Aldol addition

The aldol addition is an addition reaction from the field of organic chemistry . In the aldol addition, aldehydes and ketones react to form β-hydroxy aldehydes ( aldols ) or β-hydroxy ketones.

Overview reaction

The aldol addition is carried out here, for example, on the reaction of two molecules of acetaldehyde . The reaction is either base or acid catalyzed:

The bond marked in green is the newly established bond between the two aldehydes. This addition reaction produces 3-hydroxybutanal , which is one of the aldols.

mechanism

The base- and acid-catalyzed mechanisms of aldol addition are explained using the example reaction of two molecules of acetaldehyde.

Base-catalyzed aldol addition

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

The hydrogen atom in the α position to the carbonyl group of acetaldehyde is acidic due to the carbonyl compound ; This means that a hydrogen atom can be split off by a strong base at the α-position of the carbon atom. In this example, the acetaldehyde ( 1 ) is deprotonated by a hydroxide ion . A mesomeric-stabilized carbanion / enolate 2 is formed . The nucleophilic enolate 2 attacks the carbonyl group of another acetaldehyde. An alcoholate 3 forms , which is protonated . This forms an aldol 4 , in this case 3-hydroxybutanal .

Acid-catalyzed aldol addition

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

In the acid-catalyzed aldol addition, the oxygen atom of the carbonyl group of acetaldehyde ( 1 ) is protonated. In the presence of an acid, an enol is formed from the acetaldehyde in the course of a keto-enol tautomerism . This now attacks the protonated carbonyl group 5 nucleophilically. A protonated aldol 6 is formed . Deprotonation also produces 3-hydroxybutanal ( 4 ).

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.

[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.