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Which one of the following compounds can exist as cis-trans isomers?
A
Pent-1-ene
B
2-Methylhex-2-ene
C
1,1-Dimethylcyclopropane
D
1,2-Dimethylcyclohexane
Explanation
1,2-Dimethylcyclohexane possesses two stereocenters with restricted rotation, existing as cis- and trans-geometric isomers.
Detailed Solution
For geometrical isomerism, there must be restricted rotation and each carbon of the stereogenic unit must be attached to two different groups. In Pent-1-ene, terminal carbon has two identical H atoms; in 2-methylhex-2-ene, C-2 has two methyl groups; in 1,1-dimethylcyclopropane, C-1 has two methyl groups. In 1,2-dimethylcyclohexane, the ring provides restricted rotation and C-1 and C-2 both hold $-\text{H}$ and $-\text{CH}_3$, allowing cis- and trans-isomers.