Steric effects and steric inhibition of resonance in isopropylbenzoic acids in the gas phase and in solution

The substituent effects of the isopropyl group in 2-, 3- and 4-isopropylben zoic acids were investigated on the enthalpies of formation, gas-phase acid ities, acidities in methanol and in dimethyl sulfoxide, and on the IR spect ra in tetrachloromethane. Particular attention was given to the influence o f variable conformation on the observed steric effect. In contrast to 2-ter t-butylbenzoic acid and similarly to 2-methylbenzoic acid, 2-isopropylbenzo ic acid exists in two planar conformations in equilibrium. Due to this conf ormational freedom, the steric effects of the isopropyl group on the confor mation of the carboxyl group or on the gas-phase acidity are relatively sma ll, rather close to that of a methyl group. The gas-phase acidity can be qu alitatively described in terms of pole/induced dipole interaction in the an ion. The latter effect is observable with a lower intensity even in 3-isopr opylbenzoic 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 thu s deciding for the position of the isopropyl substituent on the scale of st eric substituent constants.