The cation shown undergoes a single 1,2-shift to give a more stable cation. Which structure is formed?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The cation shown is a secondary cation, the 3-methylbut-2-yl cation. A 1,2-hydride shift from the adjacent CH moves the charge to that carbon, giving the tertiary 2-methylbut-2-yl cation. The other structures are primary cations or cannot form by a single shift.

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