Aromaticity in compounds other than benzene
The concept of aromaticity can be extended to other cyclic compounds that contain atoms other than carbon and hydrogen. The structural requirement for these compounds is to contain one or more rings with pi electrons delocalized all the way around them. In heterocyclic aromatic compounds, at least one carbon atom is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of heterocyclic aromatic compounds are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom. Many of these heterocyclic aromatic compounds are components of important biological molecules, such as the bases of DNA and RNA, and medicinal drugs.
Figure 1. Examples of Heterocyclic Aromatic Compounds.
For a compound (or ion) to be considered aromatic, the molecule must be cyclic, planar, and must have 4n + 2 electrons in a conjugated system of p orbitals (usually on sp2-hybridized atoms. This is known as Hückel's rule.