Question: Classify each compound as aromatic, antiaromatic, or not aromatic.

The compounds which contain $\left(4n+2\right)\mathrm{\pi }$electrons are called aromatic compounds. The aromatic compounds are very much stable in nature.

The compounds which have the $4\mathrm{n\pi }$ electrons are called the anti-aromatic compounds. These are unstable in nature.

The compounds which may be non-cyclic, non-planar, or which do not have the conjugated $\mathbf{\pi }$electron system are known as the non-aromatic compounds.

a. The structure of the given compound is following:

Compound a

This compound is not an aromatic compound. It has$2\mathrm{\pi }$electrons. There are no other electrons which can participate in the conjugation.

Compound b

This compound is aromatic in nature. It has $6\mathrm{\pi }$electrons. The lone pair of the nitrogen atom with the single bond will also participate in the conjugation.

c. The structure of the given compound is following:

Compound c

This compound is aromatic in nature. It has $8\mathrm{\pi }$electrons. The double bond present in the other ring does not involve in the conjugation because of the presence of one $s{p}^3$carbon atom.