Conformation and Steric Effects in Mono- and Dimethoxybenzoic Acids

Abstract

The conformation of mono- and dimethoxybenzoic acids in solution was determined from their IR spectra and (13)C and (1)H NMR spectra. The main feature is a strong intramolecular hydrogen bond in all 2-methoxy derivatives that persists even in polar aprotic solvents but not in methanol. The methyl groups are mostly coplanar with the ring plane, the two planar conformations are almost equally abundant, and their conformation is not responsible for some enhanced values of the enthalpy of formation. The hydrogen bond (and possibly dimerization) is the reason for differences in the relative acidities (deltapK = pK - pK(H)) measured either in dimethyl sulfoxide or in methanol and also for the differences between deltapK values of methoxybenzoic acids and corresponding methylbenzoic acids. Nevertheless, it was possible to separate with good approximation the polar and steric effects on the acidity. The steric effects in methanol are then parallel for methyl- and methoxybenzoic acids despite variable conformation of the latter. Steric effects in dimethyl sulfoxide are smaller and less regular. The acidity enhancement by ortho substituents is better described as electrostatic induction in the anion rather than a steric hindrance.

Steric Effects in Isolated Molecules: Gas‐Phase Basicity of Methyl‐Substituted Acetophenones

“…The attempted separation of steric effects uses the known principle, [2][3][4][5] comparison of ortho and para derivatives. This is based on three assumptions.…”

“…Moreover, the actual magnitude of the effect was difficult to prove since solvation strongly influences the effective size of ions. Recently we evaluated the substituent effects in the isolated molecules of the neutral species and in the ions separately [2][3][4][5][6] on the basis of experimental gas-phase acidities (basicities) and gasphase enthalpies of formation. Sometimes the latter were replaced by quantum chemical calculations.…”

Towards mechanisms of bimolecular nucleophilic reactions in solution—probing the variation of the activation parameters in the reactions of aromatic compounds

“…[3][4][5][6][7]73] In most cases, substituent effects are quantified by the use of the Hammett or the Hammett-like substituent constants [68][69][70][71][74][75][76] for aromatic system. [77][78][79] The activation parameters are widely used for characterizing the TS structures. [6,23,36,37,68,72,73] For example, it was found that solvolysis of benzyl-and benzhydryl halides follows the S N 2 and S N 1 mechanism, respectively, [80,81] and S N 2 reactions show more negative values of ΔS 6 ¼ .…”

Standard molar enthalpies of formation of 2-, 3- and 4-cyanobenzoic acids

“…Fundamental research works on this class of compounds are also found in the literature [4][5][6][7][8][9][10][11][12][13][14][15]. Those studies are based on the use of several different experimental techniques and aim to understand the effect of different substituents on the structure of benzoic acid and on their gas-phase acidities, vapour pressures, enthalpies of combustion, enthalpies of sublimation, or even enthalpies of formation.…”

“…For example, Fourier-Transform Ion Cyclotron Resonance (FTICR) experiments were performed to determine the gas-phase acidities of several benzoic acid derivatives (substituted with methyl, methoxy, tert-butyl group(s)) and information on the structural effects caused by the introduction of those substituents into the ring of benzoic acid [7,9,10]. The conformations of mono-and di-methoxybenzoic acids obtained either by IR or NMR ( 13 C and 1 H) experiments were also the objective of another work [11]. Colomina et al used a differential scanning calorimeter to measure the purity, heat capacities, and phase transitions and Knudsen effusion to obtain vapour pressures of the di-, tri-, tetraand penta-methylbenzoic acids [4][5][6].…”