| Abstract: | The thermal unimolecular decomposition of hex-1-ene has been investigated over the temperature range of 915–1153 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via the competitive pathways of C3?C4 fission and retro-ene elimination, with the latter dominant at low temperatures and the former at high temperatures. This behavior results in an isokinetic temperature of 1035 K under VLPP conditions (both reactions in the unimolecular falloff regime). RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log k1 (sec?1) = (12.6 ± 0.2) -(57.7 ± 1.5)/θ for retro-ene reaction, and log k2 (sec?1) = (15.9 ± 0.2) - (70.8 ± 1.0)/θ for C-C fission, where θ = 2.303 RT kcal/mol. The A factors were assigned from the results of a recent shock-tube study of the decomposition in the high-pressure regime, and the activation energies were found by matching the RRKM calculations to the VLPP data. The parameters for C-C fission are consistent with the known thermochemistry of n-propyl and allyl radicals. A clear measure of the importance of the molecular pathway in the decomposition of a mono-olefin has been obtained. |
