Kinetics of the Thermal Isomerizations of Gaseous Cycloheptene and Cyclooctene

Article abstract of DOI:10.1021/j100020a042

Single-pulse shock tube kinetic studies of the thermal isomerizations of gaseous cycloheptene (CHEP) and cis-cyclooctene (COCT), and static reactor isomerizations of COCT at lower temperatures, have revealed a mechanistic dissimilarity in the two superficially analogous cycloalkene to, α,ω-alkadiene reactions observed.At 1035-1256 K, CHEP produced mostly vinylcyclopentane, log₁₀(k,s^-1) = 15.1 (+/- 0.7) - 69.7 (+/- 3.3) x 10³/4.576T, and some 1,6-heptadiene.From COCT, heated over the range 610-1091 K, the dominant product was 1,7-octadiene, log₁₀(k,s^-1) = 13.8 (+/- 0.2) -54.6 (+/- 0.5) x 10³/4.576T, with small amounts of vinylcyclohexane formed at the higher temperatures, log₁₀(k,s^-1) = 15.2 (+/- 0.3) - 64.4 (+/- 1.2) x 10³/4.576T.The activation energy for the COCT isomerization to 1,7-octadiene is too low to associate with formation of a diradical, but is consistent with a concerted retro-ene mechanism.The higher activation energy isomerization to vinylcyclohexane, however, passes through a diradical transition structure.In contrast, the structure of CHEP is not adaptable to a concerted retro-ene precess, and both 1,6-heptadiene and vinylcyclopentane are formed through diradical-mediated reactions.