Abstract
The keto-enol tautomeric equilibrium of 5,5-dimethyl-1,3-cyclohexanedione (dimedone) was investigated using UV-Vis spectroscopy in various solvents, including liquid acetonitrile (CH 3CN), supercritical carbon dioxide (SC CO 2) and SC CO 2–CH 3CN. The content of the enol tautomer in liquid CH 3CN is much higher than that in SC CO 2. With the addition of a small amount of cosolvent CH 3CN into SC CO 2, the enol content increases with respect to that in pure SC CO 2. The comparison of thermodynamic functions for the tautomerism was made in these solvents. It was found that the change of standard internal energy (Δ U 0) in SC CO 2–CH 3CN is close to that in liquid CH 3CN and remarkably different from that in pure CO 2. On the other hand, the maximum decrease in the change of the standard entropy (Δ S 0) was observed in SC CO 2–CH 3CN. It indicates that the cosolvent preferentially aggregates about the two tautomers. The Δ S 0 becomes the dominant thermodynamic driving force of the tautomerization in the cosolvent-modified SC CO 2.
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