JEE Main 2025 Jan 28 Shift 1, Chemistry Q8: Mechanism of Some Important Reactions

The reasoning runs as follows.

Step 1 — Acid-catalysed rearrangement of P to Q. Protonation of the hydroxyl in 1-isopropylcyclopentan-1-ol and loss of water gives a tertiary carbocation on the cyclopentane ring. A ring-expansion (alkyl shift) converts the 5-membered cation into a stabilised 6-membered carbocation. A 1,2-methyl shift followed by loss of ${\text{H}}^{+}$ produces a substituted cyclohexene bearing two methyl groups, which is compound Q.

Step 2 — Ozonolysis of Q. Cleaving the cyclohexene double bond with ${\text{O}}_3$ followed by reductive work-up (Zn / H${}_2$O) opens the ring to give an acyclic 1,6-dicarbonyl (a keto-aldehyde with appropriate methyl substitution).

Step 3 — Intramolecular aldol under alkaline reflux. The dicarbonyl undergoes an intramolecular aldol-condensation; dehydration leaves a five-membered $\alpha ,\beta$-unsaturated ketone, which matches R: 1-acetyl-2-methylcyclopent-1-ene.