Kinetic acidity of carbon acids: the hydroxide ion-catalysed ionization of chloroform and acetophenone in aqueous hexamethylphosphoric triamide

Abstract

The rate coefficients for the detritiation of [³H]chloroform and [³H]acetophenone have been measured in aqueous hexamethylphosphoric triamide (HMPTA). The rate acceleration, caused by the HMPTA in the solvent is higher in the reaction of chloroform than in that of acetophenone. The difference is even greater than that observed earlier in a less basic solvent system, aqueous dimethyl sulfoxide (DMSO). The activation entropy for the detritiation of chloroform decreases sharply from the highly positive value in pure water to negative values at mole fractions of HMPTA higher than 0.1. The observed effects may be explained in terms of changes in solvation on going from the initial to the transition state. The change in the activation entropy in the reaction of chloroform is taken to indicate a decrease in the extent of internal return.