NEET (UG) 2025 — Question 54

Geometrical isomerism requires restricted rotation and two different groups on each of the two atoms that are connected by the restricted bond. For alkenes, each carbon of the double bond must have two different substituents. For cycloalkanes, the ring restricts rotation, and at least two ring carbons must each have two different substituents. Option A: Pent-1-ene has the double bond at C1, so one carbon has two H atoms, not different groups. Hence no cis-trans isomerism. Option B: 2-Methylhex-2-ene has the double bond at C2; one carbon has two methyl groups, not different groups. Hence no cis-trans isomerism. Option C: 1,1-Dimethylcyclopropane has one ring carbon with two methyl groups, so that carbon does not have two different substituents. Hence no cis-trans isomerism. Option D: 1,2-Dimethylcyclohexane has two ring carbons each with one methyl and one hydrogen, so each has two different substituents. The ring restricts rotation, so cis and trans isomers exist (cis: both methyls on same side; trans: on opposite sides). Therefore, the correct answer is D.