The secondary alcohol shown is treated with concentrated HBr. The major bromide is obtained after rearrangement. Which structure is it?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The compound is 3,3-dimethylbutan-2-ol (pinacolyl alcohol). Protonation and loss of water gives a secondary cation next to a quaternary carbon. A 1,2-methyl shift gives the tertiary 2,3-dimethylbutan-2-yl cation, which captures bromide to give 2-bromo-2,3-dimethylbutane, CC(C)(Br)C(C)C.

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