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Among the following carbocations, which one is the most stable?

1. Cyclopentadienyl cation
2. Cycloheptatrienyl cation
3. Cyclopentenyl cation
4. None of these

Correct answer: Cycloheptatrienyl cation

Solution

The cycloheptatrienyl (tropylium) cation has 6 pi electrons in a planar, fully conjugated ring, satisfying the 4n+2 rule (n=1), making it aromatic and exceptionally stable. The cyclopentadienyl cation has 4 pi electrons (antiaromatic, unstable), and the cyclopentenyl cation is only an allylic cation.

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