August 30, 2025 | UR Gate
Cannizzaro Reaction: Theory, Mechanism, Procedure, and Applications
Cannizzaro reaction explained with mechanism, examples, and lab procedure. A
classic redox process converting aldehydes into alcohols and acids.
1. Introduction
The Cannizzaro reaction is a well-known redox process in organic chemistry
in which aromatic aldehydes or aldehydes lacking an α-hydrogen undergo
simultaneous oxidation and reduction in the presence of a strong base. The
reaction converts aldehydes into a mixture of alcohols and carboxylic acids.
For example, benzaldehyde reacts with sodium hydroxide to give benzyl
alcohol and sodium benzoate.
2. Theoretical Background
- Aldehydes that lack α-hydrogens (such as formaldehyde, benzaldehyde, and trimethylacetaldehyde) are suitable for the Cannizzaro reaction.
- In the presence of a strong base (NaOH or KOH), the aldehyde undergoes a disproportionation reaction: one molecule is reduced to an alcohol, while another is oxidized to the corresponding carboxylate salt.
- When a mixture of formaldehyde and another aldehyde is used, the process is called the Crossed Cannizzaro Reaction, where formaldehyde preferentially oxidizes to formate while the aromatic aldehyde is reduced to its alcohol.
3. Reaction Mechanism
The mechanism of the Cannizzaro reaction involves the transfer of a
hydride ion (H⁻) rather than a proton (H⁺).
- A hydroxide ion attacks the carbonyl carbon of the aldehyde, forming a tetrahedral alkoxide intermediate.
- This intermediate transfers a hydride ion to another aldehyde molecule.
- The hydride transfer results in the formation of a carboxylate ion (oxidized product) and an alkoxide ion (reduced product).
- The alkoxide ion is protonated to yield the corresponding alcohol.
4. Important Notes
- Aldehyde Requirement: Only aldehydes without α-hydrogens undergo the Cannizzaro reaction. Aldehydes with α-hydrogens typically undergo aldol condensation instead.
- Hydride Transfer: Experiments with deuterium oxide (D₂O) confirm that the mechanism involves hydride transfer, not proton transfer.
- Crossed Cannizzaro Reaction: This is especially useful when mixing an aromatic aldehyde with formaldehyde, leading to selective reduction of the aromatic aldehyde.
5. Experimental Work
Instruments
- Reagent bottle
- Beaker
- Separatory funnel
- Water bath
- Filtration apparatus
Chemicals
- Sodium hydroxide (NaOH)
- Benzaldehyde
- Ether
- Sodium bisulfite
- Sodium carbonate
- Anhydrous sodium sulfate
- Concentrated hydrochloric acid
- Ice
Procedure
- Dissolve 1.5 g NaOH in 2 mL water, cool, and transfer into a reagent bottle. Add 1.5 mL benzaldehyde with vigorous shaking, then allow the reaction to stand for a few days.
- Add 4 mL water to dissolve the sodium benzoate formed, and transfer to a separatory funnel. Extract with 1.5 mL ether.
- The aqueous layer contains sodium benzoate.
- The organic layer (ether) contains benzyl alcohol.
- Wash the organic layer with sodium bisulfite solution to remove unreacted benzaldehyde.
- Wash again with 10% sodium carbonate solution, then with water. Dry the organic layer with anhydrous sodium sulfate. Evaporate the ether using a water bath at 50 °C, then distill benzyl alcohol (bp 200–206 °C).
- Acidify the aqueous layer with HCl (20 mL) in the presence of ice (4 g) to precipitate benzoic acid. Filter under vacuum and recrystallize from hot water.
6. Discussion
- The reaction illustrates how aldehydes without α-hydrogens behave differently under basic conditions compared to aldehydes with α-hydrogens (which undergo aldol condensation).
- The simultaneous oxidation and reduction make the Cannizzaro reaction a classic example of disproportionation.
- The Crossed Cannizzaro is particularly important for producing alcohols in higher yield, since formaldehyde is preferentially oxidized.
7. Applications
- Laboratory preparation of benzyl alcohol and benzoic acid.
- Useful for studying hydride transfer mechanisms in organic chemistry.
- A fundamental example in teaching redox processes and base-catalyzed organic reactions.
8. Conclusion and Result
The Cannizzaro reaction highlights the importance of structural factors (absence of α-hydrogen) in determining the reactivity of aldehydes. It remains a valuable tool in both academic and industrial organic chemistry.
1- Morrison, R. T., & Boyd, R. N. Organic Chemistry.
2- Solomons, T. W. G., Fryhle, C. B. Organic Chemistry.
3- Carey, F. A., & Sundberg, R. J. Advanced Organic Chemistry.
