Question

Question: Label each compound as aromatic, antiaromatic, or not aromatic. Assume all completely conjugated rings are planar.

Short answer

Answer

1. Not an aromatic compound.
2. Aromatic compound.
3. Not an aromatic compound.
4. Anti-aromatic compound.

Step-by-step solution

Aromatic compounds

The compounds which are cyclic and planar in structure, and are completely conjugated with a total of $\left(4n+2\right)\mathrm{\pi }$electrons are known as aromatic compounds. The aromatic compounds are more stable than the similar acyclic compounds.

The cyclic compounds which are planar and are also completely conjugated with $4n\mathrm{\pi }$electrons are known as antiaromatic compounds. The antiaromatic compound is less stable than the similar acyclic compound with the same number of role="math" localid="1648748369735" $\mathbf{\pi }$electrons.

The compound which does not have either the $4n\mathrm{\pi }$electrons or the$\left(4n+2\right)\mathrm{\pi }$ electrons are known as the non-aromatic compounds. The non-aromatic compound is similarly stable to the acyclic compound with the same number of electrons.

Classification of the given compounds as aromatic, antiaromatic, and not aromatic compounds.

1. This compound has two carbon atoms which are not $s{p}^2$hybridized. These $s{p}^2$hybridized carbon atoms are marked with a * in the following structure. Therefore, this compound is not aromatic in nature.
2. This compound has a total of $10\mathrm{\pi }$electrons which are present in a $10$membered ring. Therefore, this compound is aromatic in nature.
3. This compound has one carbon atom marked with a * which is not $s{p}^2$hybridized. This marked carbon atom is $s{p}^3$hybridized which makes the compound non-aromatic in nature.
4. The given compound has a total of four $\mathrm{\pi }$electrons. As a result, this compound is antiaromatic in nature.