Explain Cannizzaro reaction with suitable example.

Answer: Aldehydes that have no α-hydrogen atom, when heated with concentrated aqueous (or alcoholic) NaOH or KOH, undergo self oxidation–reduction (disproportionation): one molecule is reduced to the alcohol and another is oxidised to the salt of the carboxylic acid. This is the Cannizzaro reaction.

Example 1 (formaldehyde):

2HCHO + conc. NaOH → CH₃OH (methanol) + HCOONa (sodium formate)

Example 2 (benzaldehyde):

2C₆H₅CHO + conc. NaOH → C₆H₅CH₂OH (benzyl alcohol) + C₆H₅COONa (sodium benzoate)

Aldehydes with α-hydrogen (e.g. acetaldehyde) do not give this reaction; they undergo aldol condensation instead.