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From the following set of ring species, count the number that are aromatic: cyclooctatetraene, cyclopropenyl anion, cyclopropenyl cation, 1,3-cyclohexadiene, tropylium (cycloheptatrienyl) cation, cyclopentadienyl cation, cyclopentadienyl anion, a fused bicyclic aromatic ring system, and a polycyclic aromatic ring system.

1. 3
2. 4
3. 5
4. 6

Correct answer: 5

Solution

Evaluate each: cyclooctatetraene (8 pi, tub-shaped) - not aromatic; cyclopropenyl anion (4 pi) - antiaromatic; cyclopropenyl cation (2 pi, n=0) - aromatic; 1,3-cyclohexadiene (not fully conjugated) - not aromatic; tropylium cation (6 pi) - aromatic; cyclopentadienyl cation (4 pi) - antiaromatic; cyclopentadienyl anion (6 pi) - aromatic; the fused bicyclic aromatic and the polycyclic aromatic ring systems - aromatic. Aromatic count = cyclopropenyl cation + tropylium + cyclopentadienyl anion + fused bicyclic + polycyclic = 5.

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