A saturated bicyclic hydrocarbon is shown for reference. Which of the carbocations shown is the least stable?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

The 1-norbornyl cation, a bridgehead cation, cannot become planar because of the rigid bicyclic framework, so it cannot be stabilised by effective hyperconjugation. It is far less stable than the open-chain tertiary and secondary cations and the cyclohexyl cation.

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