Cannizzaro Reaction

                              Cannizzaro Reaction

Cannizzaro Reaction – Mechanism, Examples, and Exceptions

The Cannizzaro Reaction is a fascinating transformation in organic chemistry that showcases a unique redox process involving aldehydes. Discovered by Stanislao Cannizzaro in 1853, this reaction remains a classic example of how subtle changes in molecular structure can dictate completely different chemical behaviors.

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What is the Cannizzaro Reaction?

The Cannizzaro reaction is a base-induced disproportionation of an aldehyde without α-hydrogen into a primary alcohol and a carboxylate salt.

Simply put:

One molecule gets oxidized to a carboxylate.

Another molecule gets reduced to a primary alcohol.

General Reaction:

2 RCHO \xrightarrow{\text{Conc. NaOH}} RCH_2OH + RCOONa

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Why Only Non-Enolizable Aldehydes?

For the Cannizzaro reaction to occur, the aldehyde must lack α-hydrogens (hydrogens attached to the carbon next to the carbonyl group).

If α-hydrogens are present, the aldehyde prefers the aldol condensation pathway instead, as enolate formation is easier under basic conditions.

Examples of suitable aldehydes:

Formaldehyde (HCHO)

Benzaldehyde (C₆H₅CHO)

p-Nitrobenzaldehyde

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Mechanism of the Cannizzaro Reaction

1. Nucleophilic attack

Hydroxide ion (OH⁻) attacks the carbonyl carbon, forming a tetrahedral alkoxide intermediate.

2. Hydride transfer

This intermediate donates a hydride ion (H⁻) to another molecule of aldehyde.

3. Product formation

The hydride donor becomes a carboxylate anion.

The hydride acceptor becomes a primary alcohol.

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Examples

Example 1 – Formaldehyde Reaction

2 HCHO + NaOH \longrightarrow CH_3OH + HCOONa

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Example 2 – Benzaldehyde Reaction

2 C_6H_5CHO + NaOH \longrightarrow C_6H_5CH_2OH + C_6H_5COONa

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Exceptional Cases

1. Aldehydes with α-Hydrogen

Prefer aldol condensation instead of Cannizzaro (e.g., acetaldehyde).

2. Cross Cannizzaro Reaction

Two different non-enolizable aldehydes react together—often, formaldehyde acts as the reducing agent due to its high reactivity.

3. Effect of Electron-Withdrawing Groups

Aldehydes with –NO₂, –CF₃, or similar groups react faster because these groups increase the electrophilicity of the carbonyl carbon.

4. Intramolecular Cannizzaro

In dialdehydes without α-hydrogen, both aldehyde groups can react internally to form hydroxy acids.

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Key Takeaways

Cannizzaro Reaction = Base + Non-enolizable aldehyde → Alcohol + Carboxylate salt.

Works only with aldehydes without α-hydrogen.

Can occur in intermolecular (between two molecules) or intramolecular (within the same molecule) forms.

Widely used in organic synthesis to convert aldehydes directly to alcohols and acids without external oxidizing or reducing agents.

💡 Did you know? The Cross Cannizzaro Reaction is often used in labs to prepare pure alcohols or acids when two different aldehydes are available—formaldehyde usually serves as the reducing partner.