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.