Add time:08/11/2019         Source:sciencedirect.com

Oxalacetic acid is reduced in four waves, corresponding to the reduction of the following species: i1a, keto form of the acid; i1b, enol form of the acid; i2, monoanion; i3, 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 i1a, i1b and i2, the free ester after protonation of the CO group, the monoanion without. Diethyl oxalate is also reduced in the keto (i1a) and enol (i1b) form after pre-protonation, but also in the unprotonated form in wave i2. α-Ketoglutaric acid is reduced in three waves, i1 (acid), i2 (monoanion) and i3 (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 ν12 values of equilibrium constants for the hydration-dehydration reaction (Kd) 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 Kd-values it is possible to calculate rate constants (kd) of the dehydration (Table 8).When compared with dehydration, keto-enol equilibria are slowly established. From the ratios of the currents, i1b: i1a, the value of KE = [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.

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