SOLUTE EFFECTS ON REVERSED-PHASE THIN-LAYER CHROMATOGRAPHY A LINEAR FREE-ENERGY RELATIONSHIP ANALYSIS

RM(W) values determined by Dress et al. using RP-ls precoated TLC plat es and aqueous methanol mobile phases have been analysed using our gen eral LFER to give the equation, RM(W) = 0.259 + 0.239R(2) - 0.662 pi(2 )(H) - 0.666 Sigma alpha(2)(H) - 3.006 Sigma beta(2)(O) + 3.603V(X) n = 76, rho = 0.9891, S.D. = 0.206, F = 635 The solute descriptors are, R(2) an excess molar refraction, pi(2)(H) the dipolarity/polarizabilit y, Sigma alpha(2)(H) and Sigma beta(2)(O) the overall or effective hyd rogen-bond acidity and basicity, and V-x the McGowan characteristic vo lume. The coefficients in (i) are very similar to those for the correl ation of water-octanol partition coefficients, confirming the suggesti on that RP-TLC extrapolated RM values can be used as a measure of lipo philicity. However, for a number of solutes, particularly aromatic and heterocyclic nitrogen bases, RM(W) values do not correlate either wit h i, or with log P-oct, and hence care should be taken in the applicat ion of the RP-TLC method to these compounds.More limited data by Podgo rny and Kowalska on RP-18 plates with aqueous tetrahydrofuran (THF) mo bile phases suggests that it is not always necessary to use extrapolat ed RM(W) values, but that RM values with 80% water-20% THF can be used to estimate log P-oct.