On the relative stability of the tautomers of ethyl acetoacetate

Abstract Ethyl acetoacetate is a classic β-keto ester and, like the related β-diketone acetylacetone, exists not as a single static species but as a tautomeric system comprising keto and enol forms. In this paper, we consider its gas-phase thermochemistry alongside the “shorter” and “longer” members of the series, namely ethyl pyruvate and ethyl levulinate. Using experimental enthalpies of formation and isodesmic reaction analysis, we compare these keto esters with simpler ketone and ester reference compounds, such as acetone and the corresponding saturated esters, to examine the extent to which the keto esters are stabilized or destabilized. We find that the keto form of ethyl acetoacetate is destabilized by ca. 18 kJ mol –1 , while the corresponding enol is stabilized by ca. 13 kJ mol –1 . We suggest that these two effects, acting independently and simultaneously, provide a thermochemical rationale for the predominance of the enol tautomer. By comparison, the corresponding α-keto ester is destabilized by ca. 40 kJ mol –1 , whereas the γ-keto ester is stabilized by ca. 7 kJ mol –1 . Overall, the progressively decreasing destabilization of α-, β-, and γ-keto esters is encouragingly consistent.