Enols and Enolate Anions

Specific examples of enol tautomer and enolate anion concentrations for three different compounds are shown in the following table.

Cyclohexanone is a typical monoketone. Both the enol and enolate anion concentrations are very small, even at pH=13. Phenol serves as a model for the enol tautomer of cyclohexanone, the aromaticity of the benzene ring stabilizing the hydroxyl form. The enhanced acidity of phenols was explained by charge delocalization in the conjugate base, a characteristic that is confirmed by facile electrophilic substitution of the aromatic ring. Although simple ketones have small equilibrium enol concentrations, carboxylic acid derivatives such as esters and amides have even less enol, and are weaker alpha-carbon acids.
The beta-dicarbonyl compound, 2,4-pentanedione, is remarkable in having a much higher enol concentration than monocarbonyl aldehydes and ketones. Enol concentration is solvent dependent, being greater than 90% in hexane solution. The acidity of the diketone is also increased substantially, reflecting charge delocalization over both oxygens.

The chemical behavior of beta-dicarbonyl compounds reflects their increased enol concentration and acidity. Substitution reactions, such as halogenation and isotope exchange, occur more rapidly at the central methylene group of 2,4-pentanedione than at the terminal methyl groups. Furthermore, the corresponding enolate anion may be generated in hydroxylic solvents, using common bases like sodium or potassium hydroxide.
Two other beta-dicarbonyl compounds commonly used in organic synthesis are ethyl acetoacetate, a beta-ketoester, and diethyl malonate, a diester. The weaker influence of the ester carbonyl on enolization and acidity is evident from the data in the following table. Even though diethyl malonate is the weakest acid of the three, it is easily converted to its enolate base by treatment with sodium ethoxide in ethanol. Useful nucleophilic intermediates of this kind are frequently employed in synthesis when suitable beta-dicarbonyl reactants are available.