Kinetics and mechanism of the gas-phase thermal decomposition of cis-and trans-4-chloropent-2-ene

Abstract

The thermal decomposition of a mixture of cis- and trans-4-chloropent-2-ene in the gas-phase at 520–640 K and 7–85 mmHg proceeds by clean dehydrochlorination to give HCl and penta-1,3-diene. In seasoned vessels the reactions are reproducible first-order processes and are unaffected by surface : volume ratio. Decomposition of the cis-isomer at 520–602 K, and of the trans-isomer at 564–640 K, is unaffected by radical-chain inhibitors, giving experimental Arrhenius equations (i) and (ii)(where error limits are 95% confidence limits). It is concluded that log₁₀(k_cis/s^–1)=(9.85 ± 0.46)–(131.3 ± 5.1) kJ mol^–1/RT In 10 (i), log₁₀(k_trans/s^–1)=(13.86 ± 0.26)–(196.8 ± 3.5) kJ mol^–1/RT In 10 (ii) the reactions are unimolecular. The A factor for the cis-isomer indicates a six-centre elimination involving a hydrogen atom in the 1-position. The trans-isomer probably decomposes by a normal four-centre process, with the activation energy appropriately lowered by resonance of the incipient double bond. An alternative mechanism involving rate-determining isomerisation to the cis-isomer, followed by rapid decomposition of the latter, is less likely but cannot be ruled out at present. At temperatures below 564 K the trans-isomer decomposes increasingly by a low activation energy free-radical mechanism which is strongly inhibited by but-2-ene and appears to be initiated and terminated on the walls.