Which of the following on treating with zinc gives 2–butene?

Options

A. `CH_(3)-"CH(Br)-CH(Br)"-CH_(3)`
B. `CH_(3)CHBr_(2)`
C. Both of these
D. None of these

Answer

To determine which compound, when treated with zinc, gives 2-butene, we need to analyze the structures of the given compounds and understand the reaction mechanism involved.

### Step-by-Step Solution:

1. **Identify the Structures**:
- We have two options to consider:
- **Option 1**: A compound with the structure CH3-CHBr-CH2-CH3 (1-bromo-2-butane).
- **Option 2**: A compound with the structure CH3-CHBr-CHBr-CH3 (1,2-dibromobutane).

2. **Understanding the Reaction**:
- Zinc is commonly used in organic chemistry to facilitate dehalogenation reactions. This means it can remove halogen atoms (like bromine) from the organic compounds.
- The reaction with zinc can lead to the formation of alkenes from haloalkanes.

3. **Analyzing Option 1 (1-bromo-2-butane)**:
- When 1-bromo-2-butane is treated with zinc, it can undergo dehalogenation.
- The reaction can be represented as:
\[ \text{CH}_3-\text{CHBr}-\text{CH}_2-\text{CH}_3 + \text{Zn} \rightarrow \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 + \text{ZnBr}_2 \]
- This results in the formation of 2-butene (but-2-ene).

4. **Analyzing Option 2 (1,2-dibromobutane)**:
- For 1,2-dibromobutane, the reaction with zinc can also lead to dehalogenation:
- The reaction can be represented as:
\[ \text{CH}_3-\text{CHBr}-\text{CHBr}-\text{CH}_3 + \text{Zn} \rightarrow \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 + 2\text{ZnBr}_2 \]
- This also results in the formation of 2-butene.

5. **Conclusion**:
- Both options can lead to the formation of 2-butene upon treatment with zinc. However, the most straightforward route is through the vicinal dibromide (Option 2), as it directly leads to the formation of the double bond between the second and third carbon atoms.

### Final Answer:
**Option 2 (1,2-dibromobutane) on treating with zinc gives 2-butene.**