Question

Consider the reaction: \[ \text{Ph–CH=CH}_2 \xrightarrow[\text{peroxide}]{\text{HBr}} \text{Product} \] Which of the following statements are correct?
[A.] The reaction proceeds through a more stable radical intermediate.
[B.] The role of peroxide is to generate \(\mathrm{H^\bullet}\) radical.
[C.] During this reaction, benzene is formed as a byproduct.
[D.] \(1\)-Bromo-\(2\)-phenylethane is formed as a minor product.
[E.] The same reaction in absence of peroxide proceeds via a carbocation intermediate. Choose the correct answer.

• A. A, B & D only
• B. C, D & E only
• C. A, C & E only
• D. A & E only

Hint:

The peroxide effect operates only with HBr and switches the mechanism from ionic to radical.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The addition of HBr to alkenes in the presence of peroxides (Benzoyl peroxide) is known as the Kharasch effect or the Anti-Markovnikov addition.
This reaction proceeds via a free radical mechanism.
In the absence of peroxides, the reaction follows an electrophilic addition mechanism via a carbocation intermediate (Markovnikov's rule).
Step 2: Detailed Explanation:
Mechanism in presence of Peroxide:
1. Initiation: Benzoyl peroxide undergoes homolytic cleavage to form benzoate radicals, which then decarboxylate to phenyl radicals. Phenyl radicals react with HBr to generate bromine radicals (\( \text{Br}^{\cdot} \)).
Statement B is incorrect because peroxide generates Br radicals, not H radicals.
2. Propagation: The \( \text{Br}^{\cdot} \) radical attacks the alkene to form a more stable radical. In styrene (\( \text{Ph}-\text{CH}=\text{CH}_2 \)), the bromine radical adds to the terminal carbon to form a secondary benzylic radical (\( \text{Ph}-\dot{\text{C}}\text{H}-\text{CH}_2\text{Br} \)), which is resonance stabilized.
Statement A is correct.
3. Final Product: The radical abstracts a hydrogen atom from HBr to form 1-bromo-2-phenylethane (\( \text{Ph}-\text{CH}_2-\text{CH}_2\text{Br} \)) as the major product.
Statement D is incorrect because 1-bromo-2-phenylethane is the major product, not minor.
Mechanism in absence of Peroxide:
The alkene reacts with \( \text{H}^+ \) to form the most stable carbocation (\( \text{Ph}-\text{CH}^+-\text{CH}_3 \)), followed by nucleophilic attack by \( \text{Br}^- \).
Statement E is correct.
Statement C is incorrect as benzene is not a standard byproduct of this addition reaction.
Step 3: Final Answer:
Statements A and E are the only correct statements.