The carbanion mechanism of olefin-forming elimination. Part VII. The isotope effect maximum in the dehydrochlorination of 2,2,2-trichloro-1,1-bis-(p-chlorophenyl)ethane by various bases in alcholic solvents

Abstract

When the E1cB dehydrochlorination reactions of Ar₂CH·CCl₃ and Ar₂CD·CCl₃(Ar =p-ClC₆H₄) are induced by a series of bases in ethanol ranging in strength from PhS^– to OEt^–, a maximum in K_H/K_D is observed for phenoxide as base. The kinetics of the phenoxide-induced reactions are re-examined using conditions under which buffering of the reaction solutions is ensured. The maximum is found to be real. An Arrhenius equation study of the reactions, and the reactions of PhS^–, OMe^–(in McOH), and OEt^– with these substrates fails to reveal a significant contribution from proton tunnelling to K_H/K_D values. It is proposed that the maximum is caused by variations in transition state symmetry.