The carbanion mechanism of olefin-forming elimination. Part V. Evidence from linear free energy relationships for E1cB dehydrochlorination of 1,1-diaryl-2,2,2-trichloroethanes

Abstract

Rate constants for dehydrochlorination of the title compounds by NaOMe in MeOH may be fitted to the Brøonsted equation appropriate to carbanion formation from hydrocarbons in the same medium. The identity of elimination and carbanion formation rates provides further evidence for the operation of an ‘irreversible’E1cB mechanism in the dehydrochlorination. A basicity function approach leads to the same mechanistic conclusion. The high Hammett ρ values (2–3) for eliminations in OR^––ROH (R = Me and Bu^t) are diagnostic of a carbanion-like transition state, and consideration of the effect of base strength on ρ leads to the conclusion that stronger bases cause the proton to be more extensively transferred at the transition state, contrary to predictions based on the Hammond postulate.