Keto-enol Tautomerism of hydroxy-substituted arenes: Theoretical study and experimental consequences

In this work, the chemical equilibrium between enol and keto tautomers occurring in phenol, naphthols and selected 29 hydroxy substituted polycyclic aromatic hydrocarbons classified into 4 structural types was investigated. The reaction Gibbs energies were computed using the density functional theory combined with the solvent continuum model. We have demonstrated how the consecutive condensation of benzene rings together with two-dimensional molecular arrangement and the position of the hydroxyl group modifies this equilibrium. The obtained results revealed that the prototropic rearrangement in the electronic ground state is not thermodynamically less probable between two neighbouring condensed benzene rings. The keto form is favoured in linear polycyclic aromatic hydrocarbons for substituted central moieties. The angular molecular structure has the opposite effect. Based on the theoretical energies calculated for room temperature, the tautomerisation pK_T constants and acidity pK_a constants for enols as well as corresponding keto-tautomers were predicted and compared with available experimental values for the water environment. Finally, the possible experimental consequences in respect to the chemical reactivity of studied tautomers were discussed.