JEE Advanced 2026 — Paper 2 — CHEM-I solution

JEE Advanced2026Paper 2CHEM-I

Natural rubber on complete ozonolysis (O $_{3}$ /Zn-H $_{2}$ O) gives compound $X$ as the major product. $X$ gives positive iodoform and Tollen's tests. $X$ on heating with aqueous NaOH gives $Y$ as the major product. $Y$ is

A
(A)
B
(B)
C
(C)
D
(D)

Solution

Identify X from ozonolysis of natural rubber. Natural rubber is cis-1,4-polyisoprene. Complete reductive ozonolysis cleaves every C=C and gives 4-oxopentanal (CH $_{3}$ COCH $_{2}$ CH $_{2}$ CHO) as the repeating fragment. This compound has both a methyl ketone (positive iodoform) and an aldehyde (positive Tollen's), satisfying X.

Aldol condensation to Y. 4-oxopentanal is a 1,5-dicarbonyl with an enolisable methyl ketone and an aldehyde. On heating with aqueous NaOH the methyl ketone enolate attacks the aldehyde carbonyl intramolecularly and dehydration gives the conjugated cyclic enone. Counting atoms in the new ring (the two original carbonyl carbons plus three CH/CH $_{2}$ atoms between them) yields a five-membered ring with the C=C conjugated to C=O: cyclopent-2-en-1-one.

$∴$ $Y =$ cyclopent-2-en-1-one, matching option (A).

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