Heterocyclic compound

A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s).[1] Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of these heterocycles.[2]

Examples of heterocyclic compounds include all of the nucleic acids, the majority of drugs, most biomass (cellulose and related materials), and many natural and synthetic dyes. 59% of US FDA-approved drugs contain nitrogen heterocycles.[3]


Although heterocyclic chemical compounds may be inorganic compounds or organic compounds, most contain at least one carbon. While atoms that are neither carbon nor hydrogen are normally referred to in organic chemistry as heteroatoms, this is usually in comparison to the all-carbon backbone. But this does not prevent a compound such as borazine (which has no carbon atoms) from being labelled "heterocyclic". IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.

Heterocyclic compounds can be usefully classified based on their electronic structure. The saturated heterocycles behave like the acyclic derivatives. Thus, piperidine and tetrahydrofuran are conventional amines and ethers, with modified steric profiles. Therefore, the study of heterocyclic chemistry focuses especially on unsaturated derivatives, and the preponderance of work and applications involves unstrained 5- and 6-membered rings. Included are pyridine, thiophene, pyrrole, and furan. Another large class of heterocycles are fused to benzene rings, which for pyridine, thiophene, pyrrole, and furan are quinoline, benzothiophene, indole, and benzofuran, respectively. Fusion of two benzene rings gives rise to a third large family of compounds, respectively the acridine, dibenzothiophene, carbazole, and dibenzofuran. The unsaturated rings can be classified according to the participation of the heteroatom in the conjugated system, pi system.

3-membered rings

Heterocycles with three atoms in the ring are higher in energy and 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. The C-X-C bond angles (where X is a heteroatom) in oxiranes and aziridines are very close to 60° and the peripheral H-C-H bond angles are near to 180°.

Three-membered rings with one heteroatom

Heteroatom Saturated Unsaturated
Boron Borirane Borirene
Nitrogen Aziridines Azirine
Oxygen Oxirane (ethylene oxide, epoxides) Oxirene
Phosphorus Phosphirane Phosphirene
Sulfur Thiirane (episulfides) Thiirene

Three-membered rings with two heteroatoms

Heteroatom Saturated Unsaturated
Nitrogen Diaziridine Diazirine
Nitrogen/oxygen Oxaziridine
Oxygen Dioxirane

4-membered rings

Four-membered rings with one heteroatom

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

Four-membered rings with two heteroatoms

Heteroatom Saturated Unsaturated
Nitrogen Diazetidine Diazete
Oxygen Dioxetane Dioxete
Sulfur Dithietane Dithiete

5-membered rings

Five-membered rings with one heteroatom

Heteroatom Saturated Unsaturated
Antimony Stibolane Stibole
Arsenic Arsolane Arsole
Bismuth Bismolane Bismole
Boron Borolane Borole
Nitrogen Pyrrolidine ("Azolidine" is not used) Pyrrole ("Azole" is not used)
Oxygen Tetrahydrofuran Furan
Phosphorus Phospholane Phosphole
Selenium Selenolane Selenophene
Silicon Silacyclopentane Silole
Sulfur Tetrahydrothiophene Thiophene
Tellurium Tellurophene
Tin Stannolane Stannole

Five-membered rings with two heteroatoms

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 (and partially unsaturated)
Nitrogen/nitrogen Imidazolidine
Imidazole (Imidazoline)
Pyrazole (Pyrazoline)
Nitrogen/oxygen Oxazolidine
Oxazole (Oxazoline)
Nitrogen/sulfur Thiazolidine
Thiazole (Thiazoline)
Oxygen/oxygen Dioxolane
Sulfur/sulfur Dithiolane

Five-membered rings with at least three heteroatoms

A large group of 5-membered ring compounds with three or more heteroatoms also exists. One example is the class of dithiazoles, which contain two sulfur atoms and one nitrogen atoms.

Heteroatom Saturated Unsaturated
3 × Nitrogen Triazoles
2 × Nitrogen / 1 × oxygen Furazan
2 × Nitrogen / 1 × sulfur Thiadiazole
1 × Nitrogen / 2 × oxygen Dioxazole
1 × Nitrogen / 2 × sulfur Dithiazole
4 × Nitrogen Tetrazole
4 × Nitrogen/1 × Oxygen Oxatetrazole
4 × Nitrogen/1 × Sulfur Thiatetrazole
5 × Nitrogen Pentazole

6-membered rings

Six-membered rings with one heteroatom

Heteroatom Saturated Unsaturated Ions
Antimony Stibinin[4]
Arsenic Arsinane Arsinine
Bismuth Bismin[5]
Boron Borinane Borinine Boratabenzene anion
Germanium Germinane Germine
Nitrogen Piperidine (Azinane is not used) Pyridine (Azine is not used) Pyridinium cation
Oxygen Tetrahydropyran Pyran (2H-Oxine is not used) Pyrylium cation
Phosphorus Phosphinane Phosphinine
Selenium Selenopyrylium cation
Silicon Silinane Siline
Sulfur Thiane Thiopyran (2H-Thiine is not used) Thiopyrylium cation
Tin Stanninane Stannine

Six-membered rings with two heteroatoms

Heteroatom Saturated Unsaturated
Nitrogen / nitrogen Piperazine Diazine
Oxygen / nitrogen Morpholine Oxazine
Sulfur / nitrogen Thiomorpholine Thiazine
Oxygen / oxygen Dioxane Dioxine
Sulfur / sulfur Dithiane Dithiin

Six-membered rings with three heteroatoms

Heteroatom Saturated Unsaturated
Nitrogen Hexahydro-1,3,5-triazine Triazine
Oxygen Trioxane
Sulfur Trithiane

Six-membered rings with four heteroatoms

Heteroatom Saturated Unsaturated
Nitrogen Tetrazine

Six-membered rings with five heteroatoms

Heteroatom Saturated Unsaturated
Nitrogen Pentazine

Six-membered rings with six heteroatoms

The hypothetical compound with six nitrogen heteroatoms would be hexazine.

Borazine is a six-membered ring with three nitrogen heteroatoms and three boron heteroatom.

7-membered rings

With 7-membered rings, the heteroatom must be able to provide an empty pi orbital (e.g., boron) for "normal" aromatic stabilization to be available; otherwise, homoaromaticity may be possible. Compounds with one heteroatom include:

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

Those with two heteroatoms include:

Heteroatom Saturated Unsaturated
Nitrogen Diazepane Diazepine
Nitrogen/sulfur Thiazepine

8-membered rings

Heteroatom Saturated Unsaturated
Nitrogen Azocane Azocine
Oxygen Oxocane Oxocine
Sulfur Thiocane Thiocine

9-membered rings

Heteroatom Saturated Unsaturated
Nitrogen Azonane Azonine
Oxygen Oxonane Oxonine
Sulfur Thionane Thionine


Names in italics are retained by IUPAC and they do not follow the Hantzsch-Widman nomenclature
3-Atom Ring AziridineOxiraneThiiraneAzirineOxireneThiirene
4-Atom Ring AzetidineOxetaneThietaneAzeteOxeteThiete
5-Atom Ring PyrrolidineOxolaneThiolanePyrroleFuranThiophene
6-Atom Ring PiperidineOxaneThianePyridinePyranThiopyran
7-Atom Ring AzepaneOxepaneThiepaneAzepineOxepineThiepine
8-Atom Ring AzocaneOxocaneThiocaneAzocineOxocineThiocine
9-Atom Ring AzonaneOxonaneThionaneAzonineOxonineThionine

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. Likewise, the compounds with two benzene rings fused to the central heterocycle are carbazole, acridine, and dibenzoazepine. Thienothiophene are the fusion of two thiophene rings. Phosphaphenalenes are a tricyclic phosphorus-containing heterocyclic system derived from the carbocycle phenalene.

History of heterocyclic chemistry

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry. Some noteworthy developments:[6]
1818: Brugnatelli isolates alloxan from uric acid
1832: Dobereiner produces furfural (a furan) by treating starch with sulfuric acid
1834: Runge obtains pyrrole ("fiery oil") by dry distillation of bones
1906: Friedlander synthesizes indigo dye, allowing synthetic chemistry to displace a large agricultural industry
1936: Treibs isolates chlorophyl derivatives from crude oil, explaining the biological origin of petroleum.
1951: Chargaff's rules are described, highlighting the role of heterocyclic compounds (purines and pyrimidines) in the genetic code.


Heterocyclic compounds are pervasive in many areas of life sciences and technology.[2] Many drugs are heterocyclic compounds.[7]


  1. IUPAC Gold Book heterocyclic compounds
  2. Thomas L. Gilchrist "Heterocyclic Chemistry" 3rd ed. Addison Wesley: Essex, England, 1997. 414 pp. ISBN 0-582-27843-0.
  3. Edon Vitaku, David T. Smith, Jon T. Njardarson (2014). "Analysis of the Structural Diversity, Substitution Patterns, and Frequency of Nitrogen Heterocycles among U.S. FDA Approved Pharmaceuticals". J. Med. Chem. 57: 10257-10274. doi:10.1021/jm501100b.CS1 maint: multiple names: authors list (link)
  4. "Stibinin". chemspider. Royal Society of Chemistry. Retrieved 11 June 2018.
  5. "Bismin". ChemSpider. Royal Society of Chemistry. Retrieved 11 June 2018.
  6. Campaigne, E. (1986). "Adrien Albert and the rationalization of heterocyclic chemistry". Journal of Chemical Education. 63 (10): 860. doi:10.1021/ed063p860.
  7. "IPEXL.com Multilingual Patent Search, Patent Ranking". www.ipexl.com.
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