Lipophilicity of the nitrophenols

The lipophilicity of the nitrophenols, expressed as a water-solvent partiti on coefficient, P, has been investigated using the solvation equation, log P = c + eE + sS + aA + bB + vV. It is shown that this equation accounts qua ntitatively for lipophilicity in a selection of water-solvent systems, viz: octanol, 1,2-dichloroethane, and cyclohexane. In the latter two-systems, t he 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 w ater-octanol system differs in that the a coefficient is effectively zero, so that hydrogen bond acidity of solutes plays no part, and the three monon itrophenols then have similar lipophilicities. The dinitrophenols and picri c acid are similarly discussed. The hydrogen bond acidity of 2,3-dinitrophe nol (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 sim ilar but much smaller effect occurs with 2,6-dinitrophenol (A = 0.17). Picr ic acid has a moderate hydrogen bond acidity (0.46) so that the phenolic OH is still available for external hydrogen bonding. These results are confir med by ab initio calculations which show that 2,3- and 2,6-dinitrophenol an d picric acid are significantly distorted away from planarity, which appare ntly disrupts their internal hydrogen bonding.