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= Cannizarro reaction = When reacted with concentrated NaOH (50 wt%) or other strong bases (e.g., alkoxides), aliphatic and aromatic aldehydes with no α-hydrogen undergo an intermolecular hydride-transfer reaction known as the Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro. Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium hydroxide). More typically, the reaction would be conducted with sodium or potassium hydroxide:

2 C6H5CHO + KOH → C6H5CH2OH + C6H5COOK

In this disproportionation reaction, one molecule of aldehyde oxidizes another to the corresponding carboxylic acid and is reduced to the corresponding primary alcohol in a maximum 50% yield. This reaction can be also classified as an organic redox reaction.

Mechanism
There are multiple proposed mechanisms for this reaction, but the generally accepted mechanism of the Cannizzaro reaction involves a hydride transfer. First, OH- adds across the carbonyl group, and the resulting species is deprotonated under the applied basic conditions to give the corresponding dianion. This dianion facilitates the ability of the aldehydic hydrogen to leave as a hydride ion. This leaving hydride ion attacks another aldehyde molecule in the rate-determining step (RDS) to afford the alkoxide of a primary alcohol, which gets protonated by the solvent (H2O). By running the reaction in the presence of D2O, it was shown that the reducing hydride ion came from another aldehyde and not the reaction medium, since the resulting primary alcohol did not contain a deuterium. Ashby and co-workers using resolved ESR spectra demonstrated that substituted benzaldehyde radical anions were formed in the reactions of substituted benzaldehydes with either NaOH or KOt-Bu. This observation suggested that the reaction proceeded by a single-electron transfer (SET) mechanism.

Scope
If the aldehyde has alpha hydrogens, the aldol reaction will take place faster than the Cannizzaro reaction, which causes low yield. Alternatively, high yields of alcohol can be obtained from almost any aldehyde when the reaction is performed in the presence of an excess of formaldehyde. This process is called the crossed Cannizzaro reaction. a-Keto aldehydes undergo an intramolecular Cannizzaro reaction. This method, however, has been rendered obsolete by the emergence of hydride reducing agents in 1946. In the presence of an appropriate Lewis acid catalyst, the intramolecular Cannizzaro reaction takes place with stereocontrol, yielding synthetically useful a-hydroxy esters directly from readily available glyoxals under neutral conditions. It has also been shown that the reaction rates are enhanced significantly when the Cannizzaro reaction is performed under solvent-free conditions.

Synthetic Applications
The Cannizarro reaction is used to synthesize crosslinked polymers in industry. Many linear polymers are too flexible to be of use in making everyday objects because they lack the strength, the rigidity, or the elasticity for the job. Linear polymers can be stiffened and strengthened by bonds between the chains. At this point, the cannizzarro reaction is used. Pentaerythritol is a useful industrial product in, for example, the cross- linking of polymers. This four-armed cross-linking agent known is made from acetaldehyde and formaldehyde in aqueous base by a Cannizarro reaction.

Variations
Nearly thirty years later, W.E. Tishchenko found that both enolizable and non-enolizable aldehydes can be converted to the corresponding esters in the presence of magnesium- or aluminum alkoxides. The reaction involves a hydride shift from one aldehyde to another that leads to the formation of the ester product. This transformation is known today as the Tishchenko reaction. Compared to the cannizarro reaction the base used is an alkoxide rather than hydroxide and the product is an ester rather than alcohol and carboxylate groups. Similiar to the cannizarro reaction there are also crossed Tishchenko reaction in intramolecular version. The most general catalysts in the traditional Tishchenko reaction are aluminum alkoxides, but a wide-variety of catalysts can be used such as alkali- and alkali earth metal oxides and alkoxides. One other side reaction for Tishchenko reaction rather than Cannizarro reaction is the aldol reaction but this can be turned to an advantage to synthesize monoesters of 1,3-diols. The starting aldehyde first undergoes an aldol reaction followed by the Tishchenko reaction to give the desired 1,3-diols.