Q.23 (a) Explain Cannizzaro’s reaction with the help of benzaldehyde.
(b) Write the reaction for the conversion of cyclohexene to adipic acid.
Best answer
Answer: (a) Benzaldehyde disproportionates to benzyl alcohol and sodium benzoate; (b) oxidation with hot KMnO4.
(a) Cannizzaro reaction: Aldehydes with no α-hydrogen, on heating with concentrated aqueous or alcoholic NaOH, undergo self oxidation-reduction (disproportionation): one molecule is oxidised to the carboxylate and another reduced to the alcohol.
\(\mathrm{2C_6H_5CHO + NaOH(conc.) \xrightarrow{\Delta} C_6H_5CH_2OH + C_6H_5COONa}\)
Acidifying sodium benzoate with dil. HCl gives benzoic acid.
(b) Cyclohexene is oxidised by hot alkaline KMnO4 followed by acidification (or by hot conc. HNO3/acidified KMnO4); the ring C=C bond is cleaved:
\(\mathrm{C_6H_{10} + 4[O] \xrightarrow[(ii)\ H_3O^+]{(i)\ KMnO_4,\ OH^-,\ \Delta} HOOC\text{-}(CH_2)_4\text{-}COOH}\) (adipic acid, hexanedioic acid)
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