Triadic analysis of substituent effects—gas-phase acidity of para-substituted phenols

A large variety of para-substituted phenols was examined and their acidities in the gas-phase were rationalized by a triadic formula, which is capable of delineating the initial, intermediate and final state effects in the deprotonation process. It is shown that triadic analysis is equivalent to the homodesmotic reactions approach, while being much more informative at the same time. The applied MP2(fc)/6-311+G(d,p)//B3LYP/6-31G(d) theoretical method gives acidities in very good agreement with available measured values, meaning that calculations can safely replace the missing experimental data for compounds not easily amenable to laboratory examinations. It is found that the underlying principle leading to enhanced acidity of para-substituted phenols containing strong π-electron acceptor groups is the final state effect. It reflects a more pronounced ability to accommodate the excess negative charge. Particular attention has been focused on superacidifying NO₂, SO₂CF₃ and S(O)(NSO₂CF₃)CF₃ and C(CN)C(CN)₂ moieties. It is shown that their influence on acidity is strong and that the deprotonation ability increases along the sequence of substituents NO₂<SO₂CF₃<S(O)(NSO₂CF₃)CF₃<C(CN)C(CN)₂. On the contrary, the electron releasing substituents NH₂, OCH₃, OH and CH₃ decrease acidity of phenol albeit to a small extent. Finally, it is demonstrated that pentacyano derivative of phenol is a powerful OH superacid as evidenced by ΔH_acid value of 287.5 kcal mol⁻¹.