Journal of Pharmaceutical Sciences 1994-08-01

Hydrogen bonding. 32. An analysis of water-octanol and water-alkane partitioning and the delta log P parameter of seiler.

M H Abraham, H S Chadha, G S Whiting, R C Mitchell

Index: J. Pharm. Sci. 83 , 1085, (1994)

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Abstract

A general linear solvation energy equation has been used to analyze published partition coefficients in the systems water-octanol (613 solutes), water-hexadecane (370 solutes), water-alkane (200 solutes), and water-cyclohexane (170 solutes). The descriptors used in the equation are R2, an excess molar refraction; phi H2, the solute dipolarity/polarizability; sigma alpha H2 and sigma beta H2, the effective solute hydrogen-bond acidity and basicity; and Vx, the characteristic volume of McGowan. It is shown that the water-octanol partition coefficient is dominated by solute hydrogen-bond basicity, which favors water, and by solute size, which favors octanol, but solute excess molar refraction and dipolarity/polarizability are also significant. For the water-alkane partition coefficients, the same factors are at work, together with solute hydrogen-bond acidity as a major influence that favors water. An analysis of 288 delta log P values shows that solute hydrogen-bond acidity is the major factor but that solute hydrogen-bond basicity and, to a lesser extent, solute dipolarity/polarizability and size are also significant factors that influence the delta log P parameter.