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Which of the following compounds does NOT show geometrical isomerism?

1. ClHC=CClH (each double-bond carbon bears Cl and H)
2. ClHC=C(Cl)(CH3) (one carbon bears Cl and H; the other bears Cl and CH3)
3. A cyclopropane ring fused to a double bond such that one double-bond carbon belongs to the ring and bears two ring-carbons as substituents, while the other bears Cl and H
4. One double-bond carbon bears CH3 and cyclopropyl; the other bears CH3 and H

Correct answer: A cyclopropane ring fused to a double bond such that one double-bond carbon belongs to the ring and bears two ring-carbons as substituents, while the other bears Cl and H

Solution

For a compound to show geometrical isomerism, both doubly-bonded carbons must each carry two different substituents. In an endocyclic double bond within cyclopropane, one carbon of the double bond is attached to two equivalent ring carbons, making its two substituents identical and eliminating the possibility of geometrical isomerism.

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