Abstract
RM w values determined by Dross et al. using RP-18 precoated TLC plates and aqueous methanol mobile phases have been analysed using our general LFER to give the equation, RM w = 0.259 + 0.239 R 2 − 0.662 π 2 H − 0.666 α 2 H − 3.006 Σβ 2 O + 3.603V x n = 76, ϱ = 0.9891, S.D. = 0.206, F = 635 The solute descriptors are, R 2 an excess molar refraction, π 2 H the dipolarity/polarizability, Σα 2 H and Σβ 2 O the overall or effective hydrogen-bond acidity and basicity, and V x the McGowan characteristic volume. The coefficients in (i) are very similar to those for the correlation of water-octanol partition coefficients, confirming the suggestion that RP-TLC extrapolated RM values can be used as a measure of lipophilicity. However, for a number of solutes, particularly aromatic and heterocyclic nitrogen bases, RM w values do not correlate either with i, or with log P oct, and hence care should be taken in the application of the RP-TLC method to these compounds. More limited data by Podgorny and Kowalska on RP-18 plates with aqueous tetrahydrofuran (THF) mobile phases suggests that it is not always necessary to use extrapolated RM w values, but that RM values with 80% water-20% THF can be used to estimate log P oct.
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