted by permission from Haszeldine and Smith J . 1950 2689.) The direct fluorination of organic compounds is undoubtedly a free- radical process with the usual type of chain mechanism.Products can be partially or completely fluorinated derivatives viz. of the original carbon skeleton of shorter carbon chains resulting from C-C bond rupture or of longer carbon chains due to polymerisation. Some isomerisation or cyclisation can also occur. Many hydrocarbons have been fluorinated by this technique. Ethane and methane were investigated early,s5~ss and a range of fluorocarbons was later inade87~ss from higher homologues. In this process an aromatic ring is saturated at the and benzenoid hydrocarbons give alicyclic fluoro-derivatives. Yields of fluorocarbons are reasonable reaching about 60 % in the best cases$’ e.g. perfluoro-n- heptane from n-heptane 62 % ; perfluorocyclohexane from benzene 58 %.87 Cady Grosse Barber Burger and Sheldon Ind. Eng. Chem. 1947 39 290. Musgrave and Smith J . 1949 3021 ; Haszeldine and Smith J. 1950 2689 2787. Fukuhara and Bigelow J . Amer. G e m . Soc. 1941 63 2792; Gilbert and Bigelow ibid. 1950 72,241 1. 58 QUARTERLY REVIEWS However yields are normally less than these with much by-product formation. Though different metal packings have given somewhat different yields,g0 the effect is not great and it appears that the primary function of a packing is to disperse the heat of reaction and that it is not a true catalyst. Successful fluorinations of some organic derivatives have been reported. Methyl alcohol or carbon monoxide gave trifluoromethyl hypofl~orite,~~ and methanethiol gave trifluoromethylsulphur pentafluoride.92 From acetone there were obtained hexafluoroacetone and trifluoroacetyl fluoride besides other breakdown products. 93 Perfluoro-ketones were obtained also from ethyl methyl ketone and from cyclopentanone. 94 Certain nitrogenous compounds have been fluorinated. Mono- di- and tri-ethylamine gave a range of nitrogen-containing fluorocarbon derivatives such as CF,*NF2 CF,CF,*NF, (CF,),NF etc. 95 Acetonitrile under mild conditions afforded fluorohydrocarbons and a nitrogenous polymer but no volatile fluorocarbon nitride or free use of more drastic conditions led to CF3CF,.NF2 and CF,=NF. Malononitrile afforded I -1 CF ,-CF ,CF2-NF- NF CF ,CF ,CF ,-NF , NF ,CF ,CF ,CF ,. N F , and other breakdown p r o d ~ c t s . ~ ~ Acid amidesgS are much more unstable for example acetamide was cleaved even before replacement of hydrogen to give acetyl fluoride in 52 % yield.Cyanidesg8 gave perfluoroazo-compounds and under vigorous conditions pentafluoroethyl cyanide gave hepta- fluoropropylnitrogen difluoride. Methyl thiocyanate gave pentafluoro- sulphur cyanide (F,SCN) under mild conditions but the derived nitride (F5S.CF2.NF2) together with sulphur hexafluoride under vigorous conditions. 98 In the first ap- paratusgg for this type of reaction concentric tubes were used for introduc- ing the different gas streams carbon disulphide giving inter alia CF,-SF, CF3.SF3 SF3CF,-SF, and SF,CF,.SF,. Subsequently a jet fluorination reactor was devisedlo0 in which reactor gases helped to dilute the incoming fluorine stream (see Fig. 2). Hexa- fluoroethane was formed from ethane in 85% yield.This seems to be the best method of direct fluorination found so far. More recently the process has been refined and very mild fluorinations achieved with a triple jet reactor.lol Direct vapour-phase fluorination without a catalyst. Musgrave and Smith J. 1949 3026. 91 Kellogg and Cady J. Amer. Chem. SOC. 1948 70 3986. 92 Silvey and Cady J. Amer. Chem. SOC. 1950 72 3624. 93 Fukuhara and Bigelow J. Amer. Chem. SOC. 1941 63 788. 94 Holub and Bigelow J. Amer. Chem. SOC. 1950,72 4879. 95 Gervasi Brown and Bigelow J. Amer. Chem. SOC. 1956 78 1680. 96 Cuculo and Bigelow J. Amer. Chem. SOC. 1952 74 710. 97 Avonda Gervasi and Bigelow J. Amer. Chem. SOC. 1956 78 2798. 98 Attaway Groth and Bigelow J. Amer. Chem. SOC. 1959 81 3599. gg Tyczkowski and Bigelow J.Amer. Chem. SOC. 1953 75 3523. loo Tyczkowski and Bigelow J. Amer. Chem. SOC. 1955 77 3007. lol Maxwell Detono and Bigelow J. Arner. Chern. SOC. 1960 82 5827. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 59 Kinetic studies have been carried outlo2 on the gas-phase reactions of fluorine with hydrocarbons by diluting the system with an inert gas (nitrogen) to give very low concentrations of reactants and to avoid increases of temperature. Useful information about the process was ~btained,~* but the method has not been used preparatively. To Traps FIG. 2. A Primary reactor. H Centring device. BY Secondary reactor. 1 Inlet tube (Ethane + N2). C and D Weston thermometers. J Inlet tube (F2). E Jet. K Inlet and exit tube. F Gas inlets. L Heater. G Mouth of inlet tube J.w Circular observation window. (Reproduced by permission .from Tyczkowski and Bigelow J . Amer. Chem. SOC. 1955 77 3007.) Liquid-phase fluorination. This technique though apparently simple poses many problems in practice. It is desirable to use a solvent but there are few with sufficient stability towards fluorine. Of those that have fluorocarbons are very poor solvents whilst hydrogen fluoride has a low boiling point. Much work has been done with chloro- or chlorofluoro- carbons but these often react slowly with fluorine. Further owing to the free-radical nature of the fluorination process the carbon skeletons of the starting materials are very often linked together to give polymeric products. Though fluorine is not very soluble in most organic liquids it is probable that few reactions have been true gas-liquid ones at an interface gas- vapour reactions in bubbles predominating.The first successful liquid-phase fluorinations were done in carbon lo Anson Fledricks and Tedder J. 1959,918; Fredricks and Tedder J. 1960 144; Fettis Knox and Trotman-Dickenson J. 1960 1064. 60 QUARTERLY REVIEWS tetrachloride. By using fluorine diluted with carbon dioxide and nickel apparatus n-hexadecane cyclohexane and n- and iso-butyric acid were monofluorinated.lo3 Fluorination of the acids did not occur at the a- position. Crotonic acid gave difluorobutyric acid and a dimer.lo3 It was foundlo4 that fluorine liberated chlorine from carbon tetrachloride at 0” in a sufficiently active state to bring about the chlorination of toluene. From aromatic compounds no fluoro-substitution products were formed;lo4 as with vapour-phase fluorination saturation of the ring occurred in the early stages.The fluorination of halogeno-olefins in the liquid phase has been studied by Miller.lo5 Addition of fluorine was accompanied by much dimerisation together with some replacement of chlorine by fluorine and also reaction of the displaced chlorine with organic material. Examples are CCIF=CCIF -+ CCIF2*CCIF2 (17%) + CCIF,CCIF*CCIF.CCIF (30%) (together with CCIF2.CF3 CCI,FCCIF, CCI,FCCI,F C,CI,F, and C,CI,F,) CF3*CCI=CF2 + CF,-CCIF.CF (15%) + cF3)CClCIC(cFs (62%) CF3 CFS These reactions carried out at < -50” proceed by a free-radical chain mechanism which explains the dimerisations and it has been suggested,lo6 though not proved that molecular fluorine can act as the initiator.An interesting consequence of this work is the promotion,10s by small amounts of fluorine of free-radical oxidations and chlorinations of halogeno- olefins by the respective elements. An applicationlo7 of polymerisation initiated by fluorine and ac- companied by fluorination was the production of fluoro-oils and resins from benzotrifluoride in solution in fluorocarbons even the C6 ring system undergoing dimerisation. A consequence of all this however is that no preparations of fluorocarbons by liquid-phase processes have been reported. Fluorinations of undiluted hydrocarbons at very low tempera- tures in glass apparatus have been described,lo8 in which fluorine highly diluted with nitrogen (at least N F = 4:l) and very efficient stirring are used. Whilst fluorination was fairly rapid in the dark it was accelerated by light and it was concluded that reaction occurred mainly in the gas phase of the bubbles.Another case of catalysis by ultraviolet light has been Io3 Bockemuller Annalen 1933 506 20. Io4 Bigelow Pearson Cook and Miller J. Amer. Chem. SOC. 1933 55 4614. lo5 Miller J . Amer. Chem. Suc. 1940,62 341 ; Miller Ehrenfeld Phelan Prober and Reed Ind. Eng. Chem. 1947,39,401; Miller in “Preparation Properties and Technology of Fluorine and Organic Fluoro-compounds” (eds. Slesser and Schram) McGraw-Hill New Yolk 1951 p. 567. lo6 Miller and Dittman J. Amer. Chem. Suc. 1956 78 2793; Miller Koch and McLafferty ibid. p. 4992; Miller and Koch ibid. 1957 79 3084. lo’ Smith Stacey Tatlow Dawson and Thomas J . Appl. Chem. 1952,2 97. lo8 Tedder Chem. and Ind.1955,508; Anson and Tedder J. 1957,4390. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 61 observed in the liquid-phase fluorination of certain chlorocyclohexanes in carbon tetrachloride ; y-rays and peroxides were also effective.los Fluorinations of solids. It has been mentioned above that aliphatic fluorocarbons are produced when fluorine reacts3 with various forms of carbon a mercury salt often being employed as catalyst. This reaction has been carried outl10 by a fluidised-bed technique. Reaction of carbon from an arc with fluorine at >1400°c has been claimedlll to give tetrafluoro- ethylene. There are a few examples of the fluorinations of organic salts but no advantageous synthesis of organic fluorides seems to have emerged. An interesting reaction112 was the direct fluorination of polyethylene a controlled substitution was achieved to give a polymer resembling poly- tetrafluoroethylene but the process was apparently not competitive with polymerisation.B(ii) Fluorination with High-valency Transition-metal Fluorides.-This method of exhaustive fluorination has recently been reviewed in detail.l13 The sources of fluorine are compounds such as cobalt trifluoride silver difluoride manganese trifluoride cerium tetrafluoride or lead tetrafluoride made from a lower-valency fluoride or a chloride and fluorine. They react with organic matter usually at 100-400° and revert to a fluoride of lower valency S H + 2MF -+ >CF + HF + 2MF,- :C=C< + 2MF + >CF*FC< + 2MF,- The “active” fluorine is not evolved as the element by a prior decomposi- tion of the high-valency fluorides; these react directly with organic compounds.The latter can be in either vapour or liquid phase though the vapour-phase process has been by far the more useful. As with fluorine itself aromatic compounds give alicyclic fluorides when treated with these reagents. Vapour-phase reactions with cobalt trijluoride. These were originally c ~ n d u c t e d l l ~ J ~ ~ by passing the organic compound over a thinly spread bed of cobaltic fluoride in a metal tube heated to 3OD-350”. When the high-valency fluoride is exhausted it is regenerated by passing fluorine through the tube. The latter stage can be carried out overnight giving a convenient two-stage sequence. It is advantageous to agitate the bed of fluorinating agent and this is usually a c h i e ~ e d l l ~ ~ ~ ~ in a horizontal lo9 Germano U.S.P.2,905,609/1959. 111 Farlow and Muetterties U.S.P. 2,732,411/1956. 112 Rudge B.P. 710,523/1954. 113 Stacey and Tatlow in “Advances in Fluorine Chemistry,” eds. Stacey Tatlow 11* Fowler Burf‘ord Hamilton Sweet Weber Kasper and Litant Znd. Eng. Chem. 115 Haszeldine and Smith J. 1950 3617. 116 Barbour Barlow and Tatlow J. Appl. Chem. 1952,2 127. Passino Teeters and Mantell U.S.P. 2,770,660/1956. and Sharpe Butterworths Scientific Publs. London Vol. I 1960 p. 166. 1947 39 292. 62 QUARTERLY REVIEWS tubular reactor carrying a coaxial stirrer shaft. A typical design is illus- trated in Fig. 3. Pre- heat ed C FIG. 3. Fluorination apparatus for vapour-phase fluorination with cobalt trifuoride. The tube is of nickel 3 f t . 6 in.long and of 33 in. internal diameter and wall thickness & in. holding about 3; kg. of cobalticfluoride. (Reproduced by permission from Barborrr Barlow and Tatlow J. Appl. Chem. 1952 2 127.) It has been shownll' that when cobaltic fluoride is used as a fluorinating agent the heat of an overall reaction between elementary fluorine and an organic compound is divided approximately equally between the two stages (a) formation of cobalt trifluoride from the difluoride and fluorine; and (b) reaction of the trifluoride with organic material. With compara- tively little of the fragmentation or polymerisation that fluorine itself gives open-chain and cyclic f l u o r ~ c a r b o n s ~ ~ ~ - ~ ~ ~ J ~ ~ have been produced readily from cobaltic fluoride and the appropriate aliphatic or aromatic hydrocarbon.The reactions are easy to carry out and yields are usually in the range 58-75 %. The appropriate aliphatic hydrocarbon gave perfluoro- butane,l14 -pentane,l14 -heptane,114y115 -cyclopentane,l14 and -dimethyl- cyclohexane;l15 toluene afforded perfluoro(methylcyclohexane);115.116 naphthalene and tetralin gave perfluorodecalin ;l15p116 and biphenyl gave perfluorobicyc1ohexyl.ll6 This is probably the best general synthesis of saturated fluorocarbons. Cyclic members of this series are now of syn- thetic value as precursors of perfluoro-aromatic compounds via defluorina- tion119 with heated metals. It is possible to produce fluorohydrocarbons by fluorination of hydro- carbons at temperatures too low to give full fluorination e.g. at 150-200". Most work has been done with benzene and the products120J21 were chiefly a range of fluorocyclohexanes of formula C,H,F,,_ (n = 1 4 ) including undecafluorocyclohexane,120 the six possible decafluorides (c 6H ,Fl,),l2l ,122 the four possible 1 H,2H,4Wnonafluorides (c 6H 3Fg),123 ,12* and four of the five possible lH,2H,4H,SH-octafluorides (C6H4F&125 11' Jessup Brickwedde and Wechsler J.Res. Nat. Bur. Stand. 1950,44 457. 11* Barlow and Tatlow J. 1952 4695. 119 Gething Patrick Stacey and Tatlow Nature 1959 183 588. 120 Barbour Mackenzie Stacey and Tatlow J. Appl. Chem. 1954 4 341 347. 121 Smith and Tatlow J . 1957 2505. lZ2 Evans Godsell Stephens Tatlow and Wiseman Tetrahedron 1958 2 183. 123 Godsell Stacey and Tatlow Tetrahedron 1958 2 193. lZ4 Stephens Tatlow and Wiseman J . 1959 148. 125 Nield Stephens and Tatlow J.1959 159. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 63 Preparative-scale gas chr~matographyl~~J~~ was used in order to isolate pure samples of some of these products the structures of which were established121-126 by conversion with concentrated aqueous alkali into fluoro-cyclohexenes and -cyclohexadienes followed by oxidations to fluoro-dicarboxylic acids. These structures were confirmed by stepwise fluorinations with cobaltic fluoride of each octafluoride to the expected nonafluorides,125 and of each nonafluoride to the expected decafluor- When the fluorocyclohexenes obtained by dehydrofluorination were passed over cobaltic fluoride the double bonds were saturated before the hydrogen was r e p l a ~ e d . ~ ~ * ~ ~ ~ Very small amounts of two fluorocyclohexenes and of fluoro- and p- difluoro-benzene were found113 in the mixed fluorocyclohexanes from the original benzene-cobalt trifluoride reaction.The mechanism is not yet certain; a possible path113 is addition of fluorine to benzene followed by elimination of hydrogen fluoride to give fluorobenzene which by an analogous 1,4-addition-e1imination goes to p-difluorobenzene. 1,4- Addition of fluorine to this would give 3,3,6,6-tetrafluorocyclohexa-l,4- diene from which elimination of hydrogen fluoride is difficult but which can add further to give the octafluorocyclohexanes. Comparatively little has been achieved by the reaction with cobaltic fluoride of organic compounds containing functional groups. Chlorine from a chloro-compound was largely retained overall but considerable displacement and re-entry of it occurred giving rise to much isomerisa- tion.lZ8 Bromine and iodine are removed more easily.Ketones are cleaved completely 94 and methanol gaveg1 no trifluoromethyl hypofluorite (cf. the reactions with fluorine). Amines (aliphatic and aromatic) and pyridine gavelzg perfluoronitrides (e.g. C6F,,.NF2 from aniline) but in very low yields and though trifluoromethylsulphur pentafluoride was obtainedg2 from carbon disulphide or niethanethiol other sulphur compounds suffered rupture of C-S links (cf. electrochemical fluorinations with nitrogen and sulphur-containing compounds). Fluorocarbons may be made just as with cobaltic fluoride; a series of saturated fused-ring perfluoro-deri~atives~~~ and perfluoroalkylcyclohexanes131 has been made in static reactors.Though about as reactive as cobaltic fluoride silver difluoride appears to be more difficult to hand1e.ll3 Though chlorine is removed more easily from organic structures by silver difluoride than by cobalt trifluoride there are suggestions that some ideS.122-124 Vapour-phase reactions with silver dijluoride. lZ6 Evans and Tatlow J. 1955 3779. 12’ Evans Massingham Stacey and Tatlow Nature 1958 182 591. lz9 Haszeldine J. 1950 1638 1966; 1951 102. 130 McBee and Bechtol Ind. Eng. Chem. 1947 39 380. 131 McBee and Ligett U.S.P. 2,606,212/1952. Tatlow and Worthington J. 1952 1251; Roylance Tatlow and Worthington J. 1954,4426. 64 QUARTERLY REVIEWS functional groups are retained better from fluorinations involving the former. Cyanogen chloride gave hexafluoroazomethane and trifluoro- nitro~omethane,~~~ and perfluoro-2-azapropene gave133 a dimeric form (CF3),NCF=NCF3 and the substituted hydrazine (CF,),N.N(CF,), at 15" but (CF,),NF and (CF,),N-N(CF,) at 100".Information133 has been obtained about the relative ease of reaction of silver difluoride with an unsaturated carbon a carbonyl carbon and a substituted nitrogen centre by studying the fluorination of CF,.CN C,F,.CN (CF,),NH (CF,),NCOF C2F,-NC0 CF3.C0.NH, and (CF3.CO)2NH. It was found that N-F bond formation was accompanied by rearrangement and coupling of the free-radical intermediates. Volatile fluorocarbon isocyanates were prepared from the corresponding amides and fluorocarbon azoalkanes RfCF,N=NCF,.Rf from nitriles (Rf = perfluoroalkyl). One reyort13* suggested that carbonyl groups could be retained in fluorinations with silver difluoride.Manganese trifluoride has been proposed as an alfernati~el,~ to cobaltic fluoride but is a much milder reagent. Synthesis of fluorocarbons with it would be diffi- cult and fluorination of benzene has given little if any perfluorocyclo- hexane and much unsaturated materia1136~124-a useful feature. Cerium tetrafl~oridel~~ is another mild fluorinating agent which might have advantages for synthesis of partially fluorinated compounds. Lead tetrafluoride despite handling diffi~ulties,~~~ can be used for exhaustive fluorination of hydrocarbons ;137p138 fluorocarbons can be made by its use though apparently less readily than with cobaltic and silver fluorides. It was again claimed138 that a perfluoro-ketone could be made from a ketone. The use of lead tetrafluoride for additions to olefins is mentioned below.Liquid-phase fluorinations with high-valency fluorides. These have been limited in the main to fluorinations of oils e t ~ . ~ ~ ~ 1 ~ ~ ~ with fluorocarbons as diluents and to completions of fluorination^^^^^^^^ of high-boiling fluoro- carbon material still containing residual hydrogen. Cobalt trifluoride and silver difluoride have both been used the latter being probably better but more difficult to handle. One use of silver difluoride was the conversion17 of 2,4,6-trifluoropyrimidine in solution in perfluorotributylamine into tetrafluoropyrimidine a direct replacement of hydrogen without addition to the unsaturated system. 4553. Eng. Chem. 1947,39 343. Vapour-phase fluorinations with other high-valency fluorides. 132 Glemser Schroder and Haeseler 2.anorg. Chem. 1955 282 80. 133 Young Durrell and Dresdner J. Amer. Chem. Soc. 1959 81 1587; 1960 82 134 McBee and Ligett U.S.P. 2,614,129/1952. 135 Fowler Anderson Hamilton Burford Spadetti Bitterlich and Litant Ind. 136 Fear and Thrower J . Appl. Chem. 1955,5 353. 13' McBee and Robb U.S.P. 2,533,132/1950 2,533,133/1950 2,487,820/1949. 138 McBee and Robb U.S.P. 2,567,569/1951. 139 Struve Benning Downing Lulek and Wirth Ind. Eng. Chem. 1947 39 352. lgO Stilmar Struve and Wirth Ind. Eng. Chem. 1947 39 348. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 65 Saturation of olefinic double bonds. A mixture of lead dioxide and anhydrous hydrogen fluoride will react with halogeno-olefins to give141 the difluoro-addition product (e.g. CF,CCl =CCl + CF,-CClF-CCl,F; and CCl,=CCl -+ CCl,F.CCl,F).In view of the bzhaviour of fluorine itself (p. 60) this is a useful process. The active species is probably lead tetrafluoride and it appears that if the olefin contains much hydrogen it will be attacked. Replacement of lead dioxide by cobaltic oxide or by manganic oxide has been suggested.142 B(iii) Fluorination by Halogen Fluorides. This method of fluorination has also been comprehensively reviewed quite re~ent1y.l~~ The halogen fluorides are all made from fluorine and the second halogen and their application as fluorinating agents for organic compounds is limited since this second halogen is usually introduced as well as fluorine. Controlled reactions have been de- scribedlg4 in which perchloro-compounds hydrocarbons and chloro- hydrocarbons dissolved in an inert solvent were fluorinated with chlorine monofluoride diluted with nitrogen; the vigour of the reaction decreased with increase of halogen content Mixtures of chlorofluoro-compounds were obtained.Chlorine trifluoride reacts similarly but apparently more vigorously. The catalysed reaction with benzene in carbon tetrachloride was mainly one of substitution to give chlorobenzene and fluorobenzene.lg5 A small quantity of addition products was formed also. Mercuric or cobaltous chloride or silver or cobaltous fluoride was used as catalyst. The last appeared the most effective and a mechanism was advanced involving the production of positive fluoride ions. An alternative sugges- tion involved a transition complex between cobalt trifluoride chlorine trifluoride and benzene.Benzene derivatives reacted analogously to benzene and since the entering chlorine or fluorine took up the position expected from the directive effects an electrophilic mechanism was indicated.lg6 In most of the cases studied in the liquid phase the degree of chlorination was greater than that of fluorination. Since steric compression is considerable with higher degrees of chlorination the production of hydrogen-free compounds is difficult without thermal degradation occurring. Perhalogenation was achieved14’ with 1,2,3,4-tetrachloro- butadiene however (to give chlorofluorobutanes) and obviously with unsaturated perchloro-compounds ; for example hexachlorobutadiene and octachloronaphthalene gave148 stable chlorofluoro-compounds. A range of The uses of chZovine fluorides.141 Henne and Waalkes J. Amer. Chem. SOC. 1945 67 1639; 1946 68 496. 142 Benning and Park U.S.P. 2,437,993/1948. 143 Musgrave “Advances in Fluorine Chemistry,” Butterworths Scientific Publs. Murawski and Burnett B.P. 738,289/1955; Muray and Wadsworth B.P. 760,489/ London Vol. I 1960 p. 1. 1956. 145 Ellis and Musgrave J. 1950 3608. 146 Ellis and Musgrave J. 1953 1063. 14’ Muray J. 1959 1884. 148 Burnett B.P. 695,811/1953. 3 66 QUARTERLY REVIEWS commercially important chlorofluoro-oils and -greases has been madel49 by variations of this liquid-phase process. Few compounds with functional groups have been fluorinated thus; trichloroacetic acid and its acid chloride suffered loss of the carbonyl group apparently by a free-radical mechanism.150 Vapour-phase fluorination with chlorine trifluoride has been limited to benzene.Treated as in the catalytic method of direct fluorination (p. 56) this gave143 mainly chlorofluoro-addition products with very little substi- tution. The mechanism obviously differed from that operating in the liquid phase and appeared to be of a free-radical type. Solids fluorinated have been carbon (see p. 4 9 from which chloro- fluorocarbons were and coal to give a range of chlorofluoro- oils and -greases.ljl BromineJuorides. Bromine trifluoride is very reactive towards organic material. With carbon tetra halide^,^^^ the expected products were obtained (CCI + CC1,F + CCl,F2 + CClF,; CBr -+ CBr3F + CBr,F + CBrF,; CI -+ CBr,F,). Hexachlorobenzene gave a complex mixture of addition compounds with composition approximating to C6Br2C1,F,.153 After further fluorination with antimony pentafluoride to an average composi- tion CGBrCI,F, dechlorination with zinc and alcohol gave a little hexa- fluorobenzene together with various chlorofluoro-cyclohexenes and -cyclohexadienes.Analogous reactions were carried out on pentachloro- ben~otriflu0ride.l~~ An interesting reaction154 is that of a mixture of bromine trifluoride and bromine with halogeno-olefins; bromine and fluorine are added and the active species may well be bromine monofluoride (CF,=CH + CF,CH,Br; CF,.CF=CF -+ CF,CFBr.CF,). Reactions of carbon with bromine and fluorine have been IodineJuorides. Iodine pentafluoride is the least reactive of the halogen fluorides though some hydrogen-containing compounds are attacked vigorously. The most important reaction1j2 has been that with carbon tetraiodide to give trifluoromethyl iodide.This is a source of trifluoro- methyl radicals and from it the extensive workB3 on additions to un- saturated systems and syntheses of perfluoroalkyl derivatives has originated. Tetraiodoethylene and iodine pentafluoride afforded pentafluoroethyl iodide.152 A number of perfluoro-alkyl and -cycloalkyl iodides were made from perfluoro-olefins and iodine pentafl~0ride.l~~ This addition of iodine and fluorine is best done154 with a mixture of iodine and its pentafluoride 140 Leech and Burnett B.P. 633,678/1949; Burnett B.P. 676,374/1952. 150 Cuthbertson Holmes Musgrave and Tanner J. Appl. Chern. 1958 8 390. 151 Farenden ref. 143 p. 25. 152 Banks Emelkus Haszeldine and Kerrigan J. 1948 2188. 153 McBee Lindgren and Ligett Ind.Eng. Chem. 1947 39 378. 150 Chambers Musgrave and Savory Proc. Chem. Sac. 1961 1 1 3. 155 Simons and Brice U.S.P. 2,614,131/1952. STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 67 hexafluoropropene gave heptafluoro-2-iodopropane a source of the heptafluoroisopropyl radical and Grignard reagent. Carbon disulphide and iodine pentafluoride gave bistrifluoromethyl di- and tri-su1~hide.l~~ Iodine pentafluoride was apparently the best of the halogen fluorides for the fluorination6 of carbon to give tetrafluoroethylene. In general the introduction of both fluorine and the other halogen complicates the reactions of halogen fluorides complex mixtures being formed. Simple fluorination occurs only when a perhalogeno-compound is used with the corresponding halogen fluoride. B(iv) Electrochemical Fluorination.-This process was devised by and little new work on it has been reported since the last comprehensive review.lj9 Many organic compounds especially those containing polar groups form conducting solutions in anhydrous hydrogen fluoride.When such a solution is electrolysed hydrogen is evolved at the cathode and the organic compound is fluorinated presumably at the anode. A cell such as that represented in Fig. 4 is used the electrodes -,-Coolant in FIG. 4. Cell for electrochemical fluorination. C Condenser; E mercury safety valve; F funnel for liquid additions; J condenser jacket N sodium fluoride pellets; P liquid-level probes; R rubber pellets; S . cooling spiral T polytetrafluoroethylene spacers between anodes and cathodes; V valve for addition of hydrogen fluoride; W valve for liquid addition 2 valve for draining cell.(Reproduced by permission from Gramstad and Haszeldine J. 1956 173.) being a pack of alternate anodes and cathodes insulated by spacers. Anodes. have been of nickel almost without exception cathodes of nickel or iron. Cells have varied in size so that the current they can pass has ranged from 10 to 10,000 amp.;158 voltages are usually between 5 and 7. Operating temperatures are generally close to 0” with an efficient condenser to return hydrogen fluoride (b.p. 19’) to the cell. The fluorinated product is 156 Haszeldine and Kidd J. 1953 3219. 157 Simons J. Electrochem. SOC. 1949 95 47; Simons and his co-workers ibid. 158 Simons in “Fluorine Chemistry,” Academic Press New York Vol. I 1950 p. 414; 159 Burdon and Tatlow “Advances in Fluorine Chemistry,” Buttenvorths London pp.53 55 59 64. Simons and Brice op. cit. Vol. 11 1954 p. 340 1960 voi. r p. 129. 68 QUARTERLY REVIEWS usually insoluble and either distils over or may be run off from the bottom of the cell. Hydrocarbons are difficult to fluorinate electrochemically since they are not very soluble in anhydrous hydrogen fluoride and the solutions are non-conducting. Suspensions or emulsions can bz used in the presencc of conductivity additives. These may be of two types fluoride salts or compounds (water alcohols pyridine carboxylic acids etc.) which are themselves fluorinated as electrolysis proceeds. Though claims have been made for the fluorination of h y d r ~ c a r b o n s ~ ~ ~ ~ ~ * J ~ ~ details are few and it seems that this particular fluorination is not very efficient.However though the great advantage of the electrochemical process is that many functional groups are retained during fluorination fluorocarbons are in fact formed sometimes in good yield from almost all organic compounds. Alcohols for example form conducting solutions but lose their oxygen to give fluorocarbons. With chlorohydrocarbons the same difficulties apply as with hydro- carbons but it appearslsl that hydrogen is replaced before chlorine; thus dichloromethane gave dichloro-fluoro- and -difluoro-methane. Bromine and iodine are lost very readily in electrochemical fluorination. Ethers are fluorinated easily,162 often in reasonable yields to give the corresponding perfluoro-ether { e.g. (C4Hg),0 + (C4F9)%0 ; CH,. [CH,];O + &F,.[CF,In*O (n = 1 - 4 ) >. Cyclic peduoro-ethers are also formed in the electrochemical fluorination of carboxylic acids. Though perfluoro-ethers are very inert chemically their a-fluorine atoms can be replaced by chlorine and then hydrolytic removal of the chlorine gives carbonyl g ~ 0 u p s . l ~ ~ All types of amine have been fluorinated electrochemically,164*165 all of the hydrogen atoms including those linked to nitrogen being replaced. The products perfluoronitrides have no basic properties { C6H5.NH -+ [CF,],>CF.NF,; [CH,],>NH -+ [CF,],>NF; (C4Hg)SN -+ (C,F,),N>. A detailed study166 of the electrochemical fluorination of pyridine and morpholine and their derivatives has been made with nuclear magnetic resonance spectroscopy for the establishment of structures.4-isopropyl- pyridine gave heptadecafluoro-4-isopropylpiperidine containing some of the 4-n-propyl isomer together with nitrogen trifluoride and a fluoro- carbon (C8F18). The last product was largely perfluoro-(3-ethylhexane) and contained only a little of the product expected from loss of nitrogen- 7 lSo Simons U.S.P. 2,519,983/1950. l e 2 Simons B.P. 659,251/1451; U.S.P. 2,500,388/1950; Kauck and Simons B.P. 672,72011952; U.S.P. 2,594,27211952. 163 Tiers J . Amer. Chem. SOC. 1955 77 4837 6703 6704. 164 Simons U.S.P. 2,490,098 2,490,099/1949. 165 Kauck and Simons U.S.P. 2,616,927/1952 2,631,151/1953. Wolfe U.S.P. 2,601,014/1952; B.P. 668,609/1952; B.P. 758,492/1956. Simmons Hoffmann Beck Holler Katz Koshar Larsen Mulvaney Paulson Rogers Singleton and Sparks J. Amer. Chem. SOC. 1957 79 3429.STEPHENS AND TATLOW FLUOROCARBON CHEMISTRY. PART I 69 perfluoro-(3-etliyl-2-methylpentane). Thus appreciable isomerisation of an isopropyl to an n-propyl group occurred perhaps because the fluorination is a free-radical p r 0 ~ e s s . l ~ ~ Perfluoro-nitrides are very inert chemically but reactions of them are now known. Pyrolysis of perfluoro-tertiary arnines giveP7 azomethines Heat (C,F&N -+ C2F5.N = CF2 + C2Fb.N = CF.CF3 Defluorination of perfluoropiperidines affords perfluoropyridines :Is8 CF2I5)N*F - Carboxylic acids and their anhydrides or acid fluorides give perfluoro- carboxylic acid fluorides on electrochemical fl~orination.~~*J~~J~~Best yields are given by acid fluorides. By-products are perfluoro-acid fluorides with fewer carbon atoms and fluorocarbons particularly those with the same number of carbon atoms as the starting acid and one less.From acids with sufficient carbon atoms perfluorocyclic ethers are formed171 often in good yield Though yields of perfluoro-acid fluorides have often been quite low this fluorination has been of the utmost importance in making available the homologous series of fluorocarbon carboxylic acids and thence a wide variety of fluorocarbon derivatives. Organic s ~ l p h i d e s ~ ~ ~ ~ ~ ~ and d i s ~ l p h i d e s l ~ ~ ~ ~ ~ * have given besides sulphur hexafluoride and fluorocarbons corresponding to C-S bond rupture bisperfluoroalkylsulphur tetrafluorides and perfluoroalkylsulphur penta- fluorides e.g. Carbon disulphide gave trifluoromethylsulphur pentafluoride.175 The oxidation of sulphur to the sexivalent stage occurs early in the fluorina- (CH3),S + (CF3)$F4 + CF3*SF5 167 Pearlson and Hals U.S.P.2,643,267/1953. lF8 Burdon Gilman Patrick Stacey and Tatlow Nature 1960 186 231 ; Banks 169 Kauck and Diesslin Ind. Eng. Chem. 1951 43 2332. 170 Diesslin Kauck and Simons U.S.P. 2,567,011/1951; U.S.P. 2,593,737/1952; 171 Kauck and Simons U.S.P. 2,644,823/1953; B.P. 718,318/1954. 172 Clifford El-Shamy Emelkus and Haszeldine J. 1953 2372. 173 Hoffmann and Simmons with Beck Holler Katz Koshar Larsen Mulvaney Rogers Singleton and Sparks J. Amer. Chem. SOC. 1957 79 3424; Dresdner Reed Taylor and Young J . Org. Chem. 1960,25,1464. 174 Dresdner and Young J. Amer. Chem. SOC. 1959,81,574. l i 5 Silvey and Cady J . Amer. Chem. SOC. 1952 74 5792. Ginsberg and Haszeldine J. 1961 1740. Scholberg and Bryce U.S.P.2,717,871/1955. 70 QUARTERLY REVIEWS tion.17* Mercaptoacetic acid gave176 very little of the expected product SF,CF,COF and rather more of the quadrivalent sulphur compound SF,CF,COF. Sulphonyl chlorides and fluorides afforded177 the corres- ponding perfluoroalkanesulphonyl fluorides (e.g. C8Fl7.SO2F) in quite good yield again the fluorocarbon being the main by-product. In all of these fluorinations aryl derivatives were saturated giving perfluorocycloalkyl derivatives yields were usually inferior to those obtained from aliphatic precursors and some tarry products were formed. The mechanism of electrochemical fluorination is obscure ; some possibilities have been but insufficient evidence is available at present to go beyond speculation. It seems likely that the reaction is of a free-radical type.Electrochemical fluorination is undoubtedly a valuable process since so many compounds with functional groups can be made. However yields are often poor and experimentally the method is not an easy one to apply. Summarising the fluorination methods of group B halogen fluorides are useful only if fluorohalogeno-compounds are required ; reactions with elementary fluorine are often troublesome though much valuable syn- thetic work has been done. Cobaltic fluoride is easy to use but limited in scope ; and electrochemical fluorination is relatively difficult but gives a wide range of fluorocarbon derivatives. All of these methods require specialist apparatus and techniques but so to some degree do many of the methods of group A. Methods of group B provide the only practicable synthetic routes to many perfluoro- and polyfluoro-compounds.176 Haszeldine and Nyman J. 1956 2684. 177 Brice and Trott U.S.P. 2,732,398/1956; Gramstad and Haszeldine J. 1956 173; 1957,2640; Burdon Faramand Stacey and Tatlow J. 1957,2574.

ISSN:0009-2681    

DOI:10.1039/QR9621600044

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

METAL OXIDATION By M. WYN ROBERTS (DEPARTMENT OF CHEMISTRY THE QUEEN’S UNIVERSITY OF BELFAST) IT is not intended to give an exhaustive survey of metal-oxygen reactions but to discuss some properties of the metal and oxide oxygen chemisorption and the metal-oxide and oxide-oxygen interfaces and to show how these considerations are relevant to mechanistic studies of oxidation. There is a trend towards treating surface reactions quantitatively before they are completely understood qualitatively and therefore certain aspects will be considered which at first sight seem to bear little relation to metal oxidation but which must with more detailed studies become important factors in the interpretation of such reactions. The formation of an oxide layer may be tentatively considered to take place as follows Oxygen molecules adsorbed on the metal surface are dissociated and ionised incorporation then occurs in which the oxide is formed with the simultaneous formation of cation or anion vacancies.It is therefore necessary to consider the properties of the metal its ability to chemisorb oxygen the subsequent oxide growth and the properties of that oxide. The standard free energy change for the oxidation of all metals except gold is negative thus a chemisorbed layer should be formed spontaneously on exposing a metal to oxygen even at verylow pressures (e.g. - 10-6mm.). Further oxide growth usually requires an appreciable activation energy so that the chemisorbed layer remains in a metastable state at much higher oxygen pressures. Reaction beyond the monolayer stage or incor- poration is therefore kinetically controlled and will be dependent on the characteristics of a diffusion process viz the diffusion of cations out through the oxide the diffusion of oxygen inwards (possibly a contribu- tion of both) or electron transport.Since diffusion is synonymous with the inherent defective character of the system then it is essential to consider first the existence of defects in metals and oxides. 1. Defects in Metals and Oxides The perfect solid can be regarded as an aggregate of atoms arranged in unbroken lattice array each cell of the lattice having an identical arrange- ment of the chemical constituents so that the internal structure is flawless. Solids do however exhibit a number of imperfections the primary ones being vacant lattice sites interstitial atoms positive holes and dislocations.The ratio of the number of defects to lattice atoms is given by eqn. (l) C = exp [- ( AHf - TdSf)/kT] (1) where AHf is the heat of formation of the defect and OSf is the entropy 71 72 QUARTERLY REVIEWS change associated with the defect. On the assumption that d&/k is of the order of unity and by using theoretical estimates of AHf Broom and Ham1 have derived the concentrations of vacancy vacancy pair and interstitial atom defects in copper at 300" 800" and 1300"~ (Table 1). TABLE 1. AHf Number of defects per lattice atom (kcal. mole-') 3 0 0 " ~ 800"~ 1300"~ Vacancy 22 10-17 loaa2 103.9 vacancyair 35 10-27 10-l0 10-6.2 Interstitial atom 88 1047 10-25 10-15 Thus the vacant lattice site may be present in appreciable concentration in thermodynamic equilibrium in metals since with rapid quenching as is the case during the preparation of metal films the concentration may be as high as 1 in lo4.The most mobile of these defects is the interstitial type; interstitial atoms produced by irradiation may become mobile at tempera- tures as low as 4 0 " ~ the activation energy for movement being about 2 kca1.mo1e-1.2 Single vacancies in copper and gold become mobile only above 240"~ and have an activation energy of - 20 kcal.mole-l. If the vacancies exist as clusters which they may well do with evaporated metal fihs the activation energy for vacancy-cluster movement is much smaller than for single vacancies being about 8 kcal.mole-l for a di-vacancy in copper. A value of approximately 4 kcal.mole-l for the activation energy of the sintering of iron films suggests the participation of vacancy-cluster movement in the sintering proce~s.~ Few crystals have been prepared with a dislocation density of less than lo4 per cm.2.This is probably due to vacancies present at the melting point coalescing and finally collapsing to form dislocations. Etching techniques have enabled individual dislocations to be revealed and have also demonstrated the existence of spiral growths on surfaces. The latter have been reported by Forty4 for the growth of small magnesium crystals from the vapour and also on titanium prepared by fused-salt electrolysis. Measurements of the average step height by the Tolansky multiple-beam interference techniques have a value of about 200 A. The unique properties of dislocations are of considerable significance in oxidation processes.Their r81e as paths for easy diffusion has been emphasised by Hofman and Turnbulls who showed that the rate of silver self-diffusion along sub- boundaries was a million times faster than that of lattice diffusion. The sensitivity of dislocation lines to etching is also evidence of a greater chemical reactivity along such a disarray of atoms. Since atoms associated Broom and Ham Institute of Metals Monograph 1957 No. 23. Seitz Adv. Phys. 1952 1 43. Roberts Trans. Faraday Soc. 1960 56 128. Forty Phil. Mag. 1952 43 949. Hofman and Turnbull Acta Metallurgica 1954 2 419. WYN ROBERTS METAL OXIDATION 73 with steps are less tightly bound to a surface than others then such energy differences would be expected to lead to differences in the tendency for surface atoms to react depending on their geometrical arrangement.The r61e of surface kink sites in the oxidation of metals has been discussed by Vermilyeas especially in relation to possible stresses that develop at the metal-oxide and oxide-gas interfaces. The process of self-diffusion in metals most probably occurs by a vacancy mechanism. This requires normal lattice sites from which the atom is missing to receive atoms from neighbouring sites leaving vacant the site from which the atom jumped. The vacancies may cluster in groups of two or more and therefore remain associated through many unit jumps. Two other mechanisms are possible for self-diffusion ; they involve either rotation of a coplanar group of atoms which form a closed ring (diffusion therefore occurring by rotation of the ring) or an interstitial mechanism the diffusing atoms moving through the interstitial spaces between the atoms occupying normal lattice points.For the rotation mechanism the energy barrier to self-diffusion consists of the energy to squeeze the rotating groups of atoms past the interfering atoms. In the interstitial and vacancy mechanisms the activation energy consists of the energy to form the defect plus the energy to move it from one equilibrium site to another. In oxides the exchange mechanism is less likely owing to the large repulsion energy associated with the interchange of two ions of opposite sign. If the initial oxide layer is amorphous as Eley and Wilkinson7 have suggested for aluminium oxide then electrostatic repulsion will not be an important factor and the exchange mechanism is feasible.Diffusion in oxides is however more likely to occur as a consequence of the existence of Frenkel and Schottky defects; the basic principles of such a mechanism have been developed by Wagner and his co-workers.8 Since the concen- tration of lattice defects is relatively low the laws of ideal dilute solutions can be applied although deviations mainly because of the electrostatic forces between the imperfections can occur. The two main classes of defect oxides are those involving excess of metal (n-type) and metal de- ficiency (p-type). The former include zinc and aluminium oxide in which the excess of metal is located at interstitial positions while in the latter examples of which are nickel oxide cuprous oxide and cobalt oxide there occur cation vacancies.Electrical neutrality is maintained in n- and p-type oxides by quasi-free electrons and positive holes respectively. 2. The Metal and Oxide Surfaces The most serious difficulty in the study of the early stages of oxidation is to define the metal surface. Not only do uncertainties arise from surface Vermilyea General Electric Research Laboratory Report No. 57-RL-1704 1957. Eley and Wilkinson Pvoc. Roy. SOC. 1960 A 254 327. cf. Wagner Z. phys. Chern. 1933 B 21 25. 74 QUARTERLY REVIEWS contamination but also from structural imperfections. A perfect free metal surface i.e. one containing an ideal arrangement of atoms in itself represents a defect since at the surface the continuity of the bulk atoms is absent. Owing to the high chemical activity of the surface atoms oxide is inevitably present unless special techniques have been used in surface preparation.It is only since the advent of ultra-high vacuum methods that the surface contamination factor has been minimised. A number of techniques are now available for the preparation of metal surfaces which are substantially free from presorbed gas. These include the use of flashed metal filaments metal films deposited from the vapour phase and the positive-ion bombardment of filaments. Metal filaments are usually no greater than 1 cm.2 in area and since a monolayer of oxygen could be adsorbed on a metal filament of this area in 100 sec. at a pressure of - mm. is necessary. Metal films are prepared by the thermal evaporation of a well-outgassed metal usually in the form of a filament and subsequent condensation on a cooled substrate.Such films usually have a high surface area to weight ratio and are intrinsically unstable. Atoms present in the surface of a metal will tend to rearrange in such a manner that the total free energy is a minimum. The r6le of the surface energy of solids and the consequent surface mobility of atoms has been particularly well demonstrated by Chalmers King and Shuttleworthg with silver surfaces. When heated to above 500" silver surfaces in the presence of oxygen developed a striated appearance which is attributed to the preferential exposure of (1 11) planes; heating in nitrogen caused the striations to disappear. Because of the instability of silver oxide it was not considered that an oxidation reaction supplied the motivation for the change and it was concluded that in the presence of nitrogen a smooth silver surface of any orientation is thermodynamically more stable than a stepped surface of (111) planes and that with oxygen the situation is reversed.Johnsonlo has shown that tungsten filaments heated electrically in an inert gas develop plane facets normal to the <loo> and <110> directions ; similar facets have been observed with tantalum. The migration of surface atoms has been suggested by Beckerll to occur only at tempera- tures above Tm/3 where Tm is the melting point of the metal in OK. Investigations of reaction rates as a function of crystal orientation indicate that certain crystal planes are more stable than others. Tragert and Robertson12 have used electrochemical techniques in an attempt to define the stable planes of copper.They conclude that the (1 11) plane is the only stable one and that all other planes approach the (1 11) configuration with time the process being considered to involve an etch-type mechanism. mm. a background pressure of < Chalmers King and Shuttleworth Proc. Roy. SOC. 1948 A 193 465. lo Johnson PhjJs. Rev. 1938 54 549. l1 Becker Adv. Catalysis 1955 6 135. l2 Tragert and Robertson Trans. Electrochem. SOC. 1955 102 86. WYN ROBERTS METAL OXIDATION 75 Young Cathcart and Cunningham13 have emphasised the dynamic nature of surfaces especially when used as catalysts. The faces which are stable vary markedly with the reacting gases and the experimental condi- tions the surface rearranging to expose these faces. Thus the nature and structure of a metal surface during reaction may be different from the nature and structure during adsorption although the distinction between adsorption and reaction may be difficult.Interference patterns produced by the oxidation of a copper crystal at 250” show clearly the different crystal regions. Large differences in oxidation rate with crystallographic region have been observed; for example after 50 min. at 178” the thickness of oxide on the (100) plane was 10o0 A while that on the (31 1) face was about 60 A. It is usual to regard the oxide lattice as a rigid immobile structure and this may possibly be correct at low temperature (< O”) but there is now considerable evidence from the use of isotopic and other techniques that at higher temperatures ready transfer of lattice oxygen and chemisorbed oxygen will occur.Winter14 has discussed the experimental evidence for the mobility of lattice oxygen in chemisorption and catalytic studies and there appears to be no doubt that surface oxygen is extremely mobile in oxide catalysts such as manganese dioxide cupric oxide and ferric oxide at temperatures of about 200”. Topographically the oxide surface may vary from being atomically uneven to grossly rough. In the latter instance protrusions a few hundred A in length may be observed. Even the formation of the first few oxide layers may disrupt appreciably the surface contour of the metal substrate and it could conceivably be regarded as a “surface corrosion” process. 3. Oxygen Chemisorption The precursor of oxidation is the chemisorption of oxygen. A detailed discussion will not be given of the mechanism of oxygen chemisorption.It will suffice to consider only the possible final states. If in the adsorbed oxygen a vacant energy level is present which is below the Fermi level of the metal an electron can be transferred from the metal to the oxygen. This is the case when oxygen is chemisorbed with a negative dipole pointing away from the surface. There is no unambiguous evidence for the reverse process i.e. the formation of a positive dipole during oxygen chemisorption. Suhrmann15 has suggested that the change in the resistance of a bismuth film at 77°K during the adsorption of oxygen can be attributed to the formation of Oz+. Such a conclusion may not be correct since if the bismuth film is an intrinsic semiconductor the de- crease in resistance may reflect the formation of positive holes; on the other hand surface contamination could account for the phenomenon.In l3 Young Cathcart and Cunningham Acta Metallurgica 1946,4 145. 14 Winter Adv. Catalysis 1958 10 196. l5 Suhrmann “Chemisorption” ed. Garner Butterworths London 1957. 76 QUARTERLY REVIEWS covalent bonding the chemisorbed layer is stabilised by electron exchange rather than by electrostatic forces. Such bonds would nevertheless be partially ionic as revealed by the presence of a surface dipole layer al- though the latter may only correspond to about 0.5 D. This would suggest that the metal-oxygen bond is predominantly covalent but with a small electron surfeit on the adsorbed oxygen atom. Tompkins16 concludes that transition metals exhibit definite covalent bonding with both hydrogen and carbon monoxide when these gases are adsorbed at -195” whereas with sp metals a predominantly “electrostatic” bond is formed the dipole moment of which is mainly controlled by the first ionisation potential of the adsorbate.Since the oxygen molecule has a positive electron affinity of 2-3 kcal.mole-l it is feasible that 02- ions could be formed. There is neverthe- less much speculation regarding the exact nature of a chemisorbed ion Le, whether it exists as 02- 0- or 02-. On the basis of the properties of zinc oxide surfaces Barry and Stone1’ suggest that at room temperature either 0- or 02- is prevalent but between 100” and 400” the 02- species predominates. Experiments with oxygen isotopes at low temperature strongly suggest that dissociative adsorption occurs on the oxide surface so that 0- is the more likely.The formation of the 02- species at high temperature is in agreement with the conductivity data of von Baumach and Wagner1* and of Bevan and Ander~0n.l~ As a consequence of the surface dipole at the metal-oxygen interface the apparent work function # of the metal is altered. The change in # corresponding to a surface coverage 8 is given by eqn. (2) where o m is the total number of sites available and M is the dipole moment of the adsorbed molecule. The values for the surface potentials of oxygen on metals are all negative; the only system for which there is good agreement between different workers is tungsten-oxygen. In this case values of about -1.8 v have been obtained by various investigators using different techniques and this is particularly strong evidence that the value refers to a “clean” surface.20$21 In general the variations in the surface potential can be ascribed to the use of contaminated surfaces although crystal orientation may also be a contributing factor.The presence of a potential Vat the gas-metal interface implies that a field Fexists such that F = V / X where Xis the adsorbate thickness. With a value of 1 v for V and 3 for X the field approaches 108 v/cm. Such a field would on general grounds l6 Tompkins Diskussionbeitrag Bunsengesellschaft Bad Homburg 1958. l7 Barry and Stone Proc. Roy. SOC. 1960 A 255 124. l8 von Baumach and Wagner Zphys. Cheni. 1933 B 22 199. l9 Bevan and Anderson Discuss. Furuduy Soc. 1950 8. 238. 2o Gomer and Hulm J . Chem. Phys. 1957 27 1363 21 Mignolet Rec.Trav. chim. 1955 74 685. WYN ROBERTS METAL OXIDATION 77 be expected to have a profound influence on the kinetics of the initial oxidation of metals. The formation of a chemisorbed oxide layer is usually difficult to separate and distinguish kinetically from second- or third-layer formation. In general the primary oxide layer is formed at an immeasurably fast rate at temperatures as low as -195" with a high heat of adsorption (> 50 kcal.mole-1). Oxygen chemisorption occurs with all metals; the only possible exception is with gold but Daglish and Eley22 have recently suggested that even with gold some sites possibly gold atoms at disloca- tion sites are active in chemisorption. Lanyon and T r a ~ n e l l ~ ~ measured the extent of oxygen chemisorption by comparing the volume of oxygen adsorp- tion with the uptake of gases such as hydrogen and carbon monoxide.By making assumptions concerning the nature of the chemisorbed gases the extent of the oxygen uptake could be estimated i.e. whether it involved only the formation of the first layer or whether it involved appreciable incorporation. Making use of physical adsorption techniques in conjunc- tion with chemisorption and Brennan Hayward and Trap- nel12j have estimated the extent of oxygen incorporation. The use of physical adsorption as a complementary measurement is of particular significance especially as chemisorbed gases may induce surface sintering. Table 2 summarises the extent of incorporation using various criteria of coverage and the maximum heat of adsorption for a number of metal-oxygen systems the oxygen pressure usually not exceeding 10-1 mm.It had been expected that a correlation between the d-band structure of metals and chemisorption is a means of systematising adsorption pheno- mena. Nitrogen carbon monoxide ethylene and acetylene were considered to require d-band vacancies i.e. covalent linkages are formed between the adsorbed molecule and the partly filled d-band. This fact was considered to be a distinguishing feature since oxygen chemisorption occurred on both transition and non-transition metals. There is now evidence38 that the adsorption of hydrogen atoms occurs on metals in which d-bands do not contribute significantly to the electronic structure namely on copper silver and gold. This seems to invalidate any simple correlation between d-band structure of the metal and adsorption.There may not be any simple relation between band structure and the formation of surface bonds since the mere presence of an interface may considerably alter the electrdnic distribution in its vicinity. That the surface states of metals can sometimes approximate to those of free atoms was suggested by G o ~ d w i n ; ~ ~ this would imply that activity in chemisorption will be more closely related to the properties of atoms rather than to bulk crystals. In fact 22 Daglish and Hey Preprint lliternational Congress Catalysis Paris 1960. 23 Lanyon and Trapnell Proc. Roy. SOC. 1955 A 227 387. 24 Roberts Trans. Faraday SOC. 1961 57 99. 25 Brennan Hayward and Trapnell Proc. Roy. Suc. 1960 A 256 81. 38 Pritchard and Tompkins Trans. Faraday SOC. 1960 56 540. 5 9 Goodwin Proc.Canzb. Phil. SOC. 1939 35 221. 78 Metal A1 Rh Rh Mo Mo Ta Ta Fe Fe Fe Fe Fe Fe Fe Fe Ca Ba Na Si QUARTERLY REVIEWS TABLE 2. Metal-oxygen systems. Temp. of oxidn. ("a 23 * 23 23 - 183 -183 - 183 23 23 23 23 0 - 195 - 183 - 78 - 195 23 35 23 23 Cu (100 face) 23 c u 23 c u 20 c u - 183 Ni 23 Ni 23 Ni 23 c o 25 No. of layers formed 4-5 1.0 1.0 1.5 1.0 1-0 3.0 1.5 5.0 9.0 up to 10 5-10 10.0 6.0 - -50 >200 a0 1.5 4.0 up to 6 6.0 1.0 2.5 4.0 1.0 6-0 Max. heatof adsorption (kcal. mole-l) 210 110 1 70 - - - 220 100 130 - - - 1 20 - - - - - - - - - - 105 120 and 130 150 115 Criteria for coverage b a b b a a b a b b a b b b b b b b a d d b b b b - C Ref. 25 23 25 25 23 23 25 26 25 24 23 27 28 29 24 30 31 30 32 33 23 34 34 25 35 36 37 * The temperature 23" is used for data determined at room temperature if it is not stated precisely.aSurface coverage estimated by use of chemisorption. bSurface coverage estimated by use of physical adsorption. Coverage calculated from the assumption that 4 8 of oxide is equivalent to one oxide layer. dBased on the number of molecules re- quired to form a single layer on a (1 11) copper surface. 26 Bagg and Tompkins Trans. Faraday SOC. 1955 51 1071. 27 Emmett "Structure and Properties of Solid Surfaces," ed. Gomer and Smith 28 Beebe and Stevens J. Amer. Chem. SOC. 1940 62 2134. 29 Kummer and Emmett J. Arner. Chem. SOC. 1951 73 2886. 30 Roberts and Tompkins unpublished data. 31 Bloomer Nature 1957 179 493. 32 Green and Maxwell J. Phys. and Chem. Solids 1960 13 145. 33 Rhodin J. Amer. Chem. SOC. 1951 73 3143. 34 Allen and Mitchell Discuss.Faraduy SOC. 1950 8 309. 35 Beeck Adv. Catalysis 1950 1 150. 36 Klemperer and Stone Proc. Roy. Soc. 1957 A 243 375. 37 Rudham and Stone Trans. Faraduy Soc. 1958 54 420. University of Chicago Press 1953. WYN ROBERTS METAL OXIDATION 79 Pickup and Trapnel140 have correlated the apparent inactivity of mercury and gold in oxygen chemisorption with the high ionisation potentials of the atoms rather than with the work function of the crystals. Some correlation does exist between the number of electronic states near the Fermi level and the susceptibility to oxygen chemisorption of the metalloid elements arsenic antimony bismuth selenium and tellurium. Apker Taft and Dickey41 have derived the following order of electron densities Bi>Sb>As>Se>Te. This sequence agrees with the observed order of activity in oxygen chemisorption.Such a correlation is compatible with the fact that for oxygen to be adsorbed as negative ions a large electron con- centration in the higher occupied states is probably necessary. It is never- theless a very tentative correlation since the activity of the metals was not strictly defined. Subsequent to the formation of the first oxide layer oxygen interaction will occur with a surface oxide. Although there is evidence for the occur- rence of 0- and 02- species on an oxide surface the use of such symbols to describe the surface species is undoubtedly an over simplification since the interaction of a gas with an adsorbate will rarely result in complete electron transfer. Grimley and T r a ~ n e l l ~ ~ have suggested that neutral pairs may exist on the oxide surface.These could arise from the attraction of positive holes eu and adsorbed oxygen ions their formation taking place as shown by eqn. (3) where 0- e n is the neutral pair and (US) is +02 + (US) + 0- eO (3) a possible site for adsorption on the oxide surface. 4. Oxygen Incorporation It is convenient to discuss the mechanism of oxygen incorporation in terms of the possible rate-controlling processes. Subsequent to the chemi- sorption of oxygen and by this is meant the fast initial oxygen uptake at low temperature and which may therefore include multilayer formation further oxide growth will only occur as a result of the movement of cations outwards through the oxide oxygen inwards or possibly a contribution from both processes.Electrochemical and diffusion experiments with oxides sulphides or halides have shown that in these phases cations anions and electrons are mobile but their mobilities may differ widely. It would be expected that in general owing to the smaller ionic radii of cations cation diffusion would be energetically more favourable than anion diffusion. Complications arising from the breakdown of a compact oxide lattice may however enable direct access of oxygen to the metal covered by a relatively thin oxide layer 100 8 thick. On the other hand it is possible 4 0 Pickup and Trapnell J. Chem. Phts. 1956 25 182. 41 Apker Taft and Dickey Phys. Rev. 1949 76 270. 4 2 Grimley and Trapnell Proc. Roy. SOC. 1956 A 234 405. 80 QUARTERLY REVIEWS that the flow of electrons through the oxide to the oxide-gas interface is the rate-controlling process.(a) The metal-oxide interface. Cabrera and M ~ t t ~ ~ suggested that the initial energy barrier to be surmounted by a cation entering the oxide lattice was greater than any subsequent barrier so that cation entry is rate-controlling during the formation of thin oxide films. The situation at the metal-oxide interface may be represented as in Fig. 1 which shows the change in potential energy of a cation as it leaves the metal enters the oxide and then diffuses through the oxide. The rate of oxidation at temperature T would therefore be expected to be given in the form of eqn. (4) where Xis the oxide thickness at time t E the activation energy - _ - c exp(-E/RT) dX dt (4) and c a constant. According to the theory of E ~ r i n g ~ ~ the rate of reaction is given by eqn.(9 r = (kT/h) exp( AS*/R). exp(- AH*/RT) (5) where AS* is the entropy of activation AH* the heat of activation and the other terms have their usual significance. Since the difference between the activation energy and the heat of activation is insignificant then the general expression for the activation energy of an oxidation process the rate of which is controlled by the barrier to cationic movement at the metal-oxide interface is given by eqn. (6) where W is the entry barrier E = AH* = W - AH + c‘ - qa‘FN (6) (Fig. l) AHis the heat of oxygen chemisorption c’ is the heat of formation of an anion vacancy (the site for oxygen chemisorption) the lowering of the energy barrier W by the superimposed I X I I I * I I M e t a l ! O x i d e I Oxygen I I I I I and qa‘FN is field F arising FIG.1. The potential energy of a cation on moving from a metal into the oxide. from the surface potential of the adsorbed oxygen. a’ is the jump distance for the cation N is Avogadro’s number and q is the charge on the cation. There are very few data relating to the heat of oxygen chemisorption on 43 Cabrera and Mott Reports Progr. Phys. 1948 12 163. 44 Glasstone Laidler and Eyring “The Theory of Rate Processes,” McGraw Hill New York 1941. WYN ROBERTS METAL OXIDATION 81 oxides i.e. the value of AH beyond the monolayer. In for example the oxidation of nickel oxide the slow incorporation process occurs with a heat of about 25 k ~ a l . m o l e - ~ . ~ ~ ~ ~ ~ Since the value of c’ is about 1 ev then AH - c’ beyond the monolayer and therefore E = AH* = W - qa’FN.And so eqn. (4) leads to eqn. (7). Cabrera and Mott suggested that (W - qa’FN) RT = c exp __- d X dt - (7) the constant c be given by N’Qv where N’ is the number of special sites from which a cation may enter the oxide (i-e. position S of Fig. l) Q is the volume of oxide per cation and v is the frequency of vibration of the cations in the lattice. The field 4; may be replaced by V / X where V is the surface potential of the chemisorbed oxygen on the oxide and X i s the oxide thickness. The variation of the term qa’ VN/X with oxide thickness X assuming q = 3e a’ = 2.5 A and V = 2 v is shown in Fig. 2. Thus up to an oxide thickness of approximately 30-40 A the presence of the superimposed field is likely to have a very marked influence on the ob- served kinetics.The only known attempt to detect any influence of an external field on oxidation behaviour is that of Uhlig and B~enner.*~ An electric field of 15,500 v cm.? was applied across a copper surface covered by approximately 700 A in an oxygen environment. No noticeable effect on the oxidation rate was observed which is in agreement with considera- tions similar to those of Fig. 2 the lowering of the energy barrier for cation entry being about 1 kcal.mole-I. Such a field would also be insufficient to create defects in the oxide; its r61e would therefore be merely to direct the random movement of ions already taking place in the absence of the field. 2 30 4 .& 2o 10 FIG. 2. The ‘tfield eflect” as a function of oxide thickness. The following would now appear to be relevant to the formation of thin oxide films on metals (i) The structural characteristics of the metal surface since N’ is related to surface defects.(ii) The surface potential V of the chemisorbed gas since this controls the field F ; the larger the surface potential the more extensive should be the incorporation. In the theory of 45 Uhlig and Brenner Actu Metallurgica 1955 3 108. 82 QUARTERLY REVIEWS Cabrera and Mott V is assumed to be independent of both temperature and pressure. (iii) Since W = U + H where U is the activation energy for cation diffusion through the oxide and H i s the heat of solution of the cation in the oxide then the value of W will be least for a metal-oxide system where both the heat of solution and the energy for cation diffusion are a minimum. Thus an open type oxide structure involving a cation of small ionic radius should result in a minimum of the activation energy.(iv) Electrons are able to establish an equilibrium between metal and adsorbed oxygen on the oxide in a time small with respect to that re- quired for a cation to diffuse through the oxide. That the above considerations are applicable to the initial interaction of gases with metals has been shown by a number of investigators. Bloomer,J6 studying the oxidation of barium films at pressures of about mm. showed that the initial reaction was an acceleratory one which suggests that a nucleation process was occurring which involved specific sites on the barrier surface. Above 35" oxidation continued until the metal had completely oxidised but below this critical temperature the oxide thickness approached a limiting value.Such a critical temperature is understandable since on rearranging equation (7) and substituting F = V/XL eqn. (8) is 1 W(1 - at) _ - - XL Va'qN obtained where 01 = -(R/W) In [(dXL/dt)(l/vL?n")] and dXL/dt is the experimentally defined limiting rate. Therefore when the temperature is l/a the limiting oxide thickness XL is infinite. Critical temperatures of approximately 160" and 400" have been estimated for the nitridation of calcium47 and the oxidation of iron films24 respectively. Since the field is given by F = 4vne/K where n is the number of adsorbed species per unit area e is the electronic charge and K is the dielectric constant of the oxide it would be expected that the contact potential V is a function of the oxide thickness. The assumption of Cabrera and Mott that V is constant during oxide growth implies that n varies inversely as the oxide thickness X.Since n is in general temperature-dependent then V should vary with temperature. A recent approach to gas-metal intera~tion~~ considers the electric field to be confined to narrow tubes these tubes being associated with field-creating species. Thus although the number of tubes n is temperature dependent n cc exp(- AHIRT) where AH is the heat of formation of a field-creating species; the field across any one tube is temperature independent and varies only as the inverse of the oxide thickness. The anodic oxidation experiments of Vermilyea show that with tantalum the oxide grows at the electrolyte- oxide interface. This means that the mobile entities are cations which was a priori most likely owing to the difference between the ionic radii of 46 Bloomer Nature 1957 179 173.47 Roberts and Tompkins Proc. Roy. SOC. 1959 A 251 369. WYN ROBERTS METAL OXIDATION 83 Ta5+ and 02-. The mechanism of cation transport has been recently investigated by Verkerk Wenkel and de G r ~ o t * ~ and they concluded that the mechanism was neither pure vacancy nor pure interstitial. A process termed “place exchange” was suggested by Lanyon and TrapnellZ3 to account for the initial oxidation of metals. The adsorbed oxygen is considered to change places with an underlying metal atom and this process is subsequently repeated further oxide layers being formed. Similarly it has been suggested that the initial oxidation of germanium49 occurs by a “switching” process; this is illustrated in Fig.3. 0 FIG. 3. Place-exchange mechanism. The motivation of the exchange was considered to be derived from the liberated heat of adsorption. Both nickel films and reduced nickel powder exhibit surface regeneration after oxygen chemisorption when kept in vacuo (< mm.) at room t e m p e r a t ~ r e ; ~ ~ ~ ~ ~ 1 ~ ~ whatever the detailed mechanism of the regeneration process it must involve the re-creation of surface cations. Similarly Law,53 and Eley and Wilkin~on~~ observed regeneration during the oxidation of silicon and aluminium respectively. It therefore seems unlikely that surface regeneration requires that heat be liberated during chemisorption. In the initial oxidation of aluminium place-exchange is thought only to apply during the formation of the first 6 A of oxide after which recrystallisation of the initially amorphous oxide results in a change-over of the rate-controlling process to electron transport.A recent in~estigation~~ of the amorphous oxide present on aluminium foil has shown that crystallisation is not observable with the electron microscope until a temperature of about 500” is attained. The process of place-exchange was probably first recognised by de who used photoelectric sensitivity techniques. When oxygen reacts with casium the czsium oxide becomes “buried” in the metal and the mobility of the czesium atoms is so high that the photoelectric sensitivity remains essentially unaltered until almost all the caesium is converted into oxide. This would 48 Verkerk Wenkel and de Groot Philips Res. Repts.1958 13 506. 49 Green Progr. in Semiconductors 1959 4 37. j0 Roberts and Sykes unpublished data. 61 Oda Bull. Chem. Soc. Japan 1954 27 465. 52 Anderson and Klemperer Nature 1959 183 899. j3 Law J . Phys. and Chem. Solids 1958 4 91. 64 Eley and Wilkinson Proc. Roy. Soc. 1960 A 254 327. j5 Thomas and Roberts J . Appl. Phys. accepted for publication. 56 De Boer “Electron Emission and Adsorption Phenomena,” Cambridge New York 1935. 84 QUARTERLY REVIEWS indicate that either a layer of czesium atoms remains on the oxide surface even after extensive oxidation had occurred or that the czesium surface was initially oxide and the photoelectric measurements thus referred to an oxide layer rather than to the metal. Although place-exchange suggests a process quite distinct from cation- diffusion controlled by a superimposed field the two are at least pheno- menologically identical.The place-exchange mechanism has been suggested to involve an activation energy Eexp. that increases linearly with oxide thickness X so that eqn. (9) holds. This equation clearly has only sig- nificance for small values of X . Oxidation controlled by cation diffusion in the presence of a superimposed field implies a limiting value for the activation energy since eqn. (10) is obtained from equation (7). A E = W - (qa‘VN/X) (10) distinction between two mechanisms the activation energies of which vary according to equations (9) and (lo) is not easy since the term qa‘VN/X decreases almost linearly with X . Now the kinetic equations which follow from equations (9) and (10) are (11) and (12) respectively x K log t and 1/X cc log t X being the oxide thickness at time t Over restricted ranges of oxide thicknesses both equations (1 1) and (12) may be found to describe the results equally well and a distinction is only possible if constants derived from them are shown to have a reasonable physical significance.(b) Electron transport and the logarithmic law. Electron availability and transport is an essential prerequisite for the occurrence of oxidation. According to wave mechanics a small proportion of the electrons which are incident on a potential barrier will penetrate it. This phenomenon called “the tunnel effect” is responsible for the flow of electrons between two metals the surfaces of which are inevitably covered by oxide. Up to an oxide thickness of 30 8 or so electrons are therefore considered to move through the oxide from the metal by a tunnelling process and it was Mott5’ who first suggested that the slow step in oxidation was electron transport to the oxide-gas interface by this mechanism.He later abandoned this concept for control by ionic transport. More recently Hauffe and I l ~ c h n e r ~ ~ have revived the electron transport rate-controlling process to explain the results of Scheuble for the oxidation of nickel. It is relevant to consider the possible mechanisms by which electrons may be made available. 57 Mott J . Inst. Metals 1939 65 333. 58 Hauffe and Ilschner 2. Elektrochem. 1954 58 382. WYN ROBERTS METAL OXIDATION 85 Subsequent to the formation of the initial oxide layer the energy re- quired to remove an electron from the metal is no longer related to the work function of the clean metal.The system is now a composite one involving a metal-semiconductor interface with the possibility of oxygen adsorbed on the semiconducting oxide surface resulting in electron acceptor levels being made available. The potential-energy diagrams shown in Figs. 4 5 and 6 may therefore be considered to represent the conditions for a FIG. 4. Clean metal in a vacuum. FIG. 5. “Clean” metal and chemisorbed oxygen. CONDUCT1 ON BAND ~-IFER%~;~~EL OF M E T A L ADSORBED OXYGEN LEVEL t VALENCE BAND N - O X I D E FIG. 6. Possible non-eqdlibriim electron energy levels in a metal-oxide-oxygen system. “clean” metal a “clean” metal on which oxygen has been chemisorbed and a metal in contact with oxide on which oxygen is chemisorbed.In the case of metal + chemisorbed oxygen electrons may penetrate the potential barrier #+ the work function of the metal by the tunnelling mechanism 0- ions being formed on the surface. If chemisorption and subsequent incorporation result in the formation of an n-type oxide of low work function electron transfer may occur from the oxide to the metal and if a simultaneous movement of cations in the opposite direction does not occur a space charge normally called the Schottky depletion layer is formed. Fig. 6 illustrates a possible non-equilibrium state Fig. 7 the equilibrium attained between oxide and metal and Fig. 8 the equilibrium between oxide metal and chemisorbed oxygen. The depth of the space- charge layer is related to the density of the donor states in the oxide by 86 QUARTERLY REVIEWS eqn.(13) where xo is the depth of the Schottky barrier E is the dielectric constant of the oxide e is the electronic charge and A+ is the height of Ad the barrier. In the case of a low density of donor states say 1015 per ~ r n . ~ then with E = 10 and A$ = 1 ev the space charge region is approximately lo-* cm. deep whereas for a site density of 1019 per ~111.~ the depth is only cm. Equilibrium is established when the Fermi levels in the metal and the oxide are the same. OXYGEN LEVEL N OXIDE I FIG. 7. Electron transfer from oxide to metal resulting in an energy barrier to further transfer and a space-charge layer of depth x,,. O X I D E FIG. 8. Electronic equilibrium established between metal oxide and adsorbed oxygen. Oxygen chemisorbed on an oxide surface generally results in electron levels being made available which are much lower than the conduction band of the oxide (Fig.6). At equilibrium the highest filled oxygen level will be at the height of the Fermi level in the semiconductoi (Fig. 8). If the semiconductor is an intrinsic semiconductor the effect of the positive space-charge could result in the filled band's being raised sufficiently to donate electrons to the adsorbed oxygen thus creating holes in the filled band. The latter process could continue until the top of the filled band is at the same height as the Fermi level so that the height of the barrier is restricted to Q (Fig. 8) the difference in energy between the conduction and filled bands. Oxygen-ion formation is therefore possible either by electron tunnelling from the metal into the conduction band of the oxide provided the width of the energy barrier is not prohibitive or in special circumstances from the filled band of the oxide.WYN ROBERTS METAL OXIDATION 87 Garner Gray and Stone59 have suggested that with an oxide capable of exhibiting variable valency bonding of oxygen to surface cations may take place with an attendant valency change e.g. 2Cu+ + QO + 2Cu2+ + 02-. That shallow electron-emitting centres do exist on metal surfaces un- doubtedly covered by an oxide layer is borne out by the ability of certain metals to initiate free-radical reactions at room temperature. This pheno- menon termed exoelectron emission,60 is thought to account for the forma- tion of hydrogen peroxide from water in the presence of abraded metal and illustrates the capacity of “freshly prepared” surfaces to act as an electron source.The boundary layer theory of chemisorption which is based on the presence of a surface barrier to electron transport has been applied by HauffeG1 to derive the kinetics of the uptake of oxygen by oxides. Since oxygen chemisorption increases the work function of the adsorbent the rate of oxygen chemisorption by the oxide surface is given by eqn. (14) where Xis the concentration of surface atoms at time t po is the height of the energy barrier at the beginning of chemisorption (Le. X = 0) A$ is the change of the work function of the surface as a consequence of adsorption e is the electronic charge T is the temperature and k is the Boltzmann constant. By expressing A$ in terms of the concentration of chemisorbed oxygen the concentration of holes in the oxide and the potential difference between the interior of the oxide and the surface Hauffe derived the logarithmic eqn.(15) where b = 47rea/~ E is the dielectric constant of the oxide and a is the distance between the surface of the oxide and the centres of charge of the chemisorbed atoms. Engel and Hauffe62 find that eqn. (15) is valid for the first 10 min. of the oxidation of nickel oxide at 25” after which there is a marked deviation. This logarithmic expression sometimes referred to as the Elovich or Roginski- Zeldovitch equation has been shown to be applicable to a large number of systems involving an activated process. Taylor and ThorP3 suggested that the prime function of the adsorbed gas is to create sites which over the course of the activated process decay at a bimolecular rate.A similar equation has been derived by Porter and TompkinsG4 and by Jennings and jg Garner Gray and Stone Proc. Roy. SOC. 1949 A 197 294. 6 o Grunberg Proc. Pliys. SOC. 1953 66 153. 61 Hauffe Adv. Catalysis 1955 7 213. 62 Engel and Hauffe quoted in ref. 61. 63 Taylor and Thon J . Amer. Chem. SOC. 1952 74 4169. 64 Porter and Tompkins Proc. Roy. SOC. 1953 A 217 529. 88 QUARTERLY REVIEWS Stone65 who assumed a linear increase in activation energy with surface coverage. More recently Gundry and TompkinP have invoked the presence of an intermediate chemisorbed state which must be passed through before the adsorbed molecule attains its final equilibrium state. The initial bonding to the surface is thought to involve d-orbitals only; this then transforms to a stronger hybridised dsp final state.So long as the free energy of adsorption decreases linearly with coverage the Elovich equation can be deduced. The changing hybridisation of surface bonds may play an important r81e in many slow oxidation phenomena. The Elovich type equation is however so frequently obeyed by gas- solid reactions that to invoke a particular mechanistic model merely on the basis of linearity of plots is not a satisfactory criterion of validity. There is also the difficulty of obtaining a unique value for the constant to. Landsberg6' has suggested that the ubiquitous nature of the logarithmic equation in chemisorption reflects the same basic mechanism. He derived the equation (16) where t o = l/mNapS, m is a constant 18 is the efkctive area over which the sites become invalidated by the adsorption of a single molecule N is the number of impacts of the gas molecules with the surface per unit area per unit time a is the effective contact area between a mole- cule and the surface and So is the number of sites per unit area at the commencement of the reaction.This is a denial of the Langmuir hypo- thesis that the total number of adsorption sites is constant. to would be expected to be inversely dependent on pressure. Another interpretation of the logarithmic equation is that due to Uhlig68 who considers electron flow to be controlled by a space-charge. The space-charge is envisaged as being composed of two parts (1) a uniform charge-density layer next to the metal and (2) a diffuse-charge layer beyond the uniform layer the latter arising from the presence of electrons trapped at lattice imperfections within the oxide.5. Dependence of Oxygen Uptake on Pressure The process of metal oxidation may be considered as three concurrently operating processes adsorption desorption and incorporation. The rates of these processes can be expressed by the following equations Vads. = klcxacs (17) where a is the pressure dependence of the adsorption process cs is the number of bare sites per cm.2 and k = (W,/h) (f*/fc,"fe> exp( -E,/RT) 65 Jennings and Stone Adv. Catalysis 1957 9 441. 66 Gundry and Tompkins Trans. Faruduy Sac. 1956 52 1609. 67 Landsberg J. Chem. Phys. 1955,23 1079. 68 Uhlig Actu Met. 1956 4 541. WYN ROBERTS METAL OXIDATION 89 where c is the gas phase concentration and E the activation energy for adsorption.Vdes. == k2Ca (18) where k2= (kT/h) (f+Ka) exp( -E,/RT) Ca is the concentration of adsorbed species and E the activation energy for desorption; Vim. = kinc.ca (19) where kine. is the rate constant of the incorporation process. A steady state will be established in the adsorbed layer so that lilcxacs - k2ca - ki,,.ca = 0 (20) Since c6 + Ca = N the total number of surface sites then Thus the rate of gas uptake V is given by V = Vads. - I/dea. = klcxacs - k2cs = nTklkinc.Cra/klCxa + k2 + kine. If kine. 9 k2 + klcXa then V = NklcXa which means that the rate-deter- mining step is that of adsorption. However if k,cXaBk2 + kine. then Y = khc.N and incorporation is rate controlling. On this basis alone we would expect the slow step to be adsorption at low pressures and in- corporation at high pressures.The influence of phase-boundary equilibria the nature of the adsorbed layer etc. complicate these conclusions and are discussed further. Wagner first directed attention to the pressure-dependence of the chemical composition of the oxide and according to von Baunach and Wagner18 interstitial zinc ions are formed in zinc oxide according to the equation (21) Zn2+ + 02- + Zni2+ + 2e + +O,(g) (21) where Zni2f represents an interstitial ion formed from a lattice ion Zn2+. It being assumed that the defects are in dilute solution and do not interact the equilibrium constant for the formation of interstitial ions is [Zni2+] [e-I2 p o .+ [Zn2+] [02-] K = Within the range of composition possible for the zinc oxide phase [Zn2+] and [02-] do not change appreciably so the equation can be put in the form Kl = [Zni2+] [e-]po2+ 90 QUARTERLY REVIEWS Since electrical neutrality requires that [Zni2+] = +[e-] the change in the concentration of interstitial zinc ions with oxygen pressure should be given by [Zni2+ J = (K,/4)5 po2-i Thus if incorporation proceeds by the diffusion of Zni2+ the expected pressure dependence is - 116.GrimleyGg and Grimley and T r a ~ n e l l ~ ~ have derived growth laws for oxide films by considering the equilibria that may be set up at the metal- oxide and oxide-gas interface. n- and p-type oxides with rates controlled either by cation transport or by surface reactions have been examined for two general cases (a) Surface saturated by field-creating ions. The dependence of the deiived rate laws on pressure is particularly significant.Linear laws are derived for p-type oxides when surface reaction is rate- determining similarly for an n-type oxide when the transport of interstitial cations is rate-determining. A logarithmic equation with a pressure de- pendence of 0.25 is predicted for a p-type oxide when movement of cation vacancies is rate-controlling. (b) Neutral pairs in the adsorbed layer. An equilibrium is considered to exist between neutral pairs and field-creating ions. Neutral pairs are considered to arise from the attraction of positive holes and adsorbed oxygen ions their formation taking place as follows where 0- e n is the neutral pair and (0s) is a possible site for adsorption on the oxide surface. The neutral pair may subsequently dissociate to form field-creating species,O-/ads.The position of equilibrium in the above equation is important since although the oxide surface may be apparently saturated not all the species need be of the field-creating kind. The derived pressure dependences are shown to be a function of the nature of the adsorbed species; values of -0-75 0.25 and 0 are calculated for a system where the surface species are assumed to be 02- 0- and 02- respectively. With n-type oxides the growth law is insensitive to the nature of the surface layer. thick) has been suggested to be controlled in certain cases by electron transport;58 if tunnelling is the mechanism then the oxygen uptake should be independent of pressure. If tunnelling is not feasibke and the electrons have to surmount an energy barrier E the rate of electron transport is of the form Rate cx exp(-E/RT) where E is related to the space charge at the surface.Since the space charge is a function of the concentration of adsorbed species for an unsaturated The Formation of thin oxide layers (< 50 6 9 Grimley “Chemistry of the Solid State,” ed. Garner Butterworths London 1957 336. WYN ROBERTS METAL OXIDATION 91 adsorbed layer the rate of electron tiansport will be some function of the oxygen pressure although the exact form of the pressure dependence may be difficult to predict. If the concentration q of adsorbed species is related to the pressure by q cc p n and the rate of electron trans- port is some linear function of q then the value of rz also gives the dependence of oxygen rate on pressure. Similarly in rate control by cationic diffusion in the presence of a superimposed field F the magnitude of which is a linear function of the concentration of chemi- sorbed species the oxidation rate will be related to pressure.If a saturated adsorbed layer is formed the rate will be pressure independent but if an equilibrium is set up between oxygen molecules at a pressure p and oxygen atoms the surface concentration of atoms and similarly the rate will be proportional to poe5. Table 3 gives values of n for a number of TABLE 3. Gas-metal systems. Metal Gas Temp. dependence Ref. Pressure Ca Fe Fe Fe A1 c u Mg Si U ("a 23 - 80 22 to 95 0 20 0 23 25 180 (4 1.0 0 0.28 0.2 0.6 0.75 0.8 0.52 0.15 47 24 24 23 7 23 70 71 72 gas-metal systems. There is the general difficulty of formulating a mechan- ism for a gas-metal reaction which is compatible with the dependence of rate on pressure.Eley and Wilkin~on,~ for example suggested that in the oxidation of aluminium the value 0-6 is evidence for a dilute ideal film of oxygen atoms but that owing to interaction within the monolayer there is a small increase in the value of rz over that predicted theoretically. At -80" the oxidation of iron films is independent of pressure and this has been ascribed to a saturated surface layer of some kind; in the tem- perature range 0-100" the mean values of 0.2 and 0.28 are compatible with a surface equilibrium involving species which influence cationic diffusion. 6. Nucleation and Anisotropic Oxidation The realisation that nucleation is an important factor in oxide formation stems from the work of B~5nard~~ and his collaborators.As a consequence of the microtopology of metal surfaces it would be expected that pre- 70 Sack Diskussionbeitrag Bunsengesellschaft Bad Homburg 1958. 71 Law J. Phys. and Chem. Solids 1958 4 91. 72 Anderson and Roberts J. Chem. SOC. 1955 3946. 73 Bardolle and BCnard Rev. met. 1952 49 613. 92 QUARTERLY REVIEWS ferential reaction sites exist on the surface resulting in the initiation of oxide growth at isolated points on the surface. The number of nuclei formed will depend on the activation energy for surface nucleation which may vary over the metal surface; whether nucleation is initiated as a two- dimensional monolayer or whether three-dimensional nuclei are formed is not known. Bloomer74 suggested that since the initial oxidation of barium appeared to start from a fixed number of nuclei the rate of oxygen uptake up to the commencement of the second layer would be expressed by eqn.(22) where Xis the volume of oxygen adsorbed at time t k is a constant dX/dt = ktn* (22) and n* is a non-dimensional parameter. The value of n* is considered to reflect the geometry of the nucleus and its mode of growth. For flat circular nuclei n* has the value 0-5 whereas if the oxide grows into hemi- spherical caps i.e. a second-layer process commences before the comple- tion of the monolayer a value of 0.66 would be expected. Sack70 has reported values between 0.2 and 0.7 for the oxidation of magnesium at 27" and Roberts and Tompkins4' a value of 0.19 for the nitridation of calcium films at 23". Although a small value of n* implies a large con- centration of nucleating centres it is difficult to give n* an exact physical meaning.The orientation of alien crystals by an anisotropic substrate surface termed epitaxy is important in relation to the occurrence of anisotropic oxidation and the possibility of stresses that arise in oxide films sub- sequently leading to cracking. van der M e r ~ e ' ~ developed a theory of epitaxy that involved as a necessary prerequisite the formation of a pseudomorphic layer. According to this theory such an oriented layer will occur provided the misfit is not greater than about 14%; it also predicts increasing instability of strained layers with increasing thickness. Rhodin explains the anisotropic behaviour of thin oxide layers on copper in terms of the van der M e r ~ e ~ ~ model of epitaxial growth. Oxidation is undoubtedly dependent on the crystallographic plane being oxidised.R h ~ d i n ~ ~ Gwathmey and B e n t ~ n ~ ~ and Lustman and Mehl" showed that of the most commonly occurring planes in a copper surface the (1 10) is the most readily oxidised and the (1 11) plane the least. Young Cathcart and G~athrney'~ found that at 150" the rate of oxidation of the (100) plane is four times that of the (311) although the limiting oxide thickness on the former is ten times that on the latter. Bhard and T a l b ~ t ' ~ observed at 900" that the oxidation rates for different planes of copper decreased in the following order (210~ (211) (110) ( l l l ) (loo) (123). 74 Bloomer Brit. J . App. Phys. 1957 8 321. 75 van der Merwe Discuss. Faraday SOC. 1949 5 208. 76 Gwathmey and Benton J. Phys.Chem. 1941 46 969. i i Lustman and Mehl Trans. Amer. Inst. Mining Met. Engrs. 1941 143 1. Young Cathcart and Gwathmey Acta Metallurgica 1946 4 145. BCnard and Talbot Compt. rend. 1948 225 41 1. WYN ROBERTS METAL OXIDATION 93 Such a sequence was similarly reported earlier by Gwathmey and Benton at 1OOO". The difference in behaviour at low and high temperatures suggests that different mechanisms are operating. The concept of metal atoms entering the oxide lattice only at kink sites possibly accounts for the anisotropic behaviour; a surface with a large number would be expected to oxidise faster than one with fewer. This is implicit in the theory of Cabrera and Mott. Anisotropic behaviour may in some cases be complicated by the crystallisation of an initially amorphous layer or of recrystallisa- iion.Such processes have recently been observed with copper at 150". Crystallisation of an amorphous film could result in the oxide's becoming less protective since movement can occur more easily along grain bound- aries chan within grains. Gulbransen and Coplanso have used electron optical techniques to investigate the possible influence of disloca tions defects and internal stress on the chemical reactivity of the metal surface. Thin oxide whiskers 100-150 A in diameter occur when pure iron reacts with oxygen at 400"; these whiskers may grow to a length of lo5 A. With water vapour at 400" oxide platelets are formed. These observations together with those of Pfefferkorn,81 strongly suggest a growth mechanism in which the metal structure itself determines the progress of growth the growth of a whisker taking place only at the tip where one or more screw dislocations emerge.Bknard Grarnlund Oudar and Durets2 have studied the formation of oxide and sulphide nuclei on copper. A rather surprising feature is the constant rate of formation of nuclei at 550" and an oxygen pressure of 6 x mm. With aluminium Roberts and Thomass3 have observed the formation of nuclei at temperatures around 550" the geometrical form of the nuclei being dependent on the pressure conditions. At a pressure less than mm. needle-shaped nuclei appear and at atmospheric pressure an oxide layer composed of contiguous crystals of diameter - 0-2 p are formed. Recrystallisation of the initial amorphous film is suggested since electron diffraction shows that crystalline aluminium oxide of lattice parameter 7.9 A is formed.7. Thick Oxide Films Although the general practice of discussing oxidation regions in terms of kinetic laws is an ambiguous one the parabolic growth law is correctly associated with oxidation occurring in the presence of a thick oxide layer usually 1000 A or more deep. The oxide layer must be coherent and yore- free and above about 300" the kinetics should conform to eqn. (23) dx/dt = k1/x (23) Gulbransen and Coplan Discuss. Faraday Soc. 1960 28 229. Pfefferkorn Naturwiss. 1953 40 551. 82 Benard Grmlund Oudar and Duret Diskussionbeitrag Bunsengesellschaft 83 Thomas and Roberts J. Appl. Phys. accepted for publication. Bad Homburg 1958. 94 QUARTERLY REVIEWS where x is the oxide thickness at time t and kl is a constant. If x = x, at t = 0 then This is probably the best way to express the law as it enables the initial uptake to be discarded since the origin of time may be set at any desired oxide thickness.Thus a plot of t/x against x gives a straight line of slope 2k1. It may well be that the value of y o from the intercept has little quantitative significance in view of uncertainties regarding non-isothermal conditions and departures from parabolic behaviour at small times. Wagner suggested that diffusion in oxide (or sulphide) phases may in general be interpreted as migration processes of ions and electrons whereas the migration of electrically neutral atoms or molecules can be neglected. There are therefore two limiting cases which are illustrated in Fig. 9 (1) Positively charged cations and electrons may migrate in the same direction from the metal-oxide interface to the outer surface.(2) Negatively charged anions migrate inwards and electron movement oc- curs in the opposite direction. x - XO = x = 2kl(t/x) - 2x0 C A T I O N ELECTRON P (1) ______L (2) ANION ELECTRON - FIG. 9. Diflusion in oxide phases. Wagner applied the model of a compact oxide layer with lattice defects to the derivation of the parabolic rate law assuming that cation diffusion within the oxide is rate controlling so that a concentration gradient exists between the metal-oxide and oxide-oxygen interfaces. The concentration of defects is generally so low that analytical procedures are of little value. Engel,s4 using an electrochemical method has succeeded in measuring the defect variation in an iron(@ oxide layer.He has shown a linear increase of defect concentration from the metal-oxide to the oxide-gas interface. It is significant that the defect concentration at the metal interface was not in fact 0 but at 900" amounted to 6%. Such a gradient of defect concen- tration apparently exists only when the iron(I1) oxide layer is in intimate contact with the metal phase; poor adhesion results in equilibrium being established with the Fe,O phase which leads to a constant defect con- centration in these positions of the iron@) oxide layer. If transport processes (electron cation or anion movement) within the oxide are more rapid than any of the possible phase boundary equilibria 84 Engel 2. Elektrochem. 1959 63 835. WYN ROBERTS METAL OXIDATION 95 non-equilibrium conditions then exist at the interface.The oxidation of iron under different conditions demonstrates clearly the influence of phase- boundary conditions. In oxygen at high temperature a number of oxide phases are formed and the parabolic law is obeyed; in carbon monoxide- carbon dioxide mixtures above 900" a linear rate law with only iron(I1) oxide occurs while in a hydrogen-water environment above 950" para- bolic dependence with iron@) oxide formation takes place. Evidently the high defect concentration in iron(1r) oxide ensures a high diffusion velocity compared with the surface equilibria existing in a carbon monoxide- carbon dioxide environment whereas in a water-hydrogen atmosphere the surface equilibrium proceeds sufficiently rapidly so that the diffusion of cations through the iron(I1) oxide layer represents the slower process.That diffusion through an oxide film was not solely responsible for the oxidation rate was first emphasised by Evans85 who obtained a more general equation by solving the simultaneous equations which represented the law of mass action at the boundary and transport across the film. The mixed parabolic equation (24) which expresses a reaction influenced by both diffusion and boundary processes has two obvious limiting cases the simple parabolic law and the rectilinear law. The occurrence of the latter would suggest that the replenishing of the oxidising gas cannot keep up with the rate at which oxide growth occurs so the oxygen-replenish- ment rate takes over control. GulbransensG has recently developed Wagner's diffusion picture of oxidation by combining it with the transition-state theory.This development gives an expression for the parabolic rate con- stant involving absolute physical constants two entropy and two heat of activation terms. In the case of the formation of nickel oxide a p-type oxide vacancies are formed according to equation (25); one cation vacancy n c is formed together with two positive holes and one oxygen ion 0,-. The concentration of vacancies is therefore given by where AH" and AS" are the standard heat and entropy of formation respectively N is Avogadro's number R the gas constant T the tempera- ture and po the oxygen pressure. Since the parabolic rate constant is according to Mott given by kl = 2QD(n - n,) where nl and I t 2 are the number of vacancies per ~ m . ~ at the oxide-gas and oxide-metal interfaces respectively and L2 is the volume of the oxide per cation then where AGO is the standard free energy of formation of vacancies.ay2 + py = kt (24) +O,(g) + 02- + U C + 2 0 (25) nc = (N/49)(p0,)* exp(- dH0/3RT) exp( dS0/3R) kl = [2i2DN(p0)i exp( - d G"/3RT)] /49 85 Evans Trans. Electrochem. SOC. 1924 46 247. 86 Gulbransen Proc. Gothenburg Conference on Solid State Reactions 1952. 96 QUARTERLY REVIEWS From Zener’ss7 theory of diffusion the diffusion coefficient D is related to the vibration frequency V of the lattice the free energy dG* of the diffusion process and the distance a between the jumps by D = ya2vexp(- AG*/RI’) where y is a constant characterising the nature of the jumps. Thus the rate constant is given by equation (26) k = 2ya2DvN(p,,)* exp[( AS0/3 + dS*)/R] exp[- AH0/3 + AH*/RT] Two metal-oxide systems have been considered in the light of the above theory.For nickel the agreement between the experimentally determined AS* the entropy of activation of diffusion and the calculated value is good. With cobalt the agreement is poor which possibly suggests that the assumed mechanism is not the correct one. A second model for an oxide assumes it to be composed of macroscopic as well as lattice defects. These macroscopic defects are considered to be pores cracks or blisters formed as a consequence of internal stresses in the oxide film. Pilling and Bedworth’s rule,8s which still arouses much discussion is a consequence of such considerations. It states that an oxidation process will obey a linear law if the oxide occupies a smaller volume than that of the consumed metal.A linear law can also occur if the metal forms an oxide of larger volume than that of the consumed metal but stress relief results in breakdown of the oxide film. This breakdown may be complete or partial and in the latter case a porous oxide would be formed over a compact oxide film with the result that the rate will be controlled by ionic diffusion which occurs by a defect mechanism in a com- pact oxide film of constant thickness; hence the linear law. Aylmore Gregg and Jepsonsg have recently investigated the porosity of a number of oxides. Metals such as calcium magnesium tungsten and uranium form porous oxides which oxidise at a linear rate while cobalt and copper form coherent non-porous oxides and obey the parabolic law. A phenomenon which has as yet no unambiguous explanation is that of breakaway.This has been observed during extensive studiesg0 of the high-temperature oxidation of magnesium in which a rate process which has been constant for maybe 10 hours or more suddenly accelerates. Cracking and re- crystallisation of the oxide and stress relief have been suggested as the cause. The exact origin of the stress is uncertain but factors such as thermal gradients phase changes at the metal-oxide interface and diff- erences in thermal expansion of oxide and metal may be important. Evansg1 considers that “breakdown” of an oxide scale can occur in one (26) Zener J. Appl. Phys. 1951 22 372. Pilling and Bedworth J . Inst. Metals 1923 29 529. Gregg and Jepson J. Inst. Metals. 1958 87 187. 1 3 ~ Aylmore Gregg and Jepson J. Electrochem.Soc. 1959 106 1010. 91 Evans Trans. Electrochem. SOC. 1947 91 547. WYN ROBERTS METAL OXIDATION 97 of three ways (a) Blistering which involves detachment of the oxide but no real breakage may be expected where adhesion is poor and cohesion good. Rectilinear thickening should occur dX/dt = Kl if the cracks in the blister wall admit oxygen to the cavities so that new oxidation starts at the base of each blister ; but otherwise it should obey a logarithmic growth law since the outward movement of cations although aided by rifts normal to the surface is interrupted by cavity barriers parallel to the surface. (b) Shear cracking involving breakage but no detachment which may be expected where adhesion is good and cohesion poor. This should lead to a rectilinear a parabolic or an intermediate growth law.(c) Flaking is probably rare and may be considered to be that process likely subsequent to blistering. It involves the separation of the oxide as a flap so that the oxidation should resume at the initial rate. If the assumption is made that an expression of the form of (27) de- dx - cc f(x)exp(-E/RT) dt scribes the rate of cracking at an oxide thickness x where E is the activation energy of the cracking process then for a linear growth law the rate of oxide cracking and formation must be equal. By comparing equations (27) and (7) it is seen that the experimentally determined activation energy is a composite quantity involving the terms F W and E. The ideas of Evans on scaling have been extended and equation (28) has been derived by Haycockg2 to describe the kinetics of a scaling process k k P x = - l n k1 kp - kl(x - kit) where kp and kl are the parabolic and interface reaction rate constants.The interface reaction which depletes the barrier layer may be either vaporisation of the primary reaction product or the formation of a porous scale possibly by crystallisation grain growth or mechanical cracking. If vacancies remain at the metal-oxide interface they may coalesce and fonn cavities; this would ultimately lead to considerable porosity at the interface which in turn would lead to considerable decrease in contact area. These irregularities should affect the kinetics of the reaction and Birchenallg3 has developed a number of kinetic equations based on simple models of pore growth at the interface. The ability of porosity to form is thought to be related closely to the plastic properties of both metal and oxide.A minor constituent present in the metallic phase may influence oxida- tion in the following possible ways (1) A new oxide of the minor com- ponent is formed on the outer surface and if cation diffusion is rate 92 Haycock J. Electrochem. Soc. 1959 106 771. 93 Birchenall J. Electrochem. SOC. 1956 103 619. 98 QUARTERLY REVIEWS controlling the new oxide layer will influence the oxidation rate. Quarrelg4 has suggested that the oxidation resistance of heat-resisting steels is due to the formation of a stable surface spinel. On the other hand Wagnerg5 has recently shown that if an alloy consisting of two metals A and B is oxidised one oxide only may be formed; with alloys rich in A only A 0 is formed and with alloys rich in B only BO occurs.At intermediate com- positions of the alloy the formation of an oxide does not correspond to a stable state and therefore the two oxides A 0 and BO are formed simul- taneously. Under these conditions diffusion and hence oxidation rates will depend on the spatial distribution of the two oxides in the scale. (2) The effect of substituting ions OF valency different from the cations of the parent oxide is to change the cation vacancy and positive-hole concentra- tion and hence the reaction rate. In an oxide containing cation vacancies the introduction of a minor constituent of higher valency than the parent metal will increase the number of vacancies and hence the oxidation rate (Fig. 10). Conversely in an n-type oxide a minor constituent of lower FIG.10. The influence of the addition of Cr3+ on the defect structure of nickel@) oxide. valency will increase the oxidation rate and one of higher valency will diminish it. The predicted influence of alloying elements on the concen- tration of vacancies and hence the rate of oxidation is due to Hauffeg6 and js known as the “Valency Rule”. Wagner and Ziemensg7 have shown that the addition of chromium to nickel alloys in small concentrations in- creases the oxidation rate presumably by increasing the cation vacancy Concentration in nickel oxide when some Ni“ ions are replaced by Crw. Alloying elements may decrease the oxidation rate of iron above 700” by lowering the composition range of wustite which constitutes 95% of the oxide scale and therefore most probably is the phase with the greatest natural groivth rate.Additional protection could be observed if the alloy- ing element lowers the range of defect concentration in the spinel phase or decreases the ion mobilities. If the mobilities of the cations are sufficiently 94 Quarrel Nature 1940 145 821. y5 Wagner J . Electrochem. SOC. 1952 99 369. 9G Hauffe Prog. Metnl Phjssics. 1952 4 71. 9i Wagner and Ziemens Acrn Chem. Scand. 1947 1 547. WYN ROBERTS METAL OXIDATION 99 decreased a changeover to anion control could occur. (3) If the minor component oxide is incompatible with the parent oxide embrittlement with subsequent cracking can occur thus leading to a possible “breakaway” reaction. (4) If the alloying element has a greater affinity for oxygen than the solvent metal the minor element may oxidise below the surface oxide in an area where the partial oxygen pressure is too low to cause oxidation of the parent metal. This phenomenon termed internal ~ x i d a t i o n ~ ~ has been suggested to occur when (a) oxygen is soluble in the alloy and is able to diffuse fairly rapidly within the alloy and (b) the minor constituent forms an oxide which is thermodynamically more stable than the parent metal. It is a pleasure to acknowledge many stimulating discussions with Professor F. C. Tompkins F.R.S. Rhines Trans. Amer. Inst. Min. Met. Engrs. 1940 137 246. 4*

ISSN:0009-2681    

DOI:10.1039/QR9621600071

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

RECENT ASPECTS OF SESQUITERPENOID CHEMISTRY By T. G. HALSALL (DYSON PERRINS LABORATORY OXFORD UNIVERSITY) and D. W. THEOBALD (MANCHESTER COLLEGE OF SCIENCE AND TECHNOLOGY MANCHESTER 1) THE sesquiterpenoids were last the subject of a Quarterly Review in 1957.l Since then interest in this field has continued and much progress has been made notably for sesquiterpenoids containing medium-sized rings. This Review surveys recent developments in the field of those sesquiter- penoids containing nine- ten- and eleven-membered carbocyclic rings. As is to be expected with such medium-ring compounds a rich display of transannular reactions is a prominent feature of their chemistry. A section on the biogenesis of these compounds is also included. Nine-membered Rings.-CaryophyZZene. One of the first of these compounds to be studied was the sesquiterpene caryophyllene isolated from oil of cloves and studied by Simonsen Ruzicka and their co- workers.2 After some confusion of nomenclature two substances are now distinguished caryophyllene (1) and an isomer isocaryophyllene (2) which differ in the geometry of the endocyclic double bond.Caryophyllene Cl5HZ4 is a doubly unsaturated and so bicyclic hydrocarbon. At the time of the previous review of its chemistry its structure had been established as (1) and the evidence for it was described in detai1.l Caryophyllene on treatment with acid undergoes a number of interesting cyclisations. Among the products are caryolan-l-ol(3) and the hydrocarbon clovene (4).l On more vigorous acid-treatment two other tricyclic hydrocarbons are formed isoclovene and $-clovene.$-Clovene has been tentatively given the formula (9 while recent X-ray studies3 have shown that isoclovene Barton and de Mayo Quart. Rev. 1957 11 189. Simonsen and Barton “The Terpenes,” Cambridge Univ. Press Vol. 111 1952. Clunie and Robertson Proc. Chern. SOC. 1960 82. 101 102 QUARTERLY REVIEWS can be represented by (6). The peculiar nature of the conversion of caryolan- 1-01 (3) into isoclovene has been observed and the annexed possible routes have been propo~ed.~ It is evident from their complexity that this I (3) / and related reactions in the caryophyllene series merit further investiga- tion. Betulenols. A group of compounds related to caryophyllene the betulenols isolated from the buds of white birch has been the subject of disagreement between two s c h o o l ~ .~ ~ ~ a-Betulenol C15H240 was shown to be a doubly unsaturated secondary alcohol and therefore bicyclic. Caryophyllane (7) was isolated from the products of hydrogenation and oxidative degradation gave homocaryophyllenic acid (8) showing that the secondary hydroxyl group cannot be at position 1 or 2. On hydrogenation and oxidation a-betulenol yielded a saturated ketone C,5H260 identical with that prepared from caryophyllene oxide and this proved that its hydroxyl group must be at position 4. The position of the double bonds followed from the following facts The infrared spectrum of betulenol indicated an exocyclic methylene system and the isolation of homo- caryophyllenic acid fixes the position of this at 3. The double bonds are not conjugated and there is no indication of a -CH=CH- group in the spectra.Further the secondary hydroxyl group must be allylic since it is easily hydrogenolysed. These facts enabled a-betulenol to be represented as (9).5 Similar evidence suggested the formula (10) for /3-betu1enoL5 The facts above suggest that the stereochemistry of the ring junction is that of caryophyllene and it only remains to settle that of the hydroxyl * Treibs and Lossner Annulen 1960 634 124. Sorm Holub Herout and Horhk COIL Czech. Chem. Comnt.. 1959 24. 3730. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 103 group. Possible cyclisations parallel to those observed in the caryophyllene series do not appear to have been investigated. Treibs and Lossner seem to favour the structures (1 1) and (12) for a- and fkbetulenol re~pectively.~ Ten-membered Rings.-In 1957 the time of the last Quarterly Reviews article on sesquiterpenoid chemistry,l only one sesquiterpenoid containing a ten-membered carbocyclic structure had been characterised.More than a dozen such substances have now been isolated from natural sources and have been characterised mainly by Sorm and his colleagues.s Pyrethrosin. The first of these compounds to be characterised was pyrethrosin which was isolated from Chrysanthemum cinerariaefoliurn in 1891 though its complete structure was not fully determined until r e ~ e n t l y . ~ ~ ~ This compound C,,Hz205 contains two ethylenic linkages (one conjugated with a y-lactone system) and five oxygen atoms (present as lactone ether and acetate). There was no evidence from either infrared spectra or isotopic exchange of a free hydroxyl group.Most of the evi- dence for the structure of pyrethrosin (13) comes from the results of cyclisation. Treatment with acetic anhydride and toluene-p-sulphonic acid gave cyclopyrethrosin acetate (14). Selective hydrogenation followed AcO :w;) - q J ) o Ac OAc (14) by controlled hydrolysis converted this cyclisation product into an acetoxy- alcohol which on oxidation yielded the corresponding ketone (1 5). Hydrolysis of this acetoxy-ketone resulted in two products one of which (16) could be reacetylated to give back the ketone (15) and so had the 0 I - -U Reagents CrO then base lactone system in the original position. The structure of the other product (17) was demonstrated by its conversion into the dienone (18) previously encountered in the santonin series.No dienone could be obtained by Sorm Herout and Sykora Perfumery Essent. Oil Record 1959 50 679. Barton and de Mayo J. 1957 150. Barton Bockmann and de Mayo J. 1960 2263. 104 QUARTERLY REVIEWS similar treatment of the lactone (16). Since it is this compound which can be related to the compound (14) and so to pyrethrosin the position of the lactone ring in the latter must be as shown. Pyrethrosin and sodium dichromate in aqueous acetic acid at room temperature gave two further bicyclic products (19) and (20) the latter being easily related to cyclopyrethrosin acetate (1 4). Cyclisation under these mild conditions enabled conclusions to be drawn about the structure ('3)- HO wo Ac (19) + obtained from 14) of pyrethrosin itself. The hydroxyl group in (19) established the position of one end of the ethylenic linkage in pyrethrosin while the carbonyl group marked one end of the oxide ring for it can be assumed that pro- duction of the new carbon-carbon bond was the result of electrophilic attack upon the oxide system involving participation of the n-electrons of the double bond.The other end of the oxide ring must be attached to the carbon atom bearing the methyl group at the ring junction. The fact that dihydropyrethrosin gave no formaldehyde on ozonolysis showed that the double bond involved in the cyclisations was non-terminal. Confirmation of the presence of a ten-membered ring was obtained by oxidation of a mixture of tetrahydropyrethrosins to p-methyladipic acid. The evidence for the stereochemistry of cyclopyrethrosin acetate as (14) is as follows.The absolute configuration at position 10 was established as by isolation of the known dienone (18). When the toluene-p-sulphonate of dihydrocyclopyrethrosin was heated with collidine the non-conjugated diene (21) was isolated which requires that the 1-hydroxyl group should be eq~atorial.~ The molecular rotations of the compounds (22) and (23) (2 0 showed that the 1-hydroxyl group in compound (22) must be a if the 10-methyl group is p,l0 and this implied a cis-ring junction. Pyrolysis of the benzoate of the second hydroxy-keto-lactone (17) in the gas phase conditions which favour unimolecular cis-elimination,ll led Barton Experientiu 1950 6 316; Hirschmann Snoddy and Wendler J. Amer. Chem. SOC. 1952 74 2694. lo Klyne and Stokes J. 1954 1979. l1 Barton J.1949,2174; de Puy and King Chem. Rev. 1960,60,431; J . Amer. Chem. SOC. 1961 83 2743. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 105 to the compound (24) containing three vinylic hydrogen atoms. This elimination away from position 7 is only possible if the 7-hydrogen atom and the 8-hydroxyl group are in trans-relation to one another. Further the formation of both the lactones (16) and (17) from (15) is possible only if the side-chain at position 7 is equatorial and thus p . Dehydration of the compound (25) gave an endocyclic olefin which suggests that the hydroxyl group at position 4 has an axial and so a-orientation. Provided the acid-catalysed cyclisations are stereospecific a 4whydroxyl group would arise from a cis- d4-endocyclic double bond in pyrethrosin but further work is necessary here before a final conclusion can be reached.Germacrone. The crystalline sesquiterpenoid germacrone C15HZ20 was isolated from Geranium macrorhizum L. and was given structure (26) by Treibs.12 Recent workf3 necessitates a complete revision of this formula. The rather anomalous infrared and ultraviolet spectra of this substance revealed a conjugated enone system. Hydrogenation on palladium in acetic acid gave a hexahydro-derivative a saturated ketone which was reduced to the alcohol hexahydrogermacrol C15H3,0. Germacrone is thus monocyclic. Direct reduction of germacrone gave germacrol which on hydrogena- tion in an acidic medium gave selinane (27) and on dehydration followed by hydrogenation elemane (28). The infrared spectrum of the parent hydrocarbon (germacrane) differed from that of the perhydro-derivatives of other sesquiterpenoids and the presence of a ten-membered ring was confirmed by a synthesis of germacrane (29).l2 Treibs Annalen 1952 576 116. l3 Ognjanoff Ivanoff Herout Horhk Pliva and Sorm Chem. and Ind. 1957 820; Herout and Such$ Coll. Czech. Chern. Comm. 1958 23 2169 2175; Herout HorAk Schneider and Sorm Chem. and Ind. 1959 1089. 106 QUARTERLY REVIEWS Ozonolysis of the unsaturated tetrahydrogermacrone gave one equival- ent of acetone and 2,6-dimethyldecanedioic acid and oxidation with permanganate in acetone yielded lavulic and oxalic acid. These results are compatible with representation of germacrone as either (30) or (31) for both would accommodate the lack of optical activity. The evidence now favours structure (3 1).In particular germacrone like other cyclodecane systems is susceptible to transannular cyclisations for example the forma- tion of selinane and elemane mentioned above. Germacrone is also pyrolysed to a monocyclic ketone /3-elemenone (32) during distillation,14 and when the product of acid isomerisation is hydrogenated a bicyclic ketone (33) is obtained.l5 The formation of these products is accounted for more easily on the basis of structure (31) than of (30) for in the latter case an initial double-bond shift would have to be invoked. An isomer of germacrone isogermacrone obtained by the action of alkali has recently been reported and on the basis of oxidative degrada- tion been given the structure (34).16 Costunolide. Costunolide C15H2202 is a crystalline optically active sesquiterpenoid isolated from costus root oil or from Artemisia balchan- orum a Russian wormwood species.17 It contains a y-lactone ring and three ethylenic linkages and is therefore monocarbocyclic.l* Preliminary observations for example the isolation of 1,6-dimethylnaphthalene on dehydrogenation and formaldehyde formic acid and lzvulic acid on ozonolysis suggested a partial formula (35).Costunolide and dihydro- costunolide both consumed two mol. of perbenzoic acid showing that one double bond is inert and conjugated with the y-lactone system; the infrared spectrum showed that it was exocyclic. These results indicated (36) or (37) as the most likely structure for costunolide. l4 Ohloff Farnow Philipp and Schade Annalen 1959 '625 206; Ohloff Angew. Chem. 1959,71,162; Ognjanov Herout Horak and Sorm Coll.Czech. Chem. Comrn. 1959,24 2371. l5 Ognjanov Compt. rend. Acad. bulg. Sci. 1960,13 51 ; Chem. Abs. l961,56,8455h. '13 Suchy Herout and Sorrn Coll. Czech. Chem. Comm. 1961 26 1358. l7 Rao Varma Ghosh and Dutta J. Sci. Iizd. Res. India 1958,17B 228; Bendova Rao Kelkar and Bhattacharyya Chem. and Ind. 1958 1359; Tetrahedron 1960 Sykora Herout and Sorm Clzem. and Znd. 1958 363. 9 275. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 107 The alternative (37) is favoured on the basis of ozonolysis of dihydro- costunolide to the acid (38) which has been prepared from santonin. This also established the absolute configuration of costunolide as (39). Costunolide and its derivatives readily undergo cyclisation to bicyclic compounds in acidic media.19 For example dihydrocostunolide (40) in a mixture of acetic anhydride and acetic acid gave the bicyclic lactone (41) which on hydrogenation gave the two known santonin derivatives “santanolide c” (42) and “santanolide a” (43).20 Recently the isolation of the unsaturated lactone (44) from the cyclisation has also been reported.%l These cyclisations confirm the structure of costunolide as (37) and its absolute configuration as (40).If costunolide were represented by (36) then the products would belong to the alantolactone series (45).22 From the direct pyrolysis of dihydrocostunolide a monocyclic lactone saussurea lactone (46) has been isolated.23 This is exactly parallel to the behaviour of germacrone on pyrolysis. Arctiopicrin. Arctiopicrin C19H2806 is isolated from the leaves of Arctiurn minus Bernh.(Compositae) and contains an unsaturated y-lactone system. Despite its crystallinity the tendency to oxidative polymerisation made satisfactory elemental analyses difficult to The main evidence for the structure of arctiopicrin concerns two of the four products l9 Herout and Sorm Chem. and Znd. 1959 1067; Rao Kelkar and Bhattacharyya ibid. p. 1069. 2o KovBcs HorBk Herout and Sorm Coll. Czech. Chem. Comnz. 1956 W 225; Cocker and McMurry J. 1956 4549. 21 Shaligram Rao and Bhattacharyya Chem. and Ind. 1961 671. 22 Tsuda Tanabe Iwai and Funakoshi J. Amer. Chem. SOC. 1957,79 1009 5721. a3 Rao Paul Sadgopal and Bhattacharyya Tetrahedron 1961 13 319. Suchy Herout and Sorm Coll. Czech. Chem. Comm. 1957 22 1902; Suchy Horak Herout and Sorm Chem. andZnd. 1957,894; Croat.Chem. Acta 1957,29,247; Suchy Herout and Sorm Coll. Czech. Chern. Comm. 1959,24 1542. 108 QUARTERLY REVIEWS A B C and D obtained by hydrogenation in ethanol. The compound C m.p. 134" C19H3a06 gave on hydrolysis a volatile acid identified as /3-hydroxy-a-rnethylpropionic acid and a product tetrahydroarctiolide C15H2604 m.p. 145" that was oxidised by chromium trioxide to a hydroxy-keto-lactone C15H2404 m.p. 138 O. This showed that arctiopicrin was an ester of /3-hydroxy-a-methylpropionic acid and arctiolide a mono- cyclic diol-lactone containing two double bonds. Hydrolysis of compound B gave a hydroxy-lactone Cl5Ha6O3 which was oxidised to a keto- lactone C&&. Therefore the secondary hydroxyl group of arctiopicrin is esterified while the other (tertiary) hydroxyl group is probably allylic as shown by its tendency to hydrogenolysis.These results suggest struc- tures (47) or (48) for arctiopicrin. The position of the y-lactone ring with respect to the esterified hydroxyl group was inferred by a not very convincing analogy with matricin (49) in which a characteristic shift of the infrared lactone-carbonyl absorption band is also observed;25 this occurs with a carbonyl or ester group adjacent to the lactone ring. However the alkaline conditions involved in these reactions left the positions of the esterified hydroxyl group and the lactone group undecided. The position of the tertiary hydroxyl group was de- duced from infrared spectral properties of an unsaturated keto-lactone C15H2203 m.p. 119" that was obtained by the dehydration of the hydroxy- keto-lactone C15H2404 m.p.138 ". A trisubstituted ethylenic group and an q3-unsaturated carbonyl group were detected. The decision between structures (47) and (48) in favour of the latter rested upon a chemical and stereochemical correlation. Hydrogenation (R = COCHMe.CH26-l) I c 25 Cekan Herout and Sorm Chem. and I d . 1956 1234. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 109 of arctiopicrin on palladium in acetic acid gave the hydroxy-lactone (50) which was also obtained stereospecifically from artemisin (5 1).26' This confirms the structure (48) for arctiopicrin and establishes its absolute configuration at positions 6 7 and 8 as that in artemisin. It is unlikely that the reduction of artemisin involves a configurational change at position 6 or 7 since the original configuration is preserved in the santonin derivative (52).The configuration at position 4 follows from the isolation of L(-)-methylsuccinic acid (53) on oxidation of polymeric arctiopicrin on the assumption that this acid is derived from the carbon atoms at positions 2,3,4 5 and 15. Arctiopicrin has therefore structure (54) the configuration at position 10 remaining to be determined. The cyclisation of arctiopicrin probably involves migration of the 172-double bond to the 4,5-position at some stage during the hydrogenation in an acidic medium.27 It is interesting that no such reaction has been observed with another closely related substance cnicin (55)28 or with parthenolide (56). Parthenolide. The evidence for the structures of the cyclodecane sesquiterpenoids discussed above leans heavily on the products of trans- annular cyclisation.Where these do not occur a different approach is necessary. That of oxidative degradation is illustrated in the case of parthenolide (56).29 This is a crystalline compound CI5HmO3 isolated from Chrysanthemumparthenium (L.) Bernh. It contains a y-lactone system conjugated with an exocyclic methylene group and like other such com- pounds the tendency to form polymers complicates accurate analysis. The presence of a second double bond was established by the preparation of an oxide from dihydroparthenolide itself obtained by hydrogenating parthenolide on platinum in methanol. Complete hydrogenation of parthenolide gave a hexahydro-derivative C15H2603 which contained a free hydroxyl group whereas parthenolide itself had no active hydrogen.This indicates that one oxygen atom is present as an epoxy-group and that parthenolide is monocyclic. Ozonolysis assisted in fixing the relative positions of these functional groups (57)-(60). 26 Sumi Proc. Japan Acad. 1956,32,684; J. Amer. Chem. Soc. 1958,80,4869. 27 Braude and Linstead J. 1954 3544; Fukushima and Gallagher J. Amer. Chm. 28 Sorm BeneSovB Herout and Suchy Tetrahedron Letters 1959 No. 10 5; Suchy 29 Herout Soukk and Sorm Chem. and Ind. 1959 1069; Coll. Czech. Chem. SOC. 1955 77 139. BeneSovB Herout and Sorm Chem. Ber. 1960,93,2449. Comm. 1961 803. 110 QUARTERLY REVIEWS Oxidation of parthenolide (56) and dihydroparthenolide (57) with nitric acid in the presence of vanadium salts gave a mixture of acids from which p-methyladipic acid was isolated demonstrating that at least four carbon atoms of the cyclodecane ring carry no oxygen and indicating that the oxide ring was three-membered.Y" I t H*C02H Further evidence for the location of the functional groups comes from the fact that hexahydroparthenolide (61) is easily oxidised to a saturated ketone and that the trio1 (62) obtained by the reduction of hexahydro- parthenolide with lithium aluminium hydride consumed one mol. of sodium periodate. There has been no report of cyclisation of parthenolide under the conditions used with pyrethrosin or arctiopicrin ; and the stereochemistry of parthenolide has not yet been settled. Various. Other sesquiterpenoids with a skeleton based upon a ten- membered ring include aristolactone (63),30 on which more work is needed gafrinin (64),31 acetylbalchanolide (65),32 balchanolide and iso- balchanolide (possibly a pair of geometrical isomers33) and millef~lide.~~ Spectral contributions.The geometry of the endocyclic double bonds in these sesquiterpenes is not established by the chemical evidence avail- able especially since the cyclisations reported involve acidic media and hydrogenation catalysts. It is difficult to see a purely chemical approach to this problem. If as is believed these compounds arise biogenetically from trans-farnesol then a trans-trans-geometry would be expected. The only direct evidence for this comes from nuclear magnetic resonance data. 30 Steele Stenlake and Williams Chern. and Ind. 1959 1384; J. 1959 3289. 31 de Villiers J. 1961 2049. sa Hochmannovh Herout and Sorm Coll. Czech. Chem.Comm. 1961 26 1826. 33 Herout Suchy and Sorm Coll. Czech. Chem. Comm. 1961,26,2612. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 1 1 1 It has been stated3‘ that there is a small but definite dependence of the frequencies of the methyl protons on the geometry of systems of the type ACH2CMe=CHCH2-B. The shift of frequency between the trans- and cis-relations of the vinylic hydrogen atom and methyl group is about 0.07 T. Examination of the spectra of costunolide (39) and germacrone (31) indicates that both the endocyclic double bonds are trans in these com- pounds. The abnormally high T values for the methyl groups in the tri- substituted system MeR*C= CHR’ of germacrone and costunolide are attributed to shielding of the absorbing methyl protons by the n-electrons of the second double bond; this shielding cannot occur in pyrethrosin Transannular effects are also evident in the ultraviolet spectra of these sesquiterpenes.Germacrol and costunolide (39) show extremely high end- absorptions E~~~~ and €2130 104-12 respectively. Comparison with tetrahydrogermacrone and germacrone diepoxide shows that this is due to two trisubstituted ethylene groups disposed as in the general structure (66). Compounds with only one such ethylenic linkage (67) show an end (1 3) .34 (66) a 0 CO,Me (67’ absorption typical of a trisubstituted ethylene group in a six-membered ring. The anomaly probably arises from the geometry of the cyclodecane ring in permitting electron delocalisation between adjacent but non- conjugated double b o n d ~ . ~ * ~ ~ ~ Eleven-membered Rings.-Sesquiterpenoids with eleven-membered car- bocyclic rings were well known before any ten-membered rings compounds were authenticated.The structures of these compounds humulene and the related ketone zerumbone have only recently been completely settled with the help of nuclear magnetic resonance spectroscopy. Humulene. Humulene C15H24 isolated from oil of hops is triply unsaturated and its hexahydro-derivative was shown by synthesis to be l,l,4,8-tetramethylcycloundecane.36 Location of the double bonds was a more difficult problem. Oxidative degradation of dihydrohumulene led to the isolation of aa-dimethylsuccinic acid /3/3-dimethyladipic acid and an unknown keto-acid. Similar treatment of tetrahydrohumulene gave an acid C15H2804 whose synthesis confirmed the eleven-membered ring in 94 Bates and Gale J.Amer. Chem. SOC. 1960,82 5749. 35 Jones Mansfield and Whiting J. 1956,4073. aa Sorm Mleziva Arnold and Pliva CON. Czech. Chern. Conznz. 1949 14 699; Herout Streibl Mleziva and Sorm ibid. p. 716; Sorm Streibl Pliva and Herout ibid. 1952 16 639; Sorm Streibl Jarolim Novotny DolejS and Herout ibid. 1954 19 570; Clemo and Harris J. 1951 22; 1952 655; Harris J. 1953 184. 112 QUARTERLY REVIEWS h~mulene.~' Oxidation of humulene itself gave lzevulic acid and its alde- hyde aa-dimethylsuccinic acid and f~rmaldehyde.~~ This established formula (68) for humulene. The ultraviolet spectrum of humulene shows that the double bonds are not conjugated while the infrared spectrum indicated the presence of a trisubstituted ethylene group. Careful ozonolysis of h u m ~ l e n e ~ ~ yielded aa-dimethylsuccinic acid and lavulic acid or after reduction of the ozonide with lithium aluminium hydride butane- 1,3-diol.In the latter experiment gas chromatography also revealed the two glycols corre'spond- ing to the other fragments. Though it is always wise to be cautious in accepting the results of ozonolysis the recent examination of humulene by nuclear magnetic resonance has led unequivocally to the structure (68).39 In the methyl and the methylene proton region the spectrum is almost identical with that of the related ketone zerumbone (69) whose chemistry is discussed below. In particular four methyl groups are evident and all the methylene groups are revealed as allylic. The relative areas of olefinic and saturated protons showed 3.3 olefinic protons close to the number (4) required by formula Zerumbone.Zerumbone (69) C15H220 is a monocarbocyclic crystal- line ketone isolated from the rhizomes of wild ginger.40 The ultraviolet spectrum indicated the presence of either an a/3-unsaturated carbonyl or a cross-conjugated dienone group. The latter alternative was confirmed when reduction with sodium in alcohol gave tetrahydrozerumbol C,,H2,0. Further alkaline treatment of zerumbone resulted in a reverse aldol reaction and the isolation of ethyl methyl ketone arising from the structural unit (70). Clemmensen reduction of hexahydrozerumbone gave humulane (68). (7 l) demonstrating the common skeleton of zerumbone and humulene. Ozonolysis of zerumbol obtained by reducing zerumbone with lithium 37 Harris J. 1953 184; Fawcett and Harris J.1954,2669,2673; Clarke and Ramage J. 1954 4345. 38 Hildebrand Sutherland and Waters Chem. and Ind. 1959 489. 30 Dev Tetrahedron Letters 1959 No. 7 12; Tetrahedron 1960 9 1. 40 Dev Tetrahedron 1960 8 171; Chem. and Ind. 1956 1051. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 1 13 aluminium hydride gave laevulic acid and aa-dimethylsuccinic acid. These facts suggested the structure (69) for zerumbone which has been confirmed by nuclear magnetic resonance spe~troscopy.~~. The spectrum clearly revealed the gern-dimethyl group at position 1 and the two methyl groups at positions 4 and 8. The six protons at posi- tions 2 5 and 6 were distinct from methyl protons since they are all allylic. Four olefinic protons were revealed that at position 3 which is not subjected to diamagnetic deshielding by the carbonyl group occurring at higher field strength.No chemical shift was observed between the proton at position 10 and those at positions 7 and 11 which would be expected to be subject to greater deshielding from the carbonyl by conjugative electron displacement. These results in favour of structure (69) for zerumbone were given added weight by a similar study of tetrahydrozerumbone hexa- hydrozerumbone and humulane. It is interesting that as the double bonds are progressively removed the methyl protons become more and more shielded possibly owing to increased crowding in the molecule. Biogenesis.-An interesting feature of sesquiterpene chemistry is the biogenesis of the remarkable variety of structures now known for these natural products.Initial suggestions in this field came from R ~ z i c k a ~ ~ but recently Hendrickson has considerably developed them on a stereochemical and electronic The fundamental isoprenoid unit involved in terpene biogenesis is isopentenyl pyrophosphate (72) which can condense to farnesol (73) the simplest acyclic ~esquiterpene.~~ The farnesol so formed probably has a trans central double bond and the allylic double bond can assume cis- or trans-configurations by anionotropic inter- conversion. No oxidation occurs during the cyclisation so that the most OH OH I &&OH \ \ (72) 0 OH (73) common oxidation state of cyclic sesquiterpenes is that of farnesol (com- pare the generation of triterpenes from squalene*l). It is likely42 that sesquiterpenes arise from 2,3-cis-farnesol (74) by way of the cation (75) or (76) or from all-trans-farnesol (77) by way of the cation (78) or (79).Models suggest that the cation (75) is much more strained than (76) the latter being preferentially formed with steric con- trol of cyclisation. The cation (78) is sterically and electronically pre- ferred to the cation (79). Thus the sesquiterpenes are probably generated from 2,3-cis-farnesol through cation (76) and from all-trans-farnesol through cation (78). *l Ruzicka Experientia 1953 9 364; Proc. Chem. SOC. 1959 341. 43 Hendrickson Tetrahedron 1959 7 82. 43 Bloch filling and Amdur J. Amer. Chem. Soc. 1957 79 2646; Cornforth and Popjak Tetrahedron Letters 1959 No. 19 29; Popjak ibid. p. 19; Lynen Angew. Chem. 1960 820. 114 QUARTERLY REVIEWS (78) ' (79) In the cation (76) the disposition of the double bonds does not favour cyclisation and further one of the hydrogen atoms at position 5 is turned inside the ring between the humulene skeleton positions 6 and 10.The loss of a proton yields (80) with a cis-trans-trans-geometry while the attack of the 3,4-double bond on the carbon atom at position 6 gives the caryophyllene skeleton (8 1). In the cation (78) the parent of the sesquiterpenes with a ten-membered carbocyclic ring the two double bonds are disposed favourably for cyclisa- tion and there are no hydrogen atoms turned inside the ring. The geometry of the double bonds in this cation suggests that those in the sesquiterpenes with a ten-membered ring are also trans-trans. Cyclisation of this cation can be envisaged as giving rise either to the bicyclic sesquiterpenes of the eudesmane series \82)44 or to the guaiazulene sesquiterpenes (83) for example geige~in.~~ Besides cyclisation however a direct cyclic movement of electrons may occur resulting in ring fission (84)-+(85).The fact that some reactions of these types have been realised in the 44 Cocker and McMurry Tetrahedron 1960 8 181. 45 Barton and Pinkey Proc. Chem. Suc. 1960 279; Barton and Levisalles J. 1958 45 18 ; Huffman Experientia 1960,16 120. HALSALL AND THEOBALD ASPECTS OF SESQUITERPENOID CHEMISTRY 1 15 laboratory lends weight to these biogenetic routes. The cyclisation of pyrethrosin (13) and costunolide (39) may be quoted as examples of acid- catalysed cyclisation; the cyclic electron shift is exemplified in the aprotic cyclisation of germacrone (3 1) and costunolide (39).

ISSN:0009-2681    

DOI:10.1039/QR9621600101

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

CUMULATIVE INDEXES VOLUMES I-XVI (1 947- 1962) CUMULATIVE INDEX OF AUTHORS Abrahams S. C. 10,407 Abrikosova I. I. 10,295 Addison C. C. 9 11 5 Ahrland S. 12 265 Albert A. 6 197 Allen G. 7 255 Amphlett C. B. 8 219 Anderson J. S. 1 331 Angyal S. J. 11 212 Arotsky J. 16 282 Amstein H. R. V. 4,172 Atherton F. R. 3 146 Avison A. W. D. 5 171 Bacon R. G. R. 9 287 Baddeley G. 8 355 Baddiley J. 12 152 Badger G. M. 5 147 Bagnall K. W. 11 30 Baker W. 11 15 Baltazzi E. 9 150 Barker S. A.. 7 58 Barltrop J. A. 12 34; Barnartt S. 7 84 Barrer R. M. 3 293 Barton D. H. R. 3 36; 10.44; 11 189 Bassett H. 1 247 Bateman L.. 8. 147 16 117 Battersby A R. 14,77; 15. 259 Bauhan E. C. 7 103 Baulch D. L. 12 133 Bawn C. E. H. 16,361 Bavliss N. S. 6 319 Eell R. P. 1 113; 2 132; 13 169. Bentley R. 4 172 Bergel F.2 349 Bethell D. 12 173 Bevington J. C. 6 141 Birch A. J. 4,69; 12 17 Bircumshaw L. L. 6 Bockris J. O’M. 3 173 Bolland J. L. 3 1 Bond G. C. 8 279 Bourne E. J. 7 58 Bowen E. J. 1 I ; 4,236 Bradley R. S. 5 315 Braude E. A. 4 404 Bremner J. G. M. 2 1 157 Brink N. G. 12,93 Brown B. R. 5 131 Brown R. D. 6 63 Buchanan J. G. 12 152 Buckingham A. D. 13 Bu’Lock J. D. 10 371 Bunnett J. F. 12 1 Burkin A. R. 5 1 Burnett G. M. 4 292 Burton H. 6 302 183 Cadogan J. I. G. 8,308; Caldin E. F. 7 255 Carrington A. 14,427 Challenger F. 9 255 Chatt J. 12 265 Coates G. E. 4 217 Collins C. J. 14 357 Collinson E. 9 31 1 Coltor R. 16 299 Cook A. H. 2 203 Cook J. W. 5 99 Cookson R. C. 10,44 Cooper C. F. 13 71 Cottrell T. L. 2 260 Coulson C. A. 1 144 Cowdrey W. A, 6 358 Cox E. G. 7,335 Crawford V. A. 3 226 Croft R.C. 14 1 Crofts P. C. 12 341 Crombie L. 6 101 Cross A. D. 14 317 Cruickshank D. W. J. Curran S. C. 7 1 Cuthbert J. 13 215 Dainton F. S. 12 61 Dalgliesh C. E. 5 227 Davies A. G. 9 203 Davies D. S. 6 358 Davies M. 8 250 Davies N. R. 12 265 Davies R. O. 11 134 Dawton. R. H. V. M. De Heer J. 4 94 de la Mare P. B. D. 3 43 6 16 208 14 378 7 335 9 1 126 de Mayo P. 11 189; 15. 393 Derjaguin B. V. 10,295 Dickens P. G. 11 291 Downing D. F. 16 133 Doyle W. T. 14 62 Dubinin M. M. 9 101 Duncan J. F. 2 307; Dunning W. J. 9 23 12 133 Eastham J. F. 14 221 Eley D. D. 3 181 EmelCus H. J. 2 132 Errede L. A. 12 301 Evans M. G. 4 94 6 Evans R. M. 13 61 186 Fensham P. J. 11 227 Ferrier R. J. 13 265 Fluendy M. A. D. 16 Foster A. B. 11 61 Fowles G. W. A. 16,19 Freidlina R. Kh. 10 241 330 Gascoigne R.M. 9,328 Gaydon A. G. 4 l Gee G. 1 265 Gent W. L. G. 2 383 Gibson D. T. 3 263 Gillespie R. J. 2 277; Gilman H. 13 116 Glasser F. P. 16 343 Glasser L. S. Dent 16 Glenn A. L. 8 192 Goehring M. 10 437 Gold V. 9 51 ; 12 173 Gowenlock B. G. 12 321,14,133 Gray P. 9 362 Green J. H. S. 15 125 Greenwood N. N. 8 1 Griffith J. S. 11 381 Griffith W. P. 16 188 Grove J. F. 15 56 Gundry P. M. 14 257 8,40; 11 339 343 CUMULATIVE INDEX 437 Gunstone F. D. 7 175 Gutmann V. 10 451 Halpern J. 10 463; 15 Halsall T. G. 16 101 Hamer F. M. 4 327 Hardy D. V. N. 2 25 Harman R. E. 12 93 Hams M. M. 1,299 Hartley G. S. 2 154 Hassel O. 7 221; 16 1 Hawkins E. G. E. 4 Hawkins J. D. 5 171 Haynes L. J. 2 46; 14 Heaney H. 11 109 Hey D. H. 8 308 Hickling A. 3 95 Hochstrasser R. M. 14 Hodson H. F. 14,77 Holt R.J. W. 13 327 Hughes E. D. 2 107; 5 245; 6 34 Hush N. S. 6 186 207 25 1 292 146 Ingold C. K. 6 34; Irving H. M. 5 200 Ivin K. J. 12,61 11 1 Jacobs P. W. M. 6,238 Jain A. C. 10 169 Janz G. J. 9 229 Jeffrey G. A, 7 335 Jenkins E. N. 10 83 Jennings K. R. 12,116; Jones D. G. 4,195 15,237 Kapustinskii A. F. 10 Katritzky A. R. 10 Kenyon J. 9 203 Khorana H. G. 6 340 Kipling J. J. 5,60; 10 1 Kiselev A. V. 15 99 Kitchener J. A. 13 71 283 395; 13,353 Lagowski J. J. 13 233 Lamb J. 11 134 Lamberton A. H. 5 75 Law H. D l o 230 Lea F. M. 3 82 Ledwith A. 16 361 Leech H. R. 3 22 Leisten J. A. 8 40 Levy N. 1 358 Lewis E. S. 12 230 Lewis J. 9 115 Lifshitz E. M. 10 295 Linnett J. W. 1 73; 11 291; 12,116 Lister B. A. J. 2 307 Lister M. W. 4 20 Livingston R. 14 174 Long L. H. 7 134 Longuet-Higgins H.C. 11 121; 14 427 Loudon J. D. 5 99 Luttke W. 12 321 Lythgoe B. 3 181 Maccoll A. 1 16 McCoubrey J. C. 5 MacDiarmid A. G. 10 McGrath W. D. 11 87 McKenna J. 7 231 McLaughlin E. 14,236 Maddock A. G. 5 270 Maitland P. 4 45 Manners D. J. 9 73 Marsh J. K. 1 126 Martin F. S. 13 327 Martin R. L. 8 1 Mason S. F. 15,287 Megson N. J. L. 2 25 Millar I. T. 11 109 Millen D. J. 2 277 Morgan K. J. 8 123; Morrison A. L. 2 349 Murrell J. N. 15 191 Musgrave W. K. R. 8 Nancollas G. H. 14 Nelson Smith R. 13 Nesmeyanov A. N. 10 Newth F. H. 13 30 Nicholls D. 16 19 Norrish R. G. W. 10 364; 11. 87 208 12,34 33 1 402 287 330 149 Nyhoh R. S. 3 321; 7,377; 11,339 Ollis W. D. 11 15 Orgel L. E. 8 422; 11 Orville-Thomas W. J. Overend W. G. 11 61; Owston P. G. 5 344 Page J. E. 6 262 Paneth F. A. 2 93 Parker A. J.16 163 Parsonage N. G. 13 Pauson P. L. 9 391 Payne D. S. 15 173 Peacock R. D. 16 299 Pepper D. C. 8 88 Percival E. G. V. 3,369 Phillips F. C. 1 91 Plimmer J. R. 14 292 Pople J. A. 11 273 Porter G. B. 14 146 Praill P. F. G. 6 302 Pritchard H. O. 14 46 Reid C. 12 205 Reid S. T. 15 393 Richards R. E. 10,480 Ridd J. H. 15 418 Riddiford A. C. 6 157 Riley H. L. 1,59; 3,160 Roberts H. L. 15 30 Roberts L. E. J. 15,442 Roberts M. W. 16 71 Rarmming Chr. 16 1 Rogers N. A. J. 16 117 Rose J. D. 1 358 Rowlinson J. S. 8 168 Satchell D. P. N. 9 51 Saxton J. E. 10 108 Schofield K. 4 382 Selman S. 14 221 Seshadri T. R. 10 169 Sexton W. A, 4,272 Sharpe A. G. 4 115; Shchukina L. A. 10 38 1 11 162 13,265 306 11,49 261 Sheldon J. C. 14,200 Shemyakin M. M. 10 26 1 438 QUARTERLY REVIEWS Sheppard N.6 l 7 19 Sillen L. G. 13 146 Simes J. J. H. 9 328 Simons P. 13 3 Simpson D. M. 6 1; 7 Smales A. A. 10 83 Smith B. C. 14 200 Smith E. B. 16,241 Smith H. 12 17 Smith J. A. S. 7 279 Smith M. L. 9 1 Springall H. D. 10. 230 Stacey M. 1 179 213 Staveley L. A. K. 3,65; Stephens R. 16 44 Stern E. S. 5 405 Stone F. G. A. 9 174 Sutton L. E. 2 260 Swallow A. J. 9 311 Symons M. C. R. 12 230; 13 99; 14 62; 16 282 Synge R. L. M. 3,245 Szwarc M. 5 22; 12 19 13,306 301 Tatlow J. C. 16 44 Taylor A. W. C. 4 195 Taylor H. F. W. 16 Tedder J. M. 14 336 Theobald D. W. 16 Thomas S. L. 7 407 Thomson R. H. 10,27 Thrush B. A. 10 149 Tipper C. F. H. 11 313 Tompkins F. C. 6 238; Topley B. 3 345 Trapnell B. M. W. 8 Trotman-Dickenson A. Truter E. V. 6,390 Turner E. E. 1 299 Turner H. S. 7 407 Ubbelohde A. R.4 356; 5,364; 11,246 Ulbricht T. L. V. 13,48 Uri N. 6 186 Vainshtein B. K. 14 343 101 14,257 404 F. 7 198 105 Walsh A. D. 2 73 Walton G. N. 15,71 Warburton W. K. 8,67 Warhurst E. 5 44 Waters W. A. 12 277 Weale K. E. 16 267 Weedon B. C. L. 6 380 Wells A. F. 2 185; 8 Wells R. A. 7 307 Whiffen D. H. 4 131 ; White E. A. D. 15 1 Whytlaw-Gray R. 4 Wilkins R. G. 16,3 16 Wilkinson S. 15 153 Wilson H. N. 2 1 Wittenberg D. 13 116 Woodward L. A, 10 Woolf A. A. 15 372 380 12,250 153 185 Yoffe A. D. 9 362 Zakharkin L. I. 10 330 CUMULATIVE INDEX OF TITLES Absorption spectra molecular elec- tronic 15 287 Acetylenes as natural products 10 37 1 Acetylenes infrared and Raman spectra of 6 1 Acid use of the term 1 1 1 3 Acids carboxylic anodic syntheses with 6 380 Acids carboxylic associa tion of 7 255 Acids straight-chain fatty natural and synthetic recent developments in the preparation of 7 175 Acids tetronic 14 292 Acid-base reactions simple rates of Actinide oxides.15.442 Addition polymerisation at high pres- sures 16,267 Addition polymerisation stereo- regular 16 361 Addition reactions free-radical of olefinic systems 8 308 Adsorption energy adsorption equili- bria and surface chemistry 15 99 Adsorption of non-electrolytes from solution 5,60 Affinities relative of ligand atoms for acceptor molecules and ions 12,265 Age geological determination of by radioactivity 7 1 Aldehydes polymerisation of 6 141 Aliphatic compounds saturated inter- action of free radicals with 14 336 Alkaloid biosynthesis 15 259 Alkaloids of calabash-curare and Strychnos species 14 77 Alkaloids ergot 8 192 Alkaloids indole excluding harmine and strychnine 10 108 Alkaloids steroidal 7 23 1 Alkaloids veratrum 12 34 Alkanes infrared and Raman spectra Alkanes tetra- and tri-chloro- and Analgesics synthetic 2 349 Analysis conformational principles of 10 44 Analysis inorganic applications of solvent extraction to 5 200 Analysis radioactivation 10 83 13 169 of 7 19 related compounds 10,330 Anionotropy 4.404 Anions in dipolar aprotic solvents effects of solvation on the proper- ties of 16 163 Anodic syntheses with carboxylic acids 6 380 Antibiotics newer chemistry of 12.93 Arrhenius factors (frequency factors) in unimolecular reactions 14 133 Aspects physicochernical of some recent work on photosynthesis 14# 174 Association of carboxylic acids 7,255 Asymmetry the non-conservation of parity and optical activity 13 48 Atoms in the gaseous phase produc- tion detection and estimation of 15 237 Attraction molecular direct measure- ment of between solids separated by a narrow gap 10,295 Base use of the term 1 113 Benzilic acid and related rearrange Biological reactions rBle of phosphoric Biosynthesis alkaloid 15 259 Bond aromatic 5 147 Bond chemical in crystals applica- tion of electron diffraction to the study of 14,105 Bonding chemical and nuclear quad- rupole coupling 11,162 Bonds dissociation energies of 5 22 Bonds interpretation of properties of 2 260 Bonds weak charge-transfer in solids containing chemically saturated molecules direct structural evi- dence for 16 l ments 14 221 esters in 5 171 Borazoles the 14 200 Boron hydrides chemistry of 9 174 Boron hydrides and related com- Boron trifluoride co-ordination corn- pounds 2 132 pounds of 8 1 Carbides of iron 3 160 Carbohydrate epoxides 13 30 Carbohydrate phosphates 11 61 Carbohydrate sulphates 3.369 439 440 QUARTERLY REVIEWS Carbohydrates newer aspects of stereochemistry of 13 265 Carbon amorphous and graphite 1 59 Carbon-carbon bonds oxidative hydrolysis of in organic molecules 10 261 Carbon-carbon double bonds geo- metrical isomerism about 6 101 Carbon-hydrogen bond polarity of 2 383 Carbon-hydrogen bond? mechanism of breakage of 12 230 Carbon-oxygen surface compounds of 13,287 Carbon-phosphorus bonds com- pounds containing 12 341 Carbonitrides of iron 3 160 Carbonium ions structure of 12 173 Carbons active study of porous structure of by a variety of methods 9 101 Carbons adsorbent properties and nature of 10 1 Carbonyls of metals chemistry of 1 33 1 Catalysis by metals specificity in 8 404 Catalysis of reactions involving hydro- gen mechanisms of 3,209 Catalysis and serniconductivity 11,227 Catalysts redox initiation of poly- merisations by 9 287 Cations halogen 16 282 Cations orgamc reactions of 6 302 Charcoals active study of porous structure of by a variety of methods 9 101 Charge-transfer bonds weak in solids containing chemically saturated molecules direct structural evidence for 16 1 Charge-transfer spectra theory of 15 191 Chemisorption of gases on metals 14 257 Chromatography inorganic 7 307 Chromium mechanisms of oxidation Collisions in gases energy transfer in Colloidal electrolytes state of solution Colour and constitution 1 16 by compounds of 12 277 11 87 of 2 154 Colour centres in alkali halide crystals 14,62 Combustions slow in the gas phase elementary reactions in 11 313 Complex compounds stabilities of 5 l Complexes cyanide of the transition metals 16 188 Compounds containing sulphur- fluorine bonds chemistry of 15 30 Conductance ionic in solid salts 6 23 8 Configuration of flexible organic molecules 5 364 Conformational analysis principles of 10 44 Conjugated compounds free-electron approximation for 6 319 Co-ordination compounds of boron trifluoride 8 1 Co-ordination compounds kinetics and mechanism of repIacement reactions of 16 316 Crystal structure and melting 4 356 Crystal structures of salt hydrates and complex halides 8 380 Crystalline transition-metal com- pounds electron resistance in 14,427 Crystals alkali halide colour centres in 14 62 Crystals chemical bonds in applica- tion of electron diffraction to the study of 14 105 Crystals location of hydrogen atoms in 10 480 Crystals ionic lattice energy of 10 283 Cyanide complexes of the transition metals 16 188 Cyanine dyes 4 327 Cyclohexane stereochemistry of 7 221 Deamination nitrosation and di- Decarboxylation thermal mechanism Degradation biological of trypto- Densities limiting 4 153 Di- and tri-terpenes synthesis of 16 Diazotisation nitrosation and de- Dielectric absorption 8 250 amtisation 15,418 of 5 131 phan 5 227 117 amhation 15 418 CUMULATIVE INDEX 44 1 Dihalogen compounds Grignard and organolithium compounds derived from 11 109 Disproportionation in inorganic com- pounds 2 l Diterpenoids chemistry of 3 36 Dyes effect of light on 4 236 Dyes cyanine 4,327 Dyes organic and their constitution 1 16 Earth distribution of the elements in the 3 263 Electrode processes in aqueous solu- tions mechanism of 3 95 Electrolytes effects of ultrasonic waves on 7 84 Electrolytes colloidal state of solu- tion of 2 154 Electromagnetic separation of stable isotopes 9 1 Electron correlation and chemical consequences 11 291 Electron resistance in crystalline trans- ition-metal compounds 14 427 Electron transfer and related processes in solution mechanism of 15 207 Electronic absorption spectra mole- cular 15 287 Electrons structures of molecules deficient in 1 1 12 1 Elements terrestrial distribution of 3 263 Elements heavy radioactivity of 5 270 Elements of Group VTII recent stereochemistry of 3 321 Elements of Groups IVB and IV comments on the thermochemistry of 7 103 Elements of the rare-earth series separation of 1 126 Elements transuranic chemistry of 4 20 Energy adwrption and adsorption equilibria in surface chemistry 15 99 Energy transfer of in gaseous collisions 11 87 Enzymes degradation of polysacchar- ides by 9 73 Enzymes synthesis of polysaccharides by 7 58 Epoxides of sugars 13 30 1,2-Epoxi des natural 1 y-om rr ing the chemistry of 14 317 Equilibria adsorption and adsorption energy in surface chemistry 15 99 Equilibria hydrolytic quantitative studies of 13 146 Equivalent-orbital approach to mole- cular structure 11 273 Ergot alkaloids structure of 8 192 Esters carboxylic and related com- pounds alkyl-oxygen heterolysis in 9,203 Exchange reactions of hydrogen iso- topes in solution principles of 9 51 Extraction liquid-liquid in inorganic chemistry 13 327 Ferrocene and related compounds 9 391 Fission nuclear 15 71 Flames emission spectra of 4 1 Flash photolysis and kinetic spectro- scopy 10 149 Flavones nuclear methylation of 10 169 Fluorine-sulphur bonds compounds containing 15 30 Fluorescence and fluorescenze quench- ing 1 1 Fluorine laboratory and technical production of 3 22.Fluorine compounds general aspects of the inorganic chemistry of 11 49 Fluorine compounds laboratory and technical production of 3 22 Fluorine compounds organic reac- tions of 8 331 Fluorocarbon chemistry. Part 1. Fluorination of organic com- pounds 16 44 Foaming current concepts in theory of 13 71 Force constants 1 73 Forces intermolecular and the pro- perties of matter 8 168 Free-electron approximation for con- jugated compounds 6 319 Friedel-Crafts reaction modern aspects of 8 355 Furans some aspects of the chemistry of 4 195 Gases adsorbed infrared spectra of Gases chemisorption of on metals 14 378 14,257 442 QUARTERLY REVIEWS Gases elementary reactions in slow combustions in 11 313 Gases energy transfer in collisions in 11 87 Gemstones synthetic 15 1 Gibberellins 15 56 Graphite and amorphous carbon 1 Graphite lamellar compounds of Grignard reagents derived from di- 59 14 1 halogen compounds ll 109 Halide alkali crystals colour centres in 14 62 Halides of the phosphorus group elements (P As Sb Bi) 15 173 Halides complex crystal structures of 8 380 Halides reactions of in solution 5 245 Halogens kinetics of thermal addition of to olefins 3 126 Heats of formation of simple in- organic compounds 7 134 Heteroaromatic compounds infrared spectra of 13 353 Heterogeneous reactions transport control in 6 157 Heterolysis alkyl-oxygen in carb- oxylic esters and related compounds 9 203 Hydrocarbons infrared and Raman spectra of.Part I acetylenes and olefins 6 1. Part 11 paraffins 7 19 Hydrogen molecular homogeneous reactions of in solution 10 463 Hydrogen atoms location of in crystals 10 480 Hydrogen catalysis mechanisms of 3 209 Hydrogen isotope exchange reactions in solution principles of 9 51 Hydrogen peroxide its radicals and its ions energetics of reactions involving 6 186 Hydrogenation catalytic and related reactions mechanism of 8,279 Hyperconjugation 3 226 Ice structure of 5 344 Immunochemistry aspects of 1 179 21 3 Indole alkaloids excluding harmine and strychnine 10 108 Induction asymmetric and asym- metric transformation 1,299 Infrared spectra of adsorbed gases 14 378 Inorganic chemistry and magnetism 7 377 Inorganic compounds disproportiona- tion in 2 l Inorganic compounds Raman spectra of 10 185 Inorganic compounds stereochemistry of 11 339 Inorganic compounds simple heats of formation of 7 134 Inorganic oxy-compounds topotactic reactions in 16 343 Inorganic reactions in liquid am- monia 16 19 Inositols 11 212 Insecticides synthetic structure and activity in 4 272 Interaction of free radicals with saturated aliphatic compounds 14 336 Interhalogen compounds and poly- halides 4,115 Iiitermolecular forces and some pro- perties of matter 8 168 Iodine compounds inorganic some reactions of 8 123 Ion association in aqueous solution thermodynamics of 14 402 Ion exchange 2 307 Ionisation potentials and far ultra- violet spectra their significance in chemistry 2 73 Iron carbides nitrides and carbo- nitrides of 3 160 Isoflavones 8 67 Isomerism geometrical about carbon- carbon double bonds 6 101 Isotopes exchange of between different oxidation states in aqueous solution 8 219 Isotopes synthesis of organic com- pounds labelled with 7 407 Isotopes tracer techniques involving 4,172 Isotopes stable electromagnetic separation of 9 1 Kinetics and mechanism of replace- ment reactions of co-ordination compounds 16 316.CUMULATIVE INDEX 443 Lactones physiologically active un- saturated 2 46 Lamellar compounds of graphite 14 1 Lanthanons separation of 1 126 Lattice energy of ionic crystals 10 283 Ligand atoms relative affinities of for acceptor molecules and ions 12 265 Ligand-field theory 11 381 Light absorption of and photo- chemistry 4,236 Liquids transitions in 3 65 Liquids transport properties of in relation to their structure 14 236 Liquids ultrasonic analysis of relaxa- tion processes in 11 134 Magnetic resonance absorption Magnetism and inorganic chemistry 7 377 Manganese mechanisms of oxidation by compounds of 12,277 Manganese dioxide oxidations by in neutral media 13,61 Mass spectrometry application of to chemical problems 9 23 Mass spectrometry of free radicals 13 21 5 Mechanisms of electron transfer and related processes in solution 15 207 Melting and crystal structure 4 356 Meso-ionic compounds 11 15 Metal oxidation 16 71 Metal-amine solutions reduction by; applications in synthesis and deter- mination of structure 12 17 Metal-ammonia solutions reduction of organic compounds by 4 69 Metal-transition compounds crystal- line electron resistance in 14 427 Metals chemisorption of gases on 14 257 Metals nature of solutions of 13 99 Metals specificity in catalysis by 8 Methyl radicals reactions of 7 198 Methylation biological 9 255 Methylation nuclear of flavones and related compounds 10 169 Molecular electronic absorption nuclear 7 279 404 spectra 15 287 Molecular-orbital approach to mole- cular structure 11 273 Molecular-sieve action of solids 3 293 Molecules determination of structure of by X-ray crystal analysis modern methods and their accuracy 7,335 Molecules molecular-orbital and equivalent-orbital approach to structure of 11 273 Molecules electron deficient struc- tures of 11 121 Molecules electronically excited reac- tions of in solution 13 3 Molecules flexible organic configura- tion of 5 364 Molecules organic oxidativehydro- lysis of carbon-carbon bonds in 10 26 1 Molecules simple representation of by molecular orbitals 1 144 Monte Carlo methods application of to physicochemical problems 16 24 1 Morphine synthetic approaches to structure of 5,405 Muscarine history and chemistry of 15 153 Nitramines some aspects of the chemistry of 5 75 Nitration of aromatic compounds 2 277 Nitration of heterocyclic nitrogen compounds 4,382 Nitrides of iron 3 160 Nitro-compounds aliphatic 1 358 Nitrogen active 12 116 Nitrogen compounds heterocyclic nitration of 4 382 Nitrogen dioxide-dinitrogen tetroxide system structure and reactivity of 9 362 Nitrosation diazotisation and de- amination 15 418 C-Ni troso-compounds structure and properties of 12 321 Nitrosyl group chemistry of 9 115 Non-electrolytes adsorption of from solution 5,60 Non-electrolytes theories of solutions of 13 306 444 QUARTERLY REVIEWS Nuclear chemistry quantitative 12 111 1 JJ Nuclear fission 15 71 Nuclear magnetic resonance absorp- tion.7.279 Nuclear ’ quadrupole coupling and chemical bonding 11 162 Nucleation in phase changes 5 315 Nucleotide coenzymes recent develop- ments in biochemistry of 12 152 Oceans salt deposits from 1 91 Olefinic systems free-radical addition reactions of 8,308 Olefins infrared and Raman spectra of 6 1 Olefins kinetics of oxidation of 3 1 Olefins kinetics of thermal addition of halogens to 3 126 Olefins oxidation of 8 147 Optical activity and non-conservation of parity 13 48 Orbitals molecular approach to mole cular structure through 11 273 Orbitals molecular and organic reactions 6 63 Orbitals molecular representation of simple molecules by 1 144 Organic compounds action of ionising radiations on 9 31 1 Organic compounds behaviour of in sulphuric acid 8 40 Organic compounds estimation of thermodynamic properties for 9 229 Organic compounds fluorination of 16 44 Organic compounds isotopically labelled synthesis of 7 407 Organic compounds polarography of 6 262 Organic compounds reduction of by metal-ammonia solutions 4,69 Organic compounds tervalent of phosphorus oxidation of 16 208 Organic oxygen compounds thermo- dynamic properties of 15,125 Organic reactions and molecular orbitals 6 34 Organolithium reagents derived from dihalogen compounds 11 109 Organometallic compounds of the first three periodic groups 4,217 Organosilylmetallic compounds for- mation and reactions of 13 116 5-Oxazolones chemistry of 9 150 Oxidation by compounas of chro- mium and manganese mechanisms of 12 277 Oxidation metal 16 71 Oxidation of olefins 3 1 ; 8 147 Oxidation of tervalent organic com- pounds of phosphorus 16 208.Oxidation-reduction potential of quinones relation of to chemical structure 4 94 Oxides actinide 15 442 Oxides of metals structures of 2 185 N-Oxides aromatic heterocyclic chemistry of 10 395 Oxy-compounds inorganic topotactic reactions in 16 343 Oxygen compounds organic thermo- dynamic properties of 15 125 Oxygen and carbon surface com- pounds of 13 287 Parity non-conservation of 13 48 Penicillins chemistry of 2 203 Peptides methods of synthesis and terminal-residue studies of 10 230 Peptides naturally occurring 3 245 Peptides structural investigation of 6,340 Perffuoroalkyl derivatives of metals and non-metals 13,233 Peroxides organic and their reactions 4,251 Phase changes nucleation in 5 315 Phenols tautomerism of 10 27 Phosphates of carbohydrates 11 61 Phosphates condensed 3 345 Phosphoric esters r81e of in biological reactions 5 171 Phosphorus group elements (P As Sb Bi) halides of 15 173 Phosphorus oxidation of tervalent organic compounds of 16,208 Phosphorus oxyacids some aspects of the organic chemistry of derivatives of 3 146 Photochemical rearrangements and related transformations 15 393 Photochemistry and light absorption 4,236 Photography cyanine dyes in 4 327 Photo-oxidation primary processes in 14 146 Photopolymerisation 4 236 Photosynthesis ph ysicochemical aspects of some recent work on 14 I 74 CUMULATIVE INDEX 445 Physicochemical problems application of Monte Carlo methods to 16,241 Pinacol rearrangement 14,357 Polarity of the carbon-hydrogen bond 2 383 Polarokaphy of organic compounds 6.262 Polonium chemistry of 11 30 Polyhalides and interhalogen com- pounds 4 115 Polymerisation of aldehydes 6 141 Polymerisation addition at high pressures 16 267 Polymerisation addition some thermodynamic and kinetic aspects of 12 61 Polymerisation addition stereo- regular 16 361 Polymerisation induced by light 4 236 Polymerisation initiation of by redox catalysts 9 287 Polymerisation ionic 8 88 Polymerisation radical rate constants in 4 292 Polymers based on silicon chemistry of 2,25 Polymers high thermodynamic pro- perties of and their molecular interpretation 1,265 Polysadarides; e h m i c degradation of.9. 73 Polysakharides enzymic synthesis of 7 58 Portland cement constitution of 3,82 Processes primary in photo-oxidation 14 146 Production detection and estimation of atoms in the gaseous phase 15 237 Proteins structural investigation of 6 340 Psychotomimetic substances 16 133 Pteridines 6 197 Purines some aspects of the chemistry Pyrans some aspects of the chemistry Pyrimidines some aspects of the Pyrrole pigments biogenetic origin of of 3 181 of 4 195 chemistry of 3 181 4 45 Quadrupole coupling nuclear and chemical bonding 11 162 Quadrupole moments molecular 13 Quenching of fluorescence 1 1 Quinones relation between the oxida- tion-reduction potential and chem- ical structures of 4 94 183 Radiations ionising action of on organic compounds 9,3 1 1 Radicals free electron resonance spectroscopy of 12 250 Radicals free interaction of with saturated aliphatic compounds 14 336 Radicals free mass spectrometry of 13 215 Radioactivation analysis 10 83 Radioactivity determination of gee logical age by 7 1 Radioactivity of the heavy elements 5.270 Reactions inorganic in liquid am- monia 16 19 Reactions unimolecular Arrhenius factors (frequency factors) in 14 133 Rearrangement pinacol 14 357 Rearrangements aromatic 6 34 Rearrangements benzilic acid and related 14 221 Rearrangements photochemical and related transformations 15 393 Redox potentials of quinone relation of to chemical structure 4 94 Reduction by metal-amine solutions; applications in synthesis and deter- mination of structure 12 17 Reduction by metal-ammonia solu- tions of organic compounds 4 69 Relaxation processes molecular in liquids ultrasonic analysis of 11 134 Replacement reactions of co-ordina- tion compounds kinetics and mechanism of 16 316 Rhenium chemistry 15 372 Salt hydrates crystal structures of 8 Salts basic structure of 1,247 Salts deposits of from oceans 1 91 Salts solid ionic conductance in 6 Sandmeyer reactions 6 358 Semiconductivity and catalysis 11 380 238 227 446 QUARTERLY REVIEWS Sesquiterpenoid chemistry 16 101 Sesquiterpenoids recent advances in Shock waves 14,46 Silicon chemistry of polymers con- Silyl compounds 10 208 Sodium “flame” reactions 5 44 Solids molecular-sieve action of 3 Solids thermal transformations in 11 Solids transitions in 3 65 Solids separated by a narrow gap direct measurement of molecular attraction between 10 295 Solution aqueous thermodynamics of ion association in 14,402 Solutions aqueous mechanism of electrode processes in 3 95 Solutions of non-electrolytes theories of 13.306 Solvation effects of on the properties of anions in dipolar aprotic solvents 16 163 Solvation ionic 3 173 Solvent extraction and its applications to inorganic analysis 5 200 Solvents ionising non-aqueous reac- tions in 10 451 Specificity in catalysis by metals 8 404 Spectra charge-transfer and related phenomena 8,422 Spectra charge-transfer theory of 15 191 Spectra emission of flames 4 1 Spectra far ultraviolet ionisation potentials and their significance in chemistry 2 73 Spectra infrared and Raman of hydrocarbons.Part I acetylenes and olefins 6 1. Part 11 paraffins 7 19 Spectra infrared of heteroaromatic compounds 13 353 Spectra Raman of inorganic com- pounds 10,185 Spectra rotation 4 131 Spectroscopy electron resonance of free radicals 12 250 Spectroscopy kinetic and flash photo- lysis 10 149 Stabilities of complex compounds 5 l chemistry of 11 189 taining 2 25 293 246 Stereochemistry of cyclohexane 7,221 Stereochemistry of elements of Sub- group VIB of the Periodic Table 10 407 Stereochemistry of elements of Group VIII of the Periodic Table 3 321 Stereochemistry of inorganic com- pounds 11,339 Stereoregular addition polymerisa- tion 16 361 Steric hindrance 2 107; 11 1 Steroidal alkaloids 7,23 1 Structure of liquids in relation to their transport properties 14 236 Substitutions aromatic nucleophilic mechanism and reactivity in 12 1 Sugar epoxides 13 30 Sulphur-fluorine bonds compounds containing 15 30 Sulphur nitride and its derivatives.10. 437 Sulphuric acid behaviour of organic compounds in 8 40 Surface chemistry adsorption energy and adsorption equilibria 15 99 Surface compounds the chemistry of carbon-oxygen 13,287 Sydnones 11 15 Synthetic gemstones 15 1 Tautomerism of phenols 10 27 Technetium chemistry an outline of 16 299 Terpenes di- and tri- synthesis of 16. 117 Tetronic acids 14 292 Theory of charge-transfer spectra 15 191 Thermodynamics of ion association in aqueous solution 14 402 Thermochemistry of the elements of Group IVB and IV comments on 7 103 Thermodynamic properties estima- tion of for organic compounds and chemical reactions 9,229 Thermodynamic properties of high polymers and their molecular inter- pretation 1 265 Thermodynamic properties of organic oxygen compounds 15 125 Topotactic reactions in inorganic oxy-compounds 16,343 Tracers radioactive preparation of 2 93 CUMULATIVE INDEX 447 Transformation asymmetric and asymmetric induction 1 299 Transformations related and photo- chemical rearrangements 15 393 Transformations thermal in solids 11,246 Transition metals cyanide complexes of the 16 188 Transition-metal compounds crystal- line electron resistance in 14 427 Transitions in solids and liquids 3,65 Transport control in heterogeneous reactions 6 157 Transport properties of liquids in relation to their structure 14 236 Triplet state 12 205 Triterpenes tetracyclic 9 328 Tropolones 5 99 Tryptophan biological degradation of 5 227 Ultrasonic analysis of molecular relaxation processes in liquids 11 134 Ultrasonic waves effects of on electrolytes and electrolytic pro- cesses I 84 Veratrum alkaloids 12 34 Wool wax constitution of 5 390 X-Ray crystal analysis modern methods of determination of mole- cular structure by and their ac- curacy 7,335 p-Xylylene chemistry of and of its analogues and polymers 12 301

ISSN:0009-2681    

DOI:10.1039/QR9621600435

出版商:RSC    

年代:1962

数据来源: RSC