Aldol condensation

The aldol condensation is a condensation reaction in the field of organic chemistry . During the reaction, aldehydes and ketones form α, β- unsaturated carbonyl compounds , more precisely α, β-unsaturated aldehydes or ketones, with elimination of water .

Overview reaction

The aldol condensation can take place with either acid or base catalysis. The aldol condensation is explained here using the reaction of two acetaldehyde molecules as an example .

The double bond marked in green shows the newly made double bond that is formed during the aldol condensation.

The reaction produces an α, β-unsaturated aldehyde with elimination of water .

mechanism

The base- and acid-catalyzed mechanisms of aldol condensation are explained using the example of the reaction of two acetaldehydes.

Base-catalyzed aldol condensation

In the following, the mechanism of the base-catalyzed aldol condensation is presented using the above example:

Mechanism of the base-catalyzed condensation.

In aldol condensation, as in aldol addition, 3-hydroxybutanal ( 2 ), i.e. an aldol , is initially formed from acetaldehyde ( 1 ) via a few intermediate stages . This is followed by the deprotonation of an acidic hydrogen atom by a base. A C = C double bond is formed and a hydroxide ion is released as a leaving group . The result is trans -2-butenal ( 3 ).

Acid-catalyzed aldol condensation

In the following, the mechanism of the acid-catalyzed aldol condensation is also presented using the example above:

Mechanism of acid-catalyzed condensation.

Here, too, aldol 2 is initially formed from acetaldehyde ( 1 ) via intermediate stages . In an acidic environment, this tautomerizes to cis -but-1-ene-1,3-diol ( 4 ). This is protonated to the oxonium ion 5 and water is split off by rearrangement of electron pairs . Finally, a proton is split off from the oxonium ion 6 and crotonaldehyde is formed ( 7 ).

literature

KPC Vollhardt , NE Schore: Organic Chemistry . 4th edition, Wiley-VCH, Weinheim 2005 , ISBN 978-3-527-31380-8 .
J. Buddrus, B. Schmidt: Fundamentals of organic chemistry . 4th revised and updated edition, Walter de Gruyter, Berlin / New York 2011 , ISBN 978-3-11-024894-4 .

Individual evidence

^ ^A ^b J. Buddrus, B. Schmidt: Fundamentals of organic chemistry . 4th revised and updated edition, Walter de Gruyter, Berlin / New York 2011 , ISBN 978-3-11-024894-4 , p. 616 ff.

Commons : Aldol condensation - collection of images, videos and audio files

[Buddrus_616_ff-1] A ^b J. Buddrus, B. Schmidt: Fundamentals of organic chemistry . 4th revised and updated edition, Walter de Gruyter, Berlin / New York 2011 , ISBN 978-3-11-024894-4 , p. 616 ff.