Beat Freiermuth, Bruno Hellrung, Stefan Peterli, Marie-France Schultz, David Wintgens, and Jakob Wirz, "Tautomerization of Phenols III: The Anthrone-Anthrol Equilibrium in Aqueous Solution ", Helv. Chim. Acta., 2001, 84, 3796-3808.


Abstract: The equilibrium between 10H-anthr-9-one and 9-anthrol favors the ketone, which ionizes as a carbon acid in aqueous base. Rates of equilibration were measured over the pH range of 1-13 in aqueous solution (25 C, ionic strength I = 0.1M). Five independent thermodynamic and kinetic parameters were determined by analysis of the pH-rate profile: the equilibrium constant of enolization, pK(enolization) = 2.17, the ionization quotient of anthrol, pQa(enol) = 7.84, and the rate constants of enolization catalyzed by acid, k= 2.2x10-4 M-1 s-1, base, k = 51.0 M-1 s-1, and water, k = 1.21x10-5 s-1. Structure-reactivity relationships strongly support the view that pH-independent enolization of anthrone in water proceeds by rate-determining ionization of the C-acid.