Thermal Decomposition of 3-Ethyl-3-methyloxetan and 3,3-Diethyloxetan BY ALLAN D. CLEMEN'TS? HENRY M. FREY" AND JEREMY G. FREY Chemistry Department, University of Reading, Whiteknights, Reading RG6 2AD Received 22nd April, 1975 The thermal decompositions of 3-ethyl-3-methyloxetan and 3,3-diethyloxetan have been followed in the gas phase. Both decompositions are homogeneous, kinetically first order and probably unimolecular. In the temperature range 407 to 448°C the 3-ethyl-3-methyl compound yields 2- methylbut-1-ene and formaldehyde and the rate constants fit the Arrhenius equation log kl/s-' = 15.357kO.151-(251 230f2020) J mol-l/RTInlO. Similarly it was found that 3,3-diethyloxetan gave 2-ethylbut-1-ene and formaldehyde and in the tem- perature range 402 to 463°C the rate constants fitted the Arrhenius equation log kz/s-l = 15.297 2 0.063 - (249 8502 840) J mol-l/RTlnlO.The decompositions probably proceed by a biradical mechanism. Until recently there was little published work on the pyrolysis of oxetans which contrasts sharply with the data available on cyclobutanes. Early work by Bittker and Walters on oxetan itself was followed twelve years later by a study of the 3,3-dimethyl compound.2 The fact that the rates of decomposition of these oxetans and the corresponding cyclobutanes are essentially identical has been noted. Also, the Arrhenius parameters for the decomposition of both dimethyl derivatives are identical within experimental error. However, there is a difference in the reported parameters for oxetan and cyclobutane which was felt unlikely and possibly indicated that both the reported A and Ea values for oxetan were too More recently Holbrook and Scott have studied the pyrolysis of cis- and trans- 2,3-dimethyloxetan.This is a somewhat more complex system owing to the occur- rence of a geometric isomerization and also because there are two distinct fragmenta- tion reactions. It was possible to obtain good data for the decomposition pathways. While the rates of decomposition are again close to those for the corresponding cyclobutanes, the agreement was not as strikingly close as for the other compounds mentioned. In particular, the trans-2,3-dimethyloxetan decomposes a little more slowly than oxetan itself, whereas trans-l,2-dimethylcyclobutane decomposes nearly twice as rapidly as cyclobutane. The results reported in this paper were obtained from studies on two oxetans which were carried out in an attempt to determine whether the 3,3-dimethyloxetan results were representative of this type of substitution.It was also hoped that more information about the nature of the activated complex might be obtained. EXPERIMENTAL 3-Ethyl-3-methyloxetan was prepared from diethyl carbonate and 2-ethyl-2-methylpro- pane-1,3-diol by the method of Casteignau et aL5 The oxetan was purified by fractional distillation and then by preparative gas chromatography using a column containing di- isodecyl phthalate as the liquid phase. Before kinetic runs it was dried over a molecuIar sieve type 4A. 24852486 THERMAL DECOMPOSITION OF OXETANS 3,3-Diethyloxetan was obtained from Professor G.Casteignau and, after simple distilla- tion, was found to be -99 % pure by gas chromatography. Purification by preparative chromatography yielded a product with total impurities of -0.2 %. All other materials were commercial samples. APPARATUS A conventional high vacuum static pyrolysis system was used. Teflon-glass greaseless stopcocks were employed throughout to minimise absorption problems and the entire gas handling system was maintained between 85 and 95OC. Pyrex reaction vessels both packed and unpacked were maintained at the required temperature (within 0.1OC) by immersion in a high temperature fused salt thermostat. The progress of the reaction was monitored in the majority of runs by pressure change and in a few cases by analysis using gas chromatography. For the former, a pressure transducer (Bell and Howell type 4-327-0003) connected to the reaction vessel was used and for the latter a Perkin Elmer F11 instrument equipped with a flame ionization detector and a gas sampling valve.Attempts to obtain reaction mixture compositions by direct analysis of gas samples were unsuccessful. The analytical reproducibility was very poor and we attribute this to adsorption of the relatively high boiling oxetans on the metal surfaces of the gas sampling system. Accordingly the entire reaction mixture, after a predetermined time in the heated reaction vessel, was condensed into 250 mm3 of p-xylene and aliquot por- tions of this mixture were analysed using liquid sampling. A silicone oil column gave com- pIete separation between reactants, products and the p-xylene.Chromatographic peaks were measured using either a ball and disc or an electronic integrator. RESULTS 3 -ETHYL- 3 -ME TH Y LOX E T A N The thermal decomposition of 3-ethyl-3-methyloxetan was investigated in the temperature range 407 to 448°C. Pyrolyses were carried out in a Pyrex reaction vessel that has been " aged " by treatment with 10 Torr (1 Torr = 133 N of hexamethyldisilazane at 430°C for 12 h. The only decomposition product observed by gas chromatography was 2-methylbut-1-ene. By analogy with the results of Walters on 3,3-dimethyloxetan it was assumed that one other product, formaldehyde was formed, which polymerises under our analytical conditions on the chromato- graphic column.This was confirmed by a U.V. analysis of the reaction mixture after pyrolysis, which showed the presence of formaldehyde quite unambiguously. The pressure change in the system was consistent with the stoichiometry (1). Plots of log (2Po-Pt) against time were linear up to at least 50 % decomposition (Po and P, are the initial pressure and the pressure at time t respectively). Some runs were followed for many half lives when it was found that P, = 2P0. Rate constants were obtained from a least squares analysis of the pressure plots. A series of runs was carried out at 434.1"C with initial reactant pressures in the range 7.2 to 12.5 Torr. Within experimental error these 7 runs yielded rate constants independent of the initial pressure. In another series of runs at 407.5"C, the progress of the reaction was monitored by both pressure change and gas chromatographic analysis.The pressure data yielded a rate constant of 1.187 x s-l (average from 5 runs) and the value obtained from the analytical results was 1.238 x s-l. The 4 % difference between these values is not considered significant in view of the errorsA. D . CLEMENTS, H. M. FREY AND J . G . FREY 2487 associated with the liquid sampling method employed. Finally, in another series a " packed " reaction vessel was used. This had a surface to volume ratio about 14 times that of the unpacked vessel and the reaction was monitored by gas chromatu- graphy. The measured rate constant was 1.250 x s-l. This is in good agree- ment with the value obtained by this method of analysis in the unpacked reaction vessel and indicates that there can be no appreciable surface component of the reaction.s-'. Another run with an initial oxetan pressure of 8.36 Torr but to which 20.5 Torr of propene had been added gave a rate constant of 1.570~ s-l. These results make it unlikely that there are any measurable radical decomposition pathways under the experimental conditions used. The rate constants obtained from 51 runs at 10 temperatures are given in table 1, in all cases throughout this paper the quoted errors are standard deviations. Two runs at 41 1.6"C yielded a value for the rate constant of 1.582fO.01 x TABLE 1 .-RATE CONSTANTS FOR THE DECOMPOSITION OF 3-ETHYL-3-METHYLOXETAN temperature/'C 407.5 41 1.6 415.8 420.2 104kl /s-~ 1.187f0.012 1.582+0.010 2.091+0.061 2.613&0.107 1 04k1 /s-l 3.8454 0.184 5.012k 0.139 6.273 0.171 8.667f 0.283 temperature/"C 443.3 448.1 10% 1 /s-1 lo.%+ 0.16 14.874 0.1 8 temperature/"<: 425.6 430.6 434.1 439.3 An Arrhenius plot of the results quoted in table 1 yielded a good straight line from which the Arrhenius parameters were obtained by least squares, viz., log kl/s-l = 15.357fO.151-(60 046+483) cal mol-'/RTln 10 log kl/s-' = 15.357+0.151-(251 23022020) J mol-'/RTln 10.3,3-DIETHY LOXETAN Most of the details of the decomposition of this compound were closely similar to those of the ethyl methyl compound. Initial runs showed that plots of log (2P0 -P,) against time were accurately linear up to 50 % pressure increase. Rate constants determined from such plots were independent of initial reactant pressure in the range 5 to 15 Torr. (The range of pressures that could be employed was limited at the lower end by the sensitivity of the pressure transducer and at the upper end by the relatively low volatility of the reactant.) By analogy with other oxetan decomposi- tions the expected stoichiometry for the decomposition is (2) : For this study the 2-ethylbut-1-ene was identified by gas chromatography by compari- son of retention times (with an authentic sample) on several different columns.Further quantitative analysis (using the liquid sampling technique) showed that 1 mole of the oxetan yielded 1 mole of the olefin. Formaldehyde was detected by U.V. analysis and by gas chromatographic analysis using a column packed with Poropak N, but it proved impossible to determine quantitatively (owing to poor reproducibility).How- ever, the absence of any other peaks on the chromatograms together with the observed pressure changes confirm the quantitative nature of the decomposition.A. COX AND T . J . KEMP 2493 LACTIC ACID An intense absorption was obtained at 77 K which we regard as due to a radical mixture. Its basic pattern is a 1 : 4 : 6 : 4 : 1 quintet, but the coupling constant varied somewhat from peak to peak, averaging to 1.67 mT which seems too low a figure to be attributed to CH,eHOH. Curiously, Poznyak et aZ.12 found aqueous ethylene glycol solutions of FeIrl complexes of lactate and mandelate ions to photodecompose at 77 K (indicated by loss of the e.s.r. line of FelI1) but saw no production of RcHOH radicals until the matrix was warmed to 140 K ; this " delayed action '' in radical production was rationalised by the authors in terms of an [Fell-RcHOH] adduct which they believe to show no e.s.r.absorption; this thermally dissociates at 140 K into Fe" + RCHOH. DISCUSSION Thermodynamically Fe"' is intermediate in activity between Ce" and Uv' on which we have previously concentrated. l-' Whilst CeIV photo-oxidations of organic substrates involve light absorption in the charge-transfer band of the CeIv-substrate complex (A,,, - 300 nm or 361 kJ mol-l) together with net reduction of CeIv to Ce"', for which E" = 1.70 V in HC104 solution (1 mol dm-,), equivalent to a free energy change of 164 kJ mol-', the corresponding figures for UO$+ are Amax = 400 nm (or 271 kJmol-l) and E" = 0.05 V (or AGO = 4.8 kJ mol-') and for Fe3+ in HC104 solutions with added substrate, Amax x 350 nm (or 309 kJ mol-l) and E" = 0.772 V (or AGO = 74.5 kJ mol-l).Both as regards the photochemical and electrochemical terms, then, Fe"' might be expected to show behaviour intermediate between Ce'" and Uvl in its photochemical interaction with organic substrates. In its interaction with the two simplest alcohols, excited iron(1rr) ion, denoted FeI1I4:, behaves both as CeIV* in abstracting a hydrogen atom from the hydroxylic carbon atom to give RCHOH (R = H or Me). This takes place with both FeCI, and Fe(ClO,),, indicating the process to be due to a genuine attack by FelI1* rather than by intermediate formation of a chlorine atom,6 followed by a secondary attack of the latter upon the matrix.While both of these mechanisms are conceivable for FeCl,, the CI. atom mechanism cannot operate in the case of Fe(C104),, nor can it explain the formation of readily detectable alkyl radicals as a principal path- way in the Fe"'* oxidation of tertiary alcohols which is found with both Fe(C104), and FeCl,. These originate from a C-C cleavage photoprocess previously found with CeIV*, but not with Uvl*, viz., and Uv'* R1R2R3COH + FelI1* + R1R2R3&H + Fel1 (1) (2) + - R1R2R3COH 4 H+ + R1* + R2R3C=0 Towards carboxylic acids, FelI1* shows ambivalent behaviour. [steps (1) and (2) may be concerted). The radical derived from RC02H, where R = Me, Et, (Me),CHCH, and CH2=CHCH2, is predomi- nantly or exclusively R e , which comes from a process of oxidative decarboxylation found with CeIV* and PbtV* ; l4 with isobutyric acid, however, the seven-line spectrum must be due to (Me),CC02H, i.e., the route is one of hydrogen-atom abstraction [the possibility that the primary isopropyl radical is highly reactive towards the labile tertiary hydrogen atom at 77K is ruled out by the observation of Me,CH under these conditions during CeIV* oxidation], Clearly FelI1* behaves very much like CeIV* [and unlike Uvr';] in these particular oxidations, with the exception of isobutyric acid towards which it behaves like Uvl*.A .D. CLEMENTS, H. M . FREY AND J . G . FREY 2489 The Arrhenius parameters and the rate constants (calculated from these para- meters) at 425°C for a number of oxetans and for cyclobutane and 1,l-dimethyl- cyclobutane are shown in table 3.The results in table 3 make the similarity between cyclobutane and oxetan decom- position very clear indeed. The most recent work on oxetan itself shows that the sur- mise that the previously reported Arrhenius parameters were too low was correct. The new values now fit the general pattern well. Within experimental error the parameters for cyclobutane and oxetan are the same. On the basis of a biradical mechanism this almost certainly means that in the oxetan decomposition both bi- radicals (I) and (11) contribute to the reaction. Also, the enthalpy changes associated with their formation from oxetan must be close to one another and also close to the corresponding enthalpy change for the formation of (111), the tetramethylene biradical, from cyclobutane.This is consistent with thermochemical calculations which yield values for the enthalpies of these reactions (molecule to biradical) AHzgs of 55.6, 56.2 and 55.3 kcal mol-l respectively. (1) (11) (IW (IV) (V) The substitution of geminal dimethyl groups in both cyclobutane and oxetan produces essentially the same rate enhancement. Further, in both cases, this appears to be almost entirely the result of a reduction in the energy of activation for the reaction. This is exactly what would be expected on the basis of a biradical mechanism and would be consistent with the intermediate formation of biradicals of the types (IV) and (V). Thermochemical calculations yield virtually the same enthalpy change in going from dimethyloxetan to (IV) as from dimethylcyclobutane to (V), which amounts to - 51.2 kcal mol-l.This is about 4 kcal mol-1 less than for the unsubstituted case which is greater than the observed differences in energies of activation between sub- stituted and unsubstituted molecules. If the barrier to recyclisation of the biradicals is either greater or approximately equal to that for decomposition (the most probable situation) then these calculations and comparisons with experiment indicate that the barriers are not identical for (11) and (IV) or (111) and (V). However, it is doubtful whether the thermochemical data are sufficiently precise to be certain. What is clear is that the present work has fully confirmed the earlier results on the 3,3-dimethyl- oxetan and the relationship between oxetan and cyclobutane decomposition.We thank Professor G. Casteignau for the sample of diethyloxetan. We are grateful to the S.R.C. for the award of a studentship (to A. D. C.). D. A. Bittker and W. D. Walters, J. Amer. Chem. Soc., 1955, 77, 2326. G. F. Cohoe and W. D. Walters, J. Phys. Chem., 1967,71,2326. H. M. Frey and R. Walsh, Chem. Rev., 1969, 69, 103. K. A, Holbrook and R. A. Scott, J.C.S. Faraday 1,1974,70,43. J. L. Halary, T. Yvernault and G. Casteignau, Bull. Soc. chirn. France, 1972, 12,4655. R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry (Academic Press, New York, 1970). K. A. Holbrook, personal communication ; K. A. Holbrook and R. A. Scott, J.C.S. Faraahy I, 1975,71,1849. C. T.Genaux and W. D. Walters, J. Amer. Chem. Soc., 1951,73,4497 ; R. W. Carr and W. D. Walters, J. Phys. Chem., 1963, 67, 1370. ’ J. A. Berson, J. Amer. Chem. Soc., 1972, 94, 8917 lo P. C. Rotoli, M.S. Thesis (University of Rochester, 1963). Thermal Decomposition of 3-Ethyl-3-methyloxetan and 3,3-Diethyloxetan BY ALLAN D. CLEMEN'TS? HENRY M. FREY" AND JEREMY G. FREY Chemistry Department, University of Reading, Whiteknights, Reading RG6 2AD Received 22nd April, 1975 The thermal decompositions of 3-ethyl-3-methyloxetan and 3,3-diethyloxetan have been followed in the gas phase. Both decompositions are homogeneous, kinetically first order and probably unimolecular. In the temperature range 407 to 448°C the 3-ethyl-3-methyl compound yields 2- methylbut-1-ene and formaldehyde and the rate constants fit the Arrhenius equation log kl/s-' = 15.357kO.151-(251 230f2020) J mol-l/RTInlO.Similarly it was found that 3,3-diethyloxetan gave 2-ethylbut-1-ene and formaldehyde and in the tem- perature range 402 to 463°C the rate constants fitted the Arrhenius equation log kz/s-l = 15.297 2 0.063 - (249 8502 840) J mol-l/RTlnlO. The decompositions probably proceed by a biradical mechanism. Until recently there was little published work on the pyrolysis of oxetans which contrasts sharply with the data available on cyclobutanes. Early work by Bittker and Walters on oxetan itself was followed twelve years later by a study of the 3,3-dimethyl compound.2 The fact that the rates of decomposition of these oxetans and the corresponding cyclobutanes are essentially identical has been noted.Also, the Arrhenius parameters for the decomposition of both dimethyl derivatives are identical within experimental error. However, there is a difference in the reported parameters for oxetan and cyclobutane which was felt unlikely and possibly indicated that both the reported A and Ea values for oxetan were too More recently Holbrook and Scott have studied the pyrolysis of cis- and trans- 2,3-dimethyloxetan. This is a somewhat more complex system owing to the occur- rence of a geometric isomerization and also because there are two distinct fragmenta- tion reactions. It was possible to obtain good data for the decomposition pathways. While the rates of decomposition are again close to those for the corresponding cyclobutanes, the agreement was not as strikingly close as for the other compounds mentioned.In particular, the trans-2,3-dimethyloxetan decomposes a little more slowly than oxetan itself, whereas trans-l,2-dimethylcyclobutane decomposes nearly twice as rapidly as cyclobutane. The results reported in this paper were obtained from studies on two oxetans which were carried out in an attempt to determine whether the 3,3-dimethyloxetan results were representative of this type of substitution. It was also hoped that more information about the nature of the activated complex might be obtained. EXPERIMENTAL 3-Ethyl-3-methyloxetan was prepared from diethyl carbonate and 2-ethyl-2-methylpro- pane-1,3-diol by the method of Casteignau et aL5 The oxetan was purified by fractional distillation and then by preparative gas chromatography using a column containing di- isodecyl phthalate as the liquid phase.Before kinetic runs it was dried over a molecuIar sieve type 4A. 24852486 THERMAL DECOMPOSITION OF OXETANS 3,3-Diethyloxetan was obtained from Professor G. Casteignau and, after simple distilla- tion, was found to be -99 % pure by gas chromatography. Purification by preparative chromatography yielded a product with total impurities of -0.2 %. All other materials were commercial samples. APPARATUS A conventional high vacuum static pyrolysis system was used. Teflon-glass greaseless stopcocks were employed throughout to minimise absorption problems and the entire gas handling system was maintained between 85 and 95OC. Pyrex reaction vessels both packed and unpacked were maintained at the required temperature (within 0.1OC) by immersion in a high temperature fused salt thermostat.The progress of the reaction was monitored in the majority of runs by pressure change and in a few cases by analysis using gas chromatography. For the former, a pressure transducer (Bell and Howell type 4-327-0003) connected to the reaction vessel was used and for the latter a Perkin Elmer F11 instrument equipped with a flame ionization detector and a gas sampling valve. Attempts to obtain reaction mixture compositions by direct analysis of gas samples were unsuccessful. The analytical reproducibility was very poor and we attribute this to adsorption of the relatively high boiling oxetans on the metal surfaces of the gas sampling system. Accordingly the entire reaction mixture, after a predetermined time in the heated reaction vessel, was condensed into 250 mm3 of p-xylene and aliquot por- tions of this mixture were analysed using liquid sampling.A silicone oil column gave com- pIete separation between reactants, products and the p-xylene. Chromatographic peaks were measured using either a ball and disc or an electronic integrator. RESULTS 3 -ETHYL- 3 -ME TH Y LOX E T A N The thermal decomposition of 3-ethyl-3-methyloxetan was investigated in the temperature range 407 to 448°C. Pyrolyses were carried out in a Pyrex reaction vessel that has been " aged " by treatment with 10 Torr (1 Torr = 133 N of hexamethyldisilazane at 430°C for 12 h. The only decomposition product observed by gas chromatography was 2-methylbut-1-ene.By analogy with the results of Walters on 3,3-dimethyloxetan it was assumed that one other product, formaldehyde was formed, which polymerises under our analytical conditions on the chromato- graphic column. This was confirmed by a U.V. analysis of the reaction mixture after pyrolysis, which showed the presence of formaldehyde quite unambiguously. The pressure change in the system was consistent with the stoichiometry (1). Plots of log (2Po-Pt) against time were linear up to at least 50 % decomposition (Po and P, are the initial pressure and the pressure at time t respectively). Some runs were followed for many half lives when it was found that P, = 2P0. Rate constants were obtained from a least squares analysis of the pressure plots.A series of runs was carried out at 434.1"C with initial reactant pressures in the range 7.2 to 12.5 Torr. Within experimental error these 7 runs yielded rate constants independent of the initial pressure. In another series of runs at 407.5"C, the progress of the reaction was monitored by both pressure change and gas chromatographic analysis. The pressure data yielded a rate constant of 1.187 x s-l (average from 5 runs) and the value obtained from the analytical results was 1.238 x s-l. The 4 % difference between these values is not considered significant in view of the errorsA. D . CLEMENTS, H. M. FREY AND J . G . FREY 2487 associated with the liquid sampling method employed. Finally, in another series a " packed " reaction vessel was used.This had a surface to volume ratio about 14 times that of the unpacked vessel and the reaction was monitored by gas chromatu- graphy. The measured rate constant was 1.250 x s-l. This is in good agree- ment with the value obtained by this method of analysis in the unpacked reaction vessel and indicates that there can be no appreciable surface component of the reaction. s-'. Another run with an initial oxetan pressure of 8.36 Torr but to which 20.5 Torr of propene had been added gave a rate constant of 1.570~ s-l. These results make it unlikely that there are any measurable radical decomposition pathways under the experimental conditions used. The rate constants obtained from 51 runs at 10 temperatures are given in table 1, in all cases throughout this paper the quoted errors are standard deviations.Two runs at 41 1.6"C yielded a value for the rate constant of 1.582fO.01 x TABLE 1 .-RATE CONSTANTS FOR THE DECOMPOSITION OF 3-ETHYL-3-METHYLOXETAN temperature/'C 407.5 41 1.6 415.8 420.2 104kl /s-~ 1.187f0.012 1.582+0.010 2.091+0.061 2.613&0.107 1 04k1 /s-l 3.8454 0.184 5.012k 0.139 6.273 0.171 8.667f 0.283 temperature/"C 443.3 448.1 10% 1 /s-1 lo.%+ 0.16 14.874 0.1 8 temperature/"<: 425.6 430.6 434.1 439.3 An Arrhenius plot of the results quoted in table 1 yielded a good straight line from which the Arrhenius parameters were obtained by least squares, viz., log kl/s-l = 15.357fO.151-(60 046+483) cal mol-'/RTln 10 log kl/s-' = 15.357+0.151-(251 23022020) J mol-'/RTln 10. 3,3-DIETHY LOXETAN Most of the details of the decomposition of this compound were closely similar to those of the ethyl methyl compound.Initial runs showed that plots of log (2P0 -P,) against time were accurately linear up to 50 % pressure increase. Rate constants determined from such plots were independent of initial reactant pressure in the range 5 to 15 Torr. (The range of pressures that could be employed was limited at the lower end by the sensitivity of the pressure transducer and at the upper end by the relatively low volatility of the reactant.) By analogy with other oxetan decomposi- tions the expected stoichiometry for the decomposition is (2) : For this study the 2-ethylbut-1-ene was identified by gas chromatography by compari- son of retention times (with an authentic sample) on several different columns.Further quantitative analysis (using the liquid sampling technique) showed that 1 mole of the oxetan yielded 1 mole of the olefin. Formaldehyde was detected by U.V. analysis and by gas chromatographic analysis using a column packed with Poropak N, but it proved impossible to determine quantitatively (owing to poor reproducibility). How- ever, the absence of any other peaks on the chromatograms together with the observed pressure changes confirm the quantitative nature of the decomposition.A. COX AND T . J . KEMP 2493 LACTIC ACID An intense absorption was obtained at 77 K which we regard as due to a radical mixture. Its basic pattern is a 1 : 4 : 6 : 4 : 1 quintet, but the coupling constant varied somewhat from peak to peak, averaging to 1.67 mT which seems too low a figure to be attributed to CH,eHOH.Curiously, Poznyak et aZ.12 found aqueous ethylene glycol solutions of FeIrl complexes of lactate and mandelate ions to photodecompose at 77 K (indicated by loss of the e.s.r. line of FelI1) but saw no production of RcHOH radicals until the matrix was warmed to 140 K ; this " delayed action '' in radical production was rationalised by the authors in terms of an [Fell-RcHOH] adduct which they believe to show no e.s.r. absorption; this thermally dissociates at 140 K into Fe" + RCHOH. DISCUSSION Thermodynamically Fe"' is intermediate in activity between Ce" and Uv' on which we have previously concentrated. l-' Whilst CeIV photo-oxidations of organic substrates involve light absorption in the charge-transfer band of the CeIv-substrate complex (A,,, - 300 nm or 361 kJ mol-l) together with net reduction of CeIv to Ce"', for which E" = 1.70 V in HC104 solution (1 mol dm-,), equivalent to a free energy change of 164 kJ mol-', the corresponding figures for UO$+ are Amax = 400 nm (or 271 kJmol-l) and E" = 0.05 V (or AGO = 4.8 kJ mol-') and for Fe3+ in HC104 solutions with added substrate, Amax x 350 nm (or 309 kJ mol-l) and E" = 0.772 V (or AGO = 74.5 kJ mol-l).Both as regards the photochemical and electrochemical terms, then, Fe"' might be expected to show behaviour intermediate between Ce'" and Uvl in its photochemical interaction with organic substrates. In its interaction with the two simplest alcohols, excited iron(1rr) ion, denoted FeI1I4:, behaves both as CeIV* in abstracting a hydrogen atom from the hydroxylic carbon atom to give RCHOH (R = H or Me).This takes place with both FeCI, and Fe(ClO,),, indicating the process to be due to a genuine attack by FelI1* rather than by intermediate formation of a chlorine atom,6 followed by a secondary attack of the latter upon the matrix. While both of these mechanisms are conceivable for FeCl,, the CI. atom mechanism cannot operate in the case of Fe(C104),, nor can it explain the formation of readily detectable alkyl radicals as a principal path- way in the Fe"'* oxidation of tertiary alcohols which is found with both Fe(C104), and FeCl,. These originate from a C-C cleavage photoprocess previously found with CeIV*, but not with Uvl*, viz., and Uv'* R1R2R3COH + FelI1* + R1R2R3&H + Fel1 (1) (2) + - R1R2R3COH 4 H+ + R1* + R2R3C=0 Towards carboxylic acids, FelI1* shows ambivalent behaviour.[steps (1) and (2) may be concerted). The radical derived from RC02H, where R = Me, Et, (Me),CHCH, and CH2=CHCH2, is predomi- nantly or exclusively R e , which comes from a process of oxidative decarboxylation found with CeIV* and PbtV* ; l4 with isobutyric acid, however, the seven-line spectrum must be due to (Me),CC02H, i.e., the route is one of hydrogen-atom abstraction [the possibility that the primary isopropyl radical is highly reactive towards the labile tertiary hydrogen atom at 77K is ruled out by the observation of Me,CH under these conditions during CeIV* oxidation], Clearly FelI1* behaves very much like CeIV* [and unlike Uvr';] in these particular oxidations, with the exception of isobutyric acid towards which it behaves like Uvl*.A .D. CLEMENTS, H. M . FREY AND J . G . FREY 2489 The Arrhenius parameters and the rate constants (calculated from these para- meters) at 425°C for a number of oxetans and for cyclobutane and 1,l-dimethyl- cyclobutane are shown in table 3. The results in table 3 make the similarity between cyclobutane and oxetan decom- position very clear indeed. The most recent work on oxetan itself shows that the sur- mise that the previously reported Arrhenius parameters were too low was correct. The new values now fit the general pattern well. Within experimental error the parameters for cyclobutane and oxetan are the same. On the basis of a biradical mechanism this almost certainly means that in the oxetan decomposition both bi- radicals (I) and (11) contribute to the reaction.Also, the enthalpy changes associated with their formation from oxetan must be close to one another and also close to the corresponding enthalpy change for the formation of (111), the tetramethylene biradical, from cyclobutane. This is consistent with thermochemical calculations which yield values for the enthalpies of these reactions (molecule to biradical) AHzgs of 55.6, 56.2 and 55.3 kcal mol-l respectively. (1) (11) (IW (IV) (V) The substitution of geminal dimethyl groups in both cyclobutane and oxetan produces essentially the same rate enhancement. Further, in both cases, this appears to be almost entirely the result of a reduction in the energy of activation for the reaction.This is exactly what would be expected on the basis of a biradical mechanism and would be consistent with the intermediate formation of biradicals of the types (IV) and (V). Thermochemical calculations yield virtually the same enthalpy change in going from dimethyloxetan to (IV) as from dimethylcyclobutane to (V), which amounts to - 51.2 kcal mol-l. This is about 4 kcal mol-1 less than for the unsubstituted case which is greater than the observed differences in energies of activation between sub- stituted and unsubstituted molecules. If the barrier to recyclisation of the biradicals is either greater or approximately equal to that for decomposition (the most probable situation) then these calculations and comparisons with experiment indicate that the barriers are not identical for (11) and (IV) or (111) and (V). However, it is doubtful whether the thermochemical data are sufficiently precise to be certain. What is clear is that the present work has fully confirmed the earlier results on the 3,3-dimethyl- oxetan and the relationship between oxetan and cyclobutane decomposition. We thank Professor G. Casteignau for the sample of diethyloxetan. We are grateful to the S.R.C. for the award of a studentship (to A. D. C.). D. A. Bittker and W. D. Walters, J. Amer. Chem. Soc., 1955, 77, 2326. G. F. Cohoe and W. D. Walters, J. Phys. Chem., 1967,71,2326. H. M. Frey and R. Walsh, Chem. Rev., 1969, 69, 103. K. A, Holbrook and R. A. Scott, J.C.S. Faraday 1,1974,70,43. J. L. Halary, T. Yvernault and G. Casteignau, Bull. 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