Question

Identify the anti-aromatic system from among the given options.

• A. Benzene
• B. $[14]$ - Annulene
• C. $[18]$ - Annulene
• D. Cyclo-octatetraene

Answer 1

Answer: (A), (B), (C), (D)

Benzene, $[14]$ annulene and $[18]$ annulene are aromatic. Benzene is fully planar, thought $[14]$ and $[18]$ annulene with all trans double bonds (placing the hydrogens inside the ring) can achive the planar conformation needed for aromaticity, with $[14]$ and $[18]$ annulene following Hückel's rule $(4n+2\pi$ electrons)
Cyclooctatetraene $(C_8{H}_8)$ has $8\pi$ electrons, so it is a misconception that it is anti-aromatic $(4n$ system, $n=2)$ However, actually the molecule takes a tub- shaped conformation in its native state Since, it is not a planar molecule, it becomes non-aromatic instead of anti-aromatic