The kinetics of pyrolysis of barrelene[1]: A concerted process on the C 8 H 8 energy surface

The static vessel pyrolysis of bicyclo[2.2.2]octa‐2,5,7‐triene (barrelene) has been studied between 483 and 523 K. The products were acetylene and benzene (in equal quantities) with no other detectable C 8 H 8 isomer (down to less than 1% of total). The time dependence fitted first‐order kinetics, and the data are consistent with a homogeneous unimolecular reaction close to, if not at, its high‐pressure limit at 0.25 torr. The rate constant was fitted to the Arrhenius equation\documentclass{article}\pagestyle{empty}\begin{document}$$ \log \left( {{k \mathord{\left/ {\vphantom {k {s^{ - 1} }}} \right. \kern-\nulldelimiterspace} {s^{ - 1} }}} \right)\, = \,\left( {14.27\, \pm \,0.18} \right)\, - \,{{\left( {41.71\, \pm \,0.41\,{{{\rm kcal}} \mathord{\left/ {\vphantom {{{\rm kcal}} {{\rm mol}}}} \right. \kern-\nulldelimiterspace} {{\rm mol}}}} \right)} \mathord{\left/ {\vphantom {{\left( {41.71\, \pm \,0.41\,{{{\rm kcal}} \mathord{\left/ {\vphantom {{{\rm kcal}} {{\rm mol}}}} \right. \kern-\nulldelimiterspace} {{\rm mol}}}} \right)} {RT}}} \right. \kern-\nulldelimiterspace} {RT}}\,\ln \,10 $$\end{document}These Arrhenius parameters are shown to imply a concerted single‐step process. Alternative mechanisms are discussed and a comparison is made with the retro‐Diels—Alder reactions of other bicyclo[2.2.2]octa‐olefin systems.