Simple aromatic ring
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Simple aromatic rings are aromatic organic compounds (also known as arenes or aromatics) that consist only of conjugated planar ring systems with delocalized pi electron clouds instead of discrete alternating single and double bonds. Typical simple aromatic compounds are benzene and indole.
Simple aromatic rings can be heterocyclic if they contain non-carbon ring atoms, e.g. oxygen, nitrogen, or sulfur.
Simple monocyclic aromatic rings are usually five-membered rings like pyrrole or six-membered rings like pyridine. Fused aromatic rings like naphthalene or purine consist of monocyclic rings that share their connecting bonds.
The nitrogen (N) containing aromatic rings can be separated into non-basic and basic aromatic rings:
- 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 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. Under acidic conditions these compounds get protonated and form aromatic cations (e.g. pyridinium).
In the oxygen (O) and sulfur (S) containing aromatic rings one of the electron pairs of the heteroatoms contributes to the aromatic system (similar to the non-basic nitrogen containing rings), while the second lone pair extends in the plane of the ring (similar to the basic nitrogen containing rings).
[edit] Criteria for Aromaticity
- Molecule must be cyclic.
- Every atom in the ring must have a "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 Huckel's rule: (4n+2) pi electrons, where n is an integer starting at zero.
In contrast, molecules with 4n pi electrons are antiaromatic.
[edit] See also
- Polycyclic aromatic compounds
- Polycyclic aromatic hydrocarbons (PAH)
