Question

Explain the mechanism of Cannizzaro reaction.

Hint:

To remember which aldehydes undergo Cannizzaro: "No Alpha, No Problem." Aldehydes like Benzaldehyde (C₆H₅CHO) and Formaldehyde (HCHO) lack α-hydrogens and thus cannot undergo Aldol condensation, making them perfect candidates for the Cannizzaro reaction.

Answer 1

Concept: The Cannizzaro reaction is a redox reaction (disproportionation) where two molecules of an aldehyde having no α-hydrogen atoms (like benzaldehyde or formaldehyde) react in the presence of concentrated alkali. One molecule is reduced to an alcohol, while the other is oxidized to a carboxylic acid salt.
Mechanism Steps:
• Nucleophilic Attack: The hydroxide ion (OH^-) acts as a nucleophile and attacks the carbonyl carbon of the first aldehyde molecule to form a hydrate anion. • Hydride Transfer (Rate Determining Step): The hydrate anion transfers a hydride ion (H^-) to the carbonyl carbon of a second aldehyde molecule. In this step, the first molecule is oxidized to a carboxylic acid, and the second is reduced to an alkoxide ion. • Proton Exchange: A rapid proton transfer occurs between the carboxylic acid and the alkoxide ion to produce the final stable products: an alcohol and a carboxylate salt.