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Which of the following species is non-aromatic (i.e., neither aromatic nor antiaromatic)?

1. Cyclopentadienyl anion
2. Cyclooctatetraene
3. Cyclobutadiene
4. Cyclopentadienyl cation

Correct answer: Cyclooctatetraene

Solution

COT is non-planar (tub-shaped) due to angle strain, so it is not aromatic or antiaromatic — just non-aromatic. Cyclobutadiene is antiaromatic (4 pi e), cyclopentadienyl cation is antiaromatic (4 pi e), and cyclopentadienyl anion is aromatic (6 pi e).

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