Which alkene will give optically inactive product with `Br_2//"CCl"_4` ?

Options

A. 1- butene
B. Propene
C. cis - 2 - butene
D. trans - 2 - butene

Answer

To determine which alkene will give an optically inactive product with \( \text{Br}_2/\text{CCl}_4 \), we need to analyze the structures of the given alkenes and their reactions with bromine.

### Step 1: Understand Optical Activity
Optical activity is related to the presence of chiral centers in a molecule. A chiral center is typically a carbon atom that is bonded to four different substituents. If a compound has no chiral centers, it is considered optically inactive.

**Hint:** Remember that a molecule is optically inactive if it does not have any chiral carbon atoms.

### Step 2: Analyze the Given Alkenes
1. **1-Butene (C4H8)**:
- Structure: \( \text{CH}_2= \text{CH}-\text{CH}_2-\text{CH}_3 \)
- Reaction with \( \text{Br}_2/\text{CCl}_4 \):
- The addition of bromine across the double bond will yield \( \text{CH}_2\text{Br}-\text{CH}(\text{Br})-\text{CH}_2-\text{CH}_3 \).
- This product has a chiral carbon, making it optically active.

**Hint:** Check if the product has any carbon with four different groups.

2. **Propene (C3H6)**:
- Structure: \( \text{CH}_3-\text{CH}=\text{CH}_2 \)
- Reaction with \( \text{Br}_2/\text{CCl}_4 \):
- The addition of bromine will yield \( \text{CH}_3-\text{CH}(\text{Br})-\text{CH}_2\text{Br} \).
- This product also has a chiral carbon, making it optically active.

**Hint:** Look for symmetry in the product; if there are two identical groups on a carbon, it may be chiral.

3. **Cis-2-Butene (C4H8)**:
- Structure: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 \) (cis configuration)
- Reaction with \( \text{Br}_2/\text{CCl}_4 \):
- The product will be a racemic mixture of two enantiomers, making it optically active.

**Hint:** Consider the arrangement of groups around the double bond.

4. **Trans-2-Butene (C4H8)**:
- Structure: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 \) (trans configuration)
- Reaction with \( \text{Br}_2/\text{CCl}_4 \):
- The addition of bromine will yield a meso compound, which has a plane of symmetry.
- Since meso compounds are not chiral, this product is optically inactive.

**Hint:** Identify if the product has a plane of symmetry, which indicates it may be a meso compound.

### Step 3: Conclusion
Among the given options, **trans-2-butene** is the alkene that will give an optically inactive product when reacted with \( \text{Br}_2/\text{CCl}_4 \) because it forms a meso compound.

### Final Answer
**Trans-2-butene will give an optically inactive product with \( \text{Br}_2/\text{CCl}_4 \).**