Polarographic reduction of aldehydes and ketones: Part XXX. Effects of acid-base, hydration-dehydration and keto-enol equilibria on reduction of α-ketoglutaric and oxalacetic acid and their esters

Abstract

Oxalacetic acid is reduced in four waves, corresponding to the reduction of the following species: i 1a, keto form of the acid; i 1b, enol form of the acid; i 2, monoanion; i 3, dianion. Reduction of the acid and monoanion occurs after protonation of the CO group, that of the dianion on the unprotonated CO group. Ethyl oxalacetate is reduced in corresponding waves i 1a, i 1b and i 2, the free ester after protonation of the CO group, the monoanion without. Diethyl oxalate is also reduced in the keto ( i 1a) and enol ( i 1b) form after pre-protonation, but also in the unprotonated form in wave i 2. α-Ketoglutaric acid is reduced in three waves, i 1 (acid), i 2 (monoanion) and i 3 (dianion), the first two with pre-protonation of the CO group, the third without. Diethyl α-ketoglutarate is reduced in the accessible pH-range in a single two-electron wave, where the CO group is reduced after pre-protonation. Carbanion-enolates are not reducible. Using the dependence of peak currents obtained by LSV on ν 1 2 values of equilibrium constants for the hydration-dehydration reaction ( K d) involving the CO group were determined. α-Ketoglutaric acid exists in aqueous solution 65%, its diethyl ester 75%, oxalacetic acid and its ethyl ester 87% and diethyl oxalacetate 93% in the hydrated form. Anions and dianions (with the exception of ethyl oxalacetate) are pratically unhydrated. Hydration and acid-base catalyzed dehydration results in pH-dependence of polarographic reduction currents. The limiting current is governed by the rate of dehydration and from the intensity of the current (in the region where it is pH-dependent) and K d-values it is possible to calculate rate constants ( k d) of the dehydration (Table 8). When compared with dehydration, keto-enol equilibria are slowly established. From the ratios of the currents, i 1b: i 1a, the value of K E = [enol]/[keto] has been obtained for oxalacetic acid (0.12), its monoethyl (0.47) and diethyl ester (0.42). Structural effects on reduction potentials, keto-enol equilibria and hydration are discussed.