Determination of the pK(a) of ethyl acetate: Bronsted correlation for deprotonation of a simple oxygen ester in aqueous solution

The rate constants for deprotonation of ethyl acetate by 3-substituted quinuclidines are correlated by β = 1.09 ± 0.05. The limits of k(BH) = 2-5 x 10⁹ M^-1 s^-1 for the encounter-limited reaction of the simple oxygen ester enolate with protonated quinuclidine (pK(BH) = 11.5) were combined with k(B) = 2.4 x 10^-5 M^-1 s^-1 for deprotonation of ethyl acetate by quinuclidine, to give pK(a)(K) = 25.6 ± 0.5 for ionization of ethyl acetate as a carbon acid in aqueous solution. A rate-equilibrium correlation for proton transfer from methyl and benzylic monocarbonyl compounds to hydroxide ion has been extended by 6 pK units in the thermodynamically unfavorable direction, and it is shown that the absence of curvature of this correlation is inconsistent with a constant Marcus intrinsic barrier for the enolization of simple carbonyl compounds.