Enols and enolates of carboxylic acid derivatives. A G2(MP2) ab initio study

The effect of substituents on the stability of enols and enolates relative to their oxo (C=O) forms is explored using the G2(MP2) ab initio method for CH3COX (14 substituents). Calculated enthalpies of reaction for enolate formation agree well with experimental gas-phase data. Relative to acetaldehyde, enolate formation from a carbonyl compound is favored by the presence of both sigma and sc electron-withdrawing groups and disfavored by pi-donating groups. Isodesmic reactions show that electron withdrawal stabilizes the enolate more than the oxo form. Donation from the pi system stabilizes the oxo form but has little or no effect on the enolate form. Destabilization due to electrostatic repulsion between the oxygen bound carbon and the attached atom for X = {CF3, CHO, C=N} is more severe in the oxo than enolate form. The effect of substituents on the stability of both enolates, H2CC(O-)X, and ethoxide derivatives, CH3CH(O-)X, are shown to be very similar. Relative to acetaldehyde, the heats of reaction of CH3COX for both enolate formation and hydride addition are also similar. For enols, both sigma electron-withdrawing and pi-donating substituents favor the oxo over the enol form. Isodesmic reactions show that substituents have a parallel effect on stability in the oxo and enol forms, as well as in ethene derivatives. The magnitude of these effects increases with the polarity of the double bond, i.e., carbonyls > enols > ethenes. To summarize substituent effects, enols call be thought of as ethene derivatives which are slightly polarized by the hydroxy group, while enolates can be thought of as ethoxide derivatives which are perturbed by the double bond.