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JEE Advanced levelSingle correct optionDifficulty 3 of 3o2099

3,3-Dimethylbut-1-ene is treated with HBr (i) in the dark and (ii) in the presence of peroxide. The major products are respectively

A(i) 2-bromo-2,3-dimethylbutane; (ii) 2-bromo-3,3-dimethylbutane
B(i) 1-bromo-3,3-dimethylbutane; (ii) 2-bromo-2,3-dimethylbutane
C(i) 2-bromo-3,3-dimethylbutane; (ii) 1-bromo-3,3-dimethylbutane
D(i) 2-bromo-2,3-dimethylbutane; (ii) 1-bromo-3,3-dimethylbutane

Answer: D

In the dark the ionic mechanism gives the secondary cation

(CH X_{3}) X_{3} C - CH X^{+} - CH X_{3}

, which undergoes a 1,2-methyl shift to the tertiary cation

(CH X_{3}) X_{2} C X^{+} - CH (CH X_{3}) X_{2}

and gives 2-bromo-2,3-dimethylbutane. With peroxide,

Br \cdot

adds to the terminal carbon; the radical does not rearrange and the product is

(CH X_{3}) X_{3} CCH X_{2} CH X_{2} Br

.

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