Kinetics of Primary Processes in the Pyrolysis of Cyclopentanes and Cyclohexanes

Article abstract of DOI:10.1021/j100275a013

Studies of the thermal unimolecular decomposition of cyclopentane, cylohexane, vinylcyclopentane, and vinylcyclohexane have been carried out at temperatures in the range 900-1223 K using the technique of very low-pressure pyrolysis (VLPP).The results show that ring-opening isomerization to open-chain alkenes is the major primary process.Under the experimental conditions the alkenes undergo further decomposition via C-C bond fission and/or retro-ene processes.Application of RRKM theory shows that the VLPP data (falloff regime) are consistent with the following high-pressure rate expressions (at =1100 K) for reactant decay: log(k/s^-1)=(16.8+/-0.3)-(355+/-8)/θ for cyclopentane, log(k/s^-1)=(16.9+/-0.3)-(361+/-8)/θ for cyclohexane, log(k/s^-1)=(16.0+/-0.3)-(304+/-8)/θ for vinylcyclopentane, and log(k/s^-1)=(16.0+/-0.3)-(309+/-8)/θ for vinylcyclohexane, where θ=2.303RT kJ/mol.The A factors were based on known values for C-C bond fission in alkanes and alkenes.Comparison of the activation energies for the unsubstituted cycloalkanes with those for the vinyl-substituted cycloalkanes reveals that the effect of the vinyl substituent is close to the established value for the allyl resonance energy.This evidence supports the assumption of a biradical mechanism for the ring opening of medium-sized cycloalkanes.Decomposition of pent-1-ene (the open-chain isomer of cyclopentane) under VLPP conditions yields unimolecular rate constants for overall decay that are consistent with the high-pressure rate expressions log(k/s^-1)=(12.5+/-0.4)-(238+/-10)/θ for retro-ene reaction and log(k/s^-1)=(16.0+/-0.3)-(296+/-10)/θ for C-C fission.The A factors were assigned from estimates consistent with the results of a recent shock-tube study of the decomposition in the high-pressure regime.The Arrhenius parameters are consistent with previous data on the decomposition of monoolefins via C-C fission and retro-ene pathways.