Tautomerization of Phenols,Part III,The Anthrone−Anthrol Equilibrium in Aqueous Solution

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 1 – 13 in aqueous solution (25°, ionic strength I=0. M ). Five independent thermodynamic and kinetic parameters were determined by analysis of the pH‐rate profile: the equilibrium constant of enolization, pK_E=2.17, the ionization quotient of anthrol, pQ=7.84, and the rate constants of enolization catalyzed by acid, k=2.2?10^?4 M ^?1 s^?1, base, k=51.0 M ^?1 s^?1, and water, k=1.21?10^?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.