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.