aldol condensation
An aldol condensation is the aldol reaction with one more move at the end: the beta-hydroxy carbonyl loses a molecule of water to give a conjugated enone (an alpha,beta-unsaturated aldehyde or ketone). 'Condensation' is the chemist's word for a reaction that joins two pieces and expels a small molecule — here, water. The result is a C=C double bond sitting right next to the C=O, the two conjugated together.
Why does the water leave so readily? Because the beta-hydroxy carbonyl has an acidic alpha-hydrogen between the new OH and the carbonyl. Base removes that alpha-proton, and the resulting carbanion pushes out the hydroxide (now a leaving group at the beta-position) as the C=C forms — an E1cb-style elimination. The driving force is the extra stability of the conjugated enone: putting the new double bond in conjugation with the carbonyl lowers the energy, so dehydration is thermodynamically favorable, especially on warming.
The condensation shines in its crossed and directed versions. A crossed aldol between two different carbonyls is only clean if one partner has no alpha-hydrogen (so it can only be the electrophile) and the other is a good enolate source — for example benzaldehyde (no alpha-H) plus a ketone. Run intramolecularly, an aldol condensation closes a ring and is a classic way to make five- and six-membered rings. The enone product is itself a launchpad: it is the Michael acceptor in conjugate additions and the seed of the Robinson annulation.
Benzaldehyde (PhCHO, no alpha-H) plus acetone with base gives, after dehydration, benzalacetone, Ph-CH=CH-CO-CH3 — a crossed aldol condensation. Heating drives off the water and locks in the conjugated enone.
Crossed aldol condensation: one partner lacks an alpha-H, so the product is clean.
An uncontrolled crossed aldol between two carbonyls that both have alpha-hydrogens gives a messy mixture of four products. You make it useful by removing one partner's alpha-H option or by forming one enolate first with LDA (a directed aldol).