Medium effects on the α-effect in DMSO–H₂O mixtures — Comparative studies of <i>p</i>-nitrophenyl benzoate and acetate — Dissection of ground-state and transition-state effects

Abstract

In a study of the origin of the α-effect and its dependence on the nature of the reaction medium as well as structural effects, we report herein a kinetic study of the reactions of p-nitrophenyl benzoate (PNPB) with butan-2,3-dione monoximate (Ox – , α-nucleophile) and p-chlorophenoxide (p-ClPhO – , the reference nucleophile) in dimethyl sulfoxide (DMSO)–H 2 O mixtures of varying compositions at 25.0 ± 0.1 °C. The second-order rate constants (k N ) decrease modestly on addition of DMSO to the medium up to 10 mol% DMSO but increase significantly beyond that point for both nucleophiles. Ox – is more reactive than p-ClPhO – in all solvent mixtures studied (i.e., the α-effect). The α-effect increases as the DMSO content in the medium increases up to 40 mol% DMSO and then decreases beyond that point resulting in a bell-shaped α-effect profile. The bell-shaped α-effect profile obtained for the current reaction is similar to that found previously for the corresponding reaction of p-nitrophenyl acetate (PNPA), differing notably however, in the magnitude of the α-effect beyond 40 mol% DMSO. The PNPB/p-ClPhO – reaction gains greater rate enhancement than the PNPA/p-ClPhO – reaction in the DMSO-rich region, resulting in the smaller α-effect for PNPB beyond 40 mol% DMSO. It is proposed that the observed modulation of the α-effect by the solvent medium is explicable as a ground-state effect in the H 2 O-rich region and a transition-state effect in the DMSO-rich region.Key words: medium effect, the α-effect, ground state, transition state, solvation, desolvation.