Determination of Solvent Effects on Keto-Enol Equilibria of 1,3-Dicarbonyl Compounds Using NMR - Revisiting a Classic Physical Chemistry Experiment

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The use of proton NMR to determine the equilibrium position of tautomeric 1,3-dicarbonyl compounds in various solvents has been a classic physical chemistry experiment. We are presenting an expansion of the excellent description of this experiment by Garland, Shoemaker, and Nibler. Often the assumption is made that the keto tautomer is always the more polar tautomer and hence the concentration of the keto tautomer is greater in polar solvents. However, the keto tautomers of acyclic 1,3-dicarbonyl molecules such as acetyl acetone are often not the more polar of the two tautomers, so this cannot be the only important factor that determines this equilibrium. Rather there are several factors that contribute to the tautomeric equilibrium of which solute dipole moment is just one. The study of the cyclic 1,3-diketone dimedone is especially helpful in this regard. The student is asked to analyze his/her results based on these factors. Molecular modeling is used as an aid in this analysis.