Polarographic reduction of aldehydes and ketones. Part IX. Carbanion formation accompanying the electroreduction of ethyl benzoylacetate

Abstract

The keto-form of compounds RCO·CH₂X (where R = aryl, X = electronegative group) is reducible at the dropping mercury electrode in the available potential range in solutions which contain alkali-metal cations, whereas the enol form RC(OH)CHX is electroinactive. The ambident ion RC(O^–)CHX ↔ RCO·CHX^– is reduced at more negative potentials than the keto-form. For ethyl benzoylacetate the equilibrium between the keto-form and the ambident anion was followed from the absorption of the keto-form (λ_max.= 249 nm., ε= 1·3 × 10⁴) and of the anion (λ_max.= 303 nm., ε= 1·4₅× 10⁴); the formal dissociation constant K_T= 2 × 10^–11 was found. From [Enol]/[Keto]= 0·008 it was possible to estimate the acid dissociation constants of the keto-form (K₇= 2 × 10^–11) and of the enol form (K₈= 2·5 × 10^–9). At pH < 8 ethyl benzoylacetate is reduced in the protonated form PhCOH⁺CH₂·CO₂Et.