Cannizzaro Reaction (JEE/NEET Concepts)

Definition:
Cannizzaro reaction is a disproportionation reaction in which an aldehyde without α-hydrogen undergoes self oxidation–reduction in the presence of strong base (NaOH/KOH).

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General Reaction

$$2RCHO + OH^- \rightarrow RCH_2OH + RCOO^-$$

(One molecule is reduced → alcohol, other oxidized → carboxylate salt)

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• Key Conditions (VERY IMPORTANT for JEE)

• Aldehyde must have NO α-H (alpha hydrogen)
• Strong base: conc. NaOH or KOH
• Usually occurs with:
  • Formaldehyde (HCHO)
  • Benzaldehyde (C₆H₅CHO)

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Example

$$2C_6H_5CHO + NaOH \rightarrow C_6H_5CH_2OH + C_6H_5COONa$$

(Benzaldehyde → benzyl alcohol + sodium benzoate)

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Mechanism (Conceptual Steps)

1. Nucleophilic attack of OH⁻ on aldehyde → alkoxide intermediate
2. Hydride transfer (H⁻ shift) from one molecule to another
3. Formation of:
   • Alcohol (reduction)
   • Carboxylate ion (oxidation)

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Types of Cannizzaro Reaction

1. Self Cannizzaro
   Same aldehyde reacts
   Example: benzaldehyde
2. Cross Cannizzaro (Important!)
   Two different aldehydes
   • One should be formaldehyde (HCHO) (best reducing agent)

$$HCHO + C_6H_5CHO + NaOH \rightarrow C_6H_5CH_2OH + HCOONa$$

(Formaldehyde gets oxidized, other gets reduced)

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