Kinetics and mechanism of the pyrolysis of cyclohexa-1,3-diene

Abstract

The pyrolysis of cyclohexa-1,3-diene (C₆H₈) has been studied between 512·3 and 672·5 K at pressures between 10 and 500 Torr. The products are benzene (B), cyclohexene (C), a compound with formula C₁₂H₁₈(E), hydrogen, and traces of C₂ compounds. The formations of benzene, cyclohexene, and C₁₂H₁₈ are all of the second order and their rates are unaffected by the surface-to-volume ratio of the reaction vessel or by the presence of added propene. The rate constants (in l mol^–1 s^–1) are given by equations (i)–(iii). The rate of formation of H₂ is ca. 1/7 that of B. log₁₀k_B=–(35,500 ± 100)/4·576T+(10·13 ± 0·04)(i), log₁₀k_C=–(36,400 ± 1100)/4·576T+(10·25 ± 0·40)(ii), log₁₀k_E=–(35,600 ± 1800)/4·576T+(9·52 ± 0·67)(iii)

The results are explained by a non-chain, radical mechanism where the initiation step is the bimolecular disproportionation (iv) to cyclohex-2-enyl and cyclohexadienyl radicals. The estimated value of the rate constant k_a is [graphic omitted] (iv), log₁₀k_a=–(35,500 ± 1000)/4·576T+(10·04 ± 0·40) l mol^–1 s^–1.