Kinetics of proton transfer between metasubstituted benzoic acids and the carbinol base of crystal violet in toluene: Specific solvent effect–free metasubstituent effect on the reactivity of benzoic acids

Abstract

Apolar aprotic solvents are particularly advantageous for investigating the intrinsic metasubstituent effect free from complications of specific solvent effects. A kinetic study for toluene-phase proton transfers between meta-Me, F, Cl, Br, I, OMe, OPh, COPh, CF 3 , CN, NO 2 , H benzoic acids and Crystal Violet carbinol base has shown the forward rate constant (log k 1 ) the most appropriate reactivity parameter in toluene. log k 1 (toluene) as compared to other reported reactivity parameters in benzene or toluene have been found more sensitive to the metasubstituent effect. The regression results of the correlation of log k 1 (toluene) of the acids according to the four dual substituent parameter models are statistically significant, and the best result has been obtained using Taft's model. The overall analysis reveals that a

Influence of Aprotic Solvents on the Transmission of Anomalous Substituent Effects on 13C NMR Chemical Shifts at the Carboxyl Carbon (δco) in Meta-Substituted Benzoic Acids: A Strong Evidence for π-Polarization Mechanism

“…The main mode of transmission of meta-substituent effect is direct/localized p-polarization for the reverse r I effect on d co [12,28] and direct field for the normal r I effect on logk 1 [39], whereas it is the resonance-induced p-polarization for the Table 3 Values of logk 1 for reactions of meta-substituted benzoic acids with Crystal Violet carbinol base in toluene at 28.0 ± 0.1°C Substituent logk 1 (toluene) [39] (e r = 2.38) [30] (E N T = 0.099) [ reverse r R effect on d co and field-assisted secondary resonance for the normal r R effect on logk 1 [39].…”

“…The most appropriate solvent media for measuring d co and reactivity parameters are apolar aprotic ones where specific solvation is greatly minimized and a large sensitivity to substituent effects is observed for the both. While studying reactions of metasubstituted benzoic acids with Crystal Violet carbinol base (reference base) in toluene, the forward rate constant (logk 1 ) of the reaction was found as the acids' most appropriate reactivity parameter [39].…”

Proton transfer reactions in apolar aprotic solvents

“…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 .…”