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From the following set of cyclic species, identify the one that is aromatic.

1. Cyclopentadienyl cation (4 pi electrons, 5-membered ring)
2. Cyclobutadiene dianion (square 4-membered ring carrying 2- charge)
3. Cyclopropenyl cation (2 pi electrons, 3-membered ring)
4. Protonated pyridine-type 6-membered ring with N-H and a positive charge

Correct answer: Cyclopropenyl cation (2 pi electrons, 3-membered ring)

Solution

Aromaticity requires a planar, cyclic, fully conjugated ring with (4n+2) pi electrons. The cyclopropenyl cation has 2 pi electrons (n=0), satisfying Huckel's rule, and is the classic small aromatic carbocation. The cyclopentadienyl cation has 4 pi electrons (antiaromatic), and the cyclobutadiene dianion also has 4 pi electrons (antiaromatic). Hence the cyclopropenyl cation is aromatic.

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