aldol

Meaning: An aldol is a β-hydroxy aldehyde or ketone; the name joins aldehyde and alcohol. It forms in the aldol addition. Base removes an α hydrogen to give an enolate, whose α carbon is the nucleophile. It attacks the carbonyl carbon of a second molecule, making a new C–C bond, and protonation of the alkoxide gives the β-hydroxy carbonyl. With heat, the product often loses water to give a conjugated α,β-unsaturated carbonyl; this is aldol condensation.

On the MCAT: A passage treats an aldehyde or ketone with a base such as NaOH and asks for the product or which atoms bond. Mark the α carbon of one molecule and the carbonyl carbon of the other, then join them. In a crossed aldol, a partner with no α hydrogens, such as benzaldehyde or formaldehyde, cannot form an enolate, so it acts as the electrophile. The reaction is reversible; the reverse is the retro-aldol.

Common confusion: Students join two carbonyl carbons or two α carbons. The new bond is between the α carbon of the enolate and the carbonyl carbon of the partner. The OH sits on that former carbonyl carbon, so it ends up β to the remaining C=O. Addition keeps the OH; condensation loses water.