Substituent effects on the acidity of weak acids. 3. Phenols and benzyl alcohols

The gas-phase acidities of meta- and para-substituted phenols have been calculated at the B3LYP/ 6-311+G*, MP2/6-311+G**, MP2/6-311++G**, and MP2/6-311+G(2df,2pd) theoretical levels. The larger basis sets give the more satisfactory DeltaHacid values that are correlated with the observed acidities with a slope close to unity. They are systematically about 2 kcal/mol too small. The acidities of most substituted phenols are linearly related to those of the corresponding substituted benzoic acids and benzyl alcohols. Here, the substituent effect is a Coulombic interaction (field effect) of the distributed charge in the benzene ring with the negative charge of the anionic center. The exceptions are strong para-substituted T-acceptors such as NO2 and CHO, and to a smaller extent, CN and CF3. Here there is direct charge transfer from the phenoxide oxygen to the substituent.