Rapid solvolyses of 2,6-dimethyl-and 2,4,6-trimethyl-benzoyl chlorides: model systems for solvent effects on the reactivity of acid chlorides

Abstract

Rates of solvolyses of 2,6-dimethyl- and 2,4,6-trimethyl-benzoyl (mesitoyl) chlorides are reported for aqueous binary mixtures with acetone, ethanol, and methanol. These data are compared with those obtained for solvolyses of p-methoxybenzoyl (anisoyl) chloride. A consistent definition of selectivity is proposed to remove ambiguities in published values. Selectivities (S) for formation of ester relative to acid in alcohol–water mixtures are almost independent of solvent composition and are inverse for ethanol mixtures; methanol mixtures show higher S values. Acid catalysis does not appear to be significant. The results show that, for solvolyses of acid chlorides, a solvation effect and possibly a mass law effect increase the reaction rates in methanol–water mixtures as compared with those in ethanol–water mixtures having the same Y value. Differences between the mechanisms of solvolyses of mesitoyl and anisoyl chlorides are revealed by rate enhancements with added m-nitroaniline, and by a comparison of m values. Experience of recent conductimetric studies of a wide range of relatively repid solvolytic reactions is summarised.