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. The IUPAC recommends using the Hantzsch-Widman nomenclature to name heterocyclic compounds.

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:

Heteroatom	Saturated	Unsaturated
Nitrogen	Aziridine	Azirine
Oxygen	Ethylene oxide (epoxides, oxiranes)	Oxirene
Sulfur	Thiirane (episulfides)	Thiirene

Those with two heteroatoms include:

Heteroatom	Saturated	Unsaturated
Oxygen	Dioxirane
Nitrogen		Diazirine

4-Membered rings

Compounds with one heteroatom:

Heteroatom	Saturated	Unsaturated
Nitrogen	Azetidine	Azete
Oxygen	Oxetane	Oxete
Sulfur	Thietane	Thiete

Compounds with two heteroatoms:

Heteroatom	Saturated	Unsaturated
Nitrogen	Diazetidine
Oxygen	Dioxetane	Dioxetene
Sulfur	Dithietane	Dithiete

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:

Heteroatom	Saturated	Unsaturated
Nitrogen	Pyrrolidine	Pyrroline Pyrrole
Oxygen	Tetrahydrofuran	Dihydrofuran Furan
Sulfur	Tetrahydrothiophene	Dihydrothiophene Thiophene
Phosphorus		Phosphole
Silicon		Silole
Arsenic		Arsole

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.

Heteroatom	Saturated	Unsaturated
Nitrogen	Imidazolidine Pyrazolidine	Imidazole Imidazoline Pyrazole Pyrazoline
Nitrogen/oxygen	Oxazolidine Isoxazolidine	Oxazole Oxazoline Isoxazole Isoxazoline
Nitrogen/sulfur	Thiazolidine Isothiazolidine	Thiazole Thiazoline Isothiazole Isothiazoline
Oxygen	Dioxolane
Oxygen/sulfur	Oxathiolane
Sulfur	Dithiolane

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.

Heteroatom	Saturated	Unsaturated
Nitrogen		Triazoles
Nitrogen/2-sulfur		Dithiazole
2-Nitrogen/oxygen		Furazan Oxadiazole
2-Nitrogen/sulfur		Thiadiazole

Five-member ring compounds with four heteroatoms:

Heteroatom	Saturated	Unsaturated
Nitrogen		Tetrazole

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:

Heteroatom	Saturated	Unsaturated
Nitrogen	Piperidine	Pyridine
Oxygen	Tetrahydropyran	Pyran
Sulfur	Thiane	Thiopyran

With two heteroatoms:

Heteroatom	Saturated	Unsaturated
Nitrogen	Piperazine	Diazines
Nitrogen / oxygen	Morpholine	Oxazine
Nitrogen / sulfur		Thiazine
Sulfur	Dithiane
Oxygen	Dioxane	Dioxin

With three heteroatoms:

Heteroatom	Saturated	Unsaturated
Nitrogen		Triazine
Oxygen	Trioxane

With four heteroatoms:

Heteroatom	Saturated	Unsaturated
Nitrogen		Tetrazine

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:

Heteroatom	Saturated	Unsaturated
Nitrogen	Azepane	Azepine
Oxygen	Oxepane	Oxepine
Sulfur	Thiepane	Thiepine

Those with two heteroatoms include:

Heteroatom	Saturated	Unsaturated
Nitrogen		Diazepine
Nitrogen/sulfur		Thiazepine

8-Membered rings

Compounds with one heteroatom include:

Heteroatom	Saturated	Unsaturated
Nitrogen	Azocane	Azocine
Oxygen	Oxecane
Sulfur	Thiocane

Images

	Saturated			Unsaturated
Heteroatom	Nitrogen	Oxygen	Sulfur	Nitrogen	Oxygen	Sulfur
3-Ring
Name	Aziridine	Oxirane	Thiirane	Azirine	Oxirene	Thiirene
Structure
4-Ring
Name	Azetidine	Oxetane	Thietane	Azete	Oxete	Thiete
Structure
5-Ring
Name	Pyrrolidine	Tetrahydrofuran	Tetrahydrothiophene	Pyrrole	Furan	Thiophene
Structure
6-Ring
Name	Piperidine	Tetrahydropyran	Thiane	Pyridine	Pyrylium	Thiopyran
Structure
7-Ring
Name	Azepane	Oxepane	Thiepane	Azepine	Oxepine	Thiepine
Structure

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

^ Eicher, T.; Hauptmann, S. (2nd ed. 2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications. Wiley-VCH. ISBN 3527307206. {{cite book}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)

External links

[Eicher-1] Eicher, T.; Hauptmann, S. (2nd ed. 2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications. Wiley-VCH. ISBN 3527307206. {{cite book}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)

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