Vinylcyclohexane, $C X_{6} H X_{11} - CH = CH X_{2}$ , is treated with HBr (i) in the dark and (ii) with a peroxide. The major products are respectively

A(i) (2-bromoethyl)cyclohexane; (ii) 1-bromo-1-ethylcyclohexane
B(i) 1-bromo-1-ethylcyclohexane; (ii) (1-bromoethyl)cyclohexane
C(i) 1-bromo-1-ethylcyclohexane; (ii) (2-bromoethyl)cyclohexane
D(i) (1-bromoethyl)cyclohexane; (ii) (2-bromoethyl)cyclohexane

Answer: C

In the dark,

H X^{+}

adds to the terminal carbon to give a secondary cation next to the ring carbon bearing one H. A 1,2-hydride shift gives the more stable tertiary cation in the ring, which is captured by

Br X^{-}

to give 1-bromo-1-ethylcyclohexane. With peroxide,

Br \cdot

adds to the terminal carbon and H transfer gives (2-bromoethyl)cyclohexane without rearrangement.

Practice Organic Reaction Mechanisms II: Electrophilic and Radical Reactions More on Anti-Markovnikov addition of HBr

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Vinylcyclohexane, CX6​HX11​−CH=CHX2​, is treated with HBr (i) in the dark and (ii) with a peroxide. The major products are respectively

Answer: C

More questions on Anti-Markovnikov addition of HBr

Vinylcyclohexane, $C X_{6} H X_{11} - CH = CH X_{2}$ , is treated with HBr (i) in the dark and (ii) with a peroxide. The major products are respectively