\(CH_3CH_2COCH_3\overset{Y}{\longrightarrow} CH_3CH_2CH_2CH_3\)
The reagent Y is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(NH_2NH_2, KOH,\, glycol\)
The change is butan-2-one, \( \ce{CH3CH2COCH3} \), to butane: the C=O group is reduced right down to CH2. Two reactions do this: Clemmensen reduction (Zn–Hg and conc. HCl) and Wolff-Kishner reduction (hydrazine, then KOH heated in ethylene glycol). The reagent given is the Wolff-Kishner one.
The ketone first forms a hydrazone with hydrazine; heating the hydrazone with KOH in high-boiling ethylene glycol releases nitrogen and leaves the \( \ce{CH2} \) group:
\( \displaystyle \ce{CH3CH2COCH3} \xrightarrow[\ce{-H2O}]{\ce{NH2NH2}} \ce{CH3CH2C(=NNH2)CH3} \xrightarrow[\text{ethylene glycol, heat}]{\ce{KOH}} \ce{CH3CH2CH2CH3 + N2} \)
Why the other options are incorrect
Option (2) \(LiAlH_4/ether\) — Incorrect. Lithium aluminium hydride reduces a ketone only to the secondary alcohol, butan-2-ol (\( \ce{CH3CH2CH(OH)CH3} \)), not to the alkane.
Option (3) \(NaBH_4\) — Incorrect. Sodium borohydride is a milder hydride reagent; it also gives butan-2-ol.
Option (4) \(H_2/Ni\) — Incorrect. Catalytic hydrogenation adds \( \ce{H2} \) across C=O and gives butan-2-ol; it does not remove the oxygen.
NCERT (2026-27 edition):
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).”