Structural effects on the rates of formation and the stability of enols of cyclic benzyl ketones

Abstract

The acid dissociation constants (KaK), the keto-enol equilibrium constants (K& and the rate constants for enolization of the cyclic benzyl ketones 2-indanone (la), 2-tetralone (3,4-dihydro-2( 1H)-naphthalenone, lb) and 2-benzosuberone (3,4- benzo-3-cyclohepten-I-one, IC) were measured in aqueous solution at 25 C. The rate constants for ketonization of the enols and the acid dissociation constants (KaE) for the corresponding enols were also determined. The presence of a conjugating phcnyl group provides sufficient stabilization of the negative charge to enable these ketones to ionize in the pH range. pKaK valucs wcrc determined from the rate constants for ionization of the ketones and for ketonization of the enolate ions. These valucs were confirmed by spectral titration. The acidity of the ketones decreases with increasing size; pKaK values are 12.2 (la), 12.9 (lb), and 14.9 (IC). Similarly, the acid dissociation constants of the enols decrease with increasing ring size; pKaE's are 8.3 (Za), 9.2 (2b), and 10.0 (2c). Equilibrium constants for enolization also vary with ring size; pKE values are 3.8 (la), 3.6 (lb), and 4.9 (IC).