Kinetics of the thermal unimolecular reactions of cyclohexene and 4-vinylcyclohexene behind reflected shock waves

Abstract

The thermal decomposition of cyclohexene at temperatures between 1008 and 1330 K has been studied using a single-pulse shock tube. The major products of the reaction were ethylene and buta-1,3-diene, but a little cyclohexadiene and a trace of benzene were also formed. Below 1190 K the rate coefficients for the main decyclisation (or retrodiene) reaction were found to be represented by the Arrhenius equation log(k/s^–1)= 15.16–274 kJ mol^–1/2.303 RT in good agreement with earlier work. Above 1190 K the rate coefficients increased less rapidly with temperature, although this effect was not so marked as in some other reactions.

A similar study of the thermal decomposition of 4-vinylcyclohexene between 927 and 1197 K showed that the primary product was buta-1,3-diene. Below 1050 K the rate coefficients fitted the Arrhenius equation log(k/s^–1)= 15.79–262 kJ mol^–1/2.303 RT but at higher temperatures the measured rate coefficients again fell below the extrapolation of the Arrhenius line.

These results, obtained by the absolute method, agree well with those obtained by the comparative procedure. The relative merits of the two methods are discussed.