The carbanion mechanism of olefin-forming elimination. Part 10. Isotope effects in the dehydrochlorination of 1,1-diaryl-2,2-dichloroethanes

Abstract

Intramolecular primary chlorine isotope effects have been measured for the base-induced dehydrochIorination reactions of (p-YC₆H₄)₂CH·CHCl₂(YMeO, H, orCl) in ^–OMe–MeOH, ^–OMe–MeOH–Me₂SO, and Bu^tO^––Bu^tOH. All isotope effects are small (ca. 0.14–0.37%). suggesting that the E2 transition states have nearly intact Cα–-Cl bonds. Trends in k₃₅/k₃₇ with changing substituent, base, and solvent are interpreted in terms of current theories of E2 transition-state character. An isotope effect of unity for the reaction of (p-NO₂C₆H₄)₂CH·CHCl₂ with ^–OMe–MeOH confirms an earlier suggestion that the mechanism here is E1cB with rate-determining proton transfer.