H 2 S‐promoted thermal isomerization of butene‐2 CIS to butene‐1 or butene‐2 trans around 500°C

H 2 S increases the thermal isomerization of butene‐2 cis (B c ) to butene‐1 (B 1 ) and butene‐2 trans (B t ) around 500°C. This effect is interpreted on the basis of a free radical mechanism in which buten‐2‐yl and thiyl free radicals are the main chain carriers. B 1 formation is essentially explainedby the metathetical steps:whereas the free radical part of B t formation results from the addition–elimination processes:. It is shown that the initiation step of pure B c thermal reaction is essentially unimolecular:and that a new initiation step occurs in the presence of H 2 S:. The rate constant ratio has been evaluated:\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{k_2 }}{{k_3 }} \simeq 0.4\exp ^{ - (6500 \pm 1000)/RT} (RT\,in\,cal/mol) $$\end{document}and the best values of k 1 and k 1' , consistent with this work and with thermochemical data, are\documentclass{article}\pagestyle{empty}\begin{document}$$ k_1 \simeq 10^{15.5 - 85,500/2.3RT} \sec ^{ - 1} $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ k_{1^\prime } \simeq 10^{13.6 - 53,700/2.3RT} {\rm cm}^3 /{\rm mol}\,{\rm sec} $$\end{document} . From thermochemical data of the literature and an “intrinsic value” of E −3 ⋍ 2 kcal/mol given by Benson, further values of rate constants may be proposed:\documentclass{article}\pagestyle{empty}\begin{document}$$ k_3 \simeq 10^{14 - 9,300/2.3RT} cm^3 /mol\sec $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ k_3 \simeq 10^{13.6 - 15,800/2.3RT} cm^3 /mol\sec $$\end{document}is shown to be E 4 ⋍ 3.5 ± 2 kcal/mol, of the same order as the activation energy of the corresponding metathetical step.