The carbanion mechanism of olefin-forming elimination. VIII. Theoretical analysis of substituent and isotope effects in proton transfers from 2,2-Diaryl-1,1,1-trichloroethanes

Abstract

Rate constants for the dehydrochlorination of the title compounds by anionic bases in alcoholic solvents are those for simple proton transfer. Rate and estimated equilibrium data for these reactions can be approximately analysed in terms of the Marcus theory of proton transfer. In one reaction series comprising the reactions of para-substituted Ar₂CHCC1₃ compounds with NaOMe in MeOH, the work term (w^r) and the intrinsic free energy of activation (ΔG?) are comparable and large. The change in k_H/k_D with variation in substituent can be quantitatively rationalized, and the fact that k_H/k_D does not pass through a maximum even though ΔpK = 0 within the series can be understood. Although the required rate-equilibrium parabolic relationship for a second series involving the reactions of (p-ClC₆H₄)₂CHCCl₃ with a series of bases is obtained, anomalous Marcus parameters are derived, and reasons for the anomalies are given. A revised set of approximate pK_a values for Ar₂CHCCl₃ compounds is presented, and more precise experimental values of rate constants for the reactions of (p-NO₂C₆H₄)₂CHCC1₃ and (p-NO₂C₆H₄)₂CDCCl₃ with NaOMe in MeOH are provided.