J. Phys. Chem. 1981, 85, 7787-7788
1787
Thermal Decomposition of Labeled Cyclopentene. Evidence To Preclude a 1,3-Sigmatropic Hydrogen Shift David K. Lewis,' John C. Cochran, Jeanne M. Hosseniopp, Davld M. Miller, and Terrence E. Sweeny Department of Chemistry, Colgate University, Hamilton, New York 13346 (Received: March 9, 1981)
A sample of cyclopentene-1-d was synthesized and heated to 1170-1272 K in a single-pulse shock tube (gas phase). The heating produced conversion to cyclopentadiene ranging in extent from 10 to 58%. The products were believed to be entirely H2and cyclopentadiene-dl; there was no significant loss of D from the reactant. We conclude that rearrangement of cyclopentene via 1,3-sigmatropic hydrogen shift or any other uni- or bimolecular mechanism occurs only very slowly compared with the rate of molecular hydrogen elimination at these temperatures.
Introduction We recently studied the pyrolysis of cyclopentene3,3,4,4,-d4in a single-pulse shock tube at 1100-1300 K, and found that 3,4 molecular hydrogen elimination (symmetry "disallowed" if a concerted reaction) proceeds a t a rate comparable to that of 3,5 ("allowed") elimination in that temperature range.' That conclusion depended upon the assumption that the labeled reactant did not undergo extensive rearrangement, via 1,3-sigmatropic hydrogen shift or any other uni- or bimolecular process, before conversion to cycylopentadiene. We report here the results of a brief study which confirms that assumption. In this study, a quantity of cyclopentene-1-d (hereafter, CP-1-d) was synthesized, and samples of this were heated to 1170-1272 K in a single-pulse shock tube, producing conversions to cyclopentadiene (CPD) and hydrogen ranging in extent from 10 to -60%. The CPD's and molecular hydrogen produced were analyzed by mass spectrometry to determine extents of deuteration. Our results are consistent with a lack of rearrangement of the reactant, CP1-d, before H2 elimination, and set upper limits for the rate of 1,3-sigmatropic hydrogen atom shift at these temperatures.
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Experimental Section Cyclopentene-1-d was prepared from cyclopentene by way of trans-1,2-dibromocyclopentane and l-bromocyclopentene. trans-1,2-Dibromocyclopentanewas prepared in 80% yield by the method of Weinstock et al:2 bp 75-83 "C (13 torr), n D 1.5443; lit? bp 83 "C (16 torr), nZ4D1.5444. 1-Bromocyclopentene was prepared in 25% yield by the method of Abell and Chiao:4 bp 121-126 "C, nD 1.4976; lit.4 bp 128.5 "C, n 2 b 1.4999. The NMR spectrum showed no evidence of 3-bromocyclopentene in the sample. Cyclopentene-1-d was prepared by a modification of the method of Giacomoni et al.5 To 0.89 g (0.13 mol) of Li wire in 100 mL of tetrahydrofuran (distilled from lithium aluminum hydride) was added, dropwise, 8.1 g (0.055 mol) of 1-bromocyclopentene. The reaction mixture was allowed to stir overnight with gentle heating. The cyclopentenyllithium compound was hydrolyzed by the addition (1)D. K. Lewis, M. A. Greaney, and E. L. Siebert, J. Phys. Chem., preceding article in this issue. (2)J. Weinstock, S.N. Lewis, and F. G. Bordwell, J. Am. Chem. SOC., 78, 6072 (1956). (3)M.W. Lister, J. Am. Chem. SOC., 63, 143 (1941). (4)P. E. Abell and C. Chiao, J . Am. Chem. SOC.,82, 3610 (1960). (5) J. C.Giacomoni, A. Cambon, and E. Rouvier, Bull. SOC.Chim. Fr., 3097 (1970). 0022-3654/81/2085-1787$01.25/0
of 5 mL (0.25 mol) of 99.8% DzO. After several hours with stirring, the reaction mixture was distilled until the distillation temperature exceeded 65 "C. The distillate was then redistilled through a 3-ft spinning-band column and 1.5 g (32%) of product was collected: bp 41 "C, lit.5 44 "C. The identity and isotopic enrichment of the reactant was determined by gas chromatograph (Varian 90-P and 1440-20 instruments), infrared (Perkin-Elmer 257 spectrophotometer), mass spectral (Hitachi RMU-GD), and proton magnetic resonance (Varian EM-360L) analyses. The GC analyses showed no side peaks, indicating 99.9% CP. A low (8.0 eV) ionizing voltage mass spectral analysis over the m / e range 65-72 served as the basis for determining isotopic enrichment. The dominant peak was the m l e 69 parent of CP-1-d, followed in intensity by m l e 68. Minor peaks at m / e 67 and 70 were also noted. Each peak was scanned three times on each of two sample injections; then average peak responses and their standard deviations were calculated. The ionization and sampling efficiency was assumed constant for all CP isotopic configurations (CP, CP-1-d, CP-13C1,etc.). Spectral scans of unlabeled CP were used for determining relative m+, (m 1)+(due to 13C),and (m - 1)+(due to minor fragmentation) signal intensities. The MS data on the enriched sample, corrected for contributions to all peaks, gives a CP/CP-dl ratio of 0.54 f 0.04.6 This identified the sample as 64.9 f 1.8% CP-I-d, 35.1 f 1.8% CP. We presume the undeuterated cyclopentene arose from moisture incorporated from the atmosphere during synthesis. There was no evidence of double deuteration. The assumption that the CP-1-d was deuterated entirely at the vinyl position was supported by the NMR analysis, and is confirmed by the subsequent pyrolysis experiments. A reactant mixture containing 5% labeled CP in Matheson Gold Label (ultrahigh purity) argon was prepared and mechanically homogenized. Five reactant samples ranging from 30 to 50 torr pressure were introduced into a 2.54-cm i.d. Pyrex single-pulse shock tube and heated to temperatures of 1170 to 1272 K for 800 f 100
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(6)The ratios of concentrations and their uncertainties were calculated from the formula A
E =If
[
7 4 (UE)~
uA)* +
where A / B is the ratio of concentrations of species A and B; uA, and uB are the stndard deviations of corrected mass spectral peak intensities for species A and B; and A and B are the average corrected mass spectral peak intensities.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 13, 1987
1788
TABLE I ~
~~~~
% con-
T,K
version CP- CPD
1170 1199 1224 1253 1272
10.5 18.8 29.6 46.2 58.4
av of 5 runs expected value, if no D relocation before elimination
A [HDl/
[H, 1 0.016 (-0.030) 0.024 0.024 0.039 A
0.015 0.000
A [CPDII A[CPD-d,]
0.56 (t0.04) 0.58 (t0.09) 0.50 (t0.04) 0.55 (t0.04) 0.58 (t0.04) 0.558 0.54 (~0.04)
Post-shock samples (50 cm3) were taken from the shock tube and first analyzed chromtographically for CP and CPD. Reaction temperatures were calculated from measured extents of conversion of CP to CPD assuming first-order irreversible reaction kinetics and the Arrhenius parameters log ( A , s-l) = 13.35, E, = 251 kJ/mol that we had previously obtained for elimination from unlabled CP.* The product samples were than analyzed by mass spectrometry, t h e e 8.0-eV scans over the 65-72 mass range and t h e e 70-eV scans over m / e 1-4 per sample. All peaks were corrected for ion source pressure variations by referencing to the M2+m / e = 20 peak due to the 95% argon in each sample. PS.~
Data and Results From the mass spectrometric data, apparent [HD]/ [H2] and [CPD]/[CPD-d,] ratios were calculated (see Table I). The former ratios were calculated from increases in the m / e 3 and 2 peaks over intensities recorded for an unreacted sample. The latter ratios were determined from m / e 66 and 67, with small correction applied to the 67 peak due to 13Clabeling of undeuterated CPD and (m - 1)+ions from undeuterated CP remaining after heating. Significant rearrangement of CP-I-d before elimination would have produced ratios of CPD/CPD-dl in product samples that were greater than the CPICP-I-d ratio 0.54 measured for the reactant. Furthermore, the CPD/CPD-dl ratios in product samples would likely have increased with temperature. Neither trend is apparent in Table I. The slightly higher (0.558 vs. 0.54) average ratio, though not statistically significant, could be the result of minor D enrichment of the non-vinyl positions (3, 4, 5) of the reactant; such labeling would cause a temperature-independent increase in the CPDICPD-dl product ratio. The ratio would also be increased by a combination of 3,4 elimination of C P - I d , a suprafacial two-H-atom transfer between a CP-1-d and a CPD-dl,Sand subsequent 3,4 and 3,5 elimination from the reactant CP-3-d; but such a sequence would probably be temperature dependent. Assuming the reactant ratio 0.54, a product ratio 0.558 (not statistically different) could imply that 1.1% of the reactant had rearranged such that it subsequently lost HD. A (7) The apparatus and standard techniques for its operation have been previously described: D. K. Lewis, S. E. Giesler, and M. S. Brown, Int. J. Chem. Kinet., 10,277 (1978). (8) D. K. Lewis, M. Sarr, and M. Keil, J.Phys. Chem., 78,436 (1974). (9)F. A. L. Anet and F. Leyendecker, J. Am. Chem. SOC.,95, 156 (1973).
Lewis et al.
product ratio as high as 0.60 (37.5% CPD, 62.5% CPD-dl) could imply that 2.4% of the CP-I-d had rearranged to produce CPD instead of CPD-dl. We believe this is the upper limit consistent with these data. The m / e 3/2 ratios in Table I appear (except for one run) to show HD elimination averaging 1.5% of H2 produced, and reaching nearly 4% of H2 at 1272 K. However, the small HD signals suffered from a poor signal/noise ratio and were augmented to a small but variable (with shock temperature) extent, as were the H2 peaks, by fragmentation of CP's, CPD's, and other products produced in significant quantities in the higher temperature runs (240% CP depletion). Note, for example, that the products of the 1199 K run showed slightly less apparent HD than the unreacted gas. Thus the A [HD]/A[H2]ratios are not sufficiently different from zero to require rearrangement of the reactant.
Discussion The significance of the above results is best illustrated by comparison with expected A[HD]/A[H,] ratios from various scenarios: (a) Assume (1)all molecules of the 65% enriched CP-1-d undergo one 1,3-sigmatropic shift before elimination, and (2) all elimination occurs from the 3,5 position. This results in A[HD]/A[H2] = 0.1631° ( = 0.65 X 0.50 X 0.50). (b) Assume (1)all molecules undergo a very large number of l,&sigmatropic shifts before elimination, and (2) elimination occurs by either the 3,5 or 3,4 mechanism, or both. This results in A[HD]/A[H2] = 0.163 [3,5 = (0.65 X 0.50 X 0.50); 3,4 = (0.65 X 0.50 X 0.25) (0.65 X 0.25 X 0.50)]. Thus the present data show that the rate of positional rearrangement in the CP-I-d reactant was at least a factor of 5, and most likely a factor of 210 less than the rate of H2 elimination over the range 1170-1272 K. Applying this result to our previus study of the sterochemistry of elimination from cyclopentene-3,3,4,4-d4,1 it appears that no more than 20% of the reactant molecules could have suffered a 1,3-sigmatropic shift during the 800-ps reaction time at T 5 1260 K. An average time (400 ps) would allow 510% shifted molecules, and only half of the shifted molecules ( 5 5%) would have an altered labeling pattern, CP-2,3,3,5-d4. Totally allowed elimination (3,5) from this species would produce 50% Dz and 50% HD. We conclude that the maximum m / e 68-69 ratio expected from positional rearrangement of CP-3,3,4,4-d4 followed by only 3,5 elimination is 0.025, which is more than an order of magnitude below the ratio we observed in that study. Thus the result of the present study supports our previous finding that the 3,4 and 3,5 elimination pathways are both active a t 1100-1300 K, and that the evidence for the disallowed pathway can not be ascribed instead to uni- or bimolecular rearrangement of the reactant before the elimination step.
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Acknowledgment. This research was supported by a Cottrell College Science grant from the Research Corporation and by an undergraduate fellowship grant from the Mobil Foundation. (10)(a) This analysis ignores the primary hydrogen isotope effect. (b) Simultaneous 3,4elimination would lower the expected ratio; equal rates for the 3,4and 3,5 channels would produce a ratio 0.122.
