Determination of the pKa of Ethyl Acetate: Brønsted 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 kBH = 2−5 × 109 M-1 s-1 for the encounter-limited reaction of the simple oxygen ester enolate with protonated quinuclidine (pKBH = 11.5) were combined with kB = 2.4 × 10-5 M-1 s-1 for deprotonation of ethyl acetate by quinuclidine, to give pKaK = 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.