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 (kH/kD) 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 pKa 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 kH/kD are also reported for the reactions of four compounds of the type Ar2CH·CCl3 with OMe––MeOH and ButO––ButOH. The isotope effect is independent of the nature of the para-substituent, but in all cases it is smaller for ButO– than for OMe–. This pattern is opposite to that which has been established for E2 reactions.