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].
Compound [E] in presence of dil. NaOH gives:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → But-2-enal
Working through the passage: butan-2-one [A] is reduced to butan-2-ol [B]; heating with \( \ce{H2SO4} \) dehydrates it to but-2-ene [C] (Saytzeff product); its mono-ozonide [D] on hydrolysis with zinc dust gives only acetaldehyde [E], which is oxidised to acetic acid [F], \( \ce{C2H4O2} \).
Acetaldehyde has α-hydrogens, so with dilute NaOH it undergoes aldol condensation. The aldol formed loses water on warming to give an α,β-unsaturated aldehyde:
\( \ce{2CH3CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3CH(OH)CH2CHO} \xrightarrow[-\ce{H2O}]{\Delta} \ce{CH3CH=CHCHO} \)
\( \ce{CH3CH=CHCHO} \) is but-2-enal (crotonaldehyde).
Why the other options are incorrect
Option (2) Methanol — Incorrect. A one-carbon alcohol; aldol condensation joins two molecules and builds a longer chain, it does not break acetaldehyde down.
Option (3) 2-Methylbut-2-enal — Incorrect. A \( \ce{C5} \) product, formed from ethanal and propanal (cross aldol); two \( \ce{C2} \) molecules can give only a \( \ce{C4} \) product.
Option (4) Pent-2-enal — Incorrect. Also \( \ce{C5} \); it cannot come from the self-condensation of acetaldehyde.
NCERT (2026-27 edition):
Reactions due to α-Hydrogen – “… (i) Aldol condensation: Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction. The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.”