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Aldol Condensation

Description: The aldol condensation is a reaction between an enolate (or enol) and an aldehyde or ketone that leads to the formation of a new carbon-carbon double bond. This is the product if an aldol addition reaction is heated for prolonged periods. The reaction is called a “condensation” because one molecule of water is formed from the two reactants.
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Notes: Although base is shown here, the reaction can also be catalyzed by acid.

Examples:

Notes: Example 3 shows an instance where acid is used as the catalyst. Examples 2 and 3 show “crossed” aldol reactions – that is, a reaction between two different aldehydes or ketones. In these cases only one of the two aldehydes has a carbon adjacent to the carbonyl (“alpha-carbon”) that contains a C-H bond; therefore only one enolate can be formed.
Elimination reactions are favored by heat; that is what distinguishes  this from the aldol addition reaction.
Example 4 shows an intramolecular aldol condensation.

Mechanism: Deprotonation of the aldehyde alpha carbon (Step 1, arrows A and B) leads to formation of an enolate, which attacks the carbonyl carbon in an addition reaction (Step 2, arrows C and D). Protonation of the oxygen (Step 3, arrows E and F) followed by subsequent deprotonation of the carbon adjacent to the carbonyl (Step 4, arrows G and H) leads to an enolate, which then eliminates hydroxide ion (HO-) forming an alkene.

Notes: The final elimination step can also be drawn this way (using the resonance form of the enolate):

For the mechanism of the acid-catalyzed version, see this post.

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