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Which of the following compounds will show geometrical (cis-trans) isomerism?

1. (A) A cyclopentane ring with identical T substituents on two adjacent upper carbons and a geminal dichloro group (Cl, Cl) on the topmost carbon.
2. (B) A cyclohexene ring with identical D substituents on the two carbon atoms adjacent to and on the same side as the double bond.
3. (C) A spiro arrangement of four cyclopropane rings with a central cyclopropane connected to three others at its vertices.
4. (D) A cyclohexadiene ring with identical D substituents on two adjacent carbons and two double bonds within the ring.

Correct answer: (B) A cyclohexene ring with identical D substituents on the two carbon atoms adjacent to and on the same side as the double bond.

Solution

In cyclohexene (option B), the two sp3 carbons each carry one D and one H substituent; since neither carbon has two identical groups, restricted rotation in the ring fixes their relative orientation, giving rise to cis and trans isomers. The geminal dichloro group in option A prevents G.I. at that center; the spiro polycyclopropane in option C has no appropriate stereocenters; the diene in option D places the D-substituted carbons on sp2 centers that cannot show classical cis-trans ring isomerism.

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