A heterocyclic compound is an organic compound in which one or more of the carbon atoms in the backbone of the molecule has been replaced by an atom other than carbon. Typical hetero atoms include nitrogen, oxygen, and sulfur.
Pyridine (C 5H 5N), pyrrole (C 4H 5N), furan (C 4H 4O), and thiophene (C 4H 4S) are examples of heteroaromatic compounds.
Because these compounds are monocyclic aromatic compounds, they must obey Hückel's Rule. Hückel's Rule requires 4 n + 2 π electrons, so the simplest aromatic compound should contain 6 π electrons ( n = 1). Pyrrole, furan, and thiophene appear, however, to have only 4 π electrons (2 π bonds). In systems such as these, the extra electrons needed to produce an aromatic condition come from the unshared electron pairs in sp 2 hybrid orbitals around the hetero atom.