KINETIC EFFECT OF SULFITE ION ON ESTER HYDROLYSIS

The hydrolysis of p-nitrophenyl acetate at pH 6-8 is strongly catalysed by SO32- (rate/mol l(-1) s(-1) = 1.6[ester][SO32-] at 28 degrees C; the pseudo-first-order rate constant for the uncatalysed reaction is of the order 10(-5) s(-1) at pH 8). The corresponding second-order rate constant for the reaction catalysed by HPO42- is 16 000 times smaller, despite both the conjugate acids having similar pK values. It is suggested that the large effect of sulphite ion is due to its high nucleophilicity; esters and lactones in food could become less stable in the presence of sulphite species.