Competing SN2 and carbonyl addition pathways for solvolyses of benzoyl chloride in aqueous media

Abstract

The further development of a convenient, conductimetric method for studying the rates of relatively fast solvolytic reactions in highly aqueous media is described. Rate and product data are reported for solvolyses of benzoyl chloride at 25 °C in water and in binary aqueous mixtures with acetone, ethanol, methanol, trifluoroethanol, and hexafluoropropan -2-ol. Comparison of these kinetic data with rate data for the S_N1 model, 1 -adamantyl chloride, reveals sensitivity to solvent nucleophilicity even for highly aqueous media. In contrast with other nucleophilically solvent-assisted processes, the solvent effects are not satisfactorily correlated by one linear free energy relationship. Gas-phase thermochemical data show the feasibility of a direct heterolytic cleavage of the C–Cl bond in benzoyl chloride. Rate–product correlations for hydrolysis and aminolysis in 50% w/w acetone–water are observed, providing that allowance is made for the medium effect of the added o-nitroaniline. These results are consistent with an S_N2 mechanism, not S_N1 and/or ion-pair mechanisms previously discussed. Another pathway, dominant in less aqueous media, is much less sensitive to changes in solvent ionizing power, consistent with earlier proposals for a carbonyl addition–elimination mechanism.