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==