ACID- AND WATER-CATALYZED HYDROLYSIS OF P-NITROPHENYL ESTERS

The acid-catalyzed hydrolysis of a series of p-nitrophenyl esters of aliphatic carboxylic acids has been studied in moderately concentrated HCl solutions at 30˚ with acid concentration being varied from 1.36 to 6.24 M. Reasonably linear plots of log Kobsd vs. log HCl concentration were obtained while degrees of curvature and small slopes were seen in plots of log kobsd vs. –H0. Plots of log Kobsd + H0 vs. the logarithms of the activity of water in these solutions were linear in each case. The values of w were highly positive ranging from +4.40 with the butyrate ester to +5.15 for the trimethyl acetate ester. When ionic strength was held constant at 4.80 M with LiCl linear plots of kobsd vs. HCl concentration were obtained. A plot of the logarithms of the second-order rate constants for hydrolysis at constant ionic strength vs. Es the Taft steric effects constants, was linear with a slope, δ, of 0.59. Thus, there is less sensitivity to steric hindrance in the acyl group with these p-nitrophenyl esters than in the case of ethyl esters. A facile water catalysis was observed in the hydrolysis of the p-nitrophenyl and p-methoxyphenyl esters of dichloroacetic acid. Increasing the acid concentration produces a pronounced decrease in rate with the nitro-substituted ester but only a slight decrease with the methoxy-substituted compound. Thus, the magnitude of ρ for water catalysis is highly dependent on the ionic strength of the medium. The hydrolysis of bis(4-nitrophenyl) carbonate was also studied at 50˚. There is a pH-independent reaction at pH values greater than 1. This reaction involves water catalysis and has a D2O solvent isotope effect (kH/kD) of 2.88. As with p-nitrophenyl dichloroacetate there is no apparent acid catalysis, the rate of the reaction decreasing greatly as acid concentration is increased from 1.0 to 5.29 M. © 1969, American Chemical Society. All rights reserved.