Figure 7.2. Enolization (Tautomerization)
On the left is the keto form, which is thermodynamically favored over the enol form on the right.KEY CONCEPT
Aldehydes and ketones exist in the traditional keto form (C=O) and as the less common enol
tautomer (enol = ene + ol). The deprotonated enolate form can act as a nucleophile. Note
that tautomers are not resonance structures because they have different connectivity of
atoms.Enols are important intermediates in many reactions of aldehydes and ketones. The enolate
carbanion results from the deprotonation of the α-carbon by a strong base, as described earlier.
Common strong bases include the hydroxide ion, lithium diisopropyl amide (LDA), and potassium
hydride (KH). A 1,3-dicarbonyl is particularly acidic because there are two carbonyls to delocalize
negative charge and, as such, is often used to form enolate carbanions. Once formed, the
nucleophilic carbanion reacts readily with electrophiles. We will see one example of this shortly in
the aldol condensation. Another example of this type of reaction is a Michael addition, shown in
Figure 7.3, in which the carbanion attacks an α,β-unsaturated carbonyl compound—a molecule with
a multiple bond between the α- and β-carbons next to a carbonyl.