Cannizzaro reaction
The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde.[1][2] Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide:
- 2 C6H5CHO + KOH → C6H5CH2OH + C6H5CO2K
The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol.[3] For aldehydes with a hydrogen atom alpha to the carbonyl, i.e. R2CHCHO, the preferred reaction is an aldol condensation, originating from deprotonation of this hydrogen. This reaction restricts the scope of the Cannizzaro reaction.
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[edit] Mechanism
The reaction begins with nucleophilic attack of hydroxide on the carbonyl centre. The resulting anion attacks another molecule of aldehyde, transferring hydride.[4] Overall, the reaction follows third-order kinetics. It is second order in aldehyde and first order in base.
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- rate = k[RCHO]2[OH-]
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At very high base a second path (k') becomes important that is second order in base:
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- rate = k[RCHO]2[OH-] + k'[RCHO]2[OH-]2
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[edit] Scope
The reaction is limited to aldehydes lacking alpha hydrogen centers. Under ideal conditions the reaction produces only 50% of the alcohol and the carboxylic acid. The latter is obtained only after acidification of the highly basic reaction mixture, typically 30% base.[5] To avoid the low yields, it is more common to conduct the crossed Cannizzaro reaction with a sacrificial aldehyde. In this variation, the reductant is formaldehyde, which is oxidized to sodium formate and the corresponding alcohol is obtained in a high yield, although the atom economy is still low.
A solvent-free reaction has been reported involving grinding liquid 2-chlorobenzaldehyde with potassium hydroxide in a mortar and pestle:[6] 
[edit] References
- ^ Cannizzaro, S. (1853). "Ueber den der Benzoësäure entsprechenden Alkohol". Liebigs Annalen 88: 129–130. doi:10.1002/jlac.18530880114.
- ^ List, K.; Limpricht, H. (1854). "Ueber das sogenannte Benzoëoxyd und einige andere gepaarte Verbindungen". Liebigs Annalen 90 (2): 190–210. doi:10.1002/jlac.18540900211.
- ^ Geissman, T. A. "The Cannizzaro Reaction" Org. React. 1944, 2, 94. doi:10.1002/0471264180.or002.03(Review)
- ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7, http://books.google.com/books?id=JDR-nZpojeEC&printsec=frontcover
- ^ W. C. Wilson (1941), "2-Furancarboxylic Acid and 2-Furylcarbinol", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV1P0276; Coll. Vol. 1: 276
- ^ A Facile Solvent-Free Cannizzaro Reaction Phonchaiya, Sonthi; Panijpan, Bhinyo Rajviroongit, Shuleewan; Wright, Tony; Blanchfield, Joanne T. "A Facile Solvent-Free Cannizzaro Reaction" J. Chem. Educ. 2009, volume 86, page 85. doi:10.1021/ed086p85
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