Keto–enol interconversion of 2-phenacylfuran in aqueous solution and in the presence of surfactant micelles

The equilibrium constants for the keto-enol tautomerism of 2-phenacylfuran (2PF), KT = (enol)/(ketone), have been determined in water and in aqueous solutions of CTAB (cetyltrimethylammonium bromide) by combining the pKa KH for ionization of the ketone, directly measured in aqueous NaOH solutions, and the pKa EH for ionization of the enol, obtained from the corresponding rate profile of ketonization. Comparison of these equilibrium constants with those reported for some previously studied phenylacetyl- and phenacyl- heterocycles highlights the effect of an adjacent furyl group on keto-enol tautomerism. Moreover, it has been shown that micelles from cationic (CTAB) and zwitterionic (3-N,N-(dimethyl-N- myristylammonium)propanesulfonate) surfactants decrease the rate of ketonization of the enolate of 2PF and/or increase the apparent acidity of the keto form. These effects have been related to the affinity of the surfactant micelles for the enolate of 2PF. To the contrary, anionic surfactants (SDS: sodium dodecyl sulfate) do not display appreciable effects on the acidity of the keto form or on the kinetics of ketonization of the enol form of 2PF.