Answer the questions on the basis of passage given below:
As aldehydes & ketones possess the carbonyl functional group, they show similar reactions. They exhibit nucleophilic addition reactions, reduction, oxidations, reactions due to α-hydrogen atom, and many other reactions, like Cannizzaro reaction to electrophilic substitution reactions (in case of aromatic aldehydes & ketones). These reactions help in the preparation of a number of important compounds, like acetic acid, vinyl acetate, polymers and in perfume and dye industries. Nucleophilic addition reactions form cyanohydrins, acetals, alcohols and N-substituted derivatives like imine, oxime, semicarbazone, phenylhydrazone & Schiff’s base. Reactions due to α-hydrogen atoms help in formation of aldols and α,β-unsaturated aldehydes & ketones.
Complete the following equation:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Butanoic acid, CH3CH2CH2COOH
There are two named ways to reduce the C=O of an aldehyde or ketone all the way to \( \ce{CH2} \): the Clemmensen reduction (zinc amalgam and concentrated HCl, acid medium) and the Wolff-Kishner reduction (hydrazine, then KOH in ethylene glycol on heating, basic medium). Neither reduces the C=O of a carboxylic acid.
The reagent here is Zn-Hg/HCl, so this is the Clemmensen reduction. The starting compound is 3-oxobutanoic acid, \( \ce{CH3COCH2COOH} \); only its ketone C=O is reduced:
\( \displaystyle \ce{CH3COCH2COOH + 4[H]} \xrightarrow{\ce{Zn-Hg},\ \text{conc. }\ce{HCl}} \ce{CH3CH2CH2COOH + H2O} \)
For comparison, the Wolff-Kishner route to the same kind of change:
\( \displaystyle \ce{R2C=O} \xrightarrow[-\ce{H2O}]{\ce{NH2NH2}} \ce{R2C=NNH2} \xrightarrow[\text{ethylene glycol, heat}]{\ce{KOH}} \ce{R2CH2 + N2} \)
So the product is butanoic acid.
Why the other options are incorrect
Option (1) 4-hydroxybutan-2-one — Incorrect. It keeps the ketone and turns –COOH into –\( \ce{CH2OH} \); Clemmensen does the opposite (reduces the ketone, leaves the acid).
Option (2) 3-hydroxybutanoic acid — Incorrect. The ketone is reduced only as far as an alcohol (CH–OH); this needs \( \ce{NaBH4} \) or \( \ce{H2} \)/catalyst, while Zn-Hg/HCl takes C=O all the way to \( \ce{CH2} \).
Option (4) butan-1-ol — Incorrect. The –COOH group would have to be reduced to –\( \ce{CH2OH} \), which needs \( \ce{LiAlH4} \); Clemmensen leaves the acid group alone.
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).”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 14-06 S3 12-06 S2 | – |