Simple aromatic ring

Heterocyclic aromatic rings

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The nitrogen (N)-containing aromatic rings can be separated into basic aromatic rings that are easily protonated, and form aromatic cations and salts (e.g., pyridinium), and non-basic aromatic rings.

• In the basic aromatic rings, the lone pair of electrons is not part of the aromatic system and extends in the plane of the ring. This lone pair is responsible for the basicity of these nitrogenous bases, similar to the nitrogen atom in amines. In these compounds, the nitrogen atom is not connected to a hydrogen atom. Examples of basic aromatic rings are pyridine or quinoline. Several rings contain basic as well as non-basic nitrogen atoms, e.g., imidazole and purine.
• In the non-basic rings, the lone pair of electrons of the nitrogen atom is delocalized and contributes to the aromatic pi-electron system. In these compounds, the nitrogen atom is connected to a hydrogen atom. Examples of non-basic nitrogen-containing aromatic rings are pyrrole and indole.

In the oxygen- and sulfur-containing aromatic rings, one of the electron pairs of the heteroatoms contributes to the aromatic system (similar to the non-basic nitrogen-containing rings), whereas the second lone pair extends in the plane of the ring (similar to the primary nitrogen-containing rings).

Criteria for aromaticity

Main article: Aromaticity

• Molecule must be cyclic.
• Every atom in the ring must have an occupied p orbital, which overlaps with p orbitals on either side (completely conjugated).
• Molecule must be planar.
• It must contain an odd number of pairs of pi electrons; must satisfy Hückel's rule: (4n+2) pi electrons, where n is an integer starting at zero.

In contrast, molecules with 4n pi electrons are antiaromatic.

References

References

1. Clayden, J.. (2001). "Organic Chemistry". Oxford University Press.
2. (2003). "The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications". Wiley-VCH.