15.3:
Types of Enols and Enolates
Enols and enolates vary in their stability. Enols from monocarbonyls are the least stable at equilibrium. However, enols formed in the absence of acid or base are kinetically stable, as they revert to their keto forms very slowly.
Enolate ions of β-dicarbonyl compounds, like β-diketones, β-ketoesters, and β-diesters, are thermodynamically stable because they are doubly resonance-stabilized by the adjacent carbonyls. The enols of such compounds are stabilized by intramolecular hydrogen bonding.
Phenols, which predominantly exist in the aromatic enol form, are the most stable enols.
Two types of regioisomeric enols are possible for unsymmetrical ketones. Carbonyls without an α hydrogen cannot be enolized.
Since α-hydrogen acidity is proportional to enolate stability, esters, acyl cyanides, and tertiary amides form less stable enols.
Primary or secondary amides and carboxylic acids do not form typical enolates, as deprotonation first happens at the acidic proton on nitrogen or oxygen.
15.3:
Types of Enols and Enolates
Aldehydes and ketones form enols, although only about 1% of the enol is present at the equilibrium for simple monocarbonyl compounds. The enol form is undetectable for acetaldehyde, present as only 1.5 × 10−4 % of acetone, and present as only 1.2% of cyclohexanone. Two kinds of regioisomeric enols are possible for unsymmetrical ketones, and their net composition is 1% at equilibrium. This instability is due to the lower bond energy of C=C than the C=O group. The additional instability of enols derived from esters and acids can be attributed to losing the stabilizing resonance between the carboxylate oxygen and the carbonyl p electrons present in the carbonyl form.
β-Dicarbonyl molecules with two carbonyl groups separated by a carbon atom possess more significant amounts of enol at equilibrium owing to the higher stability of the enol. For example, pentane-2,4-dione exists as 76–80% enol for two reasons. Firstly, there is extended delocalization of the conjugated double bond with the other carbonyl group. Secondly, intramolecular hydrogen bonding between the enolic hydroxyl group and the carbonyl oxygen forms a stable 6-membered ring (O⋯H separation = 166 pm). Notably, the methylene group, which two carbonyl groups flank, is preferentially involved in enolization. The alternative enol, 4-hydroxy-4-penten-2-one, is not stable and so is present negligibly at equilibrium. In acyclic ketones, the enol or enolate formed can be either geometrical isomers: (E) or (Z). Protonation on the same face of (E) and (Z) isomers produces enantiomers in solution.
The α hydrogens of esters, nitriles, and 3° amides are acidic, and the corresponding conjugate bases are resonance-stabilized enolates or carbanions. The negative charge is delocalized onto the electronegative oxygen or nitrogen atom lying adjacent to it. Although cyanides need a strong base for deprotonation, its conjugate anion is a linear system like ketene, allene, or carbon dioxide. In the case of primary and secondary amides, the N–H proton is preferentially deprotonated over a C–H proton. As a result, amides are least enolizable among the range of acid derivatives. Therefore, the pKa values of N,N-dimethylacetamide, acetonitrile, ethyl acetate, acetone, acetaldehyde, and acetylacetone are 30, 25, 25, 19.2, 17, and 9, respectively. Primary and secondary amines form enamines, the nitrogen analogs of enols. When enamines are treated with a strong base, aza-enolates are formed, the nitrogen analogs of enolates. Nitroalkanes form enolate-like anions in a weakly basic medium due to their enhanced acidity.