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.
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