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Mechanism and dynamics of the keto-enol tautomerization equilibrium of acetylacetone in binary solvent mixtures
Keto-enol tautomerism is a fundamental concept that examines the migration of a proton amongst two constitutional isomers, the keto and enol tautomer. β-diketones are a class of ketone compounds that exhibit this type of behavior. Acetylacetone (AcAc) is the simplest β-diketone, and its enol tautomer is commonly used in organic, inorganic, and medicinal syntheses. Therefore, it is of interest to separate and isolate the tautomers. Despite being one of the well-known phenomena in chemistry, the exact mechanism of keto-enol equilibrium process, as well as the dynamics of the tautomers in a binary solvent mixture, is not entirely understood. Recent studies have examined the solvent effects on the tautomerization of β-diketones and have shown that the keto tautomer is favored as the solvent polarity increases. This trend is known as Meyer’s rule, and studies have validated this rule by studying the tautomerization equilibrium of β-diketones in the presence of a single solvent. This thesis research examined the tautomerization equilibrium in binary solvent mixtures using nuclear magnetic resonance (NMR) spectroscopy, by systematically varying the molar ratio of the solvents. NMR experiments were performed using 8 solvent combinations along with diffusion measurements in 5 of them. The ability to vary the solvent polarity on a continuous basis in the binary solvent system is used to relate the difference in the solvation Gibbs energy to the dielectric constant using the Onsager-Kirkwood formalism. The dynamics of AcAc sampled using the diffusion coefficient reveals differential mobility between the keto and enol with a change in the dielectric constant of the solvent system. This research provides a novel approach to study tautomerization equilibrium, as well as giving insight on the tautomers’ structural conformations on their translational dynamics in a binary solvent mixture.