Theoretical Approach to Substituent Effects. Phenols and Phenoxide Ions

Ab initio molecular orbital calculations on substituent interactions in substituted phenols and phenoxide ions have been performed. Theoretical gas-phase acidities are generally in satisfactory agreement with available gas-phase experimental data. The effects of substituents on acidity are largely determined by effects in the phenoxide anion and only to a slight extent by those in the corresponding neutral phenol Substituents which interact favorably in the meta position of phenol generally act unfavorably at the para position and vice versa. Both σ and π charge transfer are found to be of importance in determining energies of interaction. The acceptance by a substituent stabilizes OH and O- more effectively at the para position than at the meta position by a π-inductive mechanism. Direct interactions are also more important for fiara substituents and result in stabilization by π acceptors and destabilization by donors. The net results for the π-donating and σ-accepting groups (NH₂, OH, and F) are an increase in acidity at the meta position and a decrease in acidity (with the exception of the strongly -accepting F substituent) at the para position. For the σ- and π-accepting groups (NO₂, CN, CHO, and CF₃), both meta and para substitution lead to enhanced acidity, with a larger effect at the para position.