2-Indanone and Its Enol. The Effect of a Conjugated Phenyl Group on Enol and Enolate Stability

Article abstract of DOI:10.1021/ja00232a038

Rate and equilibrium constants for the interconversion of 2-indanone with its enol and enolate ion have been determined in dilute aqueous acid and base solution (ionic strength=0.10 M) at 25 deg C.These measurements provide the keto-enol equilibrium constant, pK_E=3.84+-0.04; the acidity constant of the ketone ionizing as a carbon acid, pK_a^K=12.20+-0.08; and the acidity constant of the enol ionizing as an oxygen acid, pK_a^E=8.36+-0.09.Comparison of these results with values for acetone show that the effects of the benzene ring are large: a factor of 1E4.4 on K_E, 1E7.0 on K_a^K, and 1E2.6 on K_a^E.Considerably smaller phenyl effects are estimated for the acyclic model ketone phenylacetone; reduced coplanarity in the latter is suggested as the major reason for the reduced effects.In terms of rate constants,the greater enol content of 2-indanone relative to acetone manifests itself in the acid-catalyzed reaction mainly as a largely reduced rate of enol ketonization and in the "uncatalyzed" reaction entirely by an increased rate of enolization.For hydroxide ion promoted enolate formation the phenyl effect appears roughly equally as an increase in the rate of enolate formation and a decrease in ketonization rate.It is suggested that the effects of a conjugated benzene ring on enol and enolate stability are fully expressed by the reactions of 2-indanone.