Question

Geometrical isomerism is possible in:

• A.
• B.
• C.
• D.

Answer 1

Answer: (D)

d

Answer 2

For a molecule to exhibit geometrical isomerism (cis-trans isomerism), two conditions must be met:

1. Restricted Rotation: There must be restricted rotation around a bond (e.g., a carbon-carbon double bond or a ring structure).
2. Different Groups: Each carbon atom involved in the restricted rotation (e.g., each carbon of the double bond) must be attached to two different groups.

Cyclopentene, cyclohexene, and cyclobutene are too small to form a stable trans isomer due to severe ring strain. Only the cis configuration exists. Geometrical isomerism is not possible in these cases.

For cyclic alkenes, trans isomers become stable enough to exist when the ring size is 8 carbons or more. Both cis-cyclooctene and trans-cyclooctene are known and isolable compounds, although the cis isomer is typically more stable. Since both cis and trans forms exist, cyclooctene can exhibit geometrical isomerism.

Among the given options, only cyclooctene (d) is large enough to accommodate both cis and trans configurations of the double bond.