Partition between phases of a solute that exists as two interconverting species

Abstract

The partition between water and other solvents of a solute that exists as two rapidly interconverting species is discussed with specific reference to acetylacetone. Partition coefficients for the keto form and the enol form have been obtained from the global observed partition coefficient and the equilibrium constant for the ketoenol equilibrium in the various solvents. It is shown that global descriptors can be assigned to acetylacetone that reproduce the observed partition coefficients to 0.125 log units, and which can be used to predict further such coefficients in other solvents. In a similar way, descriptors can be assigned to the separate keto and enol forms that can be used to predict the individual partition coefficients. It is further shown that the keto form is more dipolar/polarizable than the enol form, and is a stronger hydrogen-bond base as well. Neither the keto nor the enol form have any significant hydrogen-bond acidity, which indicates that the enol must form a very strong intramolecular hydrogen-bond that remains intact in all the solvents studied; this conclusion is in accord with previous findings of Emsley and Freeman.