In this tutorial, you will learn about the Cannizzaro reaction, including its mechanism, applications, and examples.
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Cannizzaro Reaction Overview
The Cannizzaro reaction is a redox reaction that involves the simultaneous oxidation and reduction of two molecules of aldehyde that lack alpha hydrogen into carboxylic acid and alcohol. Discovered in 1853 by Italian chemist Stanislao Cannizzaro, the reaction got first observed in benzaldehyde, which gave benzyl alcohol and potassium benzoate when treated with potash.
It is is an example of a disproportionation reaction as one molecule (aldehyde) undergoes simultaneous oxidation and reduction. It is a base-induced reaction where the hydroxide group from the base act as a strong nucleophile to initiate the reaction.
Both of the alcohol and acids produced have a great biological significance and can be used in the production of food preservatives, antiseptics and antibacterial drugs.
Cannizzaro Reaction Reactants
Two aldehydes molecules which lack active hydrogen at alpha carbon, also known as non-enolizable aldehydes, act as reactant molecules. Example: Formaldehyde, Benzaldehyde, Vanillin
High concentration of strong base like sodium hydroxide and potassium hydroxide is used. The base generates electron rich hydroxide ions which attacks the carbonyl group of aldehyde and promote a disproportionation reaction.
Cannizzaro Reaction Mechanism
1.Due to the electron-withdrawing nature of bonded oxygen atoms, the carbon in the carbonyl group possesses a partial positive charge and becomes an active site for nucleophilic attack by the hydroxide ion. The incoming nucleophile attacks the carbon in the carbonyl group and displaces the electron from the pi bond onto the oxygen atom, giving it a negative charge. This forms a tetrahedral intermediate, from which the hydroxide ion again abstracts hydrogen to form an anion with two negative charges known as a dianion.
2. The negatively charged oxygen on the dianion reforms the C to O double bond, and the removal of the hydride ion as a leaving group takes place. After this, the hydride ion attacks the carbonyl group of another molecule and converts the dianion to a carboxylate anion and the neutral aldehyde to an alkoxide anion. Thus formed alkoxide anion is protonated by water molecule to give alcohol.
This hydride transfer is a slow step and hence, the rate-determining step in Cannizzaro reaction.
Cross Cannizzaro Reaction
If an aldehyde without alpha hydrogen is reacted with formaldehyde in the presence of a strong base to form formic acid and alcohol, this reaction is known as the cross Cannizzaro reaction. Here, formaldehyde acts as a reductant and is mixed with another aldehyde molecule for the production of the required alcohol. In this reaction, the complete transformation of the two different reactants takes place to give the desired product.
Cannizzaro Reaction Application
- The combination of crossed Cannizzaro reaction and aldol condensation leads to the production of Polyols
- A two stepped base catalyzed reaction involving aldol condensation and Cannizzaro reaction forms trimethylolpropane. Trimethylolpropane is a precursor to alkyd resin and utilized during the manufacture glues, dyes, silicone products, and synthetic lubricants
- The benzyl alcohol generated by the following process shows anesthetic and antipruritic properties. Due to the antibacterial and antifungal nature of benzyl alcohol, it also acts as a preservative. Similarly, benzoic acid is also an important antimicrobial preservative in the food and beverages industry.