Electronic effects on enol acidity and keto-enol equilibrium constants forring-substituted 2-tetralones

Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pK(a)(K)), for the ionization of their enols (pK(a)(E)), and for keto-enol tautomerization (pK(E)) were determined. Hammett plots of pK (a)(K) and pK(a)(E) vs. sigma are linear with slopes (-rho) of -1.66 +/- 0. 06 and -0.90 +/- 0.03, respectively, except for deviations of the points co rresponding to 6-nitro-2-tetralone (Ib) and its enol. We have previously at tributed the negative deviation of Ib from the correlation for the aciditie s of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negati ve deviation of the point for 1b from the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study re presents the first systematic study of electronic effects on equilibria amo ng ketone, enol, and enolate in aqueous solution.