JEE Main 2025 Apr 8 Shift 2, Chemistry Q18: Properties and Important Reactions of Aldehydes & Ketones

Step 1: Enolate Formation

In the presence of a base, the acidic $\alpha$-hydrogen adjacent to a carbonyl group is removed, forming a resonance-stabilised enolate ion.

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Step 2: Intramolecular Aldol Cyclisation

The enolate carbon attacks the second carbonyl carbon within the same molecule, forming a new C–C bond and closing the ring.

Five- and six-membered rings are preferred because they have minimum ring strain.

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Step 3: Dehydration

The initially formed $\beta$-hydroxy carbonyl compound readily loses water $\left(-{\mathrm{H}}_2\mathrm{O}\right)$ to form a thermodynamically stable conjugated enone.

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Conclusion

The reaction proceeds through the favourable ring-size pathway to give the fused-ring conjugated enone shown in Option (1).

$\boxed{\text{Correct Answer: Option (1)}}$