Basic aldol reaction

  When enolates are used for additions in this way, the reaction is a useful carbon-carbon bond-forming reaction known as the Aldol Reaction.  Here, an aldehyde (or ketone) forms its enolate, which then reacts with a second molecule of aldehyde (or ketone) to form a beta-hydroxy aldehyde (or ketone) by alpha C–H addition of one reactant molecule to the carbonyl group of a second reactant molecule. For this reaction to occur at least one of the reactants must have α hydrogens.

 

General Aldol reaction

If a base is added a low temperatures to an aldehyde or ketone that has an alpha hydrogen, an enolate is formed which immediately undergoes a nucleophilic addition across the C=O of another molecule of aldehyde or ketone.  As long as the reaction is kept cold, a beta-hydroxyaldehyde (often called an “aldol”) or a beta-hydroxyketone product can be isolated.  Since the only electrophile present is the aldehyde/ketone, a weaker base such as NaOH or NaOCH₃ can be used.

If the reaction is warmed, it can then lose a molecule of water to form an alkene-aldehyde or alkene-ketone, known as an alpha,beta-unsaturated aldehyde or ketone.  In this case the overall reaction is known as an aldol condensation.

Aldol condensation

An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone.

Aldol condensations are important in organic synthesis, because they provide a good way to form carbon–carbon bonds. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or “aldol” (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals.

The name aldol condensation is also commonly used, especially in biochemistry, to refer to just the first (addition) stage of the process—the aldol reaction itself—as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule.

The reaction between an aldehyde/ketone and an aromatic carbonyl compound lacking an alpha-hydrogen (cross aldol condensation) is called the Claisen-Schmidt condensation. This reaction is named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independently published on this topic in 1880 and 1881. An example is the synthesis of dibenzylideneacetone. Quantitative yields in Claisen-Schmidt reactions have been reported in the absence of solvent using sodium hydroxide as the base and plus benzaldehydes.