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Which of the following species is NOT aromatic? (A) A six-membered ring containing one nitrogen atom and two double bonds (pyridine-type) (B) A cyclooctatetraene-like eight-membered ring with four double bonds (C) A five-membered ring containing NH and two double bonds (pyrrole-type) (D) A five-membered ring containing N⁻ and two double bonds

1. A six-membered ring containing one nitrogen atom and two double bonds (pyridine-type)
2. A cyclooctatetraene-like eight-membered ring with four double bonds
3. A five-membered ring containing NH and two double bonds (pyrrole-type)
4. A five-membered ring containing N⁻ and two double bonds

Correct answer: A cyclooctatetraene-like eight-membered ring with four double bonds

Solution

Cyclooctatetraene (COT) is tub-shaped and non-planar, making it non-aromatic (8 pi electrons do not satisfy Huckel's 4n+2 rule for n=1). The other species — pyridine (6 pi), pyrrole (6 pi, lone pair from NH contributes), and isoelectronic N⁻ analogue (6 pi) — are all aromatic.

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