COSOLVENT TUNING OF TAUTOMERIC EQUILIBRIUM IN SUPERCRITICAL FLUIDS

Cosolvents offer a sensitive tool to tailor equilibria, rates or yield s of chemical reactions in supercritical fluid solvents. The tautomeri c equilibrium of a Schiff base was chosen as a model system, and small amounts of protic cosolvents shift the position of tautomeric equilib ria. The equilibrium was tuned from essentially one tautomer to anothe r by modifying the solvent of pure SCF ethane with less than 2 mol % h exafluoroisopropanol cosolvent. The equilibrium constant was a functio n of cosolvent concentration and mixture density. A chemical-physical model of equilibrium constants deviates from the measured values in th e near-critical region, which may have been caused by local compositio n enhancement of cosolvent around the Schiff base. Decrease in the deg ree of hydrogen bonding with pressure or density affects the keto-enol equilibria by decreasing the amount of the keto formation. Thus, solu tion density, as well as the degree of hydrogen bonding, is manipulate d to tune the position of tautomeric equilibria.