Aldol reaction
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The aldol reaction is an important organic reaction involving the reaction of enolate ions with a carbonyl group. Its product is an aldol (aldehyde + alcohol), specifically a β-hydroxyaldehyde or β-hydroxyketone. Indeed, the 1,3 relationship between the alcohol and carbonyl groups is characteristic of this reaction. It may be catalyzed by either base or acid. While the former is more common, both are illustrated below for ethanal:
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Mechanism
Base catalyzed
Basealdol.png
Acid catalyzed
Acidaldol.png
This reaction proceeds by specific acid catalysis. β-hydroxyaldehyde and β-hydroxyketone also often undergo dehydration following the aldol reaction. The reaction is then called a aldol condensation. This is especially common with the acid-catalyzed mechanism, as the conditions for the aldol reaction are also conducive to dehydration.
Crossed aldol reactions
Aldol reactions between two different aldehydes or ketones are generally not successful, as they tend to produce a mixture of different products. However, it is possible to perform crossed aldol reactions in two ways: when one reactant has no alpha hydrogen, or when one of the reactants is treated with LDA. in order to turn it into an enolate ion beforehand.
The first method depends on the fact that aldehydes without α-hydrogens (e.g., benzaldehyde, formaldehyde, and furfural) cannot form enolate ions, and so must act as the enolate ion acceptor. Given that such aldehydes are not very common, aldol condensations are not terribly useful, except to form the otherwise difficult-to-create five- or six-membered ring.
The second method involves treating one of the reactants with a strong base, typically LDA. This transforms it irreversibly into its enolate ion, allowing crossed aldol reactions between two reactants which both have α-hydrogens (e.g. 3-pentanone and phenylacetaldehyde). However, due to the practical difficulties of using LDA in the lab (the reaction must take place at 200 K), this method is avoided when possible.
Intramolecular aldol reactions
Intramolecular aldol reactions can be used to form five- or six-membered rings. For instance, the carbonyl groups of 2,7-Octanedione can react with each other to form five- or seven-membered rings. The five-membered ring is the major product, given that smaller-membered rings are favored, save where angle strain precludes their formation.de:Aldol-Reaktion pl:Kondensacja aldolowa