The primary carbocation shown rearranges by a single 1,2-shift to a more stable cation. Which structure shown is the product cation?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

The cation shown is the neopentyl cation. The adjacent carbon is quaternary and bears no hydrogen, so no hydride shift is possible; a 1,2-methyl shift moves a methyl group to the cationic carbon and places the charge on the tertiary carbon. The product is the 2-methylbutan-2-yl (tert-amyl) cation, a tertiary cation, much more stable than the primary neopentyl cation.

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