SUBSTITUENT AND SOLVATION EFFECTS ON GAS-PHASE ACIDITIES

The structure of a wide variety of Brønsted acids is related to their intrinsic gas-phase acidity. Polarizability is a major factor in the gas phase in the effect of all substituents on anionic centers, often reversing the “polar” effect assigned from solution phase reactivities of both electron-donating and electron-withdrawing groups. Methyl groups interact with anionic centers by a mixture of polar, polarizability, and hyperconjugative interactions. Solvation by dipolar aprotic solvents results in little compression of relative acidities, but water as solvent compresses many substituent effects to one-quarter of their intrinsic value. © 1979, American Chemical Society. All rights reserved.