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 (λmax.= 249 nm., ε= 1·3 × 104) and of the anion (λmax.= 303 nm., ε= 1·45× 104); the formal dissociation constant KT= 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.