Question

Question: Which of the following heterocycles are aromatic?

Short answer

Answer

1. This compound has $6\mathrm{\pi }$electrons and is aromatic in nature.
2. This compound has a total of $6\mathrm{\pi }$electrons and is aromatic in nature.
3. This compound has a total of $8\mathrm{\pi }$electrons and is, therefore non-aromatic in nature.
4. This compound has $10\mathrm{\pi }$ electrons and is aromatic in nature.
5. This compound is non-aromatic in nature because it has $4\mathrm{\pi }$ electrons.
6. This compound has $6\mathrm{\pi }$electrons and is aromatic in nature.
7. This compound is aromatic in nature because it has $6\mathrm{\pi }$electrons.
8. This compound has a total of $10\mathrm{\pi }$electrons and is aromatic in nature.

Step-by-step solution

Aromatic compounds

The compounds which are cyclic in nature are planar, conjugated with $\left(4n+2\right)\mathrm{\pi }$electrons, and aromatic in nature.

The aromatic compounds are highly stable. The heterocyclic compounds which have oxygen, nitrogen, sulfur, and a lone pair of electrons are aromatic in nature. The lone pair of electrons on the hetero atom may or may not be a part of the delocalized system.

The aromatic compounds among the following heterocyclic compounds

The given heterocyclic compounds can be classified as aromatic compounds if the ring has $\left(4n+2\right)\mathrm{\pi }$electrons. The lone pair of electrons on an atom that is already a part of the multiple bonds cannot be delocalized on the ring. Therefore, such a lone pair of electrons cannot be counted in determining the aromaticity of the ring.

a. This molecule has six pi electrons. The lone pair of electrons from sulfur is also included to calculate the total number of pi electrons. Therefore, the given compound is aromatic in nature.

b. The given compound has a total of six pi electrons which includes a lone pair from the oxygen atom. Therefore, this compound is also aromatic in nature.

c. This compound had a total of eight pi electrons and is, therefore, not aromatic in nature.

d. This compound has ten pi electrons and is aromatic in nature.

e. This compound is non-aromatic in nature because it has four pi electrons.

f. This compound has six pi electrons after including a lone pair of electrons from the oxygen atom. This makes this given compound aromatic in nature.

g. This compound has six pi electrons including the lone pair of electrons from the nitrogen atom. Therefore, this compound is aromatic in nature.

h. This compound has total ten pi electrons including the one from the nitrogen atom which is singly bonded. The lone pair of electrons of nitrogen atoms which are forming a double bond, are not included in the total number of pi electrons.

Therefore, this compound is aromatic in nature.