4 Gas-phase Kinetics and Mechanisms By R. WALSH Department of Chemistry University of Reading Whiteknights. Reading RG6 2AD 1 Introduction The period of this Report is limited to the year 1973 and the content to reactions of large radicals and molecules. 1972 has alas not been covered the enthusiasm of researchers and willingness of publishers having ensured too great a volume of material for inclusion. Even so it has been necessary to restrict coverage of many topics to the briefest mention such is the breadth of field covered by the title. When all this is said and bemoaning the expansion of the literature has become the reviewer’s endemic condition the author still had some anxiety that his coverage of the journals (26 were searched excluding reviews) may have failed to unearth some important contribution.He was comforted by the knowledge that another reviewer in the field’ was happy to limit his report to a ’slightly- more-conscientious-than-usual coverage of major journals’. The cataloguing of kinetic data and their critical evaluation continue and a summary of current efforts appeared this year.2 Amongst new books of interest are those on the subjects of the theory of unimolecular reactions3’ and the kinetics of addition and elimination reaction^.^' A particularly valuable series of essays on free radicals4 has appeared. 2 Theory and Experiment This year has been one of consolidation rather than dramatic development. It has seen the further extension of the molecular beam technique into this area of kinetis5s6 In a series of studies Lee Rice and co-workers5 have examined both the velocity and angular distributions of products from the decomposition A.A. Westenberg Ann. Rev. Phys. Chem. 1973 24 77. L. H. Gevantman and D. Garvin Internat. J. Chem. Kinetics 1973 5 213. (a)W. Forst ‘The Theory of Unimolecular Reactions’ Academic Press London 1973; (6) ‘Addition and Elimination Reactions of Aliphatic Compounds’ ‘Comprehensive Chemical Kinetics’ Vol. 9 ed. C. H. Bamford and C. F. H. Tipper Elsevier London 1973. ‘Free Radicals. I. Dynamics of Elementary Processes’ ed. J. K. Kochi Wiley-Inter- science 1973. J. M. Parson K. Shobatake Y. T. Lee and S. A. Rice J. Chem. Phys. 1973 59 1402 1416 1427 1435. J. T. Cheung J. D. McDonald and D.R. Herschbach J. Amer. Chem. Soc. 1973 95 7889. 69 70 R. Walsh of chemically activated fluoroalkyl radicals (and other F-containing radicals). Statistical theories are inadequate in two ways to explain their results. First for the faster decomposition pathway (methyl elimination) energy randomization is incomplete and second for light-particle (H atom) elimination considerably fewer vibrational modes than the total number available participate in energy redistribution. These results do not necessarily invalidate the RRKM* theory of unimolecular reactions except under extreme conditiqns. However Bunker' has performed trajectory calculations which suggest that RRKM theory over-estimates by more than an order of magnitude the decomposition rate of CH,NC at only 293 kJ mol-' in excess of threshold.At this energy the RRKM rate constant is only ca. 101os-l much slower than the expected rate of internal energy randomization. It is clearly too early to judge the significance of these calculations but a great deal of experimental evidence supports the validity of the RRKM theory (see for example later section on Chemical Activation) to within better than an order of magnitude. Within the framework of the theory useful papers have appeared on methods of calculation of energy level sums and densities for anharmonic oscillators and hindered rotors' as well as a simple collisional model enabling easy derivation of the average energy transferred per collision (E ) in the low-pressure limit of thermal unimolecular reactions.' Some complex trajectories have been calculated for the insertion reaction of 'CH in H, which serves as a useful reminder" that the dynamics of a reaction" usually give a much more complicated picture of reaction mechanism than a simplistic view of a reaction co-ordinate on a potential energy surface.A review has appeared of the VLPP? technique developed by Benson Golden and Spokes,i2 which gives a valuable account of the versatility of the method. A slight disappointment has been its relative inability to distinguish' between loose and tight transition-state models (high and low A factors) for unimolecular decomposithns. A number of reports have appeared of new methods for the direct 14-17 and indirect l8 spectroscopic detection of free radicals in reacting systems.Amongst them the most promising from the point of view of kinetics * RRKM = Rice-Ramsperger-Kassel-Marcus. t Very Low Pressure Pyrolysis. D. L. Bunker and W. L. Hase J. Chem. Phys. 1973 59 4621. * S. E. Stein and B. S. Rabinovitch J. Chem. Phys. 1973 58 2438. J. Troe Ber. Bunsengesellschaft phys. Chem. 1973,77 665. lo I. S. Y. Wang and M. Karplus J. Amer. Chem. SOC. 1973,95 8160. l1 J. C. Polanyi Accounts Chem. Res. 1972 5 161. l2 D. h-f. Golden G. N. Spokes and S. W. Benson Angew. Chem. Internat. Edn. 1973 12 534. I3 See for example M. J. Perona P. C. Beadle and D. M. Golden Inrernat. J. Chem. Kinetics 1973 5 495. l4 D. A. Parkes D. M. Paul C. P. Quinn and R. C. Robson Chem. Phys. Letters 1973 23 425.l5 P. D. Pacey Chem. Phys. Letters 1973 23 394. l6 G. H. Atkinson A. H. Laufer and M. J. Kurylo J. Chem. Phys. 1973,59 350. " T. A. Leggatt and D. A. Kohl J. Chem. Phys. 1973,59,611. l8 (a)E. G. Janzen 1. G. Lopp and T. V. Morgan J. Phys. Chem. 1973,77 139; (b)E. G. Janzen T. Kasai and K. Kuwata Bull. Chem. SOC.Japan 1973,46 2061. Gas-phase Kinetics and Mechanisms 71 would appear to be the modulation technique developed by Parkes and co- worker~.’~ 3 Bond Dissociation Energies The importance of reliable bond dissociation energies for kinetics remains para- mount and so this subject is again included.” It has been recently reviewed by Egger and Cocks.20 Many values for organic C-H bonds are now known and more sophisticated mass spectrometric measurements than hitherto’ are pro- ducing data in better agreement with kinetic methods.22 Three recent values21p23*24 for D(C,H-H) ca.532 kJ mol- have appeared. Amongst n-allylic stabilized radicals substituent effects vary the stabilization energy between 3 1 and 75 kJ mol-’,25-27 although there is some disagreement over chloro-ai1y1.25ay27b E.s.r. data on 2-alkanonyl radicals28 support a small n-allylic inter- action here but the thermal isomerization of acetylcy~lopropane,~~ if it occurs via a biradical mechanism suggests a figure of ca. 30 kJ mol- for this stabiliza- tion energy higher than that observed by iodination kinetic^.^' A cyano-group stabilizes a radical’centre31 by ca. 30 kJ mol-I but there is no stabilization in acryl (vinyl carbonyl) radicals.32 The CF group seems to promote a bond- strengthening effect at BC-X bonds,33 while the C-F bond in perfluorobenzene must be amongst the strongest known.34 An upward revision of the value of as well D(Me,Si-H) has been rep~rted~~.~~as a downward change in D(SiH3-H),37 which makes these Si-H bond strengths much more nearly comparable as suggested by relative abstraction rate studies.3840 Esr.19 D. C. Montague and R. Walsh Ann. Reports (A) 1971 68 175. 20 K. W. Egger and A. T. Cocks Helv. Chim. Acta 1973,56 1517 1537. 21 D. K. Sen Sharma and J. L. Franklin J. Amer. Chem. SOC.,1973.95 6562. 21 D. M. Golden and S. W. Benson Chem. Rev. 1969.69 125. 23 J. R. Wyatt and F. E. Stafford J. Phys. Chem. 1972 76 1913. 24 H.Okabe and V. Dibeler J. Chem. Phys. 1973 59 2430. 25 (a) Z. B. Alfassi D. M. Golden and S. W. Benson Internat. J. Chem. Kinetics 1973 5 155; (6)Z. B. Alfassi and D. M. Golden ibid. p. 295. Zb A. S. Rodgers and M. C. R. Wu J. Amer. Chem. SOC.,1973.95 69 13. 21 (a)A. B. Trenwith J.C.S. Faraday I 1973 1737; (6)A. B. Trenwith. Internaf.J. Chem. Kinetics 1973 5 67. 28 (a) D. M. Camaioni H. F. Walter and D. W. Pratt J. Amer. Chem. Soc. 1973. 95 4058; (6) D. M. Camaioni H. F. Walter J. E. Jordan and D. W. Pratt ibid. p. 7978. 29 A. T. Cocks and K. W. Egger J.C.S. Perkin 11 1973 197. 30 R. K. Solly D. M. Golden and S.W. Benson Internat. J. Chem. Kinetics 1970. 2. 381. 31 D. A. Luckraft and P. J. Robinson Internat. J. Chern. Kinetics 1973 5 137.32 Z. B. Alfassi and D. M. Golden J. Amer. Chem. SOC.,1973 95 319. 33 E.-C. Wu and A. S. Rodgers. Internat. J. Chem. Kinetics 1973 5 1001. 34 M. J. Krech S.J. W. Price and W. F. Yared Canad. J. Chem. 1973 51. 3662. 35 I. M. T. Davidson and A. B. Howard J.C.S. Chem. Comm. 1973 323. 36 R. Walsh and J. M. Wells J.C.S. Chem. Comm. 1973 513. 37 W. H. Deuwer and D. W. Setser J. Chem. Phys. 1973,SS. 2298. 38 E. Whittle ‘Reactions of Free Radicals’ in ‘Chemical Kinetics’ ed. J. C. Polanyi Series 1 Vol. 9 of M.T.P. International Review of Science (Physical Chemistry) Butterworths London 1972 Chap. 3 p. 75. 39 R. E. Berkley I. Safarik H. E. Gunning and 0.P. Strausz J. Phys. Chem. 1973 77 1734. 40 A. Hosaka and F. S. Rowland J. Phys. Chem.. 1973 77 705.72 R. Walsh combined kinetic and equilibrium studies in solution show promise as a tech-nique for determining the strengths of weak 0-H and N-H bonds such as occur in hydr~xylamines.~' Solvent effects largely cancel out and the values obtained should not differ significantly from those applicable to the gas phase. 4 Radical Reactions The kinetics of gas-phase radical reactions in general have been reviewed.42 Radical-Radical Reactions-Methyl radical recombination continues to excite interest and three st~dies'~*~~"*' confirm the previously accepted rate constant of ca. dm3 mol-' s-' but a produced a value greater by more than a factor of two. It is disappointing that discrepancies can still occur in such a well-studied reaction.The pressure-dependent region for the rate constant a more contentious question was shown to at pressures <0.5 Torr. Much more controversial have been recent measurements of recombination rate constants for larger alkyl radicals which are collected together in Table 1. Table 1 Recombination rate constants for alkyl radicals by different techniques log, (k/dm3 mol-s-') at (temp./K) obtained by Radical Radical bufler method VLPP Hydrocarbon pyrolysis (wp Et 10~.~-10~.~ 10'' (80(t900)d (951)' Pr' (415)b 109.7(7ocr80o)d -Bu' 105.4(373)~ 108.8(650)d 10"' (77C855)" (a) R.Hiatt and S. W. Benson J. Amer. Chem. Soc. 1972,94,25 6886. (h) R. Hiatt and S. W. Benson Infernat. J. Chem. Kinetics 1972 4 151. (c) R. Hiatt and S.W. Benson ibid.1973,5 385. (d)Ref. 12. (e) R. M. Marshall and J. H. Purnell J.C.S. Chem. Comm. 1972 764. V) P. D. Pacey and J. H. Purnell Infernat. J. Chem. Kinetics 1972. 4 657. (g) Ref. 54. Three methods have been used to obtain these numbers. The radical buffer technique which depends for its success on the establishment of an equilibrium of the type R' + R'I R'I + R2 in the system probably has a precision of only an order of magnitude since the rate constant depends on estimates of other rate constants and thermochemistry. Similar comments apply to rate constants obtained from hydrocarbon pyrolysis. The VLPP data significantly different from the other two sets are as yet only preliminary. The picture may of course be clouded by activation energy effects although no trends are apparent.It is clear that these radicals do not combine at every collision as was once thought. 41 L. R. Mahoney G. D. Mendenhall and K. U. Ingold J. Amer. Chem. Soc. 1973 95 8610. 42 J. A. Kerr ref. 4 p. 1. 43 (a) F. K. Truby and J. K. Rice Internar. J. Chem. Kinetics 1973 5 721; (b) F. Bayrakceken J. H. Brophy R. D. Fink and J. E. Nicholas J.C.S. Furaduj I 1973 69 228. 44 A. M. Bass and A. H. Laufer Internat. J. Chem. Kinetics 1973 5. 1053. Gas-phase Kinetics and Mechanisms 73 However the particularly low value for t-butyl radical recombination is also hard to swallow in view of the known consistency of radical cross-combination ratios with the geometric mean rule.45 2-Methallyl radicals43 recombine with k = dm3 mol-' s-l a value higher than that for all~l,~~ but there is evidence that for 1-methallyl and more highly substituted ally1 radicals:' recombination rates are less at the more substituted end implying the same trend as for alkyl radicals.Other recombina- tion rate constants recently determined include those for C2H3 ,48u CF :8b NH CONH ,49b CF,CCl ,50 and CH302 .I4 Disproportionation+om-bination ratios have not excited much attention this year. An expected isotope effect was observed for 2CF,H(CF,D)+ CF + CF,H,(CF2D2).51 A useful catalogue of values often individually buried in work of another concern has been p~blished.~' Radical-Molecule Reactions.-Amongst hydrogen-atom abstraction studies the year has been marked by an increasing number of claims of non-Arrhenius behaviour for these processes.The reactions of CH with H2,53 C2H6,53 iso-C4H and ne~-c,H,,~~ fall into this category. Curvature in Arrhenius plots is not usually observable in a single study over a limited temperature range but is apparently necessary to reconcile the results from low-temperature photolytic systems (300-500 K) with intermediate-temperature pyrolyses (600-900 K) and high-temperature (> 1000 K) shock-tube studies. There are a number of theoretical reasons why such curvature might occur and Clark and Dove have performed modified BEBO calculation^^^ which produce rate-constant expres- sions of the form AT" exp (-V/RT)where in practice n 21 M for the models chosen. These values of n are higher than the classical limit of transition-state theory would allow and in the opinion of this reviewer seem to imply something rather unrealistic about the frequency assignment of the transition state.It is also apparent that for some of the reactions investigated the scatter in the experi- mental data obtained within a given temperature range but by different sets of workers is enough still to permit linear Arrhenius plots within experimental error. These problems must clearly be more fully investigated before claims of significant curvature are generally accepted. 45 J. 0.Terry and J. H. Futrell Canad. J. Chem. 1968,46 664. 46 H. E. van den Bergh and A. B. Callear Trans. Faraday Soc. 1970,66,2681. 47 D. C. Montague Internat. J. Chem. Kinetics 1973 5 513. 48 (a) K. 0. MacFadden and C.L. Currie J. Chem. Phys. 1973 58 1213; (b) R. Hiatt and S. W. Benson Internat. J. Chem. Kinetics 1972,4 479. 49 (a)T. Yokota and R. A. Back Internat. J. Chem. Kinetics 1973,5 37; (b) R. A. Back and T. Yokota ibid. p. 1039. 50 R. F. Cullison R. C. Pogue and M. L. White Internat. J. Chem. Kinetics 1973 5 415. st G. 0.Pritchard and D. W. Follmer Internat. J. Chem. Kinetics 1973,5 169. 52 M. J. Gibian and R. C. Corley Chem. Reu. 1973,73 441. 53 T. C. Clark and J. E. Dove Canad. J. Chem. 1973,51. 2147,2155. 54 R. S. Konar R. M. Marshall and J. H. Purnell Internat. J. Chem. Kinetics 1973 5 1007. 55 P. D. Pacey Canad. J. Chem. 1973.51 2416. 74 R. Walsh Other hydrogen-atom abstraction studies have been reported for C,H,56 CF3:7*58 and NH2,49bas well as chlorine-atom transfer by Me3SiS9 and Me3Sn,60 and where Arrhenius parameters have been measured they seem generally reason- able.Because of the advantages of e.s.r. detection of radicals an increasing number of solution studies are appearing in this area. Where non-polar solvents are used Arrhenius parameters are probably transferable to the gas phase. One such study6' reports activation energies for halogen transfer to Bu';Sn and in BuI;Pb radicals. Substituent effects on atom transfers are usually accounted for by a judicious mixture of bond-energy and polar effects. Zavitsas6' has ques- tioned whether the latter are necessary in all cases but the answer would appear to be yes since even with non-polar radicals like Bu' polar effects are evident.63 Some new empirical schemes for predicting activation energies (within 4-8 kJ mol-') have a~peared.~~.~~ Their accuracy is as good as that of the BEBO method.Some relative rates for hot CH366*67 are reported as well as an abstrac- tion by CH at 77 K,68which if it really occurs must be a hot reaction since the activation energy otherwise implied is far too low. The debate over anchimeric assistance by /3 bromine in radical abstractions still rage^,^^.^' but the evidence from the gas phase7' at any rate suggests little effect although the conformation of /I-bromoalkyl radicals7' (and indeed other #I-halogenoalkyl radicals in general7,) is such as to inhibit backside attack in subsequent transfer processes to the radical7' A molecular-beam study of methyl with the halogens has found predominantly backward product scattering implying short-range repulsive interactions.New data on fluoroalkyl radical^'^ and CFBr,76 addition to fluoro-olefins supports earlier conclusions that addition is favoured at the least fluorinated 56 C. F. Cullis D. J. Hucknall and J. V. Shephard Proc. Roy. SOC.,1973 A335 525. 57 M. H. Arican E. Potter and D. A. Whytock J.C.S. Furuduyl 1973 69 1811. 58 N. L. Arthur and B. R. Harman Austral. J. Chem. 1973 26 1269. 59 P. Cadman G. M. Tilsley and A. F. Trotman-Dickenson J.C.S. Furaday I 1973.69 914. 60 D. A. Coates and J. M. Tedder J.C.S. Perkin 11 1973 1570. 61 J. Cooper A. Hudson and R. A. Jackson J.C.S. Perkin 11 1973 1056. 62 A.A. Zavitsas and J. A. Pinto J. Amer. Chem. SOC.,1972 94 7390. 63 W. A. Pryor W. H. Davis and J. P.Stanley J. Amer. Chem. SOC.,1973,95 4754. 64 Z. B. Alfassi and S. W. Benson Internat. J. Chem. Kinetics 1973,5 879. 65 R. R. Baldwin and R. W. Walker J.C.S. Perkin 11 1973 5 361. 66 J. K. Rice and F. K. Truby Chem. Phys. Letters 1973 19 440. 67 C.-T. Tingand R. E. Weston J. Phys. Chem. 1973 77 2257. 68 E. D. Sprague J. Phys. Chem. 1973,77 2066. 69 E. S. Lewis and S. Kozuka J. Amer. Chem. SOC., 1973,95282. 70 (a)K. J. Shea and P. S. Skell J. Amer. Chem. SOC.,1973 95 283; (b)P. S. Skell R. R. Pavlis D. C. Lewis and K. J. Shea ibid. p. 6735; (c)K. J. Shea D. C. Lewis and P.S. Skell ibid. p. 7768. 71 D. S. Ashton J. M. Tedder M. D. Walker and J. C.Walton J.C.S. Perkin II 1973 1346. 72 J. H. Hargis and P. B. Shevlin J.C.S. Chem. Comm. 1973 179. 73 (a)J. Cooper A. Hudson and R. A. Jackson Tetrahedron Letters 1973 831 ; (b)K. S. Chen I. H. Elson and J. K. Kochi J. Amer. Chem. SOC.,1973 95 5341. 74 D. L. McFadden E. A. McCullough F. Kolos and J. Ross J. Chem. Phys. 1973,59 121. 75 D. S. Ashton A. F. MacKay J. M. Tedder D. C. Tipney and J. C. Walton J.C.S. Chem. Comm. 1973,496. 76 J. P. Sloan J. M. Tedder and J. C. Walton J.C.S. Furaday I 1973 69 1143. Gas-phase Kinetics and Mechanisms 75 site and that relative rates are largely although not entirely attributable to varia- tions in activation energy. The addition of acetyl to butadiene is competitive with its thermal fragmentati~n.~~ Some results on atom and radical addition to olefin not in the gas phase but nevertheless of interest to kineticists were reported from the rotating cryo~tat.~~ A number of other addition rate constants are collected in Table 2.Table 2 Radical-addition reactions log1 0 log10 (k/dm3 (A/dm3 Reaction mol-'~-~) mol-'~-~) E/kJmol-' Temp./K Me + SO +MeSO 8.24 298" Et + CO+ EtCO 8.19 20.0 238-37gb Me0 + CO-+ Me + CO 10.2 49.4 396-426' OH + C,H,-+ 9.65 3.8 230-470d CZH + 9.08 8.8 230-470'' ,SO + cis-C,H,+ 11.21 298' ,SO + trans-C,H,+ 11.15 298' CF + CF,NO+ (CF,),NO >7.77 329/ (a)F. C. James J. A. Kerr and J. P. Simons J.C.S. Furuduy I 1973,69,2124. (b)Ref. 89 (c) E. A. Lissi and G. Massif J.C.S. Furaduy I 1973 69 346. (6)I.W. M. Smith and R. Zellner J.C.S. Furuday 11 1973 69 1617. (e) R.A. Cox J. Photochem. 1973 2 1. (f)H.-S. Tan and F. W. Lampe J. Phys. Chem. 1973,77 1335. The first example of a radical displacement (S,2) process at aliphatic un- strained saturated carbon has been observed7' in the gas-phase reaction CF + neo-C,H,,+ CF,CH + But although rate constants have yet to be measured and the process is a minor one in the system. This displacement process and its kinetics at metal and other centres are increasingly under investigation in solution.80 In the ring-opening of cyclopropane by Br stereochemical studies have proved a Walden inversion mechanism at the displacement centre.' ' Unimolecular Radical Reactions.-The intramolecular rarrangements of hydro-carbon radicals in the gas phase have been reviewed.82 Watkins' has obtained for n-hexyl- s-hexyl log (k/s-') = 9.41 -47 kJ mol-'/2.303RT.The 'A' fac-tor although low corresponds to the loss of four internal rotors in the transition state and is therefore reasonable. Not so reasonable however are values in some l7 M. V.Encina and E. A. Lissi J.C.S. Faruduy I 1973,69 1505. '~3 J. E. Bennett and B. Mile J.C.S. Furuduy I 1973 69 1398. 79 R. A. Jackson and M. Townson Tetrahedron Letters 1973 193. A. G. Davies and B. P. Roberts Accounts Chem. Res. 1972 5 381. (a)G. G. Maynes and D. E. Appelquist J. Amer. Chem. SOC.,1973,95 856; (b)K. J. Shea and P. S. Skell ibid. p. 6728. 82 A. D. Stepukhovitch and V. I. Balaban Russ. Chem. Rev. 1972,41,750. 83 K.W. Watkins J. Phys. Chem. 1973 77,2938. 76 R. Walsh cases as low as lo7-lo8 s-’,for such processes supported by Mintz and LeRoyg4 on the basis of calculations on chemically activated radicals. The entropies of the transition states implied by these numbers are significantly lower than those of known cyclic molecules of similar ~tructure,~~ and the proposal of a previously unconsidered configurational entropy adjustment84 does not avoid this dilemma. Much higher ‘A’ factors ca. 10’2.2s-’ are obtained for the similar internal H transfer in photo-excited ketones the Norrish Type I1 process.86 A calc~lation,~ based on Forst’s simplified version of thermal unimolecular reaction theory,88 has been used to show that the pressure dependence of acetyl radical decomposition is more consistent with a high-pressure ‘A’ factor ca.10’2.5s-’,rather than with the previous reported value of s-’. The kinetics of 2-methallyl decomp~sition~~” have been investigated ;those of both propiony18 9b and amid^^^^ radical decompositions have been reinvesti- gated. Methylene and Sily1ene.-The thermochemistry of methylene is still a controver- sial subect. Theoretical calc~lations~~ suggest an energy separation of 46 f.8 kJ mol- ’between triplet (3B1)and singlet (‘A’) methylene. Aclosely similar value is supported by Frey91 on the experimental grounds of approximate relative reactivities of the singlet and triplet. A smaller value of ca. 12 kJ mol-is favoured by Carr9 from the observation that 10% of the products from methylene re- actions in the 355 mm photolysis of CH2C0 come from the singlet form.But ‘CH is known to be much more reactive than 3CH2,93,94 and so this observa- tion must in fact correspond to much less than 10%of the methylene itself being in the singlet form. Indeed if Frey’s argument is correct the reactivity difference is such that at 355 nm only 3CH can be formed and the ‘CH must come entirely from collisionally induced (reverse) intersystem crossing from 3CH,. More recently Carrg5 has argued that the smaller energy difference is supported by chemical activation studies. These latter were interpreted with a stepladder (weak collision) model for deactivation. That such models are required is almost certain in one case where impossible energetics would otherwise result.96 How- ever the insensitivity of experimental parameters (curvature in D/S against pressure-’ plots) to details of the model make these chemical activation 84 K.J. Mintz and D. J. LeRoy Canad. J. Chem. 1973 51 3534. 85 H. M. Frey and R. Walsh Chem. Rev. 1969,69 103. 86 J. Grotewold D. Soria C. M. Previtali and J. C. Scaiano J. Phorochem. 1973,1 471. H. M. Frey and I. C. Vinall Internat. J. Chem. Kinetics 1973 5 523. 88 W. Forst J. Phys. Chem. 1972 76 2507. (a)W. Tsang Internat. J. Chem. Kinetics 1973 5 929; (6)K. W. Watkins and W. W. Thompson ibid. p. 523. C. F. Bender H. F. Shaefer D. R. Franceschetti and L. C. Allen J. Amer. Chem. SOC. 1972,946888. ’’ H. M. Frey T.C.S. Chem. Comm. 1972 1024. 92 R. W.Carr T. W. Eder and M. G. Topor J. Chem. Phys. 1970 53 4716. 93 W. Braun A. M. Bass and M. Pilling J. Chem. Phys. 1970,52 5131. 94 P. S. T. Lee R. L. Russell and F. S. Rowland Chem. Comm. 1971 18. 95 M. G. Topor and R. W. Carr J. Chem. Phys. 1973,58,757. 96 H. M. Frey G. R..Jackson M. Thompson and R. Walsh J.C.S. Faraday I 1973,69 2054. Gas-phase Kinetics and Mechanisms 77 at best poor guides to the thermochemistry of methylene. In one recent study9* the calculated energies quoted were actually incorrect owing to an insufficient vibrational assignment having been made for the activated molecule and its complex. In less controversial areas relative insertion rates by 'CH in alkanes have been ~ummarized.~~ A revised f numberloo for the 3A,-3B1 transition in CH means that earlier quoted absolute rate constants93 need numerical correction.Vacuum-u.v. photolytic sources101p102 produce methylene with a large spread of energy which is carried over into reaction. 1.r. chemiluminescent emission from CO in the reaction of CH with 0 has been interpreted as arising from highly excited formic acid molecule^.'^^ Emission from CH has been seen in flames.lo4 An up-to-date summary of the chemistry of methylene has recently appeared.'O5 From relative rate constant measurements in mixed silane pyrolyses and the reasonable assumption that AH (SiH,) = 242 kJ mol-' Purnell and John'06a have derived absolute rate constants for SiH insertions into H, SiH4 and Si,H,. Relative rates of insertion into a wide variety of substrates are reported.lo6' SiH will add 1,4 to butadiene.'" Si2H4 is probably produced in the pyrolysis of Si3H81°6 and the authors argue that it is more likely to be the species SiH3SiH than SiH,SiH,.Some GeH relative insertion rates have been reported."' Complex Reactions involving Radicals.-These are always difficult to assess but in the absence of a complex knowledge of likely elementary steps together with detailed computer modelling thermochemical criteria lo9 can be used to judge the validity of mechanisms proposed. This has been done in the case of some recent pyrolyses of oxalates' lo and aromatic compounds. '' Amongst paraffin pyrolyses that of iso-C4Hlo shows self-inhibition owing to rapid formation of unreactive 2-methallyl radicals arising from the initially formed product iso- C,H8.Azomethane has been commonly used as a photolytic source of methyl 97 W. L. Hase R. J. Phillips and J. W. Simons Chem. Phys. Letters 1971 12 161. 98 G. B. Kistiakowsky and B. B. Saunders J. Phys. Chem. 1973,77,427. 99 M.L. Halberstadt and 1. Crump J. Phorochem. 1973 1 295. loo J. K. Little and M.J. Pilling J. Photochem. 1973 1 337. lo' P. Ausloos R. E. Rebbert and S. G. Lias J. Photochem. 1973 2 267. lo' K. Dees and R. D. Koob J. Phys. Chem. 1973,77 759. Io3 R. J. Gordon and M. C. Lin Chem. Phys. Letters 1973 22 107. Io4 A. Jones and P. J. Padley Chem. Phys. Letters 1973 20 104. Io5 W. J. Baron M.R. Decamp M. E. Hendrick M. Jones R. H. Levin and M. B. Sohn in 'Carbenes. Vol. I' ed. M.Jones and R. A. Moss Wiley-Interscience 1973 Chap. 1. (a) P. John and J. H. Purnell J.C.S. Furaday I 1973 69 1455; (b) M. D. Sefcik and M. A. Ring J. Amer. Chem. SOC. 1973,95 5168. lo' G. P. Gennano Y.-Y.Su 0. F. Zeck S. H. Daniel and Y.-N. Tang J.C.S. Chem. Comm. 1973 637. log M. D. Sefcik and M.A. Ring J. Organometallic Chem. 1973 59 167. S. W. Benson 'Thermochemical Kinetics' Wiley 1968. 'lo R. Louw M. van den Brink and H. P. W. Vermeeren J.C.S. Perkin 11 1973. 1327. 'IL (a) R. Louw J. W. Rothuizen and R. C. Wegman J.C.S. Perkin If 1973 1635; (b)R. Louw and H. J. Lucas Rec. Trav. chim. 1973,92 55. R. Walsh radicals but its uninhibited pyrolysis has only just been reported' '' in any detail. The system is very complex producing several nitrogen-containing products as well as hydrocarbons and there is evidence for CH and C2H as well as CH as intermediates.High-temperature exchange in the system toluenedeuterium produces ;-order kinetics' ' and a relative positional reactivity for D atoms in addition to toluene of o > rn > p. The mercury-photosensitized decomposition of hydrocarbons has been an important source of alkyl radicals in the past and a unified mechanism for the primary interaction of the excited Hg (of both 63P1 and 63P0)with the alkane has been presented.' l4 In the oxidation of alkenes an oxygen-labelling experiment' l5 has shown that the peroxyl radical I >C(OH)-C-0; breaks down to ketone products without abstracting hydro- I gen intermolecularly to become a hydroperoxide.Methyl isocyanide is argued to be an important molecule in testing thermal explosion theory.'16" The anti- knock phenomenon is discussed in a shock-tube study of the decomposition of tetraethyl-lead. ' With increasing concern over air pollution it is hardly surprising that ozone- olefin reactions are increasingly receiving attention from kineticists. Niki and co-workers have recently measured some gas-phase second-order rate constants for this reaction."' In solution the reaction leads to ozonide formation (the initial molozonide having only recently been detected' l8 at low temperature) but in the gas phase ozonide is not observed and a large variety of other products largely ketones aldehydes and acids is observed.' l9 The mechanism in solu- tion although still controversial,'20 is believed to involve an aldehyde oxide the +-so-called 'Criegee Zwitterion' RCHOO.O'Neal and Blumstein' '' have argued against this mechanism being operative in the gas phase and in favour of bi- radical and monoradical intermediates. Pitts and co-workers' 22 have obtained mass spectrometric evidence for radical intermediates as well as for formation of an a-carbonyl hydroperoxide an important although not very stable product. 5 Molecular Reactions Decomposition to Radicals.-A simple classification of molecules into small and large on the basis of whether thermal fragmentations are at (or near) the low- or 'I2 (a) H. Knoll K. Scherzer and G. Geiseler Z. phys. Chem. (Leipzig) 1972 249. 359; (b) Y.Paquin and W. Forst Internat. J. Chem. Kinetics 1973 5 691. Y. Sato and A. Amano BUN. Chem. SOC. Japan 1973,46,2646. 'I4 (a) H. E. Gunning J. M. Campbell H. S.Sandhu and 0.P.Strausz J. Amer. Chem. SOC.,1973,95 740; (6)J. M. Campbell 0.P.Strausz and H. E. Gunning ibid. p. 746. D. J. M. Ray A. Redfearn and D. J. Waddington J.C.S. Perkin II 1973 540. IL6 (a) H. 0.Pritchard and B. J. Tyler Canad. J. Chem. 1973 51 4001; (b)J. B. Homer and I. R. Hurle Proc. Roy. SOC.,1972 A327,61. '17 D. H. Stedman C. H. Wu,.and H. Niki J. Phys. Chem. 1973 77 251 1. W. G. Alcook and B. Mile J.C.S. Chem. Comm. 1973 575. I l9 Y. K. Wei and R. J. Cvetanovic Canad. J. Chem. 1963 41 913. I2O For leading references see P. S.Bailey T. P. Carter C. M. Fischer and J. A.Thompson Canad. J. Chem. 1973,51 1279. H. E. O'Neal and C. Blumstein Internat. J. Chem. Kinetics 1973 5 397. R. Atkinson B. J. Finlayson and J. N. Pitts J. Amer. Chem. SOC.,1973 95 7592. Gas-phase Kinetics and Mechanisms high-pressure limits for unimolecular behaviour would appear possible. In the former category activation energies are always less than bond dissociation energies and rate measurements usually made with the shock tube have to be related to high-pressure limiting values via unimolecular reaction theory and a knowledge of the collisional characteristics of the system. In this area studies have appeared on decompositions of C02.123NOBr,'24 NOCl,124BrCN,12' NH3,126 NF3,12' CH4,12*SF,,129SF,C1,129 C2H4,13' and N2F4.131 RRKM theory has been applied to some such and methods have been given for estima- ting specific rate constants from spectroscopic data.133 In the large molecule category direct measurements of the rate as a function of temperature will give the high-pressure Arrhenius parameters ;however even here the VLPPtechnique as mentioned requires theory in order to produce them. A selection of data is given in Table 3. Not quoted in the table but of some interest was a study by Table 3 Molecular fragmentation (bond-breaking) reactions Reaction ElkJ mol - Ternp./K C2H6- 2CH3 374' 1200-1400' BusCH2C(Me)=CH C2CI6-+ 2CC13 +Bus + CH,C(Me)=CH 15.6 17.7 276 286 950-1 1 Sod 61 3-473' MeCOCOMe-2MeCO CF,CHO-* CF + CHO Et,CCEt -+ 2Et,C" 16.5 17.0 16.6 283 347 218 648-690' 7 3 3-7 939 542-570h Am'OOAm' +2Am'O 15.8' 1 52' 523433' EtN=NEt -+EtN=N + Et Pr'N=NPr' -+ Pr'N=N + Pr' Bu'N=NBu'-* Bu'N=N + Bu' 16.4' 16.6' 16.4' 208* 200' 179' 670-950' 630-820' 550-710' Me,Tl* Me,TI + Me 15.1 152 452-536k Me,SiSiMe +2Me,Si 17.5 337 7 70-8 72' (a) In solution (probably).(b)Corrected by RRKM theory. (c)A. Burcat G. B. Skinner R. W. Crossby and K. Scheller Internat. J. Chem. Kinetics 1973 5 345. (d) W. Tsang ibid. p. 929. (e)M. L. White and R. R. Kuntz ibid. p. 187. (f)H. Knoll K. Scherzer and G. Geiseler ibid. p. 271. (g)M. T. H. Liu L. F. Loucks and R. C. Michaelson Canad. J. Chem. 1973 51 2292. (h) M.-D. Beckhaus and C. Ruchardt Tetrahedron Letters 1973 197 1. (i) M. J. Perona and D. M. Golden Internat. J. Chem. Kinetics 1973,5,55.Cj Ref. 13. (k)S. J. Price J. P. Richard R. C. Rumfeldt and M.G. Jacko Canad. J. Chern. 1973 51 1397. (0 Ref. 35. lZ3 A. M. Dean J. Chem. Phys. 1973,58 5202. 124 K. K. Maloney and H. B. Palmer Internat. J. Chem. Kinetics 1973 5 1023. Iz5 P. J. Kayes and B. P. Levitt J.C.S. Furuduy I 1973 1413. IZ6 G. A. Vompe Russ.J. Phys. Chem. 1973,47 715. 12' K. 0. MacFadden and E. Tschuikow-Roux J. Phys. Chem. 1973 1475. IZ8 G. A. Vompe Russ.J. Phys. Chern. 1973,47 788. Iz9 A. P. Modica J. Phys. Chem. 1973 77 2713. P. Rost and T. Just Ber. Bunsengesekhaftphys. Chem. 1973,77 11 14. 13' E. Tschuikow-Roux K. 0. MacFadden K. H. Jung and D. A. Armstrong J. Phys. Chem. 1973,77 734. 132 W. Tsang Internat. J. Chern. Kinetics 1973 5 947. 133 M. Quack and J.Troe Ber. Bunsengesellschafr phys. Chem. 1973,77 1020. 80 R. Wulsh Crawford 34 which demonstrated that azoalkane fragmentation is a single-bond- breaking consecutive-step reaction in the gas phase. The situation is more confused in solution'35 where a spectrum of behaviour seems possible. cis-Azoalkanes which may be implicated as intermediates in trans-azoalkane pyroly- 'A' Factors in the range 10"- sis are mysteriously considerably less ~tab1e.I~~ 10" s-have been obtained using Forst theory" applied to chemically activated alcohol R-OH bond-breaking reaction^.'^' However it is not clear whether these estimates have any validity since the theory is applicable only to thermal and not to chemical activation rate constants.88 Molecular Rearrangements and Eliminations.-The pursuit of the mechanisms of these processes is still a major activity amongst organic chemists.The choice has been seen until recently as usually lying between 'concerted' (symmetry- allowed' 38 or ar~matic'~~) pathways or two-step mechanisms involving bi- radicals. Theoretical (MO type) calc~lations'~~ suggest that symmetry-forbidden processes can in some cases compete effectively with biradical pathway^.'^' The concerted symmetry-forbidden transition state is stabilized by interaction of an orbital in a migrating group with a lower occupied (subjacent) MO of the residual molecular framework and may as a result be lower in energy than the biradical. This means that the interpretation of stereochemical labelling experi- ments hitherto regarded as one of the best tests for concertedness is considerably complicated.Stereochemical randomization may result either from intervention of a biradical or from a mixture of concerted allowed and forbidden processes. Some particularly subtle stereochemical experiments have been carried out by the groups of Ber~on'~~~?'~~ Most of these centre around the 1,3- and Ba1d~in.l~~ sigmatropic carbon shift. Ber~on,'~~~ for example has been able to show that in the isomerization of trans-1,2-truns,truns-dipropenylcyclobutane, of four possible stereochemical pathways in the isomerization to 3-methyl-4-trans- propenylcyclohexene effectively only two occur. This is argued as evidence against a biradical since although the two observed processes occurring in approximately equal proportions correspond to one allowed and one forbidden pathway a freely internally rotating biradical would presumably permit all four possibilities.This may well be so but until more is known about the properties 134 R. J. Crawford and K. Takagaki J. Amer. Chem. SOC.,1972 94 7406. 135 (a)N. A. Porter L. J. Marnett C. H. Lochmuller G. L. Closs and M. Shobataki J. Amer. Chem. SOC.,1972 94 3664; (6)J. Hinz A. Oberlinner and C. Riichardt Tetrahedron Letters 1973 1975. 136 N. Porter and M. 0.Funk J.C.S. Chem. Comm. 1973 263. 13' K. J. Mintz and R. J. Cvetanovic Canad. J. Chem. 1973,51 3386. 138 R. B. Woodward and R. Hoffmann 'The Conservation of Orbital Symmetry' Verlag Chemie and Academic Press 1970.139 M. J. S. Dewar Angew. Chem. Internat. Edn. 1971 10 761. I4O (a)J. A. Berson and L. Salem J. Amer. Chem. SOC.,1972,94 8917; (b)W. T. Borden and L. Salem ibid. 1973 95 932. I4I (a)J. E. Baldwin A. H. Andrist and R. K. Pinschmidt Accounts Chem. Res. 1972,5 402; (b)J. A. Berson ibid. p. 407. 142 (a)J. A. Berson and P. B. Dervan J. Amer. Chem. SOC.,1973 95 267 269; (6)J. A. Berson and R. W. Holder ibid. p. 2037. 143 J. E. Baldwin and R. H. Fleming J. Amer. Chem. SOC.,1973 95 5249 5256 5261. Gas-phase Kinetics and Mechanisms 81 of biradicals and particularly their internal rotation rates relative to reclosure,' 44 this issue cannot be regarded as settled. Arrhenius activation energies have been another useful criterion of mechanism in the past.Two problems have recently been highlighted. The first is the dis- crepancy between theory and thermochemical calculation of biradical ener- gie~.'~~.'~~ Theoretical estimates put the energies of both 1,3 and 1,4 biradicals about 25-40 kJ mol- ' above thermochemical estimate^.'^^ The reason for this is not clear. Secondly substituent effects such as the lowering of activation energies by vinyl of 50-60 kJ mol- ',often taken in the past as evidence for a biradical process have to be treated with caution since the discovery that in concerted processes which are slightly polar in nature similar activation energy reductions can be a~hieved.'~' Interestingly phenyl substituent effects have been regarded as diagnostic for both concerted f~rbidden'~'" and biradical processes.'48 Measurements of both Arrhenius parameters for a reaction can obviously still serve as a useful guide to mechani~rn,"~ but interpretations have to be treated with some caution.Only a limited selection of the large number of studies in this area can be included and reference to the original papers should be made for rate data. Rearrangements. A competitive single-pulse shock-tube study' 49 on cyclo-propane shows that up to 1300K,the rate of isomerization to propylene fits lower-temperature data well with a slight but reasonable correction for 'fall-off '. This study casts doubt on the strange effects claimed to occur in other shock-tube studies of this rea~tion."~ Contrary to earlier reports methyl elimination from the biradical intermediate formed from tetramethylcyclopropane is not competi- tive with i~omerization'~'~ up to 1100 K.However methyl elimination does occur from 1,3-biradicals formed in other ways.' lb Substituent effects on cyclo- propane isomerization have been reported for NH ,'52 CN,31 COMe,29 MeO,lS3 and C6HS.lS3 In a series of studies of the epimerization of substituted cyclopropanes in solution Cram and co-workers have shown that both 144 (a)H. E. O'Neal and S. W. Benson J. Phys. Chem. 1968,72 1866; (6)R. G. Bergman and W. L. Carter J. Amer. Chem. SOC. 1969,91 741 1. (a) J. A. Horsley Y. Jean C. Moser L. Salem R. M. Stevens and J. S. Wright J. Amer. Chem. SOC.,1972 94 279; (6) P. J. Hay W. J. Hunt and W. A. Goddard ibid. p. 638.146 L. M. Stephenson T. A. Gibson and J. I. Brauman J. Amer. Chem. SOC. 1973 95 2849. 14' K. W. Egger Internat. J. Chem. Kinetics 1973 5 285. 148 M. J. S. Dewar and L. E. Wade J. Amer. Chem. SOC. 1973.95 290. P. Jeffers D. Lewis and M. Sarr J. Phys. Chem. 1973 77 3027. I5O (a)J. N. Bradley and M. A. Frend Trans. Faraday Suc. 1971,67 72; (6)E. A. Dorko R. W. Crossley U. W. Grimm G. W. Mueller and K. Scheller J. Phys. Chem. 1973 77 143. Is' (a) W. Tsang Internat. J. Chem. Kinetics 1973 5 651; (b) E. B. Klunder and R. W. Carr J. Amer. Chem. SOC. 1973,95 7386. 152 (a) K. A. W. Parry and P. J. Robinson Internat. J. Chem. Kinetics 1973 5 27; (b) D. A. Luckraft and P. J. Robinson ibid. p. 329. J. M. Simpson and H. G. Richey Tetrahedron Letters 1973 2545.82 R. Walsh biradi~al''~and zwitterionic"' intermediates can occur. In the gas phase bi- radicals seem to be the rule and a useful case history of trimethylene summariz- ing both theoretical and experimental work has been published.lS6 In the rearrangement of the substituted cyclopropane exo-tricycl0[3,2,1,0~~~]oct-6-ene,'" a labelling experiment shows the occurrence of an interesting [2 + 2 + 21 process involving two CT components. Vinyl carbenes are implicated in the isomerization of cyclopropenes. 58 Racemization occurs considerably faster than isomerization for an optically active cyclopropene'58b and thus the vinyl carbene seems to show the same propensity to ring-close as trimethylene in spite of the increased strain of the product.Epoxide isomerizations under favourable circumstances occur with C-C bond splitting via a carbonyl ylide,' s9 although usually rearrangement via C-0 bond-breaking predominates.' 6o Further stereochemical evidence16' and iso- tope effects '62 support the intermediacy of a non-planar trimethylenemethane in methylenecyclopropane isomerization. Disagreement over mechanism exists for vinylmethylenecyclopropane rearrangements.'63*'64 Among Cope rearrange-ments that of the long-sought-after cisdivinylcyclopropane was noteworthy. 65 The complicated mechanism situation in the vinylcy~lobutanes'~~ and methyl- enecyclobutanes' 43 has already been mentioned. A 1,3-sigmatropic shift of silicon occurs with inversion. 166 In bicyclic versions of cyclobutane ring-opening reactions alternative pathways are very close in energy.In the bicyclo[2,2,0]- hexanes stereochemical studies '" support a concerted allowed process whereas thermochemistry ' favours an intermediate biradical. The same biradical is implicated in some Cope rearrangement^.'^^ Bicyclo[2,1 llhexane isomerization is non-stereo~pecific.'~~ Hot-molecule effects were observed in rearrangements amongst some C8Hloisomers'70 in the gas phase and one of the likely interme- diates a bis-2,2-biallyl biradical (tetramethylene-ethane) was detected in solution I54 A. B. Chmurney and D. J. Cram J. Amer. Chem. SOC. 1973,95,4237. 155 N. E. Howe E. W. Yankee and D. J. Cram J. Amer. Chem. SOC. 1973 95 4230. 156 R. G. Bergman ref. 14 Chap. 5 p. 191. 157 R.B. Kimnel and R. K. Freeman Tetrahedron Letters 1973 4803. I58 (a)R. D. Streeper and P. D. Gardner Tetrahedron Letters 1973 767; (6) E. J. York W. Dittmar J. R. Stevenson and R. G. Bergman J. Amer. Chem. SOC. 1973,95,5680. 159 R. J. Crawford V. Vukov and H. Tokonaga Canad. J. Chem. 1973,51 3718. 160 M. C. Flowers D. E. Penny and J. C. Pommelet Internat. J. Chem. Kinetics 1973,5 353. 161 W. von E. Doering and L. Birladeanu Tetrahedron 1973 29 499. 162 W. R. Dolbier and J. H. Alonso J. Amer. Chem. SOC. 1973 95 4421. 163 W. E. Billups K. H. Leavell E. S. Lewis and S. Vanderpool J. Amer. Chem. SOC. 1973,95 8096. 164 J. C. Gilbert and D. P. Higley Tetrahedron Letters 1973 2075. 165 J. M. Brown B. T. Golding and J. J. Stofko J.C.S. Chem. Comm. 1973 319.I66 J. Slutky and H. Kwart J. Amer. Chem. SOC. 1973 95 8678. 167 A. Sinnema F. van Rantwijk A. J. de Konig A. M. van Wijk and H. van Bekkum J.C.S. Chem. Comm. 1973 364. 168 E. N. Cain and R. K. Solly J. Amer. Chem. SOC. 1973 95 4791 7884. 169 L. A. Paquette and M. J. Kukla Tetrahedron Letters 1973 1241. 170 (a)W. R. Roth M. Heiber and G. Erker Angew. Chem. Internat. Edn. 1973 12 504; (b)W. R. Roth and G. Erker ibid. p. 505. Gas-phase Kinetics and Mechanisms 83 by CIDNP.17 Hot-molecule effects' 72 persist in bicyclopentene isomerization even in solution.'73 Two studies of trans-cis isomerization of hexa-1,3,5-triene were in reasonable agreement.174i1 75 Some rates were reported for the rarely observed 1,7-sigma- tropic hydrogen shift.' 76 Eliminations.Interest in four-centre eliminations from alkyl halides seems to be declining apart that is from HF eliminations.' 77 Considerable disagreements have existed in the past19 between the groups of Cadman and Tschuikow-Roux. Tsang' 78 has pointed out an inconsistency between the temperatures and times reported by Cadman et al. and recalculated some of their data. However the discrepancies are not entirely removed. clcl or three-centre elimination of HF occurs to about 18% of the total from chemically activated CH3CF2H.179 Other three-centre eliminations are observed from CHF2CF2SiF3 and CHF2CF2SiMe3 to give carbenes.'*' There have been a number of studies of acetates and their derivatives.' '-" All are broadly consistent with the cyclic six-centre polar transition state.A similar though less polar transition complex applies to allyl ether' 86 and allyl amine eliminations.' Molecular H2 elimina- tions are observed from cis-but-2-ene' 88a and ~yclopentene,"~ both undoubtedly via a 1,4 mechanism as was demonstrated in the reverse D,addition to cyclo- pentadiene.' The unimolecular H2 elimination from cyclohexa- 1,3-diene observable at low pressures and high temperatures probably occurs via bicyclo- 88b [3,1,0]hex-2-ene rather than a direct W. R. Roth and G. Erker Angew. Chem. Internat. Edn. 1973 12,503. 172 H. M. Frey and M. C. Flowers J. Amer. Chem. SOC. 1972 94 8636. (a)J. I. Brauman W. E. Farneth and M. B. D'Amore J. Amer. Chem. SOC. 1973,95 5043; (b)G. D. Andrews M. Davalt and J.E. Baldwin ibid. p. 5044. 174 S. W. Orchard and B. A. Thrush J.C.S. Chem. Comm. 1973 14. 175 W. von E. Doering and G. H. Beasley Tetrahedron 1973 29 2231. 176 M. Schabel and G. W. Klumpp Rec. Trav. chim. 1973,92,605. '17 M. V. C. Sekhar G. E. Millward and E. Tschuikow-Roux Internat. J. Chem. Kinetics 1973 5 363. l8 W. Tsang Internat. J. Chem. Kinetics 1973 5 643. 179 K. C. Kim D. W. Setser and B. E. Holmes J. Phys. Chem. 1973,77 725. 180 R. N. Haszeldine P.J. Robinson and W. J. Williams J.C.S. Perkin II 1973 1013 1018. G. Chuchari S. P. de Chang and L. Lombana J.C.S. Perkin II 1973 1961. G. Chuchari I. Martin and A. Maccoll J.C.S. Perkin 11 1973 663. la3 A. Maccoll and S.S.Nagra. J.C.S. Faraduy I 1973,69 1108. Ia4 D. B. Bigley and R. E. Gabbott J.C.S.Perkin II 1973 1293. N. J. Daly G. M. Hewiston and F. Ziolkowski Austral. J. Chem. 1973 26 1259. Ia6 H. Kwart S. F. Sarner and J. Slutsky J. Amer. Chem. SOC. 1973 95 5234. Ia7 K. W. Egger J.C.S. PerkinII 1973 2007. (a)Z. B. Alfassi D. M. Golden and S. W. Benson Internat. J. Chem. Kinetics 1973 5 991 ;(b)Z. B. Alfassi S. W. Benson and D. M. Golden J. Amer. Chem. SOC. 1973 95 4784. D. A. Knecht J. Amer. Chem. SOC. 1973,95 7933. I9O F. A. L. Anet and F. Leyendecker J. Amer. Chem. SOC. 1973,95 156. 84 R. Walsh A number of cyclobutanone decompositions have been reported,'47.' '*' 92 and these are usually consistent with a zwitterionic transition state with the positive centre at C-3 and the negative one at CO although one at least occurs via a biradi~al.'~~' 1,2-Dioxetans decompose with the emission of acetone phosphores~ence,~~~" and the high efficiency of this process argues for a con- certed rather than a biradical pathway.'93b At high Concentrations in solution a second-order quantum chain process can occur.194 A curious consecutive step with concerted biradical decomposition is proposed for 1,2-dioxan (a cyclic peroxide) decomposition for which an unreasonably low 'A' factor of lo''.' s-was ~btained."~ Vinyldiazirine is believed to decompose in two steps.lg6 Molecule-Molecule Reactions.-Bimolecular hydrogen-atom transfers produce radicals which can initiate chains. Rates have been measured for propy1enelg7 and for Me2PH + C2F4.Ig8 High 'A' factors of 10" and 10" dm3 mo1-l s-' were obtained as e~pected."~ For the acid-base reactions of BF with amines very fast rates were obtained,*" whereas for NH + HCl --* NH4Cl apparently less than one collision in lo6 was effective.201 Cycloadditions of halogeno-olefins known to proceed via biradicals can occur with surprising stereo- specificity.202 Just like hydrogen halides,'03 acetic acid204 can catalyse molecu- lar elimination.6 Chemical Activation In addition to possible departures from RRKM theory uncovered in molecular- beam studies,' fluorescent lifetime and quantum yield rneas~rernents~~' reveal a non-random energy distribution in vibronic states which precede dissociation in bromo- and chloro-acetylene. However the RRKM theory seems to apply well to many chemical activation systems on large molecules at not too high excitation energies.206 Remaining uncertainties centre around the efficiency of (a) K.W. Egger J. Amer. Chem. SOC. 1973 95 175; (6) K. W. Egger Internat. J. Chem. Kinetics 1973,5 285; (c)A. T. Cocks and K. W. Egger J.C.S. Perkin II 1973 835. H. M. Frey and H. Hopf J.C.S. Perkin II 1973 2016. (a)N. J. Turro H.-C. Steinmetzer and A. Yekta J. Amer. Chem. SOC. 1973,956468; (b)N. J. Turro and P. Lechtken ibid. p. 265. (a) P. Lechtken A. Yekta and N. J. Turro J. Amer. Chern. SOC. 1973 95 3027; (6)T. Wilson M. E. Landis A. Baumstark and P. D. Bartlett ibid. p. 4765. W. Adam and J. Sanabia Angew. Chem. Internat. Edn. 1973 12 843. M. T. H. Liu and K. Toriyama Canad. J. Chem. 1973,51 2393. M. Simon and M.H. Back Canad. J. Chem. 1973,51,2934. R. Brandon R. N. Haszeldine and P. J. Robinson J.C.S. Perkin II 1973 1295. S. W. Benson Adv. Photochem. 1964 2 1. S. Glicker J. Phys. Chem. 1973,77 1093. '01 R. J. Countess and J. Heicklen J. Phys. Chem. 1973,77,444. '02 R. Wheland and P. D. Bartlett J. Amer. Chem. SOC. 1973 95 4003. '03 S. I. Ahonkhai and E. U. Emovon J.C.S. Faraday I 1973 69 183. '04 D. A. Karaitis V. R. Stimson and J. W. Tilley Austral. J. Chem. 1973 26 761. '05 K. Evans R. Scheps S. A. RiGe and D. Heller J.C.S. Faraday II 1973,69 856. '06 D. W. Setser 'Unimolecular Reactions of Polyatomic Molecules Radicals and Ions' in 'Chemical Kinetics' ed. J. C. Polanyi Series 1 Vol. 9 of M.T.P. International Review of Science (Physical Chemistry) Butterworths London 1972 Chap.1 p. 1. Gas-phase Kinetics and Mechanisms deactivating collisions. In alkyl fluorideszo7 and n-alkyl radicals2'' the strong collision assumption seems to hold whereas in some small-ring hydrocarbons' 5*96 and in photochemical systems209*210 inefficient collisions seem to be the rule. More detailed studies on simple well-defined systems such as those of Troezo9 are called for. This problem and the uncertainty in the energy carried over by methylene make some chemical activation studies on alkanes2' and silanes' rather poor guides to the nature of transition complexes for these molecules. Thus difficulties of reconciliation2" with other data2I3 for C2H,+ 2CH are probably not serious. 20' K. C. Kim and D. W.Setser J. Phys. Chem. 1973,77 2021. 208 E. A. Hardwidge B. S. Rabinovitch and R. C. Ireton J. Chem. Phys. 1973 58 340. '09 S.H. Luu and J. Troe Ber. Bunsengesellschaft phys. Chem. 1973,77 325. 210 S. W. Orchard and B. A. Thrush Proc. Roy. SOC.,1973 A329,233. 211 F. B. Growcock W. L. Hase and J. W. Simons Internat. J. Chem. Kinetics 1973 5,77. '12 W. L. Hase C. J. Mazac and J. W. Simons J. Amer. Chem. SOC. 1973 95,3454. 213 E.V. Waage and B. S. Rabinovitch Internat. J. Chem. Kinetics 1971 3 105.