Which one of the following is not aromatic?

cyclooctatetraene

The correct answer is option 3. cyclooctatetraene.

Let us delve into the aromaticity of each compound listed and why cyclooctatetraene (option 3) is not aromatic, despite having a cyclic structure.

1. Cyclopentadienyl Anion (\(C_5H_5^-\))

Cyclic and Planar: Yes, the cyclopentadienyl anion is a planar molecule with a pentagonal ring structure.

Conjugation: Each carbon atom in the cyclopentadienyl ring is sp² hybridized, allowing for a continuous overlap of p-orbitals throughout the ring.

Hückel's Rule: The cyclopentadienyl anion has 6 π-electrons (4n + 2 with n=1), satisfying Hückel's rule for aromaticity.

Conclusion: The cyclopentadienyl anion is aromatic due to its planarity, conjugation, and fulfilling Hückel's rule.

2. Cycloheptatrienyl Cation (\(C_7H_7^+\))

Cyclic and Planar: Yes, the cycloheptatrienyl cation is a planar molecule with a heptagonal ring structure.

Conjugation: Each carbon atom in the cycloheptatrienyl ring is sp² hybridized, allowing for a continuous overlap of p-orbitals throughout the ring.

Hückel's Rule: The cycloheptatrienyl cation has 6 π-electrons (4n + 2 with n=1), satisfying Hückel's rule for aromaticity.

Conclusion: The cycloheptatrienyl cation is aromatic due to its planarity, conjugation, and fulfilling Hückel's rule.

3. Cyclooctatetraene (\(C_8H_8\))

Cyclic: Yes, cyclooctatetraene has a cyclic structure.

Planarity: No, cyclooctatetraene is not planar under normal conditions. It adopts a tub-shaped or "tub puckered" conformation to relieve angle strain caused by the 90-degree bond angles in an ideal planar structure.

Conjugation: While cyclooctatetraene has alternating single and double bonds, the non-planar structure disrupts effective overlap of p-orbitals across all eight carbon atoms. The p-orbitals do not overlap continuously due to the puckered shape, preventing it from maintaining aromaticity.

Hückel's Rule: Since cyclooctatetraene is not planar and does not have effective conjugation, Hückel's rule cannot be directly applied. However, its non-planarity alone is sufficient to determine it is not aromatic.

Conclusion: Cyclooctatetraene is not aromatic due to its non-planar structure, which prevents effective conjugation and does not satisfy the conditions necessary for aromaticity.

4. Thiophene \((C_4H_4S)\)

Cyclic and Planar: Yes, thiophene is a planar molecule with a five-membered ring containing four carbon atoms and one sulfur atom.

Conjugation: Thiophene has a conjugated system of alternating single and double bonds, where the sulfur atom contributes a lone pair to the π-electron system. This conjugation allows for effective overlap of p-orbitals across the ring.

Hückel's Rule: Thiophene has 6 π-electrons (4n + 2 with n=1), satisfying Hückel's rule for aromaticity.

Conclusion: Thiophene is aromatic due to its planarity, conjugation, and fulfilling Hückel's rule.

Among the compounds listed, cyclooctatetraene (option 3) is not aromatic because it does not fulfill the requirement of being planar. Its non-planar structure disrupts the continuous overlap of p-orbitals necessary for aromatic stabilization. Therefore, the correct answer is: (3) Cyclooctatetraene