Kinetics and mechanism of the pyrolysis of 1,4-cyclohexadiene

Abstract

The homogeneous gas-phase pyrolysis of 1,4-cyclohexadiene has been shown to obey the pattern of reactivity of cyclic olefins [graphic omitted] where X is O, CH₂, CH₂—CH₂, or CHCH. The reaction was studied using mass spectrometry, u.-v. spectroscopy, gas chromatography, and pressure measurements. The process is first order to at least 80 % decomposition. The rate constant has been measured over a range of seven powers of ten, in packed and unpacked, static and flow reactors between 250 and 620°C and between 10^–3 and 10² torr. Between 330 and 390°, log k=(12.36 ± 0.13)–(43.8 ± 0.4)/θ, where θ is 2.3 RT kcal mole^–1, in good agreement with Ellis and Frey. The absence of hydrogen atoms has been shown by the lack of exchange with hexadeuterobenzene and toluene. The chain decomposition involving cyclohexadienyl radicals is, therefore, not important, probably due to the endothermicity and low A-factor of the bimolecular initiation [graphic omitted] From the Arrhenius parameters for the intramolecular elimination of hydrogen from 1,4-CH, and from estimates of the entropy and heat changes we have calculated the rate constant for the dienophilic 1,4-hydrogenation of benzene to be k= 10^8.7–50.2θ mole^–1 l. sec^–1, where θ is 2.3 RT kcal mole^–1. Butene-2 probably eliminates H₂ by a 1–4 molecular mechanism with an activation energy of 65 kcal.