EQUILIBRIA AND KINETICS OF REACTIONS BETWEEN CARBOXYLIC ACIDS AND THE CARBINOL BASE OF CRYSTAL VIOLET IN APOLAR APROTIC SOLVENTS: RELATIVE STRENGTHS OFm- ANDo-SUBSTITUTED BENZOIC ACIDS IN TOLUENE

Equilibria and kinetics of proton-transfer reactions in toluene between a set of m- and o-substituted (methyl, methoxy, chloro, nitro and hydroxy) benzoic acids and the carbinol base (colourless) of Crystal Violet were studied spectrophotometrically. The data on equilibrium and kinetic parameters were critically analysed. The rate constant for the forward step of the acid–carbinol equilibrium was found to be an appropriate criterion for determining acidities in toluene. Toluene-phase acidities were found to be significantly more susceptible than aqueous-phase acidities to the structure of the substituents, particularly for the m-substituted acids. Quantitative analysis of the substituent effect on toluene-phase acidities using the Fujita–Nishioka model for the extended Hammett equation showed that the predominant factor contributing to the "ortho effect" for benzoic acid–dye carbinol reactions in toluene is the proximity polar effect rather than the steric effect for most of the o-substituents.