JEE Advanced 2026 · Paper 2 · Chemistry Q3

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ChemistrySingle correct+3 / -1Question 39 of 54

Natural rubber on complete ozonolysis ( $\ce{O3}$ /Zn- $\ce{H2O}$ ) 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

1-(2-Hydroxycyclopropyl)ethan-1-one: a cyclopropane bearing OH on one carbon and an acetyl group on the next.

1,4-Diacetylcyclohexa-1,4-diene: a cyclohexa-1,4-diene with COCH₃ on each of the two opposite alkene carbons.

A cycloheptene ring fused to a tetrahydrofuran: the ring-fusion carbon next to O bears CH₃, the furan carbon next to O bears OH (a cyclic hemiacetal), and the seven-membered ring carries a C=O conjugated with its C=C.

Answer and worked solution

Official answer: A

X. Natural rubber is cis-1,4-polyisoprene, $\ce{[-CH2-C(CH3)=CH-CH2-]_n}$ . Cutting every C=C gives 4-oxopentanal, $\ce{CH3COCH2CH2CHO}$ : a methyl ketone (iodoform positive) and an aldehyde (Tollens positive).

Y. In aqueous NaOH the enolate of the $\ce{CH3}$ group attacks the CHO carbon (intramolecular aldol, five-membered ring). Dehydration then gives the conjugated enone, cyclopent-2-en-1-one: option (A).

Topics:Aldol and related condensations Alkenes Named reactions

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JEE Advanced 2026 · Paper 2, Chemistry question 3