Kinetic isotope effects in some reactions of tri-(4-nitrophenyl)-methane with alkoxide bases in various alcoholic media

Abstract

Rate constants and Arrhenius parameters have been determined for proton-transfer and deuteron-transfer reactions of tri-(4-nitrophenylmethane) with alkoxide ions in alcohol + toluene mixtures. The kinetic isotope effects (k^H/k^D) at 25°C are considerable (8.4 in ethanol + toluene, 8.9 in propan-2-ol + toluene, 23 in t-butanol + toluene) and indicate that the rate-limiting step is a proton transfer. For the forward reactions, the results suggest that the importance of tunnelling increases with the size of the alkoxide group. For the backward reactions, the rates for proton- and deuteron-transfer are different, although they would be expected to be equal by reason of rapid exchange with the solvent; this suggests that in the product the proton remains firmly bonded to the carbanion.