Pyrolysis of 1-methylcyclohexene and methylenecyclohexane

Abstract

At 495º the decomposition of both 1-methylcyclohexene and methylenecyclohexane is homogeneous and first order overall, the rate constants (with 90% confidence limits) being (6.01 ± 0.40) x 10^-4 and (6.53 ± 0.23) x 10^-4 s^-1 respectively. Pressure is not a good measure of the rate of decomposition of either isomer. Toluene inhibits the decomposition of both olefins, although the effect is more marked for the exo-isomer. The two main reaction pathways for 1-methylcyclohexene are a reverse Diels-Alder reaction, giving 2-methylbuta-1,3-diene and ethene, and a radical chain dehydrogenation to give toluene and benzene. Methylenecyclohexane cannot react through a reverse Diels-Alder reaction and ring fission gives a mixture of C₃ and lower hydrocarbons. Dehydrogenation to give benzene and toluene is also important. Isomerization is a much more favoured pathway for methylenecyclohexane as expected from the relative thermodynamic stabilities of the isomers.