The carbanion mechanism of olefin-forming elimination. Part IV. Isotope effects in the dehydrochlorination of 1,1-diaryl-2,2,2-trichloroethanes by anionic bases in alcoholic solvents

Abstract

Deuterium isotope effects (k_H/k_D) have been measured for the dehydrochlorination of (p-ClC₆H₄)₂CH·CCl₃(DDT) induced by the bases PhS^–, p-NO₂·C₆H₄·O^–, PhO^–, and OEt^–(in EtOH), and OMe^–(in MeOH). The isotope effect passes through a maximum as the base strength increases. The maximum corresponds to the estimated pK_a value of DDT (17) and it is suggested that the rate-determining step is simple proton transfer, so that the mechanism is ‘irreversible’E1cB. Values of k_H/k_D are also reported for the reactions of four compounds of the type Ar₂CH·CCl₃ with OMe^––MeOH and Bu^tO^––Bu^tOH. The isotope effect is independent of the nature of the para-substituent, but in all cases it is smaller for Bu^tO^– than for OMe^–. This pattern is opposite to that which has been established for E2 reactions.