KINETICS OF THE CATALYZED HYDROLYSIS OF p-NITROPHENYL ACETATE

Abstract

Catalytic constants have been determined for the hydrolysis at 20 °C of p-nitrophenyl acetate in 9.56% (w/w) dioxane–water mixtures; catalysts used were hydroxide ions, imidazole and 2-methylimidazole, various substituted pyridines, serine, histidine and histidylhistidine. In the case of hydroxide ion catalysis, the rate constant falls off at high catalyst concentrations, and this is attributed to the establishment of an equilibrium involving the anion of the substrate. The results with the various catalysts indicate that there is no simple correlation between catalytic efficiency and pK or nucleophilicity. In the imidazole series there is evidence of steric hindrance when a group is present in a position next to the functional group. There is an indication of bifunctional catalysis in the hydrolysis by 2-(β-hydroxyethyl)pyridine and 4-(γ-hydroxypropyl)pyridine, both of which are much more effective than pyridine.