Kinetics of the hydrolysis of acetic anhydride and the reaction of 4-nitrophenyl acetate with imidazole in aqueous-organic mixed solvents

Abstract

With the general aim of elucidating the role of water structure in the kinetics of hydrolysis, a comparative study has been made of the kinetics of the hydrolysis of acetic anhydride and the reaction of 4-nitrophenyl acetate with imidazole in mixtures of water with ethanol, t-butyl alcohol, dimethyl sulphoxide and dioxan. Both rate constants were always reduced by the addition of organic solvent. Transition state activity coefficients were measured for both reactions in dimethyl sulphoxide-water mixtures and compared with the activity coefficient of phenylalanine (a model zwitterionic transition state). Activation parameters were measured for the hydrolysis of aceticanhydride in t-butyl alcohol-water and dimethyl sulphoxide-water mixtures. Semilogarithmic plots of rate constant against Winstein's Y-value were non-linear for some of the solvents and this fact, coupled with the effect of dimethyl sulphoxide on the transition state activity coefficients, leads to the conclusion that specific interactions of the solvent with the reactants and with the transition state could be of major importance in controlling the reaction rate.There was no obvious relationship between the effect of an organic solvent on the kinetics of hydrolysis of acetic anhydride and its effect on the structure of water.