Chemistry:Heterocyclic compound

Short description: Molecule with one or more rings composed of different elements

Structures and names of common heterocyclic compounds

Pyridine, a 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). Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of organic heterocycles.

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. More than half of known compounds are heterocycles. 59% of US FDA-approved drugs contain nitrogen heterocycles.

Classification

The study of organic heterocyclic chemistry focuses especially on organic unsaturated derivatives, and the preponderance of work and applications involves unstrained organic 5- and 6-membered rings. Included are pyridine, thiophene, pyrrole, and furan. Another large class of organic heterocycles refers to those fused to benzene rings. For example, the fused benzene derivatives of pyridine, thiophene, pyrrole, and furan are quinoline, benzothiophene, indole, and benzofuran, respectively. The fusion of two benzene rings gives rise to a third large family of organic compounds. Analogs of the previously mentioned heterocycles for this third family of compounds are acridine, dibenzothiophene, carbazole, and dibenzofuran, respectively.

Heterocyclic organic compounds can be usefully classified based on their electronic structure. The saturated organic heterocycles behave like the acyclic derivatives. Thus, piperidine and tetrahydrofuran are conventional amines and ethers, with modified steric profiles. Therefore, the study of organic heterocyclic chemistry focuses on organic unsaturated rings.

Inorganic rings

Some heterocycles contain no carbon. Examples are borazine (B₃N₃ ring), hexachlorophosphazenes (P₃N₃ rings), and tetrasulfur tetranitride S₄N₄. In comparison with organic heterocycles, which have numerous commercial applications, inorganic ring systems are mainly of theoretical interest. IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.

Notes on lists

• "Heteroatoms" are atoms in the ring other than carbon atoms.
• Some of the names refer to classes of compounds rather than individual compounds.
• Also no attempt is made to list isomers.

3-membered rings

Although subject to ring strain, 3-membered heterocyclic rings are well characterized.

Three-membered rings with one heteroatom

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

Three-membered rings with two heteroatoms

Heteroatoms	Saturated	Unsaturated
2× Nitrogen	Diaziridine	Diazirine
Nitrogen + oxygen	Oxaziridine	Oxazirine
2× Oxygen	Dioxirane (highly unstable)

4-membered rings

Four-membered rings with one heteroatom

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

Four-membered rings with two heteroatoms

Heteroatoms	Saturated	Unsaturated
2× Nitrogen	Diazetidine	Diazete
2× Oxygen	Dioxetane	Dioxete
2× 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.

Heteroatoms	Saturated	Unsaturated (and partially unsaturated)
2× nitrogen	Imidazolidine Pyrazolidine	Imidazole (Imidazoline) Pyrazole (Pyrazoline)
Oxygen + sulfur	1,3-Oxathiolane 1,2-Oxathiolane	Oxathiole (Oxathioline) Isoxathiole
Nitrogen + Oxygen	Oxazolidine Isoxazolidine	Oxazole (Oxazoline) Isoxazole
Nitrogen + sulfur	Thiazolidine Isothiazolidine	Thiazole (Thiazoline) Isothiazole
2× oxygen	Dioxolane
2× sulfur	Dithiolane	Dithiole

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 atom.

Heteroatoms	Saturated	Unsaturated
N N N		Triazoles
N N O		Furazan Oxadiazole
N N S		Thiadiazole
N O O		Dioxazole
N S S		Dithiazole
N N N N		Tetrazole
N N N N O		Oxatetrazole
N N N N S		Thiatetrazole
N N N N N		Pentazole

6-membered rings

Six-membered rings with one heteroatom

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

Six-membered rings with two heteroatoms

Heteroatom	Saturated	Unsaturated
Nitrogen / nitrogen	Diazinane	Diazine
Oxygen / nitrogen	Morpholine	Oxazine
Sulfur / nitrogen	Thiomorpholine	Thiazine
Oxygen / Sulfur	Oxathiane	Oxathiin
Oxygen / oxygen	Dioxane	Dioxine
Sulfur / sulfur	Dithiane	Dithiin
Boron / nitrogen		1,2-Dihydro-1,2-azaborine

Six-membered rings with three heteroatoms

Heteroatom	Saturated	Unsaturated
Nitrogen	Triazinane	Triazine
Oxygen	Trioxane
Sulfur	Trithiane

Six-membered rings with four heteroatoms

Heteroatom	Saturated	Unsaturated
Nitrogen		Tetrazine

Carborazine is a six-membered ring with two nitrogen heteroatoms and two boron heteroatom.

Six-membered rings with five heteroatoms

Heteroatom	Saturated	Unsaturated
Nitrogen		Pentazine

Six-membered rings with six heteroatoms

The hypothetical chemical compound with six nitrogen heteroatoms would be hexazine.

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

7-membered rings

In a 7-membered ring, the heteroatom must be able to provide an empty π-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

Borazocine is an eight-membered ring with four nitrogen heteroatoms and four boron heteroatoms.

9-membered rings

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

Images of rings with one heteroatom

Names in italics are retained by IUPAC and they do not follow the Hantzsch-Widman nomenclature

	Saturated			Unsaturated
Heteroatom	Nitrogen	Oxygen	Sulfur	Nitrogen	Oxygen	Sulfur
3-atom ring	Aziridine	Oxirane	Thiirane	Azirine	Oxirene	Thiirene
		Structure of oxirane	Structure of thiirane	Structure of azirine	Structure of oxirene
4-atom ring	Azetidine	Oxetane	Thietane	Azete	Oxete	Thiete
		Structure of oxetane	Structure of thietane	Structure of azete	Structure of oxete
5-atom ring	Pyrrolidine	Oxolane	Thiolane	Pyrrole	Furan	Thiophene
		Structure of oxolane			Structure of furan
6-atom ring	Piperidine	Oxane	Thiane	Pyridine	Pyran	Thiopyran
		Structure of oxane			Structure of pyran
7-atom ring	Azepane	Oxepane	Thiepane	Azepine	Oxepine	Thiepine
		Structure of oxepane	Structure of thiepane	Structure of azepine	Structure of oxepine
8-atom ring	Azocane	Oxocane	Thiocane	Azocine	Oxocine	Thiocine
		Structure of oxocane	Structure of thiocane	Structure of azocine	Structure of oxocine
9-atom ring	Azonane	Oxonane	Thionane	Azonine	Oxonine	Thionine
		Structure of oxonane	Structure of thionane	Structure of azonine	Structure of oxonine

Fused/condensed 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:

• 1818: Brugnatelli makes 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 chlorophyll 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.

Uses

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

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)