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From the species listed below, identify which are aromatic: (A) a benzene ring bearing an exocyclic =CH2 (methylenecyclohexadiene type) (B) benzene (C) cyclopentadienyl anion (D) cyclopentadienyl cation Select the correct combination.

1. (A) and (B) only
2. (B) and (C) only
3. (B), (C) and (D) only
4. (A), (B) and (C) only

Correct answer: (B) and (C) only

Solution

Benzene has 6 pi electrons (n=1) and is aromatic. The cyclopentadienyl anion has 6 pi electrons in a planar fully conjugated ring -> aromatic. The cyclopentadienyl cation has only 4 pi electrons (4n, n=1) -> antiaromatic, not aromatic. The exocyclic methylene structure breaks full ring conjugation (one ring carbon is sp2 with the double bond exocyclic), so it is not aromatic. Hence only (B) and (C).

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