Cannizzaro reaction 1734220 221668395 2008-06-25T15:20:02Z Chem-awb 4806773 [[Category:Name reactions]] using [[Project:AutoWikiBrowser|AWB]] 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]] [[de:Cannizzaro-Reaktion]] [[fr:Réaction de Cannizzaro]] [[it:Reazione di Cannizzaro]] [[nl:Cannizzaro-reactie]] [[ja:カニッツァーロ反応]] [[pl:Reakcja Cannizzaro]] [[ru:Реакция Канниццаро]] [[Category:Name reactions]]