The carbanion mechanism of olefin-forming elimination. Part 9. Proton tunnelling and isotope effects in the dehydrochlorination of 1,1-diaryl-2,2-dichloroethanes

Abstract

Rate constants for the dehydrochlorination of (p-ClC6H4)2CH·CHCl2 and its deuterio-analogue induced by NaOMe–MeOH have been obtained at five temperatures in the 0–45 °C range. The values of the respective Arrhenius parameters substantiate the earlier view that proton tunnelling is important in this E2 reaction. Tunnelling is less significant in the E2 reactions of Ph2CH·CHCl2 and Ph2CD·CHCl2 and is apparently of no consequence in the dehydrochlorination of (p-NO2·C6H4)2CH·CHCl2. The latter substrate reacts at a rate equal to the estimated rate of carbanion formation, and an E1cB mechanism is induced. Isotope effects decrease as dimethyl sulphoxide is added to MeOH for the dehydrochlorination of Ph2CH·CHCl2.