Which of the following alkenes on treatment with HBr in presence or absence of peroxide give the same product ?

Options

A. Cyclohexene
B. But-2-ene
C. Hex-3-ene
D. Ethene

Answer

To determine which alkenes give the same product when treated with HBr in the presence or absence of peroxide, we need to analyze the reactions of the given alkenes.

### Step-by-Step Solution:

1. **Identify the Alkenes**:
- The alkenes mentioned are cyclohexene, butene, hex-3-ene, 2-pentene, and ethene.

2. **Understand the Reaction Mechanism**:
- When alkenes react with HBr, they can undergo two different pathways:
- **Without Peroxide**: The reaction follows Markovnikov's rule, where the bromine atom attaches to the more substituted carbon atom.
- **With Peroxide**: The reaction follows the anti-Markovnikov rule, where the bromine atom attaches to the less substituted carbon atom.

3. **Analyze Each Alkene**:
- **Cyclohexene**:
- Without peroxide: Br attaches to the more substituted carbon.
- With peroxide: Br attaches to the less substituted carbon.
- However, due to the symmetry of cyclohexene, both methods yield the same product.

- **Butene (1-butene)**:
- Without peroxide: Br attaches to the second carbon.
- With peroxide: Br attaches to the first carbon.
- Different products are formed.

- **Hex-3-ene**:
- Without peroxide: Br attaches to the more substituted carbon.
- With peroxide: Br attaches to the less substituted carbon.
- Different products are formed.

- **2-Pentene**:
- Without peroxide: Br attaches to the more substituted carbon.
- With peroxide: Br attaches to the less substituted carbon.
- Different products are formed.

- **Ethene**:
- Ethene is symmetrical, so whether you apply HBr with or without peroxide, it will yield the same product (bromoethane).

4. **Conclusion**:
- The alkenes that yield the same product when treated with HBr in the presence or absence of peroxide are cyclohexene and ethene.

### Final Answer:
Cyclohexene and ethene give the same product when treated with HBr in the presence or absence of peroxide.