Self-consistent reaction field study was performed to rationalize the selective formation of 3,7-dimethyloctanal by hydrogenation of citral (3,7-dimethyl 2,6-octadienal) exclusively in supercritical carbon dioxide medium. We have used density functional theory (DFT) to calculate and compare the reactivity of the atoms present in the reactant molecule for a range of solvent with variable dielectric constant. Solvent with low dielectric constant was found to be reducing the electrophilicity of the sites present and hence enhances the selectivity of an isolated and conjugated C=C bond-breaking phenomenon. In contrast to that, solvents with high dielectric constant prefer to hydrogenate the conjugated C=C. The geometric and electronic aspects were compared to propose a plausible explanation for the unprecedented selectivity observed in supercritical carbon dioxide medium. (C) 2004 Elsevier B.V. All rights reserved.