The secondary carbocation shown undergoes a single 1,2-methyl shift. Which structure shown is the cation formed?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

The cation shown is the 3,3-dimethylbutan-2-yl cation. A methyl group from the adjacent quaternary carbon migrates with its electron pair to the cationic carbon, so the charge ends up on the former quaternary carbon. The product is the 2,3-dimethylbutan-2-yl cation, which is tertiary and more stable than the secondary starting cation.

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