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Styrene (Ph-CH=CH2) is treated with BrCCl3 in the presence of a peroxide initiator. Which of the following correctly describes the mechanism and/or the product of this reaction?

1. The peroxide generates RO• which abstracts Br from BrCCl3 to give ROBr and •CCl3
2. The •CCl3 radical adds to styrene to give the intermediate Ph-CH•-CH2-CCl3
3. The reaction follows anti-Markovnikov addition and gives Ph-CH(Br)-CH2-CCl3 as the major product
4. The reaction follows Markovnikov addition and gives Ph-CH(CCl3)-CH2Br as the major product

Correct answer: The •CCl3 radical adds to styrene to give the intermediate Ph-CH•-CH2-CCl3

Solution

The peroxide generates RO• which abstracts Br from BrCCl3 giving •CCl3. This adds to the less hindered terminal carbon of styrene forming the stable benzylic radical Ph-CH•-CH2-CCl3. The benzylic radical then abstracts Br from another BrCCl3 molecule to give the anti-Markovnikov-like product Ph-CH(Br)-CH2-CCl3 and regenerates •CCl3.

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