Solvent Effect on the α-Effect: Ground-State versus Transition-State Effects; a Combined Calorimetric and Kinetic Investigation

Abstract

In a study of the solvent effect on the α-effect, second-order rate constants (kNu−) have been determined spectrophotometrically for reactions of a series of substituted phenyl acetates with butan-2,3-dione monoximate (Ox-, α-nucleophile) and p-chlorophenoxide (p-ClPhO-, reference nucleophile) in DMSO−H2O (DMSO = dimethyl sulfoxide) mixtures of varying compositions at 25.0 ± 0.1 °C. The magnitude of the α-effect, kOx−/kp-ClPhO−, increases as the DMSO content in the medium increases up to 40−50 mol %, reaching 500, one of the largest α-effect values, and then decreases on further addition of DMSO, resulting in a bell-shaped α-effect profile regardless of the nature of the substrates. The magnitude of the α-effect is found to be significantly dependent on the substrates (or, more quantitatively, on βnuc). Thus, βnuc is an important predictor of the magnitude of the α-effect. The bell-shaped α-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 βnuc values, it is shown that the transition-state effect is more dominant than the ground-state effect as the origin of the α-effect in the present system.