Solvent Effects on Keto-Enol Equilibria: Tests of Quantitative Models

Article abstract of DOI:10.1021/jo00208a014

The effect of solvent on the equilibrium constants between selected keto-enol isomer pairs has been determined.The protomeric systems studied are 5,5-dimethyl-1,3-cyclohexanedione (1), 2-methyl-1,3-cyclohexanedione (3), 3-oxabicyclo<4.3.0>-2,9-dioxononane (5), 2,4-pentanedione (7), ethyl 3-oxobutanoate (9), and 9-anthracenone (11).A wide variety of theoretical and empirical solvation parameters have been tested in a multiparameter linear free energy format to model the changes in equilibria.The most successful correlations are obtained with the Kamlet-Taft polarity-polarizability and hydrogen-bonding terms, although the Swain A and B factors have advantages in same cases.In general, for the isomer pairs in which the enol cannot form an internal hydrogen bond (1-2, 3-4, and 11-12), the equilibria appear to be controlled almost completely by the hydrogen-bonding basicity of the solvent.For the isomer pairs 5-6, 7-8, and 9-10, in which intramolecular hydrogen bonding is possible, the polarity-polarizability effect dominates, although differential stabilization of the isomers by hydrogen bonding remains significant.