Stoichiometric solvation effects. Part 1. New equations relating product selectivities to alcohol–water solvent compositions for hydrolyses of p-nitrobenzoyl chloride

Abstract

Rate constants at 25 °C are reported for solvolyses of p-nitrobenzoyl chloride (1) in water, D₂O, and in acetonitrile–water, ethanol–water and methanol–water mixtures, and activation parameters are reported for solvolyses in water and in 5% ethanol– and methanol–water. Product selectivities are reported at 25 °C for a wide range of ethanol–water and methanol–water solvent compositions. A general theory is developed for third order solvolytic reactions in alcohol–water mixtures, for which there are four competing reactions with the following rate constants: k_aa where alcohol is nucleophile and general base; k_aw where alcohol is nucleophile and water is general base; k_wa where water is nucleophile and alcohol is general base; and k_ww where water is nucleophile and general base. Values of k_aa, and k_ww can be obtained from the observed first order solvolysis rate constants in the pure solvents. Two independent equations including product composition and solvent stoichiometry are devised to evaluate the two ‘hidden’ rate constants, k_wa and k_aw, and first order rate constants for solvolyses of 1 in methanol– and ethanol–water mixtures are then calculated from the four third order rate constants and the solvent stoichiometry. The results indicate that medium effects, other than those determined by solvent stoichiometry, make only a minor contribution to the solvent dependence of the first order solvolysis rate constants. Stoichiometric solvation effects also account for solvolyses in acetone– and acetonitrile–water mixtures, if it is assumed that water is nucleophile and either water or the cosolvent acts as general base.