Cannizzaro reaction & Crossed Cannizzaro reaction

• The Cannizzaro and Crossed Cannizzaro reaction 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:

   • If 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:

1. 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.
2. 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.

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