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

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

J & K CETJ & K CET 2018Hydrocarbons

Solution:

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