Simple aromatic ring

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[edit] Table of simple aromatic rings
  Fused four-membered rings:
  Image:Benzocyclobutene_simple_structure.png
Benzocyclobutene		  
 
Five-membered rings: Fused five-membered rings:
  Image:Pentalene_simple_structure.png
Pentalene		  

Furan		 Image:Benzofuran_simple_structure.png
Benzofuran		 Image:Isobenzofuran_simple_structure.png
Isobenzofuran
Image:Pyrrole_simple_structure.png
Pyrrole		 Image:Indole_simple_structure.png
Indole		 Image:Isoindole_simple_structure.png
Isoindole
Image:Thiophene_simple_structure.png
Thiophene		 Image:Benzothiophene_simple_structure.png
Benzothiophene		 Image:Benzo-c-thiophene_simple_structure.png
Benzo[c]thiophene
Image:Imidazole_simple_structure.png
Imidazole		 Image:Benzimidazole_simple_structure.png
Benzimidazole		 Image:Purine.svg
Purine
Image:Pyrazole_simple_structure.png
Pyrazole		 Image:Indazole_simple_structure.png
Indazole		  
Image:Oxazole_simple_structure.png
Oxazole		 Image:Benzoxazole_simple_structure.png
Benzoxazole		  
Image:Isoxazole_simple_structure.png
Isoxazole		 Image:Benzisoxazole_simple_structure.png
Benzisoxazole		  
Image:Thiazole_simple_structure.png
Thiazole
Benzothiazole		  
 
Six-membered rings: Fused six-membered rings:  
Image:Benzene_simple_structure.png
Benzene		 Image:Naphthalene_simple_structure.png
Naphthalene		 Image:Anthracene_simple_structure.png
Anthracene
Image:Pyridine_simple_structure.png
Pyridine		 Image:Quinoline_simple_structure.png
Quinoline		 Image:Isoquinoline_simple_structure.png
Isoquinoline
Image:Pyrazine_simple_structure.png
Pyrazine		 Image:Quinoxaline_simple_structure.png
Quinoxaline		 Image:Acridine_simple_structure.png
Acridine
Image:Pyrimidine_simple_structure.png
Pyrimidine		 Image:Quinazoline_simple_structure.png
Quinazoline		  
Image:Pyridazine_simple_structure.png
Pyridazine		 Image:Cinnoline_simple_structure.png
Cinnoline		  


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

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