Thermodynamics of hydration of 3-substituted and 3,5-disubstituted phenols

Abstract

The enthalpies of transfer of 3-chloro-, 3-nitro-, and 3-t-butyl-phenol and their 3,5-disubstituted analogues from the gas phase to aqueous solution are reported together with the corresponding free energies and entropies of transfer for 3-nitro-, 3,5-dinitro-, 3-t-butyl-, and 3,5-di-t-butyl-phenol. The results support previous conclusions that the entropies of ionization of phenols in water are not solely explicable in terms of the electronic effects of ring substituents on the hydration of phenoxide anions. The hydration of the neutral phenols must also be considered and in some cases becomes the predominant factor influencing the effect of substituents on entropies of ionization. Contrary to current theories the solvent ordering effect of phenoxide anions does not always become weaker if substituents are replaced by other substituents which withdraw electronic charge more strongly from the ring position at which the phenoxide oxygen is sited.