aldol condensation

Meaning: Base removes an acidic α hydrogen from an aldehyde or ketone to make an enolate. The enolate carbon attacks the carbonyl carbon of a second molecule, forming a new C–C bond and a β-hydroxy aldehyde or ketone (the aldol, "aldehyde + alcohol"). That step is the aldol addition. Dehydration, often helped by heat, then removes the OH and an α hydrogen as H₂O. This gives an α,β-unsaturated carbonyl, and the C=C conjugated with C=O makes the product stable. The loss of water is the "condensation."

On the MCAT: A passage gives one or two carbonyl compounds with NaOH or a similar base, and you predict the product. Find the α-carbon of the nucleophile, bond it to the carbonyl carbon of the electrophile, then decide whether water is lost. A crossed aldol gives a cleaner result when one partner has no α hydrogen, as with benzaldehyde. An intramolecular aldol of a dicarbonyl usually forms a five- or six-membered ring. The reverse reaction is called retro-aldol.

Common confusion: Students mix up aldol addition and aldol condensation. The addition product still has the β-OH. The condensation product has lost H₂O and has a C=C. Only the nucleophilic partner needs an α hydrogen. The electrophilic partner does not.