15.2: Reactivity of Enolate Ions
Enolate ions are formed by the acid–base reaction of a carbonyl compound with a base. This leads to deprotonation of the α hydrogen atom, leading to a resonance-stabilized enolate ion where one of the contributing structures is an oxyanion, which imparts additional stability. Therefore, the proton on the α carbon is more acidic in nature than that of other sp3-hybridized C–H bonds but less acidic than those in O–H bonds where the negative charge in the conjugate base is localized on the oxygen atom. This is reflected in their trend of pKa values. For example, acetic acid, ethanol, acetone, 1-propene, and ethane have pKa values of 4.8, 16, 19.2, 43, and 50, respectively.
The enolate ion is an example of an ambident nucleophile—i.e., a nucleophile with two reactive sites. The contributing structures of enolate ions show that both carbon and oxygen atoms can bear the negative charge. Hence, the enolate ion is the conjugate base of both keto and enol forms. In theory, it can react with a particular electrophile to form two different products by bond formation at the two different sites. However, an enolate ion usually reacts at the carbon end, as this is more nucleophilic than the oxygen site.
As enolate ions are Brønsted bases, they react with Brønsted acids, like protons. This leads to hydrogen exchange at the α position of carbonyl compounds with that of solvent, leading to isotope exchange in the presence of D2O and an aqueous base. An optically active aldehyde or ketone undergoes racemization if there is an asymmetric α carbon in the molecule. The loss in stereogenicity owes to the formation of an achiral enolate intermediate where all three atoms are trigonal planar due to sp2 hybridization and conjugation through p-orbital overlap. Since the pKa of an α hydrogen is very high in the case of esters, the various consequences of enolate ion formation is observed specifically for aldehydes and ketones.
Enolate ions also react as Lewis bases, where they act as nucleophiles. Therefore, they can undergo two types of reaction leading to the formation of new bonds at the α carbon:
- Substitution reactions with electrophiles to yield halogenated and alkylated products with molecular halogen (X2) in the presence of an acid or base and an alkyl halide (RX) or sulfonate ester (RSO3−), respectively.
- Addition reactions with carbonyl groups at the electrophilic carbon center followed by nucleophilic acyl substitution reactions depending on the structure of the carbonyl group.