- Cannizzaro reaction
The Cannizzaro reaction, named after its discoverer
Stanislao Cannizzaro , is achemical reaction that involves the base-induceddisproportionation of analdehyde lacking a hydrogen atom in the alpha position. [cite journal
author = Cannizzaro, S.
title = Ueber den der Benzoësäure entsprechenden Alkohol
journal =Liebigs Annalen
year = 1853
volume = 88
pages = 129–130
doi = 10.1002/jlac.18530880114] [cite journal
author = List, K.; Limpricht, H.
title = Ueber das sogenannte Benzoëoxyd und einige andere gepaarte Verbindungen
journal =Liebigs Annalen
year = 1854
volume = 90
pages = 190–210
doi = 10.1002/jlac.18540900211] Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and benzoic acid from the treatment of benzaldehyde withpotash (potassium carbonate).The
oxidation product is acarboxylic acid and the reduction product is analcohol . For aldehydes with a hydrogen atom alpha to the carbonyl, i.e. RC"H"R'CHO, the preferred reaction is analdol condensation , originating from deprotonation of this hydrogen. Reviews have been published. [Geissman, T. A. "Org. React." 1944, "2", 94. (Review)]Reaction mechanism
The first reaction step is
nucleophilic addition of the base (for instance thehydroxy anion) to the carbonyl carbon of thealdehyde . The resulting alkoxide is deprotonated to give a di-anion , known as the Cannizzaro intermediate. Formation of this intermediate requires a strongly basic environment.Both intermediates can react further with aldehyde to transfer a
hydride , "H"−. The hydridic character of the C-"H" is enhanced by the electron-donating character of the alpha oxygen anion. This hydride transfer simultaneously generates a hydroxyl anion and a carboxylate. Further evidence for the hydridic character of the Cannizzaro intermediate is provided by the formation of H2 by its reaction with water.Only aldehydes that cannot form an
enol ate ion undergo the Cannizaro reaction. The aldehyde cannot have an enolizable proton. Under the basic conditions that facilitate the reaction, aldehydes that can form an enolate instead undergoaldol condensation . Examples of aldehydes that can undergo a Cannizaro reaction includeformaldehyde andaromatic aldehydes such asbenzaldehyde .Variations
A special condition is the crossed Cannizzaro reaction. This variation is more common these days because the original Cannizzaro reaction yields a mixture of alcohol and carboxylic acid. For example any aldehyde with no alpha hydrogens can be reduced when in the presence of
formaldehyde . Formaldehyde is oxidized toformic acid and the corresponding alcohol is obtained in a high yield although theatom economy is still low.References
ee also
*
Meerwein-Ponndorf-Verley reduction
*Oppenauer oxidation
*Tishchenko reaction
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