Geometrical Isomerism In Alkenes
Restricted rotation about a carbon-carbon double bond can give rise to geometrical isomers, and the compound capable of showing cis-trans isomerism must be identified.
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About This Question
- Subject
- chemistry
- Chapter
- hydrocarbons
- Topic
- geometrical isomerism in alkenes
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
But-2-ene
Geometrical or cis-trans isomerism arises when there is restricted rotation about a carbon-carbon double bond and each doubly bonded carbon bears two different groups. If either alkene carbon carries two identical groups, the cis and trans arrangements become superimposable and isomerism vanishes. In but-2-ene, CH3−CH=CH−CH3, each double-bond carbon bears one hydrogen and one methyl group, two different substituents, so distinct cis and trans forms exist. But-1-ene, CH2=CH−CH2CH3, has a terminal =CH2 carbon carrying two identical hydrogens, so it cannot show geometrical isomerism, making it wrong. 2-Methylpropene, (CH3)2C=CH2, similarly has one carbon with two identical methyls and another with two identical hydrogens, so it is incorrect. Propene, CH3−CH=CH2, has a terminal carbon bearing two hydrogens, so it also fails the requirement and does not apply. The essential condition is that both alkene carbons must hold two unlike groups. This follows the NCERT criterion for geometrical isomerism. A plausibility check: only but-2-ene has each sp2 carbon bonded to two different atoms or groups, confirming it alone shows cis-trans isomers.
This easy difficulty chemistry question is from the chapter hydrocarbons, covering the topic of geometrical isomerism in alkenes. It appeared in the 2025 exam.
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