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Cannizzaro Reaction and Crossed Cannizzaro Reaction

An aldehyde is disproportionately reacted with a base and a carboxylic acid to yield carboxylic acid and a primary alcohol in the Cannizzaro reaction.

Cannizzaro reaction

An aldehyde is disproportionately reacted with a base and a carboxylic acid to yield a carboxylic acid and a primary alcohol in the Cannizzaro reaction. From two molecules of an aldehyde, one molecule of alcohol is obtained, and from one molecule of alcohol, one molecule of carboxylic acid is the result. Cannizzaro was the first to isolate and get potassium benzoate and benzyl alcohol from benzaldehyde in 1853. The reaction is carried out by the leaving group attacking another aldehyde in a nucleophilic acyl substitution. The formation of a tetrahedral intermediate occurs when hydroxide attacks carbonyls. By collapsing this tetrahedral intermediate, it reforms the carbonyl and transfers a hydride which attacks another colony. As a result, an acid ion and an alkoxide ion exchange proton. An anion with a charge of 2 forms when an aldehyde is introduced to a base of high concentration. This leads to the formation of carboxylate and alkoxide ions from a hydride ion on a second molecule of aldehyde. In addition, alkoxide ions receive proton ions from the reaction solvent.


Mechanism

Step 1

The carbonyl group of a given aldehyde is attacked by a nucleophile like a hydroxide ion, resulting in a disproportionation reaction and producing an anion with two negative charges.

Step 2

It is now possible to use the resulting intermediate as a hydride reducer. The intermediate is unstable, and it releases an anion of hydride. Another aldehyde molecule is attacked by this hydride anion. An aldehyde is converted into an alkoxide anion while the doubly charged anion becomes a carboxylate anion.


Step 3

A proton is given to the alkoxide anion by water in this final step, resulting in the final alcohol product. Due to the alkoxide's greater basicity than water, the reaction can proceed. The carboxylate ion is transformed into carboxylic acid by acid workup (the carboxylate ion is less basic than water, so it cannot obtain a proton from water).



Crossed Cannizzaro reaction

Combining formaldehyde and another aldehyde that does not have an α-hydrogen atom is an example of condensation. Alkali leads to carboxylic acid oxidation and alcohol reduction of formaldehyde. In essence, this reaction can be compared to that of Cannizzaro.

For example, when benzaldehyde and formaldehyde are treated with conc. NaOH, benzyl alcohol and sodium formate are formed.



Mechanism

An example of cross cannizzaro reaction would involve benzaldehyde and formaldehyde:

Step 1

The anion of formaldehyde is formed by the nucleophilic attack of the hydroxide ion on its carbonyl group in the first step.



Step 2

This step involves the transfer of an ionic hydride to the electron-deficient carbonyl carbon of benzaldehyde molecules, yielding carboxylic acid and an alkoxide ion.



Step 3

At this step, a proton exchange occurs between the carboxylic acid and the alkoxide ion, resulting in sodium formate (the carboxylic acid salt) and alcohol (the benzyl alcohol).



Advantages

Formaldehyde is used in this reaction to treat an aldehyde. The oxidation of formaldehyde to sodium formate and the reduction of another aldehyde to alcohol results in the formation of sodium formate. Using both aldehydes result in complete conversion to the desired chemical products, so no valuable chemicals are wasted.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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