JEE Advanced Chemistry Question (2026) — Solution

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Natural rubber is cis-1,4-polyisoprene, with the repeating unit [- CH _2- C ( CH _3)= CH - CH _2-]_n. Complete reductive ozonolysis ( O _3/Zn- H _2 O ) of natural rubber cleaves the double bonds to give 4-oxopentanal (levulinaldehyde) as compound X. [- CH _2- C ( CH _3)= CH - CH _2-]_n O _3/ Zn-H _2 O n CH _3- C (= O )- CH _2- CH _2- CHO Compound X ( CH _3- CO - CH _2- CH _2- CHO ) has a methyl ketone group, which gives a positive iodoform test, and an aldehyde group, which gives a positive Tollen's test. When X is heated with aqueous NaOH, it undergoes an intramolecular aldol condensation. The base abstracts an -proton from the terminal methyl group to form an enolate, which then attacks the aldehyde carbonyl carbon to form a stable five-membered ring. The intermediate formed is 3-hydroxycyclopentanone, which upon heating undergoes dehydration to yield cyclopent-2-en-1-one as the major product Y. Answer: