Effects of charge and intramolecular structure on the lipophilicity of nitrophenols

The lipophilicity of a series of phenolic compounds was studied in the 1,2-dichloroethane (1,2- DCE)/water system. Cyclic voltammetry at the ITIES was used to study the transfer characteristics of the charged species, and their partition coefficient was deduced from their formal transfer potential. For the neutral species, log PDCE values were measured by a two-phase pH-metric method. The results are compared to those previously obtained in octanol/water and by linear solvation energy relationships (LSER) in the two solvent systems. It is shown that nitrophenols with intramolecular H-bonding deviate from the solvatochromic equation for the 1,2-DCE/water system, and discrepancies between both approaches are discussed on the basis of conformational and steric effects. When charged however, all the species have approximately the same partition coefficients because the effect of the intramolecular H-bond disappears and the differences in measured lipophilicity arise from the variation of the intramolecular charge delocalization due to resonance equilibria. Some biological implications of these properties are discussed.