J. Phys. Chem. 1981, 85, 1783-1786
1783
ARTICLES Stereochemistry of Molecular Hydrogen Elimination from Cyclopentene at 1100-1 300 K Mark A. Greaney, and Edwin L. Slbert, 111
David K. Lewis;
Department of Chemistry, Colgete University, Hamllton, New York 13346 (Received: August 12, 1980; In Final Form: January 31, 1981)
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The mechanism of the homogeneous gas-phase reaction cyclopentene cyclopentadiene + H2has been studied via thermal activation at 1100-1300 K, by use of an isotopically labeled reactant. Samples containing 5% cyclopentene-3,3,4,4-d4in argon diluent were heated for -800 ps in a single-pulseshock tube. Mass spectrometric analyses of product samples, to determine relative extents of HD and D2 eliminationfrom the labeled reactant, indicated that the symmetry “disallowed” (assuming a concerted reaction and thermal excitation)3,4 elimination and the symmetry “allowed” 3,5 elimination mechanisms both occur at 1100-1300 K, in about a 1/1to 2 / 1 ratio. The apparent difference in activation energies is -34 kJ. This result contrasts with reports of lower temperature, static reactor experiments in which both the forward and reverse reactions appeared to proceed predominately via the symmetry allowed pathway; but the result is consistent with previously reported trends toward higher net activation energy for cyclopentene hydrogen elimination at higher temperatures.
Introduction The stereochemistry of the homogeneous gas-phase elimination of molecular hydrogen from cyclopentene (CP) has received considerable theoretical and experimental attention. Application of Benson and Haugen’s’ electrostatic model involving a four-center transition state appears to favor elimination from adjacent ring positions:2 I
5
I
0 4
2
(1.4) s4Q;
t
H2
(1)
3
However, Woodward and Hoffman3 proposed that elimination from across the molecule, via a six-center transition state, should be dominant since this is a concerted symmetry allowed process: I
1
The latter prediction was supported by experimental work of Anet and Leyendecker; who found evidence of only 3,5 addition in the reaction of D2with cyclopentadiene (CPD) at 573 K. However, Baldwin5earlier had reported that the ratio of 3,5 to 3,4 plus 4,5 elimination from cyclopentene-4-dl at 823 K is approximately 6:l. From these experimental results one can speculate that the 3,4 rate may compete or even dominate a t still higher temperatures. The present report describes the results of our test of that speculation a t temperatures accessible in a single-pulse shock tube. Reactant samples containing 5% cyclopentene-3,3,4,4-d4in argon diluent were heated to (1)S. W. Benson and G. R. Haugen, J. Am. Chem. Soc., 87, 4036 (1965). (2)To avoid confusion, we adhere to the ring-numbering convention shown in eq 1and 2,in our discussion of the present and earlier published work. (3) R. B. Woodward and R. Hoffman, “The Conservation of Orbital Symmetry”, Verlag Chemie, Weinheim Bergstr., Germany, 1970,p 141. (4)F. A. L. Anet and F. Leyendecker, J. Am. Chem. Soc., 96, 156 (1973). (5)J. E.Baldwin, Tetrahedron Lett., 2953 (1966). 0022-3654/81/2085-1783$01.25/0
1088 IT I1280 K for 800 f 100 ps behind reflected shocks, and extents of Dz (3,4) and HD (4,5 and 3,5) elimination were determined from mass spectral analyses of post-shock gas samples.
Experimental Section The labeled reactant, obtained from Merck Sharpe and Dohme, Ltd., had an advertised purity of >98%. Gas chromatographic analysis (Varian Aerograph 1440-20 with hydrogen flame detector) indicated >99% CP. A variety of NMR analyses were run on the bulk sample: a proton spectrum (Varian EM-390), a deuterium spectrum (JEOL JNM-PFT-loo), and two carbon-13 spectra, one with protons fully decoupled and the second with protons partially decoupled (both on the JEOL instrument). This combination of spectra confirmed the unique identity of each CP ring position, and established upper limits for H/D mislabeling at each carbon position ranging from 0.2 to 0.5%. A low-voltage (10 eV) mass spectrum (HitachiPerkin Elmer RMU-6D) gave the expected dominant peak rn/e 72. The mass 73 and 71 peaks were slightly higher than the corresponding mass 69 (rn + 1)+13Cisotope and mass 67 (rn - 1)’ fragment ion peaks from unlabeled CP, suggesting the presence of trace CP-d6and CP-d3 impurities. From the above analyses, the reactant appeared to exceed the specified purity of 98% CP-3,3,4,4-d4 For purposes of later calculations, we define the sample as follows: 1%CP-d3, assumed 0.5% 3, 3, 4 and 0.5% 3, 4, 4; 98% CP-d4,assumed all 3,3,4,4; 1% CP-d,, assumed all 3, 3, 4, 4, 5. Before use, the reactant was frozen and degassed, then expanded into a 5-L glass bulb fitted with a Teflon stopcock, and diluted with Matheson Ultra High Purity grade argon (299.999%). The mixture was allowed to mix 3 days before use. The 2.54-cm i.d. Pyrex shock tube used for this study has been previously described.6 The driver gas was helium. Post-shock samples (50 cm3) were extracted from (6)D.K.Lewis, S. E. Giesler, and M. S. Brown, Int. J. Chem. Kinet., 10,277 (1978).
0 1981 American Chemical Society
1704
Lewis et al.
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981 TEMPERATURE
O K
b , 4 8r 4,5 ELIMINATION ONLY
[L
L
maximum possible concentrations of various CP-d3 and CP-d5impurities in the reactant sample. These expectation values have not been corrected for kinetic isotope effects or possible isotopic scrambling reactions among reactants and/or products (see Discussion). Mass ratios 73/72 and 71/72 were not observed to deviate from the values expected from normal isotopic abundances and residual fragmentation, respectively, over the experimental range of conversions. However, significant production of mass 67 and 70 products was observed at >50% conversion, with both the 67/68 and the 70/69 ratios rising to -0.15 at -75% conversion. The experimental data in Figure 1suggest that both the 3,5 and the 3,4 plus 4,5 elimination mechanisms are effective channels for hydrogen loss from CP at 1100 K. However, for this result to be conclusive, contributions to or losses from the 68 and 69 mass peaks due to isotopic scrambling must be ascertained (see Discussion).
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W
000 00
oko
025
1
075
EXTENT OF CONVERSION ( I -
Hj
10
Figure 1. Experimental68/69 mass intensity ratios from CPDd, and CPD-d3. Also shown are the values expected for certain assumed (see text). ratios of k3,5/k3,4/k4,5
the tube and first analyzed chromatographically to quantify CP, CPD, and side products. The column was a '/8 in. X 10 f t length of 20% polypropylene glycol saturated with AgN03, on 60-80 mesh Chromosorb W. The chromatograph was calibrated with 5%-in-argon samples of CP, CPD, and various C1-C.I species expected as side products, a t intervals during each day on which shocked samples were analyzed. Reaction temperatures were calculated from CPD/CP ratios by utilizing the Arrhenius parameters previously determined for the elimination of H2from unlabeled CP,7p8log (Ais-') = 13.35, E, = 251 kJ/mol. We estimate that temperature values calculated in this manner have a precision of &5K and an accuracy of A15 K. Under conditions of this study, the reaction is essentially irre~ e r s i b l e .The ~ post-shock samples as obtained from the shock tube were then mass analyzed (several scans of each sample) over the mass range 67-74, at an ionizing voltage of 10 eV. Preliminary analyses of unlabeled CP and CPD at 10 eV were also run. CP gave only a small (-5%), reproducible (rn - 1)+peak due to H. loss below the parent ion mass. CPD gave no observable peak below the parent ion. This indicates that 10-eV spectra of product samples yield reliable concentration ratios of the various possible product CPD's at m/e values of 67-70. In the analysis, it was assumed that ionization and collection efficiencies were identical for all CPD's.
Data and Results Figure 1displays the relative extents of D2 and HD loss from labeled CP in seven shock tube runs. Each data point represents the average 68/69 mass ratio corrected for background, determined from two or three analyses of a product sample. The error bars indicate the scatter of individual analyses. Also shown with the experimental ratios are lines representing the expected values for (a) k3,5 >> k3,4 = k4.5 (68/69 0); (b) k35 = k3.4 + k4,5,k3,4 = k4,5 (68/69 0.33); (c) k3,5 = k3,4 = k,, (68/69 0.50); and (d) k3,5 1000 K nor the relative barrier heights were avaliable for comparison with the results of laser pumping. We suggest that the availability of this information for the hydrogen eliminations from cyclopentene (this study) and cycloherene (Tardy et al.17) make these molecules attractive candidates for laser-induced pyrolysis studies in the infrared region.
Conclusions This study yielded estimates of the relative rates of 3,5 vs. 3,4 and 4,5 elimination of molecular hydrogen from cyclopentene. Ratios of cyclopentadienes of masses 68 and 69 produced from cyclopentadiene-3,3,4,4-d4 are consistent with (k3,4.+ k4,5) 1:k3,5,at T 1100 K. Since corrections for the kinetic isotope effect and for possible occurrence of reaction 4 have not been made, the (k3,4+ k4,5)/k33ratio -1.0 is believed to be a lower limit. This ratio for unlabeled CP may be considerably larger, possible k3,4 = k4,5 1 :k35. It is unlikely that the observed mix of produch was produced via a single elimination mechanism, preceded or followed by isotopic exchange reactions, especially at the lower end of the temperature (and percent conversion) range covered. Further study of exchange processes among reactants and/or products at 1150-1300 K will be needed before this conclusion can be applied with certainty to the higher conversion experiments. Comparison of the activation energies previously deduced for hydrogen loss from unlabeled cyclopentene a t various temperatures suggests that the activation energy increases with temperature. This is consistent with the indication, from this study, that the 3,4 + 4,5 mechanism becomes increasingly competitive with the 3,5 mechanism at higher temperatures, and that the difference in E, values for the mechanisms is -34 kJ. This difference is similar to that reported for the corresponding H2eliminiations from cyclohexene, but is considerably lower than reported M , ’ s for disallowed vs. allowed channels in other systems. Because of this relatively small difference and the existence of thermal activation data on the relative rates of the competing mechanisms, we speculate that cyclopentene and cyclohexene might be attractive candidates for multiphoton pumping in the infrared region to assess the extent of nonthermal (nonrandom) channeling of the pumped energy. Acknowledgment. We are indebted to Mr. Martin Ashley and the NMR facility a t the Department of Chemistry, University of Colorado at Boulder, for obtaining and interpreting the NMR spectra on the labeled reactant. We also thank Dr. Peter Jeffers of State University College, Cortland, New York for performing the reactant sample transfer and mixture preparation, and Dr. Fred McLafferty of Cornel1 University for helpful suggestions related to the mass spectral analysis of cyclopentene/cyclopentadiene mixtures. This work has been supported by a Cottrell College Science Grant from the Research Corporation and by Colgate University.
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