Micellar charge effects as mechanistic criteria in spontaneous hydrolyses of acid chlorides

Abstract

AbstractReaction rates in aqueous micelles, treated in terms of a pseudophase model, depend on transfer equilibria between water and micelles and rate constants in each pseudophase. Rate constants of spontaneous reactions of fully micellar‐bound substrates are independent of transfer and are those in the micellar pseudophase. They depend on its properties as a reaction region and the sensitivity of the reaction to medium effects. For a wide range of spontaneous reactions there are simple relationships between rate constants and properties of the micellar interfacial region, e.g. apparent polarity and head‐group charge, which depend on the reaction mechanism. Aqueous micelles inhibit nucleophilic substitutions at alkyl centers, but rate constants, k+, in cationic micelles are higher than those, k−, in anionic micelles for SN2 reactions and lower for SN1 reactions. Except for nitro derivatives, hydrolyses of substituted benzoyl chlorides are micellar inhibited and k+/k−≈18 for nitro derivative and ca 0.04 for 4‐OMe and Me derivatives, indicating different extents of bond making and breaking in the transition state, but there is no evidence of rate‐limiting dissociation in hydrolyses of the latter. For hydrolyses of substituted benzenesulfonyl chlorides, values of k+/k− decrease from 21 for the 4‐NO2 derivative to 1.1 for the 4‐OMe derivative, indicating dominant bond making. These mechanism‐related charge effects are ascribed to charge asymmetries at micelle–water interfaces and zwitterionic sulfobetaine micelles behave very similarly to cationic micelles in these hydrolyses. Copyright © 2004 John Wiley & Sons, Ltd.