Eliminations promoted by weak bases. Part VI. Isotope, basicity, and substituent effects on the dehydrochlorination of 1,1-diaryl-2,2,2-trichloroethanes in dimethylformamide

Abstract

Primary hydrogen isotope effects on the rates of the dehydrochlorination reactions of Ar₂CH·CCl₃ with the bases Cl^–, Br^–, p-NO₂C₆H₄S^–, and 2,4-(NO₂)₂C₆H₃O^– in dimethylformamide are reported. The values of k_H/k_D are in the range 5·1–5·6 and are largely independent of ring substituents in the substrate and of the base employed. It is considered that they are too high to admit the possibility of an E2C mechanism, and an E2H explanation is favoured. Brønsted correlations of log k₂vs. pK_a are not linear. The effect of ring substituents on the equilibrium constants for the reversible dehydrochlorinations promoted by Buⁿ₄NCl in acetone is negligible, which indicates that the transition states are not olefin-like, but rather have an excess of negative charge on C_β.