Heterocyclic compound
Overview
 
A heterocyclic compound is a cyclic compound
Cyclic compound
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...

 which has atoms of at least two different elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 as members of its ring(s). The counterparts of heterocyclic compounds are homocyclic compounds, the rings of which are made of a single element.

Although heterocyclic compound
Compound
Compound may refer to:* Chemical compounds, combinations of two or more elements* Compound , a cluster of buildings having a shared purpose, usually inside a fence or wall...

s may be inorganic
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

, most contain at least one carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom, and one or more atoms of elements other than carbon within the ring structure, such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 or nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

.
Since in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 non-carbons usually are considered to replace carbon atoms, they are called heteroatom
Heteroatom
In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...

s, meaning 'different from carbon and hydrogen' (rings of heteroatoms of the same element are homocyclic).
Encyclopedia
A heterocyclic compound is a cyclic compound
Cyclic compound
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...

 which has atoms of at least two different elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 as members of its ring(s). The counterparts of heterocyclic compounds are homocyclic compounds, the rings of which are made of a single element.

Although heterocyclic compound
Compound
Compound may refer to:* Chemical compounds, combinations of two or more elements* Compound , a cluster of buildings having a shared purpose, usually inside a fence or wall...

s may be inorganic
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

, most contain at least one carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom, and one or more atoms of elements other than carbon within the ring structure, such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 or nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

.
Since in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 non-carbons usually are considered to replace carbon atoms, they are called heteroatom
Heteroatom
In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...

s, meaning 'different from carbon and hydrogen' (rings of heteroatoms of the same element are homocyclic). The IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.

Heterocyclic chemistry is the branch of chemistry dealing with synthesis, properties, and applications of heterocycles.

Classification based on electronic structure

Heterocyclic compounds can be usefully classified based on their electronic structure. The saturated heterocycles behave like the acyclic derivatives. Thus, piperidine
Piperidine
Piperidine is an organic compound with the molecular formula 5NH. This heterocyclic amine consists of a six-membered ring containing five methylene units and one nitrogen atom...

 and tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...

 are conventional amines and ethers, with modified steric profiles. The study of heterocyclic chemistry therefore 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
Quinoline
Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown...

, benzothiophene
Benzothiophene
Benzothiophene is an aromatic organic compound with a molecular formula C8H6S and an odor similar to naphthalene . It occurs naturally as a constituent of petroleum-related deposits such as lignite tar. Benzothiophene has no household use...

, indole
Indole
Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an...

, and benzofuran
Benzofuran
Benzofuran is the heterocyclic compound consisting of fused benzene and furan rings. This colourless liquid is a component of coal tar. Benzofuran is the "parent" of many related compounds with more complex structures. For example, psoralen is a benzofuran derivative that occurs in several...

, respectively. Fusion of two benzene rings gives rise to a third large family of compounds, respectively the acridine
Acridine
Acridine, C13H9N, is an organic compound and a nitrogen heterocycle. Acridine is also used to describe compounds containing the C13N tricycle....

, dibenzothiophene
Dibenzothiophene
Dibenzothiophene is the organosulfur compound consisting of two benzene rings fused to a central thiophene ring. It is a colourless solid that is chemically somewhat similar to anthracene...

, carbazole
Carbazole
Carbazole is an aromatic heterocyclic organic compound. It has a tricyclic structure, consisting of two six-membered benzene ring fused on either side of a five-membered nitrogen-containing ring...

, and dibenzofuran
Dibenzofuran
Dibenzofuran, is a heterocyclic organic compound with the chemical structure shown at right. It is an aromatic compound that has two benzene rings fused to one furan ring in the middle. All of the numbered carbon atoms have a hydrogen atom bonded to each of them...

. The unsaturated rings can be classified according to the participation of the heteroatom in the pi-system.

3-Membered rings

Heterocycles with three atoms in the ring are more reactive because of ring strain
Ring strain
In organic chemistry, ring strain is the tendency of a cyclic molecule, such as cyclopropane, to destabilize when its atoms are in non-favorable high energy spatial orientations...

. Those containing one heteroatom are, in general, stable. Those with two heteroatoms are more likely to occur as reactive intermediates.

Common 3-membered heterocycles with one heteroatom are:
Heteroatom Saturated Unsaturated
Nitrogen Aziridine
Aziridine
Aziridines are organic compounds containing the aziridine functional group, a three-membered heterocycle with one amine group and two methylene groups...

Azirine
Oxygen Oxirane (ethylene oxide
Ethylene oxide
Ethylene oxide, also called oxirane, is the organic compound with the formula . It is a cyclic ether. This means that it is composed of two alkyl groups attached to an oxygen atom in a cyclic shape . This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered...

, epoxide
Epoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...

s)
Oxirene
Oxirene
Oxirene is a heterocyclic hypothesized chemical compound which contains an unsaturated three-membered ring containing two carbon atoms and one oxygen atom. As the configuration is extremely strained, no consensus exists whether the structure constitutes a molecule or whether it is merely a...

Sulfur Thiirane
Thiirane
Thiirane, more commonly known as ethylene sulfide, is the cyclic chemical compound with the formula C2H4S. It is the smallest sulfur-containing heterocycle. Like many organosulfur compounds, this species has a stench. Thiirane is also used to describe any derivative of the parent ethylene...

 (episulfide
Episulfide
Episulfides are a class of organic compounds that contain a saturated heterocyclic ring consisting of two carbon atoms and one sulfur atom. It is the sulfur analogue of an epoxide or aziridine. They are also known as thiiranes, olefin sulfides, thioalkylene oxides, and thiacyclopropanes.The parent...

s)
Thiirene
Thiirene
Thiirene is a heterocycle....



Those with two heteroatoms include:
Heteroatom Saturated Unsaturated
Nitrogen Diazirine
Diazirine
Diazirines are a class of organic molecules consisting of a carbon bound to two nitrogen atoms, which are double-bonded to each other, forming a cyclopropene-like ring...

Nitrogen/oxygen Oxaziridine
Oxaziridine
An oxaziridine is an organic molecule that features a three-membered heterocycle containing oxygen, nitrogen, and carbon.-History:Oxaziridine derivatives were first synthesized in the mid 1950s by Emmons and subsequently by Krimm and Horner and Jürgens...

Oxygen Dioxirane
Dioxirane
A dioxirane is a molecule containing a three-membered ring composed of one carbon and two oxygens. Somewhat unstable, they are used in organic synthesis as oxidizing reagents. The only dioxirane in common use is dimethyldioxirane , the oxirane derived from acetone....


4-Membered rings

Compounds with one heteroatom:
Heteroatom Saturated Unsaturated
Nitrogen Azetidine
Azetidine
Azetidine is a heterocyclic organic compound. It belongs to the class of four membered rings and it contains a nitrogen atom.-External links:* *...

Azete
Azete
Azete is a heterocyclic chemical compound consisting of an unsaturated four-membered ring with three carbon atoms and one nitrogen atom.-See also:* Azetidine, the saturated analog* Cyclobutadiene, the carbocyclic analog...

Oxygen Oxetane
Oxetane
Oxetane, or 1,3-propylene oxide, is an heterocyclic organic compound with the molecular formula C3H6O, having a four-membered ring with three carbon atoms and one oxygen atom....

Oxete
Sulfur Thietane
Thietane
Thietane is a heterocyclic compound containing a saturated four-membered ring with three carbon atoms and one sulfur atom....

Thiete
Thiete
Thiete is a heterocycle. It is more commonly encountered not on its own, but in anellated derivatives, several of which have been synthesized. Thietes are generally not very stable.- Configuration :...



Compounds with two heteroatoms:
Heteroatom Saturated Unsaturated
Nitrogen Diazetidine
Oxygen Dioxetane
Dioxetane
The chemical substance 1,2-dioxetane is an heterocyclic organic compound with formula C2O2H4, containing a ring of two adjacent oxygen atoms and two adjacent carbon atoms...

Dioxete
Sulfur Dithietane
Dithietane
Dithietanes are saturated heterocyclic compounds that contain two sulfur atoms and two sp3-hybridized carbon centers. Two isomers are possible for this class of organosulfur compounds:* 1,2-dithietanes, where the sulfur atoms are adjacent...

Dithiete
Dithiete
Dithietes are unsaturated heterocyclic compounds that contain two sulfur atoms and two sp2-hybridized carbon centers. Having 6 π electrons, dithietes are aromatic. Two isomers are possible for this small class of organosulfur compounds, 1,2-dithietes, where the sulfur atoms are adjacent, and...


5-Membered rings

With heterocycles containing five atoms, the unsaturated compound
Unsaturated compound
In organic chemistry, a saturated compound is a chemical compound that has of a chain of carbon atoms linked together by single bonds and has hydrogen atoms filling all of the other bonding orbitals of the carbon atoms. Alkanes are an example of saturated compounds...

s are frequently more stable because of aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

.

Five-membered rings with one heteroatom:
Heteroatom Saturated Unsaturated
Nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

Pyrrolidine
Pyrrolidine
Pyrrolidine, also known as tetrahydropyrrole, is an organic compound with the molecular formula C4H9N. It is a cyclic secondary amine with a five-membered heterocycle containing four carbon atoms and one nitrogen atom...

(Azolidine is not used)
Pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

(Azole is not used)
Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

Tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...

(Oxolane is rare)
Furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....

(Oxole is not used)
Sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

Thiolane Thiophene
Thiophene
Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene...

(Thiole is not used)
Boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

Borolane Borole
Borole
Borole is a - for the moment - theoretical heterocyclic organic compound, a five-membered ring with the formula C4H4BH. It is classified as a metallole. It can be viewed as an structural analog of pyrrole, with boron replacing the nitrogen atom of pyrrole. The unsubstituted compound has not been...

Phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

Phospholane Phosphole
Phosphole
Phosphole is the organic compound with the chemical formula C4H4PH; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle...

Arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

Arsolane Arsole
Arsole
Arsole, also called arsenole or arsacyclopentadiene, is an organoarsenic compound with the formula C4H4AsH. It is classified as a metallole and is isoelectronic to and related to pyrrole except that an arsenic atom is substituted for the nitrogen atom...

Antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

Stibolane Stibole
Stibole
Stibole is a theoretical heterocyclic organic compound, a five-membered ring with the formula C4H4SbH. It is classified as a metallole. It can be viewed as an structural analog of pyrrole, with antimony replacing the nitrogen atom of pyrrole...

Bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

Bismolane Bismole
Bismole
Bismole is a theoretical heterocyclic organic compound, a five-membered ring with the formula C4H4BiH. It is classified as a metallole. It can be viewed as an structural analog of pyrrole, with bismuth replacing the nitrogen atom of pyrrole. The unsubstituted compound has not been isolated due to...

Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

Silolane Silole
Tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

Stannolane Stannole
Stannole
Stannole is a organotin compound with the formula 4SnH2. It is classified as a metallole, i.e. an unsaturated five-membered ring containing a heteroatom. It is a structural analog of pyrrole, with tin replacing the nitrogen...



The 5-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are collectively called the azole
Azole
An azole is a class of five-membered nitrogen heterocyclic ring compounds containing at least one other non-carbon atom of either nitrogen, sulfur, or oxygen. The parent compounds are aromatic and have two double bonds; there are successively reduced analogs with fewer...

s. Thiazole
Thiazole
Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen; the term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular formula C3H3NS...

s and isothiazole
Isothiazole
An isothiazole is a type of organic compound containing a five-membered aromatic ring that consists of three carbon atoms, one nitrogen atom, and one sulfur atom. Isothiazole is a member of a class of compounds known as azoles...

s contain a sulfur and a nitrogen atom in the ring. Dithiolane
Dithiolane
A dithiolane is a sulfur heterocycle derived from cyclopentane by replacing two CH2 groups with thioether groups. The parent compounds are 1,2-dithiolane and 1,3-dithiolane.1,2-Dithiolanes, such as lipoic acid, are cyclic disulfides....

s have two sulfur atoms.
Heteroatom Saturated Unsaturated (and partially unsaturated)
Nitrogen/nitrogen Imidazolidine
Imidazolidine
Imidazolidine is a heterocyclic compound formally derived by the addition of four hydrogen atoms to imidazole. The intermediate, resulting from the addition of only two hydrogen atoms is called dihydroimidazole...


Pyrazolidine
Pyrazolidine
Pyrazolidine is a heterocyclic compound.-See also:* Pyrazole* Pyrazoline...

Imidazole
Imidazole
Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents...

(Imidazoline
Imidazoline
Imidazoline is a nitrogen-containing heterocycle with formula C3H6N2, derived from imidazole. The ring contains an imine bond, and the carbons at the 4 and 5 positions are singly bonded, rather than doubly bonded for the case of imidazole...

)
Pyrazole
Pyrazole
Pyrazole refers both to the class of simple aromatic ring organic compounds of the heterocyclic diazole series characterized by a 5-membered ring structure composed of three carbon atoms and two nitrogen atoms in adjacent positions, and to the unsubstituted parent compound...

(Pyrazoline)
Nitrogen/oxygen Oxazolidine
Oxazolidine
Oxazolidine is a five-membered ring compound consisting of three carbons, a nitrogen, a hydrogen, and an oxygen. The oxygen and NH are the 1 and 3 positions, respectively. In oxazolidine derivatives, there is always a carbon between the oxygen and the nitrogen . All of the carbons in oxazolidines...


Isoxazolidine
Oxazole
Oxazole
Oxazole is the parent compound for a vast class of heterocyclic aromatic organic compounds. These are azoles with an oxygen and a nitrogen separated by one carbon. Oxazoles are aromatic compounds but less so than the thiazoles...

 (Oxazoline
Oxazoline
Oxazoline is both the five-membered ring heterocyclic chemical compound with the formula C3H5NO and the class of compounds containing this ring.- See also :* Desoxazoline * Oxazole* Oxazolidine* Oxazolidinedione...

)
Isoxazole
Isoxazole
Isoxazole is an azole with an oxygen atom next to the nitrogen. It is also the class of compounds containing this ring.Isoxazole rings are found in some natural products, such as ibotenic acid. Isoxazoles also form the basis for a number of drugs, including the COX-2 inhibitor valdecoxib...

Nitrogen/sulfur Thiazolidine
Thiazolidine
Thiazolidines are a class of heterocyclic organic compounds with a 5-membered saturated ring with a thioether group and an amine group in the 1 and 3 positions, respectively. It is a sulfur analogue of oxazolidine. The drug pioglitazone contains a thiazolidine ring. It is a drug usually indicated...


Isothiazolidine
Thiazole
Thiazole
Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen; the term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular formula C3H3NS...

 (Thiazoline
Thiazoline
Thiazoline is a heterocyclic compound containing both sulfur and nitrogen in the ring. Although thiazoline itself is rarely encountered, its derivatives are often bioactive. For example, in a common post-translational modification, cysteine residues are converted into...

)
Isothiazole
Isothiazole
An isothiazole is a type of organic compound containing a five-membered aromatic ring that consists of three carbon atoms, one nitrogen atom, and one sulfur atom. Isothiazole is a member of a class of compounds known as azoles...

Oxygen/oxygen Dioxolane
Dioxolane
Dioxolane is a heterocyclic acetal with the chemical formula 2O2CH2. It is related to tetrahydrofuran by interchange of one oxygen for a CH2 group. The corresponding saturated 6-membered C4O2 rings are called dioxanes. The isomeric 1,2-dioxolane is an peroxide...

Sulfur/sulfur Dithiolane
Dithiolane
A dithiolane is a sulfur heterocycle derived from cyclopentane by replacing two CH2 groups with thioether groups. The parent compounds are 1,2-dithiolane and 1,3-dithiolane.1,2-Dithiolanes, such as lipoic acid, are cyclic disulfides....



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
3 × Nitrogen Triazole
Triazole
Triazole refers to either one of a pair of isomeric chemical compounds with molecular formula C2H3N3, having a five-membered ring of two carbon atoms and three nitrogen atoms.The two isomers are:*1,2,3-Triazole 100px*1,2,4-Triazole 100px...

s
2 × Nitrogen / 1 × oxygen Furazan
Oxadiazole
Oxadiazole
Oxadiazole is a heterocyclic aromatic chemical compound with the molecular formula C2H2N2O. There are four isomers of oxadiazole:1,2,4-Oxadiazole, 1,2,5-oxadiazole, and 1,3,4-oxadiazole are known, but the 1,2,3-isomer is unstable and reverts to the diazoketone tautomer...

2 × Nitrogen / 1 × sulfur Thiadiazole
1 × Nitrogen / 2 × sulfur Dithiazole


Five-member ring compounds with four heteroatoms:
Heteroatom Saturated Unsaturated
4 × Nitrogen Tetrazole
Tetrazole
Tetrazoles are a class of synthetic organic heterocyclic compound, consisting of a 5-member ring of four nitrogen and one carbon atom . The simplest is tetrazole itself, CN4H2. They are unknown in nature...



With 5-heteroatoms, the compound may be considered inorganic rather than heterocyclic. Pentazole
Pentazole
Pentazole is an aromatic chemical molecule consisting of a five-membered ring with all nitrogen atoms, one of which is bonded to a hydrogen atom. It has a molecular formula of HN5. Its SMILES structure is...

 is the all nitrogen heteroatom unsaturated compound.

6-Membered rings

Six membered rings with a single heteroatom:
Heteroatom Saturated Unsaturated
Nitrogen Piperidine
Piperidine
Piperidine is an organic compound with the molecular formula 5NH. This heterocyclic amine consists of a six-membered ring containing five methylene units and one nitrogen atom...

(Azinane is not used)
Pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

(Azine
Azine
Azines are a functional class of organic compounds, formed from the condensation reaction of two equivalents of an aldehyde or ketone with one equivalent of hydrazine...

 is not used)
Oxygen Oxane
Oxane
Oxane may refer to any of several distinct chemical compounds:*Tetrahydropyran*Oxirane...

Pyran
Pyran
In chemistry, a pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds...

(2H-Oxine is not used)
Sulfur Thiane
Thiane
Thiane is a heterocyclic compound containg a saturated six-membered ring with five carbon atoms and one sulfur atom....

Thiopyran
Thiopyran
Thiopyran , or thiopyrylium is a heterocyclic compound, a pyran whose oxygen atom has been replaced with a sulphur atom....

(2H-Thiine is not used)
Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

Salinane Siline
Germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

Germinane Germine
Tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

Stanninane Stannine
Boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

Borinane Borinine
Phosphorus Phosphinane Phosphinine
Arsenic Arsinane Arsinine


With two heteroatoms:
Heteroatom Saturated Unsaturated
Nitrogen / nitrogen Piperazine
Piperazine
Piperazine is an organic compound that consists of a six-membered ring containing two opposing nitrogen atoms. Piperazine exists as small alkaline deliquescent crystals with a saline taste....

Diazine
Diazine
Diazine refers to a group of organic compounds having the molecular formula C4H4N2. Each contains a benzene ring in which two of the C-H fragments have been replaced by isolobal nitrogen...

s
Oxygen / nitrogen Morpholine
Morpholine
Morpholine is an organic chemical compound having the chemical formula O2NH. This heterocycle, pictured at right, features both amine and ether functional groups. Because of the amine, morpholine is a base; its conjugate acid is called morpholinium...

Oxazine
Sulfur / nitrogen Thiomorpholine
Thiomorpholine
Thiomorpholine, C4H9NS , is a heterocyclic compound containing nitrogen and sulfur. It can be considered a thio derivative of morpholine. Thiomorpholine derivatives are commonly developed as pharmaceuticals....

Thiazine
Thiazine
Thiazines are organic compounds containing a ring of four carbon, one nitrogen and one sulfur atom. Chemicals that include thiazine are used for dyes, tranquilizers and insecticides.-See also:*Methylene blue*Phenothiazine*Heterocyclic compound*Morpholine...

Oxygen / oxygen Dioxane Dioxine
Sulfur / sulfur Dithiane
Dithiane
A dithiane is a heterocyclic compound composed of a cyclohexane core structure wherein two methylene units are replaced by sulfur centres. The three isomeric parent heterocycles are 1,2-dithiane, 1,3-dithiane and 1,4-dithiane....

Dithiine


With three heteroatoms:
Heteroatom Saturated Unsaturated
Nitrogen Triazine
Triazine
A triazine is one of three organic chemicals, isomeric with each other, whose molecular formula is 333 and whose empirical formula is CHN.- Structure :...

Oxygen Trioxane
Trioxane
Trioxane refers to a pair of isomeric organic compounds having the molecular formula C3H6O3. Each contains a six membered ring with three carbon atoms and three oxygen atoms.The two isomers are...



With four heteroatoms:
Heteroatom Saturated Unsaturated
Nitrogen Tetrazine
Tetrazine
Tetrazine is an unstable compound that consists of a six-membered aromatic ring containing four nitrogen atoms with the molecular formula C2H2N4. The name tetrazine is used in the nomenclature of derivatives of this compound...



The hypothetical compound with six nitrogen heteroatoms would be hexazine
Hexazine
Hexazine is a hypothetical allotrope of nitrogen composed of 6 nitrogen atoms arranged in a ring-like structure analogous to that of benzene. It would be the final member of the azabenzene series, having all of the methylidyne groups of the benzene molecule replaced with nitrogen atoms...

.

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
Homoaromaticity
Homoaromaticity in organic chemistry refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom...

 may be possible. Compounds with one heteroatom include:
Heteroatom Saturated Unsaturated
Nitrogen Azepane
Azepane
Azepane is a saturated heterocycle, containing one nitrogen atom in seven-membered ring.A well known azepane derivative is caprolactam....

Azepine
Oxygen Oxepane
Oxepane
Oxepane is a heterocyclic chemical compound with the formula .Oxepane can be polymerized by cationic initiators such as to form a crystalline solid with a melting point around 56–58 °C....

Oxepine
Sulfur Thiepane
Thiepane
Thiepane is a heterocycle....

Thiepine


Those with two heteroatoms include:
Heteroatom Saturated Unsaturated
Nitrogen Homopiperazine Diazepine
Diazepine
1,2-Diazepine is a seven-membered heterocyclic compound with two nitrogen atoms and three double bonds.-External links:*...

Nitrogen/sulfur Thiazepine
Thiazepine
Thiazepines are a substituted thiepins, with a nitrogen replacing a carbon in the seven-membered heterocyclic compound.Types include:* 1,3-thiazepine* 1,4-thiazepine...


8-Membered rings

Heteroatom Saturated Unsaturated
Nitrogen Azocane
Azocane
Azocane is a heterocyclic organic compound with the molecular formula C7H15N. It consists of a saturated eight-membered ring having seven carbon atoms and one nitrogen atom attached to a single hydrogen atom. The fully unsaturated analog of azocane is azocine....

Azocine
Azocine
Azocine is a heterocyclic organic compound with the molecular formula C7H7N. It consists of an unsaturated eight-membered ring having seven carbon atoms, one nitrogen atom and four double bonds....

Sulfur

Images

Names in italics are retained by IUPAC and they do not follow the Hantzsch-Widman nomenclature
Saturated Unsaturated
Heteroatom Nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 
Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 
Sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 
NitrogenOxygen Sulfur
3-Ring
Name Aziridine
Aziridine
Aziridines are organic compounds containing the aziridine functional group, a three-membered heterocycle with one amine group and two methylene groups...

 
Oxirane  Thiirane
Thiirane
Thiirane, more commonly known as ethylene sulfide, is the cyclic chemical compound with the formula C2H4S. It is the smallest sulfur-containing heterocycle. Like many organosulfur compounds, this species has a stench. Thiirane is also used to describe any derivative of the parent ethylene...

 
Azirine  Oxirene
Oxirene
Oxirene is a heterocyclic hypothesized chemical compound which contains an unsaturated three-membered ring containing two carbon atoms and one oxygen atom. As the configuration is extremely strained, no consensus exists whether the structure constitutes a molecule or whether it is merely a...

 
Thiirene
Thiirene
Thiirene is a heterocycle....

Structure
4-Ring
Name Azetidine
Azetidine
Azetidine is a heterocyclic organic compound. It belongs to the class of four membered rings and it contains a nitrogen atom.-External links:* *...

 
Oxetane
Oxetane
Oxetane, or 1,3-propylene oxide, is an heterocyclic organic compound with the molecular formula C3H6O, having a four-membered ring with three carbon atoms and one oxygen atom....

 
Thietane
Thietane
Thietane is a heterocyclic compound containing a saturated four-membered ring with three carbon atoms and one sulfur atom....

 
Azete
Azete
Azete is a heterocyclic chemical compound consisting of an unsaturated four-membered ring with three carbon atoms and one nitrogen atom.-See also:* Azetidine, the saturated analog* Cyclobutadiene, the carbocyclic analog...

 
Oxete Thiete
Thiete
Thiete is a heterocycle. It is more commonly encountered not on its own, but in anellated derivatives, several of which have been synthesized. Thietes are generally not very stable.- Configuration :...

Structure
5-Ring
Name Pyrrolidine
Pyrrolidine
Pyrrolidine, also known as tetrahydropyrrole, is an organic compound with the molecular formula C4H9N. It is a cyclic secondary amine with a five-membered heterocycle containing four carbon atoms and one nitrogen atom...

Oxolane  Thiolane  Pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

Furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....

Thiophene
Thiophene
Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene...

Structure
6-Ring
Name Piperidine
Piperidine
Piperidine is an organic compound with the molecular formula 5NH. This heterocyclic amine consists of a six-membered ring containing five methylene units and one nitrogen atom...

Oxane
Oxane
Oxane may refer to any of several distinct chemical compounds:*Tetrahydropyran*Oxirane...

 
Thiane
Thiane
Thiane is a heterocyclic compound containg a saturated six-membered ring with five carbon atoms and one sulfur atom....

 
Pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

Pyran
Pyran
In chemistry, a pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds...

Thiopyran
Thiopyran
Thiopyran , or thiopyrylium is a heterocyclic compound, a pyran whose oxygen atom has been replaced with a sulphur atom....

Structure
7-Ring
Name Azepane
Azepane
Azepane is a saturated heterocycle, containing one nitrogen atom in seven-membered ring.A well known azepane derivative is caprolactam....

 
Oxepane
Oxepane
Oxepane is a heterocyclic chemical compound with the formula .Oxepane can be polymerized by cationic initiators such as to form a crystalline solid with a melting point around 56–58 °C....

 
Thiepane
Thiepane
Thiepane is a heterocycle....

 
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
Indole
Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an...

 or isoindole
Isoindole
Isoindole in heterocyclic chemistry is a benzo fused pyrrole. The compound is an isomer of indole and its reduced form is an isoindoline.In solution its tautomer without full aromaticity over the whole ring system is predominant:...

 depending on the orientation. The pyridine analog is quinoline
Quinoline
Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown...

 or isoquinoline
Isoquinoline
Isoquinoline is a heterocyclic aromatic organic compound. It is a structural isomer of quinoline. Isoquinoline and quinoline are benzopyridines, which are composed of a benzene ring fused to a pyridine ring. In a broader sense, the term isoquinoline is used to make reference to isoquinoline...

. For azepine, benzazepine
Benzazepines
Benzazepines are heterocyclic compounds consisting of a benzene ring fused to an azepine ring. Examples include fenoldopam....

 is the preferred name. Similarly, the compounds with two benzene rings fused to the central heterocycle are carbazole
Carbazole
Carbazole is an aromatic heterocyclic organic compound. It has a tricyclic structure, consisting of two six-membered benzene ring fused on either side of a five-membered nitrogen-containing ring...

, acridine
Acridine
Acridine, C13H9N, is an organic compound and a nitrogen heterocycle. Acridine is also used to describe compounds containing the C13N tricycle....

, and dibenzoazepine.

History of heterocyclic chemistry

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

. Some noteworthy developments:

1818: Brugnatelli isolates alloxan
Alloxan
Alloxan is an oxygenated pyrimidine derivative. It is present as alloxan hydrate in aqueous solution.-History:...

 from uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...



1832: Dobereiner produces furfural
Furfural
Furfural is an organic compound derived from a variety of agricultural byproducts, including corncobs, oat, wheat bran, and sawdust. The name furfural comes from the Latin word , meaning bran, referring to its usual source....

 (a furan) by treating starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 with sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...



1834: Runge obtains pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

 ("fiery oil") by dry distillation of bones

1906: Friedlander synthesizes indigo dye
Indigo dye
Indigo dye is an organic compound with a distinctive blue color . Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today — several thousand tons each year — is synthetic...

, allowing synthetic chemistry to displace a large agricultural industry

1936: Treibs
Alfred E. Treibs
Alfred E. Treibs was a German organic chemist who is credited with founding the area of organic geochemistry. He received his PhD under Hans Fischer at the Technical University of Munich. Fischer had received the Nobel Prize in Chemistry for elucidating the structures of porphyrins. In the...

 isolates chlorophyl derivatives from crude oil, explaining the biological origin of petroleum.

1951: Chargaff's rules
Chargaff's rules
Chargaff's rules state that DNA from any cell of all organisms should have a 1:1 ratio of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. This pattern is found in both strands of the DNA...

 are described, highlighting the role of heterocyclic compounds (purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....

s and pyrimidine
Pyrimidine
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...

s) in the genetic code.

Commercial Exploitation

Leading companies with a vast amount of patents related to heterocyclic compounds are Bayer, Merck, Ciba-Geigy, Pfizer, Eli Lily, BASF, Hoffmann La Roche, ER Sqibb, Warner Lambert and Hoechst.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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