Keto–Enol Tautomerization of Acetylacetone in Mixed Solvents by NMR Spectroscopy. A Physical Chemistry Experiment on the Application of the Onsager-Kirkwood Model for Solvation Thermodynamics

Keto–enol tautomerism is a fundamental concept that examines the migration of a proton between two constitutional isomers, the keto and enol tautomer. Many experiments have been proposed to understand the concept behind Meyer’s Rule, which states that the keto tautomer is favored as the solvent polarity increases. However, all the experiments have only been performed in a single solvent. A new physical chemistry experiment is introduced to determine tautomerization equilibrium in binary solvent mixtures using nuclear magnetic resonance (NMR) spectroscopy, by systematically varying the molar ratio of the solvents. This approach allows the students to explore the variation of the polarity of the solvent system as a continuous variable. Furthermore, using an empirical method, the solvent polarity at any binary solvent condition can be converted to an effective dielectric constant of the sample. Because of this ability, the keto–enol tautomerization equilibrium constant can be directly related to the solvation Gibbs energy using the Onsager–Kirkwood theory. Detailed experimental procedures for the direct adaptation of the experiment using a high-resolution or desktop NMR spectrometer is provided in the Supporting Information.