Steric Effects and Steric Hindrance to Resonance in tert -Butylbenzoic Acids in the Gas Phase and in Solution

Substituent effects of the tert-butyl group in isomeric tert-butylbenzoic acids have been investigated on the basis of enthalpies of formation, gas-phase acidities, acidities in methanol and in dimethyl sulfoxide, and the IR spectra in tetrachloromethane. In contrast to 2-methylbenzoic acid, 2-tert-butylbenzoic acid must adopt a non-planar conformation, as is confirmed by its IR spectrum. Enhanced acidity in the gas phase has been found for all isomers and attributed to a pole-induced dipole interaction in the anion (polarizability effect); in the ortho isomer the steric inhibition of resonance is only responsible for a minor part of the observed effect. In solution, the electrostatic interaction is attenuated, but remains strong in the case of the ortho isomer.The support of the Grant Agency of the Czech Republic, grant No. 203/99/1454 (to P. F. and O. E.) and of the Conseil Ge´ne´ral des Alpes Maritimes (to M. D., J.-F. G. and P.-C. M.) is gratefully acknowledged.Peer reviewe