Acid—base, hydration—dehydration and keto—enol equilibria in aqueous solutions of α-ketoacids: study by spectroscopy, polarography and linear sweep voltammetry

Abstract

In aqueous buffered solutions of oxalacetic and α-ketoglutaric acids and their esters, the most reliable values of acid dissociation constants Ki were found using spectrophotometry, potentiometry, changes in wave-heights and shifts in polarographic half-wave potentials. Keto—enol equilibrium constants KE were obtained by spectrophotometry and polarography, and dehydration constants Kd by linear sweep voltammetry. The combination of constants Ki, KE, and Kd in some cases enabled separation of specific dissociation constants of the enol, keto, and hydrated forms. These constants made possible calculation of the distribution of individual ionic forms at various pH values in dilute (10−4 M) solutions, under conditions closer to those existing in biological systems and used in kinetic studies than information obtained by nuclear magnetic resonance in concentrated (greater than 1 M) solutions. In their acid—base and hydration—dehydration equilibria, structural effects operating in oxalacetic acid and its esters resemble those for α-ketoglutaric acid, the differences in their behavior in physical methods being predominantly due to the stability of enol forms of oxalacetic acid resulting from their β-ketoacid character. Whereas the monoanion −OOCCOCH2COOH predominates between pH 5.1 and 12.3 in aqueous solutions of oxalacetic acid, the anion HOOCCOCH2COO− is the reactive species in decarboxylation. A neglect of decarboxylation may account for some dubious results between pH 2 and 7 reported in the literature.