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The aldol reaction forms a carbon-carbon bond when an enol or enolate adds to an aldehyde or ketone. The initial addition product is a beta-hydroxy carbonyl compound, containing the combined carbon frameworks.

For example, acetaldehyde can add to another acetaldehyde molecule to give 3-hydroxybutanal. Subsequent loss of water gives an alpha,beta-unsaturated carbonyl compound, the condensation product. Keeping addition and dehydration distinct explains why an aldol reaction need not itself eliminate water. The reaction is useful for constructing longer carbon skeletons with predictable functional groups.

Benzaldehyde is ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO}$, an aldehyde whose carbonyl carbon is attached directly to a phenyl group. It combines aromatic-ring chemistry with reactions of an aldehyde carbonyl.

Addition of an appropriate nucleophile at the carbonyl carbon gives a new tetrahedral centre. Because benzaldehyde has no ordinary enolizable alpha carbon, its role in an aldol reaction commonly differs from that of an enolate-forming partner.

A carbon acid is a chemical acid that loses a proton from a carbon-hydrogen bond, producing a carbanion or a resonance-stabilized related species. The name refers to the atom bearing the acidic hydrogen, not to every compound containing carbon.

For example, deprotonation of acetone next to its carbonyl group gives an enolate. Delocalization of the negative charge makes this hydrogen much more acidic than an ordinary alkane hydrogen. Carbon acidity therefore explains how carbon nucleophiles arise in reactions such as the aldol reaction, while the actual acid strength depends strongly on substituents and solvent.

A condensation reaction joins molecular units while eliminating a small molecule such as water or an alcohol. Formation of an ester from a carboxylic acid and an alcohol is a familiar example.

The term classifies the overall transformation, not one universal mechanism. In polycondensation, repeated joining can build a polymer while small molecules are released. An aldol condensation combines carbon-carbon bond formation with a subsequent elimination, so its full outcome must be distinguished from the initial aldol reaction.

An aldol reaction between two reactive sites of the same molecule. A tethered enol or enolate attacks a carbonyl group to close a ring; the tether, conformation and competing chemical reactions influence the product.

A Robinson annulation combines a Michael addition with an intramolecular aldol reaction and dehydration. Tracking the atoms shows which carbonyl survives and which new bond closes the six-membered ring.