Substituent effects in the gas‐phase thermolysis of aryl tert‐butyl ethers and sulfides

The pyrolyses of p ‐nitrophenyl t ‐butyl ether, p ‐methoxyphenyl t ‐butyl ether, p ‐aminophenyl t ‐butyl ether, p ‐nitrophenyl t ‐butyl sulfide, and propargyl t ‐butyl sulfide have been studied in a stirred‐flow reactor over the temperature range 430–530°C and pressures in the range 7–14 torr, using toluene as carrier gas. The reactions yielded between 90 and 99% isobutene plus the corresponding phenol, thiophenol, or thiol as products. The first order rate coefficients for reactant consumption, based on isobutene production, followed the Arrhenius equations p ‐nitrophenyl t ‐butyl ether\documentclass{article}\pagestyle{empty}\begin{document}$$ k({\rm s}^{ - 1}) = 10^{12.79 \pm 0.24} {\rm exp}(- 179 \pm 3{\rm kJ/mol}RT) $$\end{document}p ‐methoxyphenyl t ‐butyl ether\documentclass{article}\pagestyle{empty}\begin{document}$$ k({\rm s}^{ - 1}) = 10^{14.50 \pm 0.30} {\rm exp}(- 210 \pm 4{\rm kJ/mol}RT) $$\end{document}p ‐aminophenyl t ‐butyl ether:\documentclass{article}\pagestyle{empty}\begin{document}$$ k({\rm s}^{ - 1}) = 10^{14.59 \pm 0.28} {\rm exp}(- 208 \pm 4{\rm kJ/mol}RT) $$\end{document}p ‐nitrophenyl t ‐butyl sulfide\documentclass{article}\pagestyle{empty}\begin{document}$$ k({\rm s}^{ - 1}) = 10^{12.12 \pm 0.27} {\rm exp}(- 185 \pm 4{\rm kJ/mol}RT) $$\end{document}propargyl t ‐‐butyl sulfide:\documentclass{article}\pagestyle{empty}\begin{document}$$ k({\rm s}^{ - 1}) = 10^{13.79 \pm 0.24} {\rm exp}(- 204 \pm 3{\rm kJ/mol}RT) $$\end{document}The results support a unimolecular elimination of the isobutene involving polar, four‐center cyclic transition states.