Charge and Delocalisation Effects on the Lipophilicity of Protonable Drugs

Abstract

The transfer mechanisms of ionisable compounds of pharmaceutical interest were studied by cyclic voltammetry at the water/1,2-dichloroethane interface. The partition coefficients of the various ions were deduced from the voltammograms which were monitored as a function of aqueous pH. The dissociation constants and the partition coefficients of the neutral species were determined by a pH-metric titration technique, and the results obtained are displayed in the form of ionic partition diagrams which define the predominance domains of each species in both phases. These diagrams afford an easy interpretation of the mechanisms governing ion transfer and show how neutral species can facilitate the passage of protons from water into an organic phase and thus how ionisable compounds can modulate the pH. The change in lipophilicity between charged and neutral forms of a given compound is discussed in terms of an intramolecular stabilisation of the charge. The nature of the substituents surrounding the charged atom as well as the degree of delocalisation of the charge are shown to contribute markedly to the stabilisation of ionic species in the organic phase. Born's solvation model is also used to illustrate qualitatively the effect of the molecular radius on the lipophilicity and to show that ions retain more water molecules when they transfer into octanol than into 2,2-dichloroethane