Kinetics of reaction of picryl bromide with substituted biphenyl-4-carboxylate ions

Abstract

Second-order rate constants have been measured for the reactions of picryl bromide with 2′-, 3′-, and 4′-substituted biphenyl-4-carboxylate ions in 80% methanol–water at several temperatures and activation parameters have been calculated. The Hammett equation applies very well to the reactions of 3′- and 4′-substituted biphenyl-4-carboxylate ions with σ values calculated by the FMMF method. The effect of 2′-substituents is understandable in terms of π-electron steric effects. The rate constants correlate with the pK_a values of the correspondingly substituted biphenyl-4-carboxylic acids. In the case of 3′- and 4′-substituted biphenyl-4-carboxylate ions such a correlation leads to an α-value of 0.52 indicating substantial bonding in the transition state.