aldol reaction

The aldol reaction is the workhorse that joins two carbonyl molecules together. One molecule is turned into an enolate (or enol) and becomes a carbon nucleophile; the second molecule keeps its intact carbonyl and acts as the electrophile. The nucleophilic alpha-carbon of the first attacks the carbonyl carbon of the second, and a new carbon-carbon bond appears. The product has BOTH an alcohol (from the attacked carbonyl) and an aldehyde or ketone (the original one) — which is exactly why it is called an 'aldol', a contraction of aldehyde and alcohol.

Step by step under base: hydroxide removes an alpha-hydrogen to make the enolate; the enolate's carbon end attacks the carbonyl carbon of a second molecule, just like any nucleophilic addition, breaking the C=O pi bond and putting negative charge on oxygen; finally that alkoxide grabs a proton from water to become a neutral alcohol. The result is a beta-hydroxy carbonyl: a hydroxyl on the beta-carbon, with the carbonyl still present. It can also be run with acid catalysis through the enol.

Why does this matter? Because nature and the laboratory both build big carbon skeletons from small ones, and the aldol is the prime tool for stitching them at the alpha-beta junction. The simplest case is one aldehyde reacting with itself; the more powerful cases are the crossed aldol (two different partners) and the intramolecular aldol (which closes a ring). Often the freshly made beta-hydroxy carbonyl is then dehydrated to a conjugated enone — that follow-on step is the aldol condensation.