Answer the question on basis of passage given below:
A ketone [A] which undergoes haloform reaction gives compound [B] on reduction. Compound [B] on heating with sulphuric acid gives compound [C] which forms mono-ozonide [D]. The compound [D] on hydrolysis in the presence of zinc dust gives compound [E]. Compound [E] on oxidation gives \(C_2H_4O_2\) [F].
Identify compound [A]
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Butan-2-one
Work back from [F]. \( \ce{C2H4O2} \) is acetic acid, so [E] is acetaldehyde. A mono-ozonide whose hydrolysis gives acetaldehyde alone must come from the symmetrical alkene but-2-ene [C], \( \ce{CH3CH=CHCH3} \). But-2-ene is the dehydration product of butan-2-ol [B], and butan-2-ol is obtained by reducing butan-2-one.
\( \ce{CH3COCH2CH3} \xrightarrow{\text{reduction}} \ce{CH3CH(OH)CH2CH3} \xrightarrow[\Delta]{\text{conc. }\ce{H2SO4}} \ce{CH3CH=CHCH3} \)
\( \ce{CH3CH=CHCH3} \xrightarrow[\text{(ii) Zn}/\ce{H2O}]{\text{(i) }\ce{O3}} \ce{2CH3CHO} \xrightarrow{[\ce{O}]} \ce{2CH3COOH} \)
Butan-2-one has a \( \ce{CH3-CO} \)– group, so it does give the haloform reaction, as the passage requires:
\( \ce{CH3COCH2CH3 + 3NaOI -> CH3CH2COONa + CHI3 v + 2NaOH} \)
Why the other options are incorrect
Option (2) 3-Methylpentan-2-one — Incorrect. A methyl ketone, but the chain would give 3-methylpent-2-ene, whose ozonolysis gives butan-2-one and acetaldehyde, not acetaldehyde alone.
Option (3) Propane — Incorrect. A hydrocarbon, not a ketone; it cannot give the haloform reaction or be reduced to an alcohol.
Option (4) Acetophenone — Incorrect. A methyl ketone, but it gives 1-phenylethanol and then styrene, whose ozonolysis gives benzaldehyde and HCHO.
NCERT (2026-27 edition):
Oxidation of Methyl Ketones by Haloform Reaction – “Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound.”