heterocyclic

Pyridine, a simple heterocyclic compound

Heterocyclic compounds are organic compounds containing at least one atom of carbon, and at least one element other than carbon, such as sulfur, oxygen or nitrogen within a ring structure.^[1] These structures may comprise either simple aromatic rings or non-aromatic rings. Some examples are pyridine (C₅H₅N), pyrimidine (C₄H₄N₂) and dioxane (C₄H₈O₂).

Note that compounds such as cyclopropane, an anaesthetic with explosive properties, and cyclohexane, a solvent, are not heterocyclic; they are cycloalkanes. The stem '-cyclic' implies a ring structure, whereas 'hetero' refers to an atom other than carbon, as above. Many heterocyclic compounds, including some amines, are carcinogenic.

Heterocyclic chemistry is the chemistry branch dealing exclusively with synthesis, properties, and applications of heterocycles. IUPAC name is Hantzsch-Widman nomenclature.

3-Membered rings

Heterocycles with three atoms in the ring are more reactive because of ring strain. Those containing one heteroatom are, in general, stable. Those with two heteroatoms are more likely to occur as reactive intermediates. Common 3-membered heterocycles are:

Those with two heteroatoms include:

4-Membered rings

Compounds with one heteroatom:

Compounds with two heteroatoms:

5-Membered rings

With heterocycles containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.

Five-membered rings with a single heteroatom:

The 5-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are collectively called the azoles. Thiazoles and isothiazoles contain a sulfur and a nitrogen atom in the ring. Dithiolanes have two sulfur atoms.

A large group of 5-membered ring compounds with three heteroatoms also exists. One example is dithiazoles that contain two sulfur and a nitrogen atom.

Five-member ring compounds with four heteroatoms:

With 5-heteroatoms, the compound may be considered inorganic rather than heterocyclic. Pentazole is the all nitrogen heteroatom unsaturated compound.

6-Membered rings

Six membered rings with a single heteroatom:

With two heteroatoms:

With three heteroatoms:

With four heteroatoms:

The hypothetical compound with six nitrogen heteroatoms would be hexazine.

7-Membered rings

With 7-membered rings, aromatic stabilization is not available. Compounds with one heteroatom include:

Those with two heteroatoms include:

Fused rings

Heterocyclic rings systems that are formally derived by fusion with other rings, either carbocyclic or heterocyclic, have a variety of common and systematic names. For example, with the benzo-fused unsaturated nitrogen heterocycles, pyrrole provides indole or isoindole depending on the orientation. The pyridine analog is quinoline or isoquinoline. For azepine, benzazepine is the preferred name. Similarly, the compounds with two benzene rings fused to the central heterocycle are carbazole, acridine, and dibenzoazepine.

References

1. ^ Eicher, T.; Hauptmann, S. (2nd ed. 2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications. Wiley-VCH. ISBN 3527307206. 

External links

• Hantzsch-Widman nomenclature, IUPAC
• Heterocyclic amines in cooked meat, US CDC
• List of known and probable carcinogens, American Cancer Society
• List of known carcinogens by the State of California, Proposition 65 (more comprehensive)