17.11 : 五元杂环芳香族化合物：概述

Cyclic aromatic compounds having one or more heteroatoms are termed heterocyclic aromatic compounds.

Five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole, having nitrogen, oxygen, and sulfur as heteroatoms.

In these planar compounds, all atoms are sp² hybridized, including the heteroatom.

As evident from the π bonding structure of pyrrole, one π electron from each carbon and one lone pair of electrons from the heteroatom occupy the unhybridized 2p orbitals.

These five orbitals overlap, forming a closed-loop of 6 π electrons, making pyrrole aromatic.

In furan and thiophene, the additional lone pairs of electrons occupy an sp² hybrid orbital and are not  part of the π  system.

Imidazole has two nitrogen atoms. The lone pair on one of the nitrogens is part of the aromatic sextet. The lone pair on the second nitrogen occupies an sp² hybrid orbital and is not involved in the π system.