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Which of these species is the most stable?

1. Cyclopropenyl cation (three-membered ring with one double bond and a positive charge)
2. Cyclopropyl anion (three-membered ring carrying a negative charge)
3. Cyclopentadienyl cation (five-membered ring with two double bonds and a positive charge)
4. Cyclohexadiene (six-membered ring with two double bonds)

Correct answer: Cyclopropenyl cation (three-membered ring with one double bond and a positive charge)

Solution

The cyclopropenyl cation has 2 pi electrons (4n+2 with n=0), so it is aromatic and especially stable. The cyclopentadienyl cation has 4 pi electrons (antiaromatic), the cyclopropyl anion is not aromatic, and cyclohexadiene is an ordinary non-aromatic diene. Hence the cyclopropenyl cation is the most stable.

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