Heterocyclic Compounds, 3-ring
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A heterocyclic compound or ring structure is a
cyclic compound A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...
that has atoms of at least two different elements as members of its ring(s). Heterocyclic organic chemistry is the branch of
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
dealing with the synthesis, properties, and applications of organic heterocycles. Examples of heterocyclic compounds include all of the
nucleic acids Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic a ...
, the majority of drugs, most
biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
(
cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...
and related materials), and many natural and synthetic dyes. More than half of known compounds are heterocycles. 59% of US
FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
-approved drugs contain
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
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 Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
,
thiophene Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a planar five-membered ring, it is aromatic as indicated by its extensive substitution reactions. It is a colorless liquid with a benzene-like odor. In most of its reacti ...
,
pyrrole Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula . It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-methylpyrrol ...
, and
furan Furan is a Heterocyclic compound, heterocyclic organic compound, consisting of a five-membered aromatic Ring (chemistry), ring with four carbon Atom, atoms and one oxygen atom. Chemical compounds containing such rings are also referred to as f ...
. Another large class of organic heterocycles refers to those fused to
benzene ring Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal ring with one hydrogen atom attached to each. Because it contains only carbon and hydr ...
s. For example, the fused benzene derivatives of pyridine, thiophene, pyrrole, and furan are
quinoline Quinoline is a heterocyclic aromatic organic compound with the chemical formula C9H7N. It is a colorless hygroscopic liquid with a strong odor. Aged samples, especially if exposed to light, become yellow and later brown. Quinoline is only sl ...
,
benzothiophene Benzothiophene is an aromatic organic compound with a molecular formula C8H6S and an odor similar to naphthalene (mothballs). It occurs naturally as a constituent of petroleum-related deposits such as lignite tar. Benzothiophene has no househol ...
,
indole Indole is an organic compound with the formula . Indole is classified as an aromatic heterocycle. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. Indoles are derivatives of indole ...
, and
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 structural nucleus (parent compound) of many related compounds with more complex stru ...
, 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 Acridine is an organic compound and a nitrogen heterocycle with the formula C13H9N. Acridines are substituted derivatives of the parent ring. It is a planar molecule that is structurally related to anthracene with one of the central CH groups ...
,
dibenzothiophene Dibenzothiophene (DBT, diphenylene sulfide) is the organosulfur compound consisting of two benzene rings fused to a central thiophene ring. It has the chemical formula C12H8S. It is a colourless solid that is chemically somewhat similar to anth ...
,
carbazole Carbazole is an aromatic Heterocyclic compound, heterocyclic organic compound. It has a tricyclic structure, consisting of two six-membered benzene rings fused on either side of a five-membered nitrogen-containing ring. The compound's structure is ...
, and
dibenzofuran Dibenzofuran (DBF) is a heterocyclic organic compound with the chemical structure shown at right. It is an aromatic compound that has two benzene rings fused to a central furan ring. All the numbered carbon atoms have a hydrogen atom bonded to each ...
, respectively. Heterocyclic organic compounds can be usefully classified based on their electronic structure. The saturated organic heterocycles behave like the acyclic derivatives. Thus,
piperidine Piperidine is an organic compound with the molecular formula (CH2)5NH. This heterocyclic amine consists of a six-membered ring containing five methylene bridges (–CH2–) and one amine bridge (–NH–). It is a colorless liquid with an odor de ...
and
tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
are conventional
amines In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
and
ethers In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R′ r ...
, 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 Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...
(B3N3 ring),
hexachlorophosphazene Hexachlorophosphazene is an inorganic compound with the chemical formula . The molecule has a cyclic, unsaturated backbone consisting of alternating phosphorus and nitrogen atoms, and can be viewed as a trimer of the hypothetical compound (p ...
(P3N3 ring), and
trithiazyl trichloride Trithiazyl trichloride is the inorganic compound with the formula . A white solid, it is a precursor to other sulfur nitrides, but has no commercial applications. Structure The molecule is a 6-membered ring of alternating nitrogen and sulfur ato ...
(S3N3 ring). In comparison with organic heterocycles, which have numerous commercial applications, inorganic ring systems are mainly of theoretical interest.
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.


Notes on lists

* "Heteroatoms" are atoms in the ring other than
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
atoms. * Names in italics are retained by
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
and do not follow the Hantzsch-Widman nomenclature. * Some of the names refer to classes of compounds rather than individual compounds. * Also no attempt is made to list
isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the existence or possibili ...
.


3-membered rings

Although subject to
ring strain In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles ar ...
, 3-membered heterocyclic rings are well characterized.


4-membered rings


5-membered rings

The 5-membered ring compounds containing ''two'' heteroatoms, at least one of which is nitrogen, are collectively called the
azole Azoles are a class of five-membered heterocyclic compounds containing a nitrogen atom and at least one other non-carbon atom (i.e. nitrogen, sulfur, or oxygen) as part of the ring. Their names originate from the Hantzsch–Widman nomenclature. Th ...
s.
Thiazole Thiazole (), or 1,3-thiazole, is a 5-membered 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 ...
s and
isothiazole Isothiazole, or 1,2-thiazole, is an organic compound consisting with the formula . The ring is unsaturated and features an S-N bond. The isomeric thiazole, where the S and N are not directly bonded are far more common. Isothiazones are produced ...
s contain a sulfur and a nitrogen atom in the ring. Dithioles have two sulfur atoms. 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.


6-membered rings

The 6-membered ring compounds containing ''two'' heteroatoms, at least one of which is nitrogen, are collectively called the azines.
Thiazine Thiazine is an organic compound containing a ring of four carbon, one nitrogen and one sulfur atom. There are three isomers of thiazine, 1,2-thiazine, 1,3-thiazine, and 1,4-thiazine, which differ by the arrangement of the nitrogen and sulfur ato ...
s contain a sulfur and a nitrogen atom in the ring. Dithiines have two sulfur atoms. Six-membered rings with five heteroatoms
The
hypothetical chemical compound A hypothetical chemical compound is a chemical compound that has been conceived of, but is not known to have been synthesized, observed, or isolated (identified or shown to exist). Some hypothetical compounds cannot form at all, due to steric ef ...
with five nitrogen heteroatoms would be
pentazine Pentazine is a hypothetical chemical compound that consists of a six-membered aromatic ring containing five nitrogen atoms with the molecular formula C H N5. The name ''pentazine'' is used in the nomenclature of derivatives of this compound. P ...
. Six-membered rings with six heteroatoms
The
hypothetical chemical compound A hypothetical chemical compound is a chemical compound that has been conceived of, but is not known to have been synthesized, observed, or isolated (identified or shown to exist). Some hypothetical compounds cannot form at all, due to steric ef ...
with six nitrogen heteroatoms would be
hexazine Hexazine (also known as hexaazabenzene) is a hypothetical allotrope of nitrogen composed of 6 nitrogen atoms arranged in a ring-like structure analogous to that of benzene. As a neutrally charged species, it would be the final member of the azabe ...
.
Borazine Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...
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 Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally though ...
may be possible.


8-membered rings

Borazocine Borazocine is a polar inorganic compound with the chemical formula B4H8N4. In this cyclic compound, the four BH units and four NH units alternate. Related compounds Borazine is a six-membered aromatic ring with three boron atoms and three nitroge ...
is an eight-membered ring with four nitrogen heteroatoms and four boron heteroatoms.


9-membered rings


Images of rings with one heteroatom


Fused/condensed rings

Heterocyclic rings systems that are formally derived by fusion with other rings, either
carbocyclic In organic chemistry, an alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached. Cycloa ...
or heterocyclic, have a variety of common and systematic names. For example, with the benzo-fused unsaturated nitrogen heterocycles, pyrrole provides
indole Indole is an organic compound with the formula . Indole is classified as an aromatic heterocycle. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. Indoles are derivatives of indole ...
or
isoindole In organic chemistry and heterocyclic chemistry, isoindole consists of a benzene ring fused with pyrrole. The compound is an isomer of indole. Its reduced form is isoindoline. The parent isoindole is a rarely encountered in the technical lit ...
depending on the orientation. The pyridine derivative is
quinoline Quinoline is a heterocyclic aromatic organic compound with the chemical formula C9H7N. It is a colorless hygroscopic liquid with a strong odor. Aged samples, especially if exposed to light, become yellow and later brown. Quinoline is only sl ...
or
isoquinoline Isoquinoline is an individual chemical specimen - a heterocyclic aromatic organic compound - as well as the name of a family of many thousands of natural plant alkaloids, any one of which might be referred to as "an isoquinoline". It is a struc ...
, and the class of analogues with two nitrogen atoms is known as the
benzodiazine Diazanaphthalenes are a class of aromatic heterocyclic chemical compounds that have the formula C8H6N2. They consist of a naphthalene double ring in which two of the carbon atoms have been replaced with nitrogen atoms. There are ten positional isom ...
s. For the azepine derivative,
benzazepine Benzazepines are heterocyclic chemical compounds consisting of a benzene ring fused to an azepine ring. Examples include: File:Benazepril structure.svg, Benazepril File:Fenoldopam.svg, Fenoldopam File:GSK-189,254.svg, GSK-189,254 File:Ivabrad ...
is the preferred name. Likewise, the compounds with two benzene rings fused to the central heterocycle are
carbazole Carbazole is an aromatic Heterocyclic compound, heterocyclic organic compound. It has a tricyclic structure, consisting of two six-membered benzene rings fused on either side of a five-membered nitrogen-containing ring. The compound's structure is ...
,
acridine Acridine is an organic compound and a nitrogen heterocycle with the formula C13H9N. Acridines are substituted derivatives of the parent ring. It is a planar molecule that is structurally related to anthracene with one of the central CH groups ...
, and
dibenzoazepine Dibenzazepine (iminostilbene) is a chemical compound with two benzene functional group, rings fused to an azepine ring (chemistry), ring. Many pharmaceuticals, such as carbamazepine, oxcarbazepine, and depramine, are based on a dibenzazepine struc ...
.
Heptazine Heptazine, or tri-''s''-triazine or cyamelurine, is a chemical compound with formula , that consist of a planar triangular core group or three fused triazine rings, with three hydrogen atoms at the corners. It is a yellow, weakly fluorescent soli ...
is a tricyclic nitrogen-containing heterocyclic system derived by fusion of three triazine rings, and analog of the carbocycle
phenalene 1''H''-Phenalene, often called simply phenalene is a polycyclic aromatic hydrocarbon (PAH). Like many PAHs, it is an atmospheric pollutant formed during the combustion of fossil fuels. It is the parent compound for the phosphorus-containing phos ...
.


History of heterocyclic chemistry

The history of heterocyclic chemistry began in the 1800s, in step with the development of
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
. Some noteworthy developments: * 1818: Brugnatelli makes
alloxan Alloxan, sometimes referred to as alloxan monohydrate, is an organic compound with the formula . It is classified as a derivative of pyrimidine. The anhydrous derivative is also known, as well as a dimeric derivative. These are some of the earlie ...
from
uric acid Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the Chemical formula, formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the meta ...
. * 1832: Dobereiner produces
furfural Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occu ...
(a furan) by treating
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...
with
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
. * 1834: Runge obtains
pyrrole Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula . It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-methylpyrrol ...
("fiery oil") by dry distillation of bones. * 1906: Friedlander synthesizes
indigo dye Indigo dye is an organic compound with a distinctive indigo, blue color. Indigo is a natural dye obtained from the leaves of some plants of the Indigofera#Uses, ''Indigofera'' genus, in particular ''Indigofera tinctoria''. Dye-bearing ''Indigofer ...
, 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 Chargaff's rules (given by Erwin Chargaff) state that in the DNA of any species and any organism, the amount of guanine should be equal to the amount of cytosine and the amount of adenine should be equal to the amount of thymine. Further, a 1:1 st ...
are described, highlighting the role of heterocyclic compounds (
purine Purine is a heterocyclic aromatic organic compound that consists of two rings (pyrimidine and imidazole) fused together. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which include substituted puri ...
s and
pyrimidine Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The oth ...
s) in the genetic code.


Uses

Heterocyclic compounds are pervasive in many areas of life sciences and technology.Thomas L. Gilchrist "Heterocyclic Chemistry" 3rd ed. Addison Wesley: Essex, England, 1997. 414 pp. . Many drugs are heterocyclic compounds.


See also

*
Spiroketals In chemistry, spiroketals are structural motifs composed of two Heterocyclic compound, heterocycles sharing one central carbon which makes them a subclass of spiro compound. Their structural specificity lays on the presence of one oxygen atom i ...


References


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 Proposition 65 (formally titled The Safe Drinking Water and Toxic Enforcement Act of 1986, and also referred to as Prop 65) is a California law passed by direct voter initiative in 1986 by a 63%–37% vote. Its goals are to protect drinking wate ...
(more comprehensive) {{DEFAULTSORT:Heterocyclic Compound