NUCLEAR MAGNETIC RESONANCE MEASUREMENTS IN SOLUTIONS OF ACETYLACETONE: THE EFFECT OF SOLVENT INTERACTIONS ON THE TAUTOMERIC EQUILIBRIUM

Abstract

A modified assignment of the PMR signals in acetylacetone is confirmed. The changes in intensity of selected signals with temperature are used to calculate an enthalpy of conversion of 2700 ± 100 cal. between keto and enol forms in pure acetylacetone.Interactions, which perturb the equilibrium between the tautomeric forms in dilute solution by formation of solution complexes, are studied by observing dilution chemical shifts in various solvents. The ratio of keto to enol forms is estimated from measurements of signal intensities at several dilutions in each solvent. The deviations from the correlations of Bernstein and Powling (5) between solvent dielectric constant and molar volume, and the position of the tautomeric equilibrium in dilute solutions, have been used as a criterion of solvent interaction. They are consistent with the present measurements.Typical basic, acidic, amphoteric, and neutral solvents have been chosen to investigate possible types of interaction. Cyclohexane and acetic acid do not perturb the equilibrium by any interactions. Triethylamine forms a hydrogen bonded complex through the enolic—OH group and the nitrogen lone pair, thus converting acetylacetone completely to enol form. Pyrrole forms a weakly hydrogen bonded complex through the carbonyl oxygens of the keto form. Freezing diagrams in the interacting systems are consistent with the complexes suggested by the PMR measurements.