The Cannizzaro and Crossed Cannizzaro reactions are significant in organic chemistry, involving the transformation of aldehydes without alpha-hydrogens under basic conditions.
These reactions yield alcohols and carboxylic acids or their salts.
Cannizzaro Reaction
The Cannizzaro reaction is a redox reaction where two molecules of a non-enolizable aldehyde are transformed into a primary alcohol and a carboxylic acid (or its corresponding salt if the reaction is carried out in the presence of a base) under basic conditions.
This reaction does not occur with aldehydes that have α-hydrogens because such compounds typically undergo aldol condensation under basic conditions.
Mechanism:
1.Nucleophilic Attack:
The reaction starts with the deprotonation of the aldehyde by a strong base, leading to the formation of an alkoxide ion.
This ion then acts as a nucleophile, attacking another molecule of the aldehyde at the carbonyl carbon.
2.Hydride Transfer:
A hydride ion (H-) is transferred from the alkoxide ion to the carbonyl carbon of the second aldehyde molecule.
This step is crucial and involves the simultaneous reduction of one aldehyde molecule to an alcohol and the oxidation of the other to a carboxylate anion.
3.Protonation:
f the reaction is carried out under strictly basic conditions, the process ends with the formation of a carboxylate salt and an alcohol.
However, if acidic workup is performed, the carboxylate anion is protonated to yield the corresponding carboxylic acid.
Crossed Cannizzaro Reaction
Overview:
A variation where two different aldehydes react under basic conditions.
One aldehyde is reduced to an alcohol, and the other is oxidized to a carboxylic acid or its salt.
Key Considerations:
Reactivity: A significant difference in reactivity between the two aldehydes is essential. The less reactive aldehyde is typically reduced, while the more reactive one is oxidized.
Product Formation: Selectivity is crucial to avoid a complex mixture of products. The more reactive aldehyde is preferentially oxidized.
Key Differences and Challenges
1.Selectivity:
The main challenge in the Crossed Cannizzaro reaction is achieving selectivity when two different aldehydes are present. Without a significant difference in reactivity, the reaction can lead to a complex mixture of products.
2.Application:
While the Cannizzaro reaction is straightforward when a single aldehyde is used, the Crossed Cannizzaro reaction offers a pathway for the synthesis of more complex molecules by judicious choice of reactant aldehydes.