Reversed-phase thin-layer chromatography in the parametrization of lipophilicity of some series of arylaliphatic acids

Abstract

In the series of arylacetic acid and β-aryl- n-butyric acids, chromatography was carried out on a thin layer of silica gel impregnated with silicone oil, with 50% acetone as mobile phase. A separation mechanism in this system was evaluated using relationships between R M values and the concentration of the lipophilic solvent in the silica-gel layer. It was found, that both partition and adsorption mechanisms participate, and that the adsorption effect increases with decreasing lipophilicity of the acids. The dichotomy of the mechanism manifests itself in the non-linear course of the relationships between R M values and π parameters, or fragmental constants f derived from the partition system n-octanol—water. Such relationships can be expressed by a single quadratic dependence between lipophilic parameters and R M values, or by two separate linear expressions with different slopes for different regions of substituent lipophilicities. The linear dependence between R M and π at the lower range of lipophilicity is most probably made possible by significant linear dependence between π parameters and molecular surface areas of the substituents.