Question: Which heterocycles are aromatic?

a.

b.

c.

d.

Heterocycles are organic molecules containing rings that are made up of carbon and other atoms. The heteroatoms can be nitrogen, oxygen, sulfur, etc.

• Heterocycles are aromatic if they contain conjugated double bonds and follow Huckel’s rule of aromaticity (4n+2 pi / non-bonding electrons).
• Heterocycles are non-aromatic if they do not contain a conjugated double bond.

Heteroatoms contain lone pairs of electrons. If the atom is attached to multiple bonds (double or triple), the lone pair is not included in the count for aromaticity.

(a)

Aromatic heterocycle

4 pi electrons from two double bonds and one lone pair from an O atom.

Total 6 non-bonding electrons.

$$\begin{gathered} 4n+2=6 \\ 4n=4 \\ n=1 \end{gathered}$$

Since the n value is not fractional, Huckel’s rule is followed.

Therefore, the heterocycle is aromatic.

(b)

Aromatic heterocycle

6 pi electrons from two double bonds and no lone pair from O atom.

Total 6 non-bonding electrons.

$$\begin{gathered} 4n+2=6 \\ 4n=4 \\ n=1 \end{gathered}$$

Since the n value is not fractional, Huckel’s rule is followed.

Therefore, the heterocycle is aromatic.

(c)

Non-aromatic heterocycle

4 pi electrons from two double bonds and one lone pair from each O atom.

Total 8 non-bonding electrons.

$$\begin{gathered} 4n+2=8 \\ 4n=6 \\ n=1·5 \end{gathered}$$

The n value is fractional so Huckel’s rule is not followed.

Therefore, the heterocycle is non-aromatic.

(d)

Aromatic heterocycle

6 pi electrons from two double bonds and no lone pair from N atom.

Total 6 non-bonding electrons.

$$\begin{gathered} 4n+2=6 \\ 4n=4 \\ n=1 \end{gathered}$$

The n value is not fractional. Therefore, the heterocycle is aromatic.