Kinetics of the hydrolysis of acetic anhydride in concentrated salt solutions

Abstract

The kinetics of the hydrolysis of acetic anhydride have been investigated in concentrated salt solutions at 20°. Sine salts were used in concentrations of up to 5 mol 1^-1; all inhibited the reaction.     The salt effect was resolved into its component effects on the reactants and the transition state by use of the Bronsted-Bjerrum equation to calculate transition state activity coefficients from rate constants and measured activity coefficients of acetic anhydride. The effect of a salt on the free energy of the reactants was always significant and in some cases it was the major component of the effect of the salt on the free energy of activation. The enthalpy and entropy of transfer from water to 1 mol l^-1 sodium chloride, for both acetic anhydride and the transition state, show the enthalpy-entropy compensation effect which is typical of aqueous solutions.     These salt effects are considered to be part of the general phenomenon of the effect of salts on the activity coefficients of non-electrolytes. The inhibition is not caused by formation of a complex between salt and acetic anhydride. Rate constants could not be correlated with dielectric constant and ionic strength, using Gold's equation, and changes in water structure which occur in these salt solutions were shown to have no direct effect on the reaction rate.