Cannizzaro reaction
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The '''Cannizzaro reaction''', named after its discoverer [[Stanislao Cannizzaro]], is a [[chemical reaction]] that involves the [[base (chemistry)|base]]-induced [[disproportionation]] of an [[aldehyde]] lacking a hydrogen atom in the alpha position.<ref>{{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}}</ref><ref>{{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
}}</ref> Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and benzoic acid from the treatment of benzaldehyde with [[potash]] (potassium carbonate).
[[Image:Benzaldehyde Cannizzaro reaction.png|500px|center|The Cannizzaro reaction]]
The [[oxidation]] product is a [[carboxylic acid]] and the [[Redox|reduction]] product is an [[alcohol]]. For aldehydes with a hydrogen atom alpha to the carbonyl, i.e. RC''H''R'CHO, the preferred reaction is an [[aldol condensation]], originating from deprotonation of this hydrogen. Reviews have been published.<ref>Geissman, T. A. ''Org. React.'' '''1944''', ''2'', 94. (Review)</ref>
==Reaction mechanism==
[[image:Cannizzaro1.png|center|The mechanism of the Cannizzarro reaction]]
The first reaction step is [[nucleophilic addition]] of the base (for instance the [[hydroxy]] anion) to the carbonyl carbon of the [[aldehyde]]. The resulting alkoxide is deprotonated to give a di-[[anion]], known as the Cannizzaro intermediate. Formation of this intermediate requires a strongly basic environment.
[[image:Cannizzaro2.png|center|The mechanism of the Cannizzarro reaction]]
Both intermediates can react further with aldehyde to transfer a [[hydride]], "H"<sup>−</sup>. 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 H<sub>2</sub> by its reaction with water.
[[image:Cannizzaro3.png|center|The mechanism of the Cannizzarro reaction]]
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 undergo [[aldol condensation]]. Examples of aldehydes that can undergo a Cannizaro reaction include [[formaldehyde]] and [[aromatic]] aldehydes such as [[benzaldehyde]].
==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 to [[formic acid]] and the corresponding alcohol is obtained in a high yield although the [[atom economy]] is still low.
==References==
{{Reflist}}
==See also==
*[[Meerwein-Ponndorf-Verley reduction]]
*[[Oppenauer oxidation]]
*[[Tishchenko reaction]]
[[Category:organic redox reactions]]
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[[Category:Name reactions]]