Lipophilicity of the nitrophenols

The lipophilicity of the nitrophenols, expressed as a water-solvent partition coefficient, P, has been investigated using the solvation equation, log P = c + eE + sS + aA + bB + vV. It is shown that this equation accounts quantitatively for lipophilicity in a selection of water-solvent systems, viz: octanol, 1,2-dichloroethane, and cyclohexane. In the latter two-systems, the major factor in the increased lipophilicity of a-nitrophenol over 3- and 4-nitrophenol is the lack of hydrogen bond acidity of 2-nitrophenol. The water-octanol system differs in that the a coefficient is effectively zero, so that hydrogen bond acidity of solutes plays no part, and the three mononitrophenols then have similar lipophilicities. The dinitrophenols and picric acid are similarly discussed. The hydrogen bond acidity of 2,3-dinitrophenol (0.67) is very much larger than that of 2,4- or 2,5-dinitrophenol (0.09 and 0.11), indicating a very much reduced internal hydrogen bonding. A similar but much smaller effect occurs with 2,6-dinitrophenol (A = 0.17). Picric acid has a moderate hydrogen bond acidity (0.46) so that the phenolic OH is still available for external hydrogen bonding. These results are confirmed by ab initio calculations which show that 2,3- and 2,6-dinitrophenol and picric acid are significantly distorted away from planarity, which apparently disrupts their internal hydrogen bonding.