Keto-enol equilibria in the pyruvic acid system: Determination of the Keto-enol equilibrium constants of pyruvic acid and pyruvate anion and the acidity constant of pyruvate enol in aqueous solution

Article abstract of DOI:10.1021/ja00034a053

Keto-enol equilibrium constants for the pyruvic acid system in aqueous solution at 25°C were determined by Meyer halogen titration and also by another method that evaluates these constants as ratios of enolization to ketonization rate constants, K_E = k^E/k^K. Measurements by each method were made in both acidic and basic solution, and enol required for the ketonization rate measurements was supplied by hydrolysis of a silyl derivative and also by an equilibrated DMSO solution in which the enol content is greater than it is in water. The various methods gave nicely consistent results, which nevertheless differed between acidic and basic solutions, in accord with the different states of ionization of pyruvic acid in the two media; the values obtained were pK_E = 3.21 for pyruvic acid in the carboxylic acid form and pK_E = 5.03 for the pyruvate ion. The latter gives a free energy change for the ketonization of pyruvate enol that is 47% of the free energy liberated by the hydrolysis of the high-energy molecule, phosphoenolpyruvate; this shows that nearly half of the high energy content of this molecule resides in its masked enol function. An acidity constant for ionization of the enol hydroxyl group of pyruvate enol, pK_a^E = 11.55, was also determined, and this, when combined with pK_E for this species, gives pK_a^K = 16.58 as the acidity constant of the pyruvate ion ionizing as a carbon acid.