Steric Effects and Steric Inhibition of Resonance in Isopropylbenzoic Acids in the Gas Phase and in Solution

Abstract

The substituent effects of the isopropyl group in 2-, 3- and 4-isopropylbenzoic acids were investigated on the enthalpies of formation, gas-phase acidities, acidities in methanol and in dimethyl sulfoxide, and on the IR spectra in tetrachloromethane. Particular attention was given to the influence of variable conformation on the observed steric effect. In contrast to 2-tert-butylbenzoic acid and similarly to 2-methylbenzoic acid, 2-isopropylbenzoic acid exists in two planar conformations in equilibrium. Due to this conformational freedom, the steric effects of the isopropyl group on the conformation of the carboxyl group or on the gas-phase acidity are relatively small, rather close to that of a methyl group. The gas-phase acidity can be qualitatively described in terms of pole/induced dipole interaction in the anion. The latter effect is observable with a lower intensity even in 3-isopropylbenzoic acid: in this action from a more remote position, the isopropyl group resembles more the tert-butyl than the methyl group. In methanol and dimethyl sulfoxide solutions, the steric effects and particularly the pole/induced dipole interaction are attenuated and effect of isopropyl acquires intermediate values between methyl and tert-butyl. Solvent effects are thus deciding for the position of the isopropyl substituent on the scale of steric substituent constants.