Solvent effect on the α-effect:: Ground-state versus transition-state effects;: a combined calorimetric and kinetic investigation

In a study of the solvent effect on the a-effect, second-order rate constants (k(Nu)-) have been determined spectrophotometrically for reactions of a series of substituted phenyl acetates with butan-2,3-dione monoximate (Ox(-), a-nucleophile) and p-chlorophenoxide (p-ClPhO-, reference nucleophile) in DMSO- H2O (DMSO = dimethyl sulfoxide) mixtures of varying compositions at 25.0 +/- 0.1 degrees C. The magnitude of the alpha-effect, k(Ox)(-)/k(p)-(ClPhO-), increases as the DMSO content in the medium increases up to 40-50 mol %. reaching 500, one of the largest a-effect values, and then decreases on further addition of DMSO, resulting in a bell-shaped a-effect profile regardless of the nature of the substrates. The magnitude of the a-effect is found to be significantly dependent on the substrates (or, more quantitatively, on beta(nuc)). Thus, beta(nuc) is an important predictor of the magnitude of the a-effect. The bell-shaped a-effect profile found in the present system is attributed to the differential change in the sensitivity of the medium effect on the Ox- and p-ClPhO- systems but not due to a change in the reaction mechanism or to a drastic change in the basicity of the two nucleophiles on addition of DMSO to the medium. Through application of calorimetric measurements of ground-state solvation combined with the diagnostic beta(nuc) values, it is shown that the transition-state effect is more dominant than the ground-state effect as the origin of the a-effect in the present system.