Quantitative structure—activity relationships for herbicides : Reversed-phase liquid chromatographic retention parameter, log kw, versus liquid-liquid partition coefficient as a model of the hydrophobicity of phenylureas, s-triazines and phenoxycarbonic acid derivatives

Abstract

The retention behaviour of phenylureas, s-triazines and phenoxycarbonic acid derivatives in a reversed-phase high-performance liquid chromatographic (RPLC) system has been examined. Using methanol—water or acetonitrile—water as the mobile phase, a linear relationship between the volume fraction of the organic modifier, ϕ, and the logarithm of the capacity factor, log k′, is established for each solute. The different correlation curves for each compound indicate selective effects upon retention due to solute—solvent and solute—stationary phase interactions. It is shown that log k w, a theoretical capacity factor obtained by extrapolation of retention data in binary solvent systems to pure aqueous eluent, is suitable for eliminating the selective effects and thereby for quantitatively describing the hydrophobic nature of solutes in a way which is strongly related to the partition coefficient, log P, of the standard n-octanol—water partitioning system. The dependence of log k w on the nature of the organic modifier and an analysis of functional group behaviour in different eluents reveal that log P and log k w are not completely interchangeable, because certain substituents, i.e., methylthio and trifluoromethyl groups, behave differently in RPLC and a true liquid—liquid partitioning system. The consequences of this non-polar group selectivity in RPLC on the quality of quantitative structure—activity relationships for electron transport-inhibiting herbicides are demonstrated. The results suggest that log k w might be a better model for the assessment of the hydrophobicity of drugs in biological systems.