Analysis of the dependence on temperature of kinetic solvent isotope effects. Application to kinetic data for the solvolysis of organic halides and carboxylic acid anhydrides

Abstract

The implications are explored of both a single- and a two-stage mechanism for solvolytic reactions on the observed kinetic solvent isotope effect, k_sie(=k[D₂O]/k[H₂O]). The analysis identifies three types of behaviour for the dependence of k_sie on temperature. In the first class, k_sie changes regularly with temperature towards a limiting value. In a second class, k_sie passes through either a maximum or minimum, the nature of the extremum depending on the difference between the apparent heat capacities of activation for reaction in the two solvents at the temperature of the extremum, Finally, and in exceptional cases, both a maximum and a minimum may be observed if the plots showing the dependence of apparent enthalpies of activation on temperature in the two solvents cross in two places. This analysis is used in an examination of the kinetic data for the hydrolysis of several organic carboxylic acid anhydrides where the case for a two-stage mechanism is well established. A similar mechanism for the solvolysis of t-butyl chloride, 2-chlorobutan-1-ol, 2,2-dibromopropane and 2-bromo-2-chloropropane clarifies the trends in previously reported activation parameters. The analysis reveals that no simple relationship exists between k_sie and the apparent heat capacity of activation for solvolysis in water.