The stability of the keto form of ethyl acetoacetate compared to the enol form can be attributed to several factors, including electronic and steric effects.
Electronic effects: In the keto form, the carbonyl group (C=O) is stabilized by resonance, where the electrons of the carbonyl oxygen can delocalize into the adjacent carbonyl carbon. This resonance forms a stable, conjugated system that helps distribute electron density and lower the overall energy of the molecule. In contrast, the enol form lacks this resonance stabilization, and the presence of an sp2 hybridized carbon with a lone pair of electrons makes it less stable.
Steric effects: In the enol form, the presence of a bulky alkyl group attached to the α-carbon (the carbon adjacent to the carbonyl carbon) can lead to steric hindrance. The alkyl group may experience repulsive interactions with neighboring groups, resulting in higher energy and decreased stability. In the keto form, the alkyl group is not positioned in a similar steric hindrance environment, thereby providing greater stability.
Tautomeric equilibrium: Ethyl acetoacetate exists in dynamic equilibrium between its keto and enol forms, with the keto form being predominantly favored due to its greater stability. The stability difference between the two forms results in a higher population of the keto form at equilibrium.
Overall, the resonance stabilization of the carbonyl group and the absence of steric hindrance contribute to the increased stability of the keto form of ethyl acetoacetate compared to the enol form.