Heterocyclic compound

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.[4]

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 refers to those fused to benzene rings. For example, the fused benzene analogs 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 compounds. Analogs of the previously mentioned heterocycles for this third family of compounds are acridine, dibenzothiophene, carbazole, and dibenzofuran, respectively. The unsaturated rings can be classified according to the participation of the heteroatom in the conjugated pi system.

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°.

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:

Those with two heteroatoms include:

Borazocine is an eight-membered ring with four nitrogen heteroatoms and four boron 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
4-Atom Ring	Azetidine	Oxetane	Thietane	Azete	Oxete	Thiete
5-Atom Ring	Pyrrolidine	Oxolane	Thiolane	Pyrrole	Furan	Thiophene
6-Atom Ring	Piperidine	Oxane	Thiane	Pyridine	Pyran	Thiopyran
7-Atom Ring	Azepane	Oxepane	Thiepane	Azepine	Oxepine	Thiepine
8-Atom Ring	Azocane	Oxocane	Thiocane	Azocine	Oxocine	Thiocine
9-Atom Ring	Azonane	Oxonane	Thionane	Azonine	Oxonine	Thionine

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.

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry. Some noteworthy developments:[7]
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.[8]

Wikimedia Commons has media related to heterocyclic compounds.

Wikiquote has quotations related to: Heterocyclic compound

• 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)