Cannizzaro Reaction
The Cannizzaro Reaction is a special type of organic reaction shown by aldehydes that do not have an α-hydrogen. These aldehydes react with strong alkali to produce two different products at the same time — one molecule gets oxidized to an acid and the other gets reduced to an alcohol. This “self-redox” process is what makes the reaction unique.
Where the Cannizzaro Reaction Occurs
- It works best with aromatic aldehydes (like benzaldehyde).
- Mostly seen in aldehydes without α-hydrogen.
- Some aldehydes with α-hydrogens may show the reaction under strong alkaline conditions, but it is uncommon.
Crossed Cannizzaro Reaction
This happens when two different aldehydes are used, as long as both lack α-hydrogens. The most important example is:
- Formaldehyde + another aldehyde (without α-hydrogen)
In this pair, formaldehyde gets oxidized and the other aldehyde gets reduced. This is because the nucleophile attacks formaldehyde more easily than larger aldehydes.
Mechanism of Cannizzaro Reaction
In simple terms, the reaction proceeds as follows:
- Hydroxide ion attacks one aldehyde molecule to form a negatively charged intermediate.
- This intermediate then transfers a hydride ion (H−) to another aldehyde molecule.
- One molecule becomes an acid, and the other becomes an alcohol.
- Finally, proton transfer makes both products stable.
Under very strong alkali, a double-charged species can form, which again donates a hydride ion to a second aldehyde molecule.
Evidence Supporting the Mechanism
The reaction mechanism is supported by experimental findings such as:
- Hydride transfer observation
- Formation of expected alcohol and acid products
- Behavior under strong alkali conditions
Applications of Cannizzaro Reaction
1. Crossed Cannizzaro Reaction
Aldehydes with α-hydrogens first undergo aldol condensation, then react with formaldehyde in a Cannizzaro step. The products may contain one or more –CH2OH groups.
One key industrial product from this reaction is Pentaerythritol, widely used in:
- Resins for surface coatings
- Explosives (PETN — pentaerythritol tetranitrate)
2. Intramolecular Cannizzaro Reaction
Dialdehydes and α-keto aldehydes can undergo this internal version of the reaction. They form valuable alcohol and acid combinations within the same molecule.
3. Synthesis of Alcohols and Acids
This reaction is used to prepare various primary alcohols and carboxylic acids depending on the aldehyde used.
The Cannizzaro Reaction is an important tool in organic chemistry because it allows aldehydes without α-hydrogen to undergo useful transformations in the presence of strong alkali.
Detailed Notes:
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