Question

Which one of the following compounds shows cis-trans isomerism?

• A. Pent-1-ene
• B. But-2-ene
• C. But-1-ene
• D. Propene
• E. Ethene

Hint:

Rule: Each carbon of C=C must have two different groups for cis-trans isomerism.

Answer 1

The Correct Option is B

Concept: Cis-trans isomerism (geometrical isomerism) is a type of stereoisomerism that arises due to restricted rotation around a double bond. For a compound to exhibit cis-trans isomerism, the following condition must be satisfied:
• Each carbon of the double bond must have two different substituents

Step 1: Understand restriction in double bond A double bond consists of:
• One sigma bond (free rotation possible)
• One pi bond (restricts rotation) The presence of $\pi$ bond prevents free rotation, making different spatial arrangements stable.

Step 2: Analyze each option carefully (A) Pent-1-ene: \[ CH_2 = CH - CH_2 - CH_2 - CH_3 \] First carbon has two identical H atoms → does not satisfy condition → no isomerism (B) But-2-ene: \[ CH_3 - CH = CH - CH_3 \] Each carbon has two different groups:
• Left carbon → CH$_3$ and H
• Right carbon → CH$_3$ and H Thus two distinct arrangements possible:
• Cis: both CH$_3$ on same side
• Trans: CH$_3$ on opposite sides Hence shows geometrical isomerism. (C) But-1-ene: \[ CH_2 = CH - CH_2 - CH_3 \] First carbon has two H → no isomerism (D) Propene: \[ CH_2 = CH - CH_3 \] Again, first carbon has identical H → no isomerism (E) Ethene: \[ CH_2 = CH_2 \] Both carbons have identical substituents → no isomerism

Step 3: Final deduction Only But-2-ene satisfies required condition.

Step 4: Visualization insight Cis-trans isomerism arises because groups cannot rotate around double bond, leading to fixed spatial arrangements. Final Answer: \[ \boxed{\text{But-2-ene}} \]