Chlorophosphate solvolyses. Evaluation of third-order rate laws and rate–product correlations for diphenyl phosphorochloridate in aqueous alcohols†

Abstract

Rate constants and product selectivities for solvolyses of diphenyl phosphorochloridate in aqueous ethanol and methanol have been determined, along with additional kinetic data for solvolyses in acetone–water, D₂O, MeOD, 2,2,2-trifluoroethanol–water, and CF₃CH₂OH–EtOH. Kinetic data for solvolyses of bis(4-chlorophenyl) phosphorochloridate in the above solvents have also been obtained. The results show that these solvolyses have the following features: (i) no evidence for mechanistic changes over the solvent range ethanol to water; (ii) the largest kinetic solvent isotope effect (KSIE in MeOH/MeOD) yet reported for a chloride solvolysis; (iii) large rate decreases in CF₃CH₂OH-rich solvents, indicating a very high sensitivity to solvent nucleophilicity. The large KSIE and the product selectivities are well explained by the accepted S_N2(P) mechanism, extended to incorporate two solvent molecules in the rate-determining step; i.e. reactions are third-order, with one molecule of solvent acting as nucleophile and the other acting as general base. This relatively simple theory accounts well for several important features of these solvolyses (including solvolyses in trifluoroethanol–water and –ethanol), but the third-order rate constants derived from product selectivities lead to calculated first-order rate constants which are not always in agreement with experimental values. The unexpected failure of the rate–product correlation may be due to initial-state effects, reducing values of third-order rate constants as alcohol is added to water.