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[[Image:Pyridine simple structure.png|frame|left|[[Pyridine]] a simple heterocyclic compound]]

==Overview==
'''Heterocyclic compounds''' are organic [[Chemical compound|compounds]] that contain a ring structure containing atoms in addition to [[carbon]], such as [[sulfur]], [[oxygen]] or [[nitrogen]], as part of the ring. They may be either [[simple aromatic ring]]s or non-aromatic rings. Some examples are [[pyridine]] (C<sub>5</sub>H<sub>5</sub>N), [[pyrimidine]] (C<sub>4</sub>H<sub>4</sub>N<sub>2</sub>) and [[dioxane]] (C<sub>4</sub>H<sub>8</sub>O<sub>2</sub>).

Note that compounds such as ''[[cyclopropane]]'', an anaesthetic with explosive properties, and ''[[cyclohexane]]'', a solvent, are not heterocyclic, they are merely [[cycloalkanes]]. The suffix '-cyclic' implies a ring structure, while 'hetero' refers to an atom other than carbon, as above.  Many heterocyclic compounds, including some [[amines]], are [[carcinogenic]].

'''Heterocyclic chemistry''' is the chemistry branch dealing exclusively with synthesis, properties and applications of heterocycles especially vital to [[drug]] design. 

==3-membered rings==
Heterocycles with three atoms in the ring are more reactive because of [[ring strain]].  Those containing one heteroatom are generally stable.  Those with two heteroatoms are more likely to occur as reactive intermediates.  Common 3-membered heterocycles are:

{| class="wikitable"
|-
! heteroatom
! saturated
! unsaturated
|-
| Nitrogen
| [[aziridine]]
| 
|-
| Oxygen
| [[ethylene oxide]] ([[epoxide]]s, [[oxirane]]s)
| [[oxirene]]
|-
| Sulphur
| [[thiirane]] ([[episulfide]]s)
| 
|}

==4-membered rings==
{| class="wikitable"
|-
! heteroatom
! saturated
! unsaturated
|-
| Nitrogen
| [[azetidine]]
| 
|-
| Oxygen
| [[oxetane]]
|
|}

==5-membered rings==
With heterocycles containing five atoms, the [[unsaturated compound|unsaturated compounds]] are frequently more stable because of [[aromaticity]].

{| class="wikitable"
|-
! heteroatom
! saturated
! unsaturated
|-
| Nitrogen
| [[dihydropyrrole]] ([[pyrroline]]) & [[tetrahydropyrrole]] ([[pyrrolidine]])
| [[pyrrole]]
|-
| Oxygen
| [[dihydrofuran]] & [[tetrahydrofuran]]
| [[furan]]
|-
| Sulphur
| [[dihydrothiophene]] & [[tetrahydrothiophene]]
| [[thiophene]] ([[thiole]])
|-
| Arsenic
| 
| [[arsole]]
|}

With two heteroatoms:
* The [[azole]]s:
** Two N: [[pyrazole]] & [[imidazole]]
** Three N: [[triazole]]
** Four N: [[tetrazole]]
** One N and one O: [[oxazole]] & [[isoxazole]]
** One N and one S: [[thiazole]] & [[isothiazole]]
* Two S: [[Dithiolane]]

==6-membered rings==
{| class="wikitable"
|-
! heteroatom
! saturated
! unsaturated
|-
| Nitrogen
| [[piperidine]]
| [[pyridine]]
|-
| Oxygen
| [[tetrahydropyran]]
| [[pyran]]
|-
| Sulphur
| [[Thiane]]
| [[Thiine]] aka [[thiapyrane]]
|}

With two heteroatoms:
* Two N:  [[Pyridazine]], [[Pyrimidine]], and [[Pyrazine]] are the 1,2-, 1,3-, and 1,4-isomers, respectively.
* Two N:  [[Piperazine]]
* One N and one O: [[Oxazines]]
* One N and one S: [[Thiazine]]
* Two S: [[Dithiane]]
* Two O: [[Dioxane]]

==Heterocyclic amines and cancer==
Some heterocyclic amines (HCAs) found in cooked meat are known [[carcinogen]]s.  Research has shown that cooking certain meats at high temperatures creates chemicals that are not present in uncooked meats. For example, heterocyclic amines are the carcinogenic chemicals formed from the cooking of muscle meats such as [[beef]], [[pork]], [[fowl]], and [[fish]]. HCAs form when [[amino acids]] and [[creatine]] (a chemical found in muscles) react at high cooking temperatures. Researchers have identified 17 different HCAs resulting from the cooking of muscle meats that may pose human cancer risk.<ref>{{cite web | title = Heterocyclic Amines in Cooked Meats | date = 15 Sep 2004 | publisher = National Cancer Institute | accessdate = 2007-08-09 | url = http://www.cancer.gov/cancertopics/factsheet/Risk/heterocyclic-amines}}</ref>  NCI's Division of Cancer Epidemiology and Genetics found a link between individuals with [[stomach cancer]] and the consumption of cooked meat, and other studies for [[colorectal]], [[pancreatic]], and [[breast cancer]] is associated with high intakes of well-done, fried, or [[barbecue]]d meats.  Other sources of [[protein]] ([[milk]], [[egg (food)|eggs]], [[tofu]], and organ meats such as [[liver]]) have very little or no HCA content naturally or when cooked.

==References==
<references/>

==External links==
* [http://www.cancer.gov/cancertopics/factsheet/Risk/heterocyclic-amines  Heterocyclic amines in cooked meat, US CDC]
* [http://www.cancer.org/docroot/PED/content/PED_1_3x_Known_and_Probable_Carcinogens.asp List of known and probable carcinogens, American Cancer Society]
* [http://www.scorecard.org/chemical-groups/one-list.tcl?short_list_name=p65 List of known carcinogens by the State of California, [[Proposition 65]]]  (more comprehensive)

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[[Category:Heterocyclic compounds| ]]

[[ar:حلقة غير متجانسة]]
[[ca:Compost heterocíclic]]
[[cs:Heterocyklické sloučeniny]]
[[de:Heterocyclen]]
[[es:Compuesto heterocíclico]]
[[fr:Hétérocycle]]
[[it:Composto eterociclico]]
[[he:תרכובת הטרוציקלית]]
[[mk:Хетероциклично соединение]]
[[nl:Heterocyclische verbinding]]
[[ja:複素環式化合物]]
[[pl:Związki heterocykliczne]]
[[ru:Гетероциклические соединения]]
[[sk:Heterocyklická zlúčenina]]
[[fi:Heterosyklinen yhdiste]]
[[sv:Heterocykliska föreningar]]
[[uk:Гетероциклічні сполуки]]
[[zh:杂环化合物]]

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