Claisen condensation
The Claisen condensation or Claisen-Geuter condensation is a reaction from the field of organic chemistry . It was named after its discoverer, the German chemist Ludwig Claisen (1851–1930). The reaction is understood to be the base-mediated acylation of an ester with a second ester molecule to form a β- ketocarboxylic acid ester.
Overview reaction
The Claisen condensation is carried out here, for example, on the reaction of two molecules of ethyl acetate with ethanolate as the base.
The bond drawn in red stands for the newly formed CC single bond within the β-ketocarboxylic acid ester formed.
As bases , other sodium can alcoholates , sodium amide or sodium hydride are used.
Reaction mechanism
The following mechanism of Claisen condensation is explained using the example reaction of two molecules of ethyl acetate with ethanolate as the base.
First, an ethyl acetate ( 1 ) is deprotonated in the α- position to the carbonyl group by added sodium ethanolate . The enolate 2 is formed . This then reacts with another molecule of ethyl acetate via an intermediate stage to form the β-ketocarboxylic acid ester, in the example ethyl acetoacetate ( 3 ). The β-ketocarboxylic acid ester reacts with another alcoholate to form a mesomeric-stabilized anion 4 . After aqueous-acidic work-up, the desired β-ketocarboxylic acid ester 3 is ultimately obtained . The last step is the only step in the whole mechanism that is irreversible. Since ethanol is a stronger acid than the CH-acidic ester 1 , the equilibrium between 1 and 2 is on the side of 1 . In contrast, the β-ketocarboxylic acid ester 3 is a stronger acid than ethanol. Therefore, the equilibrium between 3 and 4 is on the side of the mesomeric-stabilized anion 4 and stoichiometric amounts of the base - i.e. the ethanolate - are required. As a result, the β-ketocarboxylic acid ester 3 is removed from the reaction mixture with formation of 4 in the last step, thus enabling the Claisen condensation. Protonation during work-up of 4 then yields the β-ketocarboxylic acid ester 3 .
The cyclic, intramolecular Claisen condensation is called the Dieckmann condensation .
Individual evidence
- ^ Siegfried Hauptmann : Organic chemistry . 2nd edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1985, ISBN 3-342-00280-8 , p. 454.
- ^ Hans Beyer , Wolfgang Walter : Organic chemistry . S. Hirzel Verlag, Stuttgart 1984, ISBN 3-7776-0406-2 , p. 279.
- ^ László Kürti , Barbara Czakó: Strategic Applications of Named Reactions in Organic Synthesis . Elsevier Academic Press, Burlington / San Diego / London 2005, ISBN 0-12-369483-3 .
- ^ Siegfried Hauptmann : Organic chemistry . 2nd edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig 1985, ISBN 3-342-00280-8 , p. 219.
