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ChemistryMediumClass 12
Cannizzaro Reaction — Non-enolizable aldehydes
Aldehydes, Ketones & Carboxylic Acids
17
JEE Qs
8%
Hard
50
min
Always check for the absence of alpha-hydrogens on the aldehyde and the presence of concentrated strong base to identify a Cannizzaro reaction and predict the correct redox products.
🧮 Key Formulas
2 RCHO + Conc. NaOH/KOH --> RCH2OH + RCOONa (where R is a group such that there are no alpha-hydrogens)
HCHO + RCHO + Conc. NaOH/KOH --> CH3OH + RCOONa (Cross-Cannizzaro, formaldehyde is preferentially oxidized)
✅ Key Points for JEE
- 1The Cannizzaro reaction is specific to aldehydes that lack alpha-hydrogens (non-enolizable) and requires a strong concentrated base (e.g., 50% NaOH or KOH).
- 2It is a disproportionation (redox) reaction where one molecule of the aldehyde is oxidized to a carboxylic acid salt, and another molecule is reduced to an alcohol.
- 3The mechanism involves nucleophilic attack of hydroxide on the carbonyl carbon, followed by a rate-determining hydride transfer to a second aldehyde molecule.
- 4In a cross-Cannizzaro reaction between two different non-enolizable aldehydes, the more reactive aldehyde (typically formaldehyde) is preferentially oxidized to the carboxylic acid salt (formate), while the other aldehyde is reduced to its corresponding alcohol.
- 5Intramolecular Cannizzaro reaction can occur if a single molecule possesses two non-enolizable aldehyde groups (e.g., glyoxal forming glycolic acid salt).
⚠️ Common Mistakes
- ✕Confusing Cannizzaro reaction with Aldol condensation; Aldol requires alpha-hydrogens for enolate formation, while Cannizzaro explicitly requires NO alpha-hydrogens.
- ✕Attempting the reaction with ketones or aldehydes possessing alpha-hydrogens.
- ✕Incorrectly identifying which aldehyde is oxidized and which is reduced, especially in cross-Cannizzaro reactions (e.g., assuming formaldehyde is reduced).
- ✕Not recognizing the requirement for a strong, concentrated base and often forgetting the role of the second aldehyde molecule.
NCERT Chapters
- Class 12 Chemistry Part II Ch 12: Aldehydes, Ketones and Carboxylic Acids