Which of the following alkene will give the same product when treated with HBr in presence and absence of peroxide ?

Question

Accepted Answer

Option 2, Butene-2, is the correct answer because it will give the same product when treated with HBr in the presence and absence of peroxide. This is due to the nature of the reaction mechanism involved.

1. **Explanation for Option 2 (Butene-2):**
   - Butene-2 is a symmetrical alkene, meaning that the double bond is located between two identical groups (CH3-CH=CH-CH3). When HBr is added to Butene-2, the reaction follows Markovnikov's rule in the absence of peroxide, leading to the formation of 2-bromobutane. In the presence of peroxide, the reaction follows the anti-Markovnikov rule due to the free radical mechanism, but since Butene-2 is symmetrical, the product remains the same, 2-bromobutane. Thus, in both scenarios, the product is identical.

2. **Explanation for why other options are incorrect:**

- **Option 1 (Propene):**
     - Propene is an asymmetrical alkene. In the absence of peroxide, HBr adds according to Markovnikov's rule, resulting in 2-bromopropane. In the presence of peroxide, the addition follows the anti-Markovnikov rule, leading to 1-bromopropane. Therefore, the products are different in the presence and absence of peroxide.

- **Option 3 (2-methyl propene):**
     - 2-methyl propene is also an asymmetrical alkene. Without peroxide, HBr adds according to Markovnikov's rule, forming tert-butyl bromide. With peroxide, the addition follows the anti-Markovnikov rule, resulting in isobutyl bromide. Thus, the products differ in the presence and absence of peroxide.

- **Option 4 (Butene-1):**
     - Butene-1 is an asymmetrical alkene. In the absence of peroxide, HBr adds according to Markovnikov's rule, producing 2-bromobutane. In the presence of peroxide, the reaction follows the anti-Markovnikov rule, yielding 1-bromobutane. Therefore, the products are not the same in both conditions.

In conclusion, Butene-2 is the only alkene among the options that produces the same product, 2-bromobutane, regardless of whether the reaction is carried out in the presence or absence of peroxide. This is due to its symmetrical structure, which leads to identical products under both reaction conditions.