Question

How can we identify aromatic,non aromatic and anti aromatic structure?

Answer 1

Well, what does the Huckel criterion say?

Explanation:

Aromaticity demands `4n+2` `pi` electrons delocalized around a ring....and benzene, toluene, cyclopentadienyl anion, and aromatic heterocycles fulfil this criterion. So too does `"COT dianion"`, `C_8H_8^(2-)`. And non-aromatic cyclic compounds do not meet this requirement.

On the other hand, so-called `"antiaromatic"` compounds have `4n` `pi` electrons...cyclobutadiene WOULD meet this criterion, however, its antiaromaticity means that it is an inherently unstable beasty, and requires special experiments to give evidence of its independent existence.