Solvent effects on the tautomeric equilibrium of 2,4-pentanedione

Abstract

The influence of solvent on the equilibrium position of the tautomeric forms of 2,4-pentanedione was studied by calorimetric and nmr spectroscopic techniques. For solvents such as CCl4 and cyclohexane the intramolecular bond of the enol form persists and bulk solvent effects account for the equilibrium enol–keto content. In solvents such as DMSO, disruption of the intramolecular bond occurs and the percentage of enol falls due to unfavorable entropy changes. The enol intramolecular bond is disrupted by the solvents water and methanol. Enol hydrogen bond formation through self-association and with the solvent accounts for the entropy changes upon enolization in these solvents. The thermodynamic parameters for enolization in neat 2,4-pentanedione are rationalized by the disruption of the enol intramolecular hydrogen bond through consequent polymerization of the enol tautomer.