Cannizzaro Reaction

Cannizzaro Reaction

Aldehydes having no α hydrogen atom (i.e. nonionizable) undergoes disproportionation reaction in the presence of strong base. This reaction is called Cannizzaro Reaction. The Cannizzaro reaction takes place slowly when electron-donating groups are present and occurs at faster rates when electron withdrawing groups are present.
Aromatic aldehydes, heterocyclic aldehydes, formaldehydes etc does not have α hydrogen atom, so all these species undergoes self oxidation reduction reaction with strong base. This reaction can also occurs between two different aldehydes having no α hydrogen atom and is called crossed cannizzaro reaction.
When formaldehyde undergoes crossed cannizzaro reaction with other aldehydes without α hydrogen atom, formaldehyde is oxidized as nucleophile attack more readily on formaldehyde than other aldehydes and the other aldehyde gets reduced.

Mechanism of Cannizzaro Reaction

In the presence of strong base, aldehyde first formed doubly charged anion from which a hydride anion is transffered to the second molecule of the aldehyde to form acid and an alkoxide ion. Subsquently, the alkoxide ion aquire a proton from the solvent.

Examples of Cannizzaro Reaction

Why does 2-Methylpropanal (also called isobutylaldehyde or isobutyral) give Cannizzaro reaction?

Due to +I effect of two alkyl group, carbanion of this aldehyde is not stable, so does not undergo expected aldol condensation instead aldehyde prefer to undergo nucleophilic attack of OH^- ion at carbonyl carbon giving an intermediate which acts as a hydride donar to the second molecule of the aldehyde leading to the formation of disproportionation product.

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