Steric hindrance and acidity. Part 3.—Enthalpies and entropies of ionization of phenols in methanol

The enthalpies and entropies of ionization in methanol of 4-t-butylphenol, phenol, 4-bromophenol, 4-formylphenol, 4-nitrophenol and their 2,6-di-t-butyl substituted analogues have been measured. 3-Nitrophenol, 2-t-butylphenol and 2,4- and 3,5-di-t-butylphenol have also been studied. The influence of substituents on the acidity of the unhindered phenols is primarily governed by the changes in the phenol OH bond dissociation energy. However, the effect of solute-solvent interactions on acidity is considered to be more important in methanol than in water. Solute-solvent interactions are probably the predominant factor which decides the increment in pKa produced on substituting ortho t-butyl groups in a phenol.