A kinetic study of the nucleophilicity of substituted arenethiols in reaction with p-nitrophenyl acetate

Abstract

The nucleophilic reactivity of a series of substituted arenethiols towards a single ester substrate, p-nitrophenyl acetate, has been determined by spectrophotometric measurements of reaction rates in ethanol at 22°. Conversion of a thiol function into a thiolate anion is a prerequisite for nucleophilic attack on the sp³ carbon atom. The second-order rate constants were correlated with the pK_a values of the corresponding arenethiols via the Brönsted equation giving a β value of 0·61; comparison of proton basicity, carbon basicity, and nucleophilicity of the arenethiolate ions has been made. The reaction was found to be little influenced by variation of ionic strength. A three-fold increase of reactivity was observed on changing the solvent from methanol to ethanol. The activation parameters for the reaction of p-nitrophenyl acetate and sodium benzenethiolate in ethanol and in methanol as solvents at 22° have been calculated : ΔH= 11·22 kcal mol^–1, ΔS=–23·15 cal mol^–1 K^–1 in ethanol, ΔH= 12·98 kcal mol^–1, ΔS=–19·33 cal mol^–1 K^–1 in methanol.