Gas-phase Unimolecular Pyrolyses of cis- and trans-2,3- Dimethyloxetan BY KENNETH A. HOLBROOK AND ROBERT A. SCOTT Department of Chemistry, University of Hull, Hull HU6 7RX Received 18th June, 1973 The gas phase pyrolyses of both cis and trans isomers of 2,3-dimethyloxetan have been studied at temperatures from 415483°C and at initial pressures from 2 to 32 Torr. Both isomers undergo two modes of decomposition producing either propene and acetaldehyde or but-2-ene (cis and trans) and formaldehyde. The cis-trans isomerisation of the starting material is relatively unimportant. The reactions all appear to be unimolecular processes in their first-order regions and the following rate expressions were obtained : cis isomer+C3H6+CH3CH0 : logl0(kl/s-l) = (15.70f0.22)-(63 216f621) K/4.576T cis isomer+C4H8 +CH20 : loglo(kZ/s-l) = (15.24f0.25)-(62 493 If: 715) K/4.576T trans isomer+C3H6+ CH3CH0 : l0glO(k3/s-’) = (15.91 f0.25)- (64 652f 664) K/4.576T trans isomer+C4H8+CHz0 : 1og10(k4/s-1) = (15.49&0.26)-(63 676f745) K/4.576T.Possible mechanisms involving 1 +biradical intermediates are discussed. The pyrolysis of oxetans is important since these compounds occur as intermediates in the oxidation of hydr0carbons.l Relatively few fundamental studies have been made of these pyrolyses apart from those of oxetan itself which decomposes primarily by a unimolecular mechanism to give ethylene and formaldehyde,2 and of 3,3- dimethyloxetan which similarly yields isobutene and f~rmaldehyde.~ It therefore seemed of interest to study the pyrolysis of an unsymmetrical oxetan in some detail.The pyrolysis of 2,3-dimethyloxetan was chosen, since this is capable of undergoing the following concurrent unimolecular reactions : CH3CHzCHZ + CHJCHO ’QH3 0 CH3 trans + CH20 This paper describes the pyrolysis of both cis- and trans-isomers of the starting material and the results enable the rate constants kl to k, to be derived. The cis-trans isomerisation of the reactant was negligibly slow under the chosen conditions. 4344 ? PYROLYSIS OF DIMETHYLOXETAN EXPERIMENTAL MATERIALS 2,3-Dimethyloxetan was prepared by ring-closure of the chloroester obtained from the reaction of acetyl chloride with 2-methylbutane-l,3-diol. The method differed from that of Searles et aL4 in that the ring closure involved slow addition of the chloroester (1 mol) to a rapidly stirred mixture of potassium hydroxide (2 mol) in ethylene glycol at 120°C and removal of the volatile oxetan product as it was formed. The isomers of 2,3-dimethyloxetan so obtained were later shown to be in the cis : trans ratio of 4.4 : 1.After distillation from sodium metal the cis and trans isomers of 2,3-dimethyloxetan were each obtained in ca. 95 % purity by distillation through a 1 m Nester-Faust spinning band column. The isomers were then separately purified by preparative g.1.c. employing a 6 mx 10 mm i.d. glass column packed with 20 % di-isodecyl phthalate on Celite at 90°C. (cis b.p. 86°C (760mmHg), trans b.p. 84°C (760 mmHg)). The cis and trans isomers were shown to be chromatographic- ally pure (2 99.4 %) on the following columns : 5 ft Porapak Q (SO/lOO) at 150"C, 5 ft 30 % squalane on Celite at 70°C and 2 m 10 % polypropylene glycol on Celite at 50°C.Infra-red and n.m.r. spectroscopic studies revealed no impurity. cis- and tvans-2,3-dimethyloxetan were unambiguously identified by proton and I3C n.m.r.5 Propene, nitric oxide, cis- and trans-but-2-ene were obtained from Matheson Co. Ltd., and had stated purities of at least 99 %. This was confirmed chromatographically. Acet- aldehyde (May and Baker Ltd.) was doubly distilled and dried before use. G.1.c. analysis showed no impurity. Formaldehyde was obtained from paraformaldehyde by the method of Spence and Wild and stored at liquid air temperature. APPARATUS A N D PROCEDURE The reaction vessels used were cylindrical Pyrex vessels of length 20 cm and internal radius 4 cm.The unpacked vessel had a surface/volume ratio = 0.94 cm-l, and the packed reaction vessel contained Pyrex tubes to give a surface/volume ratio of 10.6cm-l. A conventional high-vacuum static apparatus with greaseless stopcocks was used. The furnace temperature was controlled to +O.l"C and the maximum variation along the reaction vessel length was kO.75"C. The dead-volume was approximately 2 % of the reaction vessel volume and corrections were applied in the calculation of rate constants. At the end of a run, the reaction mixture was expanded down a heated line into a 1 cm3 g.1.c. sample loop placed within the column oven of a Pye 104 gas-liquid chromatograph. The columns used were 2 m 10 % polypropylene glycol on Chromosorb W at 50°C (for oxetan and acetaldehyde analysis) and 4 m 13 % bis-2-methoxyethyl adipate 7 % di-2- ethylhexyl sebacate on Chromosorb P at 40°C (for analysis of propene and butenes). All analyses were performed in triplicate.Analyses of calibration mixtures of reaction products were then accurate to +2 % of the amount of each component. Formaldehyde was analysed by the method of Rayner and Jephcott.' The formaldehyde was condensed into slightly acidified water and determined by a colorimetric procedure using acetone and Schiffs reagent. RESULTS At temperatures between 415 and 483°C the decompositions of cis- and trans-2,3- dimethyloxetan are adequately described by the concurrent reactions (1) to (4). This is confirmed by the equivalence of the pressure change and the amounts of propene + but-2-ene determined by g.1.c.starting from either isomer (table I). Further confir- mation is provided by the equivalence of the amounts of propene and acetaldehyde formed and by the amounts of but-Zene and formaldehyde formed (table 2). No products other than propene, but-2-ene, acetaldehyde and formaldehyde were de- tected. In some runs which were allowed to go to completion, the final pressure was found to be very close to twice the the initial pressure.K . A . HOLBROOK AND R. A . SCOTT 45 For each isomer the rate constants for the concurrent paths producing propene ( facetaldehyde) and but-2-ene (+formaldehyde) were obtained from overall rate constants determined from the first-order log plots and the ratios of propenelbut-2-ene determined chromatographically at 15-25 % decomposition.Thus for the cis isomer : kl = (propene)/(propene + but-2-ene)k' k2 = (but-2-ene)/(propene + but-2-ene)k' k3 = (propene)/(propene + but-2-ene)k" k4 = (but-2-ene)/(propene + but-2-ene)k" where k' and k" are overall rate constants determined from pressure change. The rate of reaction determined by pressure change was found to be first-order up to 30 % decomposition (fig. 1). The small amount of curvature after this extent of ,reaction is probably due to the occurrence of some cis-trans isomerisation of the reactant. In general, this was very slow under the conditions reported here and it was estimated and for the trans isomer : TABLE 1 .-COMPARISON OF AP AND (PROPENE+ BUT-2-ENE) FORMATION Po/Torr 16.03 18.16 16.05 16.30 16.59 8.73 14.83 32.50 6.37 13.79 :is-isomer at 4.66.0"C propene + but-2-eneI APITorr Torr 5.42 5.83 4.99 5.76 4.86 2.59 4.76 11.36 2.21 5.23 5.40 5.86 5.09 5.65 4.64 2.58 4.68 11.10 2.19* 5.177 1 Po/Torr 7.44 7.51 8.29 7.69 7.30 7.89 8 .oo 8.09 24.33 frans-isomer at 466.2"C APITorr 2.32 2.36 2.43 2.29 2.16 2.21 1.36 1.42 3.65 propene+ but-Zene/ Torr 2.25 2.26 2.41 2.20 2.19 2.16 1.30 1.35" 3.61 * 25 % NO added ; 9.8 % NO added.TABLE EQUIVALENCE OF PROPENE TO ACETALDEHYDE AND BUT-2-ENE TO FORMALDEHYDE cis-isomer at 466.0"C trans-isomer at 446.5OC Po/Torr C3H&H3CH0 Po/Torr C~HG/CH~CHO 32.50 1.03 4.22 1.03 18.54 1.03 4.25 1.02 16.08 1.02 4.20 1.03 14.83 1.02 4.06 1.03 8.73 1.02 2.24 1.03 6.37 1.01 2.39 1.03 cis-isomer at 457.6"C trans-isomer at 446.5"C Po/Tarr C4Hs/Torr CH20/Torr Po/Torr CH20IAP C4HsIA.P 32.31 3.73 3.52 2.24 0.42 0.427 18.70 2.46 2.32 4.25 0.41 0.435 18.56 2.36 2.22 4.06 0.42 0.43 8 18.53 2.99 2.96 2.33 0.4 1 0.432 8.53 1.90 1.85 4.13 0.42 0.43 146 PYROLYSIS OF DIMETHYLOSETAN that at 15 % decomposition the extent of cis to trans isomerisation is less than 2 %, and trans to cis isomerisation is less than 1 %.The first-order rate-constants obtained were independent of initial pressure in the range 2-32 Torr and were unaffected by the addition of up to 50 % of the radical-inhibitor nitric oxide. Virtually no change occurred in the values of the rate-constants on changing the surfacelvolume ratio of the vessel by a factor of over ten (table 3). 1.7 Y, Y .” I 5 z a +a $ 8 --.1.6 0 0 4 - 1.5 0 IOG 2 0 0 3 0 0 time/s FIG. 1.-First-order log plots. A, cis and B, trans isomcr. TABLE 3 .-EFFECT OF INITIAL PRESSURE, ADDED NITRIC OXIDE AND SURFACE/VOLUME RATIO cis-isomer 446.4 8 unpacked none 4.81 3.09 1.72 446.4 5 unpacked 14-50 % 4.84 3.1 1 1.73 466.0 lo* unpacked none 16.2 10.4 5.82 466.0 5 packed none 16.5 9.70 6.80 NO added 104 k’ls-1 104 /cl/s-l lo4 k2lS-l T/”C no. of runs vessel 466.0 4 unpacked 10-50 % 16.4 10.5 5.90 trans-isomer 7’PC no. of’ runs vessel NO addcd 1otk‘’/s 1 Io4k3/S-1 1O4k4/S 446.5 10-1 unpacked none 3.34 1.90 1 . 4 4 446.5 6 unpacked 13-50 ”/, 3.39 1.94 I .45 466.2 7 unpacked none 10.8 6.19 4.58 466.2 5 packed none 10.9 6.05 4.80 * Runs with initial pressures 2.4-32.5 Torr ; runs with initial pressures 2.0-24.0 Torr.K.A . HOLBROOK AND R. A. SCOTT 47 TABLE 4.-MEAN FIRST-ORDER RATE CONSTANTS FOR THE PYROLYSIS OF 2-3-DIMETHYLOXETAN rqT 483.2 475.6 473.3 466.0 457.6 446.4 435.1 418.8 T/"C 476.5 466.2 458.0 446.5 436.8 424.8 415.7 no. of runs 8 6 7 9 6 7 7 6 no. of runs 5 6 5 7 5 5 5 cis-isomer IOS k'ls-1 432 288 242 162 97.1 48.1 23.8 9.20 trans-isomer 105 k"/s-1 205 108 65.0 33.4 18.3 8.42 4.37 105 lills- 1 272 187 1 60 104 63.1 30.9 15.2 5.78 105 k31s-l 117 61.9 37.7 19.0 10.4 4.75 2.42 105 kz/s 161 .O 103 86.1 58.2 35.2 17.6 8.70 3.42 105 k41s-l 89.0 45.8 27.3 14.6 7.90 3.62 1.96 The effect of teinperature upon the rate constants is given in table 4. The rate constants were fitted to least squares Arrhenius plots with the result that the rate constants are expressed by the equations 10glo(kl/S-l) = (15.70*0.22)-(63 216+621) K/4.576T lOglo(k2/s-l) = (15.24f0.25)-(62 493 +715) K/4.576T lOglo(k3/s-l) = (15.91 kO.25)-(64 652+664) K/4.576T 10g10(k4/S-1) = (1 5.49 & 0.26) - (63 676 * 745) K/4.576T. Error limits quoted are the 95 % confidence limits.The product distribution, expressed by the rztio of propeiie to but-2-enc, was found to vary very little with temperature; this variation is reflected in the difference in activation energies between reactions (1) and (2) for the cis starting material and B 8 C 4 4 0 46c) 480 TIT FIG. 2.Variation of (cis-but-2-ene/tbut-2-ene) with temperature A, from cis-2,3-dimethyl- oxetan ; B, thermal equilibrium ratio ; C, from frans-2,3-dimethyloxetan.48 PYROLYSIS OF DIMETHYLOXETAN between reactions (3) and (4) for the trans starting material.The ratios of cis- to trans-but-2-ene obtained were also very little dependent on temperature as is shown in fig. 2. DISCUSSION From the data presented above, it is clearly established that both the cis and trans isomers of 2,3-dimethyloxetan undergo parallel ring-cleavage reactions to give either propene and acetaldehyde (reactions (1) and (3)) or but-2-ene and formaldehyde (reactions (2) and (4)). All of these reactions appear to be unimolecular, homogene- ous ht-order processes under the experimental conditions used and to be unaffected by additions of nitric oxide. and of 3,3-dimethyloxetan have shown that these also are mainly unimolecular processes and we have confirmed this for oxetan in a recent re-examination of the pyrolysis at low pressures.* Ring-cleavage of 2,3-dimethyloxetan can occur either by a concerted process involving the simultaneous rupture of two ring bonds or by rupture of a single ring bond initially to lead to the formation of an intermediate 1,4-biradical.The former process would be expected to produce complete retention of configuration for the product but-2-ene i.e., cis-2,3-dimethyloxetan would produce entirely cis-but-2-ene and trans-2,3-dimethyloxetan would produce entirely trans-but-2-ene. Our results (fig. 2) show partial retention of configuration i.e., cis-oxetan-+68 % cis-butene and trans-oxetan-76 % trans-butene and in neither case are these the equilibrium proportions of cis- and trans-b~t-2-em.~ It was independently shown that the rate of cis-trans isomerisation of but-2-ene is negligible under these conditions.These facts are consistent with the formation of a l74-biradical intermediate capable of existing for a few rotations before undergoing reaction to the cleavage products. The observation of some cis-trans isomerisation of the reactant, although this is slow compared with ring cleavage, is also evidence for the biradical mechanism. In addition, concerted mechanisms would be forbidden for the ring-cleavage reactions from considerations of conservation of orbital symmetry.1o The magnitude of the A-factors and the resulting calculated entropies of activation are consistent with processes involving ring opening via a biradical. It is instructive to compare the results for the pyrolysis of cis- and trans-2,3- dimethyloxetan with those for the pyrolysis of cis- and trans- 1,2-dimethylcyclobutane obtained by Gerberich and Waiters.'' Previous work on the pyrolyses of oxetan The comparable reactions for l,2-dimethylcyclobutane are : trans and the kinetic data are compared with those from this work in table 5.In both systems the cis-isomer decomposes faster than the trans-isomer which could be due to the release of stereochemical repulsion between the cis-methyl groups. The Arrhenius parameters for all eight reactions in this table are very similar, taking into account experimental errors, and the small differences between them for the 2,3-dimethyloxetan decomposition are not too significant.K. A . HOLBROOK AND R . A. SCOTT 49 1 TABLE 5 cis- ly2-dimethylcyc1obutane cis-2,3 -&met hyloxet an 60.4 15.5 (1) 63.2 15.7 15.2 reaction Elkcal mol-1 log10(A/s-9 reaction E/kcal mol-1 lol?lo(~/s-9 62.5 (5) (6) 63 .O 15.6 (2) ks/k6(43O0C) = +[propene]/[butene] = 5.2 kl/k2(4300C) = [propene]/[butene] = 1.7 trans- 1 ,Zdimet hylcyclo butane trans-2,3 -dime t hyloxe t an 61.6 15.4 (3) 64.7 15.9 15.5 63.7 (7) (8) 63.4 15.5 (4) k7/ks(43O0C) = *[propene]/[butene] = 3.5 k3/k4(4300C) = [propene]/[butene] = 1.3 A better comparison is obtained by considering the rate constant ratios at a given temperature for the alternative paths available to a given isomer.From table 5 it is seen that whereas both for dimethylcyclobutane and dimethyloxetan, the production of propene is always favoured over that of butene, the preference is less marked in the di methyloxe tan case.A possible explanation for this is obtained if the various 1,4-biradical inter- mediates which may be formed are considered in detail. From cis- 1 ,2-dimethylcyclobutane, three possible biradicals may be formed by C-C bond fission i.e., 4 3 CiS \ Evidence that the initial ring-cleavage is rate-determining is provided by the fact that decomposition is faster than the cis-trans isomerisation of the reactant which must imply that a biradical, if formed, has a lower energy path for decomposition .than for recyclisation. Of these biradicals, (c) which has both electrons located on secondary carbon atoms is the most favoured and the observed activation energies for the forma- tion of propene and butene of 60.4 and 63.0kcalmol-l respectively reflect the probable bond dissociation energies for C( 1)-C(2) and C(2)-C(3).(Bond dissocia- tion energies for the open-chain analogues 2,3-dimethylbutane and 2-methylbutane are 78 +2 and 80+2 kcal mol-1 respectively 12). From cis-2,3-dimethyloxetan, there are four possible biradicals, (d) and (e) from C-C fission and (f) and (9) from C-0 fission.50 PYROLYSIS OF DIMETHYLOXETAN CHj CH3 C3H6fCH3CH0 _.__) C,H,+CH,O -> C,H,+CH,O -> C3H6 + CHJCHO (9) Estimates based on group-additivity calculations and known heats of formation of alkoxy radicals l 2 show that in an unstrained ring the dissociation energies of the C-C and C-0 bonds in this compound are close to 80+ 1 kcal mol-l. It is therefore probable that all four biradicals (4-(g) should be considered and the 'observed smaller difference in activation energies between the reactions producing propene and butene in the case of the 2,3-&methyloxetan decomposition is reasonable. These arguments account qualitatively for the fact that (kl/k2) .c (k,/k,) for the cis-isomers and (k3/k4) < (k,/k,) for the trans-isomers as is shown in table 5. P. Barat, C. F. Cullis and R. T. Pollard, 13th Int. Combustion Symposium, Combustion Institute, 1971, p. 179. D. A. 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