Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom, respectively. Both possess two lone pairs of electrons. Imidazole differs from the other three in having two nitrogens as heteroatoms with one lone pair of electrons on each nitrogen atom. In all four heterocyclic compounds, all the atoms are sp2 hybridized, including the heteroatoms.
Consequently, all the atoms have one unhybridized p orbital perpendicular to the ring plane. In each case, all the carbon atoms contribute one π electron, and heteroatoms contribute one lone pair of electrons, occupying the unhybridized p orbital. This completes the aromatic sextet and makes a closed loop of 6 π electrons.
The additional lone pair of electrons of oxygen and sulfur heteroatom in furan and thiophene, respectively, occupy the sp2 hybrid orbital, which lies perpendicular to the p orbital, and do not participate in the π system. In the case of imidazole, the lone pair of electrons on the second nitrogen atom occupies the sp2 hybrid orbital and does not participate in the aromatic π system. Therefore, the lone pair of electrons of only one nitrogen atom is involved in the aromatic sextet.
