Kinetics of Proton Transfer between Ortho Substituted Benzoic Acids and the Carbinol Base of Crystal Violet in Toluene. Ortho Effect on the Reactivity of Benzoic Acids in Apolar Aprotic Solvents

Abstract

Apolar aprotic solvents are particularly advantageous for investigating the intrinsic ortho effect free from complications of specific solvent effects. A kinetic study for toluene-phase proton transfers between ortho F, Cl, Br, I, OMe, OEt, OPh, OAc, Me, NO(2), COMe, COPh, OH, NH(2), and H benzoic acids and crystal violet carbinol base has shown the forward rate constant (log k(+1)) is the most appropriate reactivity parameter in toluene. log k(+1) (toluene) as compared to other reported reactivity parameters in benzene, toluene, or chlorobenzene has been found more sensitive to the ortho substituent effect. The regression results of the correlation of log k(+1) (toluene) of the acids (except OH and NH(2) substituted ones) according to seven ortho effect models are all very significant, and the best result is given by Fujita-Nishioka's model. The overall analysis reveals that a substituent's ortho effect pattern is a 58:24:18 ratio of its ordinary electrical, proximity electrical, and steric effects and that the proximity electrical effect is the major component to account for the peculiarity of the substituent's ortho effect. The results further favor the transmission of this effect mainly through the molecular cavity. The effect may, however, be outweighed by the steric component for bulky enough substituents, e.g., Me. The enhanced strength exhibited by salicylic acid in toluene has been quantitatively described using Pytela-Liška's σ(HB)(i) parameter. The abnormally high log k(+1) observed for anthranilic acid in toluene has been ascribed to a very extensive homoconjugation in its acid-acid anion complex induced by the acid's three hydrogen bond donors.