Influence of the entropic terms on the tautomeric equilibria of hydroxypyrones

Abstract

A theoretical study of the tautomeric equilibria of the hydroxypyrones, in the vapour phase and in the presence of water molecules, was performed for two series of these compounds, using AMI calculations and the supermolecule approach. The ketonic form is the most stable in the gas phase. In contrast, in the “aqueous solvent”, one enolic form becomes the most stable. The calculations show that the entropic contribution to the equilibrium constants is negligible in the vapour phase, whereas it is an important factor in solution. Our results permit explanation of the displacement of the hydroxypyrones' keto-enol equilibrium observed on passing from the gas phase to aqueous solution.