Transition state imbalance in proton transfer from phenyl ring-substituted 2-tetralones to acetate ion

Rate constants for the acetate ion-catalyzed ketonization of phenyl-substituted 2-tetralone enols have been determined by stopped-flow UV spectroscopy. From these rate constants and the keto-enol equilibrium constants, the rate constants (k(-2)) for enolization were calculated. A Bronsted plot of these rate constants (log k(-2)) vs the acidity of the appropriate 2-tetralone (pK(a)(K)) is linear, with a slope (-alpha(E)) of -0.78 +/- 0.03, except for the point Corresponding to 6-nitro-2-tetralone (4b). Rate constants for the ionization of 2-tetralone by substituted acetates were;determined directly by NMR, giving a corresponding Bronsted beta(E) of 0.54 +/- 0.03. Both the negative deviation of the-point for 4b from the correlation line for alpha(E) and the inequality between alpha(E) and beta(E) indicate an imbalanced transition state for the proton abstraction of 2-tetralone by acetate ion. This reaction is impeded by a thermodynamic barrier of 11 kcal/mol, along with an intrinsic kinetic barrier of 14 kcal/mol. A comparison of the transition states for proton abstraction of 2-tetralone by hydroxide ion and by acetate ion shows similar transition state imbalance and intrinsic kinetic, barriers for both reactions. The relevance of these results to the mechanism of enzymatic acceleration of enolization is discussed.