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Difficulty: MediumStructural Isomerism and Stereoisomerism

But-2-ene exhibits geometric (cis-trans) isomerism because each carbon atom involved in the double bond is bonded to two non-identical groups, whereas but-1-ene does not exhibit geometric isomerism.

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Answer

The statement is true because geometric isomerism requires restricted rotation about the C=CC=C bond together with two distinct groups attached to each double-bonded carbon atom—a condition satisfied by but-2-ene but not by but-1-ene.
The statement correctly describes the structural rule for geometric isomerism in alkenes. But-2-ene meets the condition because both double-bonded carbons carry two non-identical groups (−H-H and −CH3-CH_3), whereas but-1-ene fails the condition because its terminal carbon carries two identical hydrogen atoms.

Step-by-Step Solution

1
Identify the structural requirements for geometric (cis-trans) isomerism.
Geometric isomerism in alkenes requires a rigid C=CC=C double bond where each of the two unsaturated carbon atoms is attached to two non-identical substituents.
If either carbon atom of the double bond carries two identical groups, swapping those groups produces an identical molecule rather than a distinct stereoisomer.
2
Analyze the substituent groups attached to the double-bonded carbons in but-2-ene (CH3−CH=CH−CH3CH_3-CH=CH-CH_3).
Carbon-2 is attached to −H-H and −CH3-CH_3, and Carbon-3 is also attached to −H-H and −CH3-CH_3.
Since both double-bonded carbons have two different groups attached, but-2-ene exists as two stereoisomers: cis-but-2-ene and trans-but-2-ene.
3
Analyze the substituent groups attached to the double-bonded carbons in but-1-ene (CH2=CH−CH2−CH3CH_2=CH-CH_2-CH_3).
Carbon-1 is attached to two identical hydrogen atoms (−H-H and −H-H).
The presence of two identical hydrogen atoms on Carbon-1 prevents the formation of cis-trans isomers for but-1-ene.

Key Concept

Structural Criteria for Geometric (Cis-Trans) Isomerism
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Topics

Structural Isomerism and Stereoisomerism

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