Isomerism

Which one of the following will show geometrical isomerism?

1. [figure]
2. [figure]
3. [figure]
4. [figure]

Answer

(1) [figure]

Solution

Exocyclic C=C shows geometrical isomerism only if BOTH ends carry two different groups.

(2), (3): =CH2 end has two H ⇒ (2), (3) show no geometrical isomerism

(4): Br on C4 ⇒ the two ring arms of C1 are identical ⇒ (4) shows none

(1): =CHBr end (H ≠ Br) and Br on C3 makes the two ring arms of C1 different ⇒ two isomers:

[figure]

⇒ (1) shows geometrical isomerism

Answer: (1)