Theoretical Study of Substituent Effects on the Gas‐Phase Acidities of Benzoic and Phenylacetic Acids

Abstract

The relative gas‐phase acidities of 25 ring‐substituted benzoic and phenylacetic acids were theoretically determined with proton‐transfer reactions. The energies and geometries of the acids and their corresponding anions, which were involved in the reactions, were calculated at the B3LYP/6‐311+G(2d,p) level of theory. The obtained substituent effects were compared with each other and those of phenols. The acidities of the benzoic and phenylacetic acids were governed by three kinds of electronic effects (inductive, resonance, and saturation effects), which confirmed that the acidities were reflected in the nature of the benzoate and phenylacetate anions, respectively. Substituent effect analyses with an extended Yukawa–Tsuno equation, ${ - {\rm{\Delta }}E_X = \rho \left( {\sigma ^0 + r^ - {\rm{\Delta }}\bar \sigma _{\rm{R}}^ - + s{\rm{\Delta }}\bar \sigma _{\rm{S}} } \right)}$, gave excellent linear correlations for both anionic systems. The degrees of through‐resonance and saturation effects in the phenylacetate anion, as reflected by the resultant r− and s values, were unexpectedly larger than those in the benzoate anion, which were mainly attributed to hyperconjugation and through‐space interactions between the anionic moiety and the benzene π‐electron system, respectively. The acidities of the benzoic acids (or stabilities of the benzoate anions) constituted a better system than the acidities of phenylacetic acids (or stabilities of the phenylacetate anions) for a standard of the normal substituent constants (σ0) for anions in the gas phase, in contrast to solution‐phase results.

Computational study of the substituent effects on the gas-phase stabilities of phenylboranylmethyl anions

“…We have reported some applications of eq. 2 previously [8][9][10][11][12]. In the course of our studies, a through-resonance effect was found in 1, although it was slight [12].…”

“…The normal substituent constant σ 0 , measures the general stabilizing or destabilizing capability of ring substituents, which were defined by the gas-phase stabilities of benzoate anions (1) [9]. The resonance substituent constant ( R ∆σ − ) and the saturation substituent constant ( S ∆σ ) measure the additional stabilizing capabilities by the through-resonance effect for para +R groups and by the saturation effect for electrondonating groups (EDGs), respectively.…”

“…The resonance substituent constant ( R ∆σ − ) and the saturation substituent constant ( S ∆σ ) measure the additional stabilizing capabilities by the through-resonance effect for para +R groups and by the saturation effect for electrondonating groups (EDGs), respectively. Both substituent constants were defined by subtracting σ 0 from σ − defined by the gas-phase stabilities of phenoxide anions (2) [9]. The resultant r − and s values reveal the degree of through-resonance and saturation effects, respectively.…”

Formation and Phase Selection of CaCO3 in the Intervention of Lignin Monomer Model Compounds

“…The increased ratio of vaterite in the F‐ACCs was mainly due to the adjacent methoxyls, which enhanced the reactivity of the hydroxyls in the FA. [ 26,27 ] The SA held the most methoxyls. However, the steric hindrance made the reactivity of the hydroxyls decrease in the SA.…”

Formation and Reactivity of Gold Carbene Complexes in the Gas Phase

“…Systems such as substituted benzoic acids give ρ values greater than 10 in the gas phase. 21 The value suggests that relatively little positive charge is developing on the olefin and points to an early transition state. To probe this system in the absence of substituents, we allowed the benzylidene to react with a mixture of styrene and d 8 -styrene.…”