Aldol reaction

The aldol reaction forms a carbon–carbon bond when an enol or enolate adds to a carbonyl compound. Its initial product is a β-hydroxy carbonyl compound, which can lose water to form an α,β-unsaturated carbonyl compound.

Enolate: A resonance-stabilized anion formed by removing an α-hydrogen from a carbonyl compound. It is the common nucleophile that attacks the carbonyl partner in base-promoted aldol reactions.
Carbonyl group: A functional group consisting of a carbon atom double-bonded to an oxygen atom. Aldol reactions depend on the electrophilic carbonyl carbon and carbonyl-derived nucleophiles.
Crossed aldol reaction: An aldol reaction between two different carbonyl compounds. It extends the reaction to distinct partners but can produce mixtures without selectivity control.
Claisen condensation: A carbon–carbon bond-forming reaction in which an enolate adds to an ester, followed by alkoxide elimination. It also uses carbonyl-derived enolates but forms a β-keto ester or related product.
Enol: A compound with a carbon–carbon double bond and a hydroxyl group on one alkene carbon. It is the neutral tautomer that can act as the nucleophile in acid-catalyzed aldol reactions.
α,β-Unsaturated carbonyl compound: A carbonyl compound with a carbon–carbon double bond between its α and β carbons. This is the characteristic product when an aldol adduct dehydrates.
Claisen–Schmidt condensation: A crossed aldol condensation, commonly between an aromatic aldehyde and a ketone lacking α-hydrogens. It provides a selective route to conjugated carbonyl compounds from unlike partners.
Michael addition: Conjugate addition of a nucleophile to the β carbon of an α,β-unsaturated electrophile. It forms carbon–carbon bonds by attack at an alkene rather than direct carbonyl addition.
Aldol addition: The formation of a β-hydroxy carbonyl compound by addition of an enol or enolate to a carbonyl group. It names the bond-forming stage before any dehydration.
β-Hydroxy carbonyl compound: A carbonyl compound bearing a hydroxyl group on the β carbon relative to its carbonyl group. It is the defining product of the aldol bond-forming addition.

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