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.

Proton transfer reactions in apolar aprotic solvents

“…Further, log k α as well as log k cat versus log K for all these benzoic acids follows significantly the Brønsted relationship. The degree of proton transfer in the transition state thus found is 50–60% when the acid acts as a monomer but may rise to 80–90% when it acts as some hyperacidic homoconjugated acid complex . The result is in agreement with the observed increase in log k α with solvent dielectric constant.…”

“…Using Crystal Violet carbinol as the reference base, the authors studied detailed kinetics of equilibria for 34 variously substituted benzoic acids: m ‐methyl, fluoro, chloro, bromo, iodo, methoxy, phenoxy, benzoyl, trifluoromethyl, cyano, and nitrobenzoic acids in toluene; o ‐methyl, fluoro, chloro, bromo, iodo, methoxy, ethoxy, phenoxy, acetoxy, acetyl, benzoyl, nitro, hydroxy, and aminobenzoic acids in toluene; and 2,3‐difluoro‐benzoic, 2,5‐difluoro‐benzoic, 2,6‐difluoro‐benzoic, and 3,5‐difluoro‐benzoic and dichloro‐benzoic acids along with 2‐monofluoro‐benzoic, 3‐monofluoro‐benzoic, and monochloro‐benzoic acids in chlorobenzene …”

“…In Eqn , k α is the catalytic constant for the monomer acid, and k β and k γ are the constants for net catalytic action by the dimeric and trimeric acid species, respectively, which may be positive or negative, with the contribution of the successive terms decreasing rapidly. The catalytic behavior is alternatively expressible as Eqn , where k cat is the composite catalytic constant and p is an acid exponent (generally p ≈ 1 except for strong enough acids for which p > 1) …”

“…Among these, those used by earlier researchers, for example, log K BHA (benzene) by Davis et al . and log k (chlorobenzene) by Chapman et al ., log k +1 (toluene) derived from pseudo‐first‐order reaction kinetics for mono‐substituted benzoic acids, and log K (chlorobenzene) derived from the equilibria for di‐substituted benzoic acids, are the most structure sensitive and rank as the most appropriate for defining acidity scales for the respective class of benzoic acids in inert solvents. Results of these parameters are given in Tables and .…”

“…The result is in agreement with the observed increase in log k α with solvent dielectric constant. The activation parameters evaluated (∆H # = 5–10 kJ/mol and T∆S # = −30 to −35 kJ/mol at 28 °C) for HA–D reactions in an apolar aprotic solvent attribute their abnormally “slow” kinetics to this large negative entropy of activation, which is contributed not only by restrictions on the freedom of motions of the reactant molecules in the transition state but also by an increase in the orientation of apolar solvent molecules around the dipolar transition state …”