Keto–enol tautomerism

Keto–enol tautomerism is the interconversion of a carbonyl compound and its enol by transferring a proton and rearranging a double bond. The two forms are constitutional isomers in dynamic equilibrium.

Acid-catalyzed enolization: A pathway in which carbonyl oxygen is protonated before an α-carbon loses a proton to form an enol. It shows how protonation makes the carbonyl easier to convert into an enol.
Carbonyl group: A functional group containing a carbon atom double-bonded to oxygen. The keto form contains the carbonyl group that participates in tautomerization.
Aldol reaction: A carbon–carbon bond-forming reaction in which an enolate or enol adds to a carbonyl compound. Enol formation provides the nucleophilic partner that initiates aldol bond formation.
Keto–enol equilibrium: The equilibrium distribution between keto and enol forms of a compound. It describes the resulting population balance, not the interconversion mechanism itself.
Base-catalyzed enolization: A pathway in which a base removes an α-proton to form an enolate, which is then protonated at oxygen. It reaches the enol through an enolate intermediate rather than a protonated carbonyl.
Enol: A compound with a hydroxyl group directly attached to a carbon–carbon double-bonded carbon. It is the specific product form paired with the keto form.
Alpha halogenation: Replacement of an α-hydrogen in a carbonyl compound by a halogen atom. The reaction often proceeds after tautomerization creates an enol or enolate.
Resonance isomer: A hypothetical distinct molecule represented by a resonance contributor, rather than a separately interconverting constitutional structure. Tautomers are real structures that interconvert; resonance contributors are bookkeeping descriptions.
Enolate: A resonance-stabilized anion formed when a base removes an α-proton from a carbonyl compound. It is the key intermediate in base-catalyzed tautomerization.
Tautomer: One of two constitutional isomers that interconvert by moving a proton and shifting a double bond. Keto and enol structures are a prominent example of tautomeric forms.

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