tailieunhanh - Effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical study

The proton transfer between interconversion tautomers is of importance in synthetic chemistry, such as: keto-enol, imine-enamine, oxime-nitroso [1, 2] . The keto-enol tautomerization, especially in the -diketone compounds is a common one. Acetyl acetone, one of the -diketone compound, was studied experimentally early and thoroughly [1, 2]. This compound usually exists an equilibrium mixture of enol and keto tautomers with equilibrium | Journal of Chemistry Vol. 44 2 P. 249 - 254 2006 EFFECT OF SOLVENTS ON TAUTOMERIC EQUILIBRIUM OF ACETyL ACETONE A THEORETICAL STUDy Received 22 June 2005 NGUyEN THANH CUQNG LE KIM LONG AND DANG UNG VAN Center for computational chemistry Faculty of Chemistry College of Natural Sciences Vietnam National University Hanoi SUMMARy The tautomeric equilibrium of acetyl acetone in gas and different solvents were studied by electronic structure calculations. The mechanism of reaction was proposed. In gaseous phase the results of B3LYP level are more agreement with experiments than that of HF level. The cis-enolic form was found to be the most thermodynamic stable structure. The solvent effects on tautomeric equilibrium are estimated by performing self-consistent reaction field SCRF -Onsager and PCM models at B3LYP level. The latter is shown of a better model for solvation. The solvents are shown to be not effecting on the thermodynamic stabilities of enol or keto form but have more influence on the transition state. In solvents the activation energy decreases ca. 20 . The enol keto concentration ratio in different solvents were calculated and compared with experimental data. In more polar solvent the more content of keto form was found. I - INTRODUCTION The proton transfer between interconversion tautomers is of importance in synthetic chemistry such as keto-enol imine-enamine oxime-nitroso 1 2 . The keto-enol tautomerization especially in the P-diketone compounds is a common one. Acetyl acetone one of the P-diketone compound was studied experimentally early and thoroughly 1 2 . This compound usually exists an equilibrium mixture of enol and keto tautomers with equilibrium 0 CH3 7 - 0 CHf h XOH CHf enol constant Ke Ke kt Enol form of acetyl acetone exists mainly in gaseous phase however the enol keto ratio depends on strongly the polarization of solvents. The equilibrium constant measured by 1H-NMR spectroscopy indicates a higher enol content in apolar aprotic .