Effects of Solvents and Substituents for Tautomeric Equilibria of Methyl p-Substituted Benzoylacetates

Seizo MASUDA*, Keiji MINAGAWA, Hideaki OSAKA and Masami TANAKA

Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima; Minamijosanjima, Tokushima-shi 770-8506 Japan
Faculty of Pharmaceutical Science,Tokushima Bunri University; Yamashiro, Tokushima-shi 770-8514 Japan

Tautomeric equilibria of methyl p-substituted benzoylacetate were studied by 1H-NMR spectroscopy. Equilibrium constants (Kt) depend greatly on solvents and substituents. Regression analysis of Kt for methyl benzoylacetate with solvatochromic parameters proposed by Kamlet and Taft revealed that the polarity and the hydrogen bond donor acidity of the solvent are the major factors of the Kt-variations as a result of preferential solvation of the ketonic tautomer. The equilibrium constants for p-substituted benzoylacetates were evaluated in chloroform and acetonitrile. Hammett's plot of the constants have negative ρ values in both solvents, indicating that the equilibrium shifts to the enolic direction as electron-withdrawing power of the substituents on phenyl ring increases.

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