Nucleophilicity of phenolates in the reaction with p-nitrophenyl acetate in ethanol

Abstract

The rate of release of p-nitrophenol in the reaction of nine substituted phenolates with p-nitrophenyl acetate has been determined by spectrophotometric measurements in absolute ethanol at 22°. The phenolate anion is the reactive species and competes with ethoxide anion, arising from solvolysis of the phenolate, for nucleophilic attack on the ester carbonyl carbon atoms. From the observed reaction rates the solvolysis constants and the pK_a values of the phenols were obtained. The second-order rate constants for phenoxide anions were correlated with the pK_a values of the corresponding phenols giving a Brönsted β value of 0.57. A comparison with the nucleophilic reactivity of arenethiolates towards the same substrate in ethanol has been made. The rate-determining step is probably the expulsion of the leaving group in the reaction of arenethiolates, whereas it is the nucleophile attack in the reaction of phenoxides.