Question

Which one of the following is not aromatic?

• A. Cyclopentadienyl anion
• B. Cycloheptatrienyl cation
• C. Cyclooctatetraene
• D. Thiophene

Answer 1

Answer: (C)

Cyclooctatetraene

Answer 2

Cyclooctatetraene is not aromatic in nature. In cyclooctatetraene, delocalisation of $\pi -$ electrons takes place but Huckel's rule is not followed. According to Huckel's rule, it should contains $10\pi (4n+2)$ electrons but it contains only $8\pi$ electrons.
$\pi$ electron $(8)\ne (4n+2)=10$ (i.e., Huckel rule is not followed), hence it is not aromatic.