Consider the following statements about the reduction of aldehydes and ketones.
(A) Sodium borohydride reduces aldehydes to primary alcohols and ketones to secondary alcohols.
(B) In the Clemmensen reduction, the carbonyl group is reduced to a CH2 group by zinc amalgam and concentrated nitric acid.
(C) In the Wolff-Kishner reduction, the carbonyl compound is treated with hydrazine and then heated with potassium hydroxide in ethylene glycol.
(D) Ketones are reduced to secondary alcohols by catalytic hydrogenation.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (C) and (D) only
Aldehydes and ketones can be reduced in two ways: to alcohols (the >C=O group becomes >CH–OH) or to hydrocarbons (the >C=O group becomes >CH2). The reagent decides which product is formed.
(A) Sodium borohydride reduces aldehydes to primary alcohols and ketones to secondary alcohols. — True. For example:
\( \displaystyle \ce{CH3CHO + 2[H]} \xrightarrow{\ce{NaBH4}} \ce{CH3CH2OH} \)
\( \displaystyle \ce{CH3COCH2CH3 + 2[H]} \xrightarrow{\ce{NaBH4}} \ce{CH3CH(OH)CH2CH3} \)
(B) In the Clemmensen reduction, the carbonyl group is reduced to a CH2 group by zinc amalgam and concentrated nitric acid. — False. The Clemmensen reduction uses zinc amalgam and concentrated hydrochloric acid, not nitric acid:
\( \displaystyle \ce{CH3COCH2CH3 + 4[H]} \xrightarrow{\ce{Zn-Hg},\ \text{conc. }\ce{HCl}} \ce{CH3CH2CH2CH3 + H2O} \)
(C) In the Wolff-Kishner reduction, the carbonyl compound is treated with hydrazine and then heated with potassium hydroxide in ethylene glycol. — True. The hydrazone formed first is heated with KOH (or NaOH) in ethylene glycol, a high boiling solvent, and the >C=O group ends up as >CH2:
\( \displaystyle \ce{CH3COCH2CH3 + NH2NH2 -> (CH3)(C2H5)C=NNH2 + H2O} \)
\( \displaystyle \ce{(CH3)(C2H5)C=NNH2} \xrightarrow[\text{ethylene glycol, heat}]{\ce{KOH}} \ce{CH3CH2CH2CH3 + N2} \)
(D) Ketones are reduced to secondary alcohols by catalytic hydrogenation. — True. Hydrogen with a metal catalyst such as Pt, Pd or Ni gives the same alcohols as NaBH4 and LiAlH4:
\( \displaystyle \ce{CH3COCH2CH3 + H2} \xrightarrow{\ce{Pt}/\ce{Pd}/\ce{Ni}} \ce{CH3CH(OH)CH2CH3} \)
NCERT (2026-27 edition):
8.4 Chemical Reactions – “(i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation…”
“(ii) Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).”