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.
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