Cannizzaro reaction

The Cannizzaro reaction involves self oxidation-reduction between two molecules of aromatic or aliphatic aldehydes having no alpa-hydrogens...

The Cannizzaro reaction involves self oxidation-reduction between two molecules of aromatic or aliphatic aldehydes having no alpa-hydrogens when treated with a strong base. In. this reaction, one molecule of aldehyde reduces the other to a primary alcohol and is itself oxidised to, the corresponding carboxylic acid salt normally in equimolar amounts.
Cannizzaro reaction

Cannizzaro reaction of benzaldehyde formaldehyde trialkylacetaldehyde

Reaction Mechanism of Cannizzaro reaction:

The Cannizzaro reaction proceeds through hydride ion transfer mechanism ------
aldehydic hydrogen of one molecule of the aldehyde is transferred along with its bonded electron pair to the carbonyl carbon of the other one. The kinetic study of the reaction revealed that though the reaction in general follows third order kinetics (Rate [aldehyde]2[ OH]1, there are evidences of maintaining fourth order kinetics (Rate  [aldehyde] [ OH])') particularly at higher base concentrations.

Rapid addition of OH to one molecule of aldehyde results in the formation of a hydroxy alkoxide ion which like aluminium-isopropoxide acts as a hydride-ion donor to the second molecule of aldehyde.(In the final step of the reaction, the acid and the alkoxide ion exchange proton for reasons of stability.

In the presence of a very strong concentration of alkali, aldehyde first forms a doubly charged anion (I)
from which a hydride anion is transferred to the second molecule of the aldehyde to form acid and an alkoxide ion. Subsequently, the alkoxide ion acquires a proton from the solvent.

The reaction can also occur between two different aldehydes having no alpa-hydrogens when it is called crossed cannizzaro reaction.
cannizzaro reaction of benzaldehyde and formaldehyde
fig: cannizzaro reaction of benzaldehyde and formaldehyde
When formaldehyde undergoes crossed Cannizzaro reaction with other aldehydes without alpa-hydrogens, it is seen that formaldehyde is oxidized and the other is reduced. This is because the nucleophilic attack occurs more readily on formaldehyde than on other aldehydes.



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Advanced Chemistry: Cannizzaro reaction
Cannizzaro reaction
Advanced Chemistry
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