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Among the following ring systems, identify the aromatic species: (a) a seven-membered carbocyclic cation with a complete cycle of conjugation, (b) a seven-membered carbocyclic anion, (c) a fused bicyclic anion, (d) a four-membered ring containing two double bonds. Which one is aromatic?

1. (a) the seven-membered ring cation
2. (b) the seven-membered ring anion
3. (c) the fused bicyclic anion
4. (d) the four-membered ring with two double bonds

Correct answer: (a) the seven-membered ring cation

Solution

The cycloheptatrienyl (tropylium) cation is a seven-membered, planar, fully conjugated ring. Removing one electron to form the cation leaves 6 pi electrons distributed over 7 carbons, satisfying Huckel's (4n+2) rule with n=1. This makes it aromatic and unusually stable. The seven-membered anion would carry 8 pi electrons (antiaromatic / non-aromatic), the four-membered ring with two double bonds (cyclobutadiene) has 4 pi electrons (antiaromatic), and the fused bicyclic anion does not meet the planar monocyclic Huckel count.

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