Benzene is the archetypical aromatic compound. It is planar, bond angles=120º, all carbon atoms in the ring are sp² hybridized, and the pi-orbitals are occupied by 6 electrons. The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavenging protons. Furan and pyrrole have heterocyclic five-membered rings, in which the heteroatom has at least one pair of non-bonding valence shell electrons. By hybridizing this heteroatom to a sp2 state, a p-orbital occupied by a pair of electrons and oriented parallel to the carbon p-orbitals is created. The resulting planar ring meets the first requirement for aromaticity, and the π-system is occupied by 6 electrons, 4 from the two double bonds and 2 from the heteroatom, thus satisfying the Hückel Rule.

Four illustrative examples of aromatic compounds are shown above. The sp2 hybridized ring atoms are connected by brown bonds, the π-electron pairs and bonds that constitute the aromatic ring are colored blue. Electron pairs that are not part of the aromatic π-electron system are black. The first example is azulene, a blue-colored 10 π-electron aromatic hydrocarbon isomeric with naphthalene. The second and third compounds are heterocycles having aromatic properties. Pyridine has a benzene-like six-membered ring incorporating one nitrogen atom. The non-bonding electron pair on the nitrogen is not part of the aromatic π-electron sextet, and may bond to a proton or other electrophile without disrupting the aromatic system. In the case of thiophene, a sulfur analog of furan, one of the sulfur electron pairs (colored blue) participates in the aromatic ring π-electron conjugation. The last compound is imidazole, a heterocycle having two nitrogen atoms. Note that only one of the nitrogen non-bonding electron pairs is used for the aromatic π-electron sextet. The other electron pair (colored black) behaves similarly to the electron pair in pyridine.

Heterocycles – cyclic structures in which the ring atoms may include oxygen or nitrogen – can also be aromatic.  Pyridine, for example, is an aromatic heterocycle.  In the bonding picture for pyridine, the nitrogen is sp²-hybridized, with two of the three sp² orbitals forming sigma overlaps with the sp² orbitals of neighboring carbon atoms, and the third nitrogen sp² orbital containing the lone pair.  The unhybridized p orbital contains a single electron, which is part of the 6 pi-electron system delocalized around the ring.

another image of orbitals in pyridine

Why do we not assume that the nitrogen in pyrrole is sp³-hybridized, like a normal secondary amine? The answer is simple: if it were, then pyrrole could not be aromatic, and thus it would not have the stability associated with aromaticity.  In general, if a molecule or group can be aromatic, it will be, just as water will always flow downhill if there is a downhill pathway available.

Imidazole is another important example of an aromatic heterocycle found in biomolecules – the side chain of the amino acid histidine contains an imidazole ring.

In imidazole, one nitrogen is ‘pyrrole-like’ (the lone pair contributes to the aromatic sextet) and one is ‘pyridine-like’ (the lone pair is located in an sp² orbital, and is not part of the aromatic sextet).