Bond dissociation energies of O-H bonds in substituted phenols from equilibration studies

Bond dissociation energies (BDE) of several phenolic compounds have been determined by studying the equilibration of couples of phenols and of the corresponding phenoxyl radicals by means of EPR spectroscopy. Measurements were carried out in highly concentrated solutions submitted to continuous photolysis in the presence of di-tert-butyl peroxide. Since under this experimental condition the decay of the phenoxyl radicals was slower than the hydrogen transfer reaction from phenols to phenoxyls, equilibrium concentrations of the two radicals were actually measured. Due to the fact that the radical species are continuously generated in solution, phenols giving short-lived phenoxyl radicals could also be investigated by this method. All of the examined phenols are characterized by O-H bonds weaker than that of the parent compound, PhOH, and their BDE values can be calculated to a good approximation by an additive rule using fixed contributions for the various substituents. Only in the case of the sterically crowded 4-methoxytetramethylphenol (5b), where the p-methoxy substituent is compelled to stay out of the plane of the aromatic ring, is the experimental BDE remarkably different from the calculated value.