Editing Chlorine (section)

===Organochlorine compounds===
{{main|Organochlorine compound}}
[[File:Phosphorus pentachloride mechanism.png|thumb|upright=2.25|right|Suggested mechanism for the chlorination of a carboxylic acid by phosphorus pentachloride to form an [[acyl chloride]]]]
Like the other carbon–halogen bonds, the C–Cl bond is a common functional group that forms part of core [[organic chemistry]]. Formally, compounds with this functional group may be considered organic derivatives of the chloride anion. Due to the difference of electronegativity between chlorine (3.16) and carbon (2.55), the carbon in a C–Cl bond is electron-deficient and thus [[electrophilic]]. [[Chlorination reaction|Chlorination]] modifies the physical properties of hydrocarbons in several ways: chlorocarbons are typically denser than [[water]] due to the higher atomic weight of chlorine versus hydrogen, and aliphatic organochlorides are [[alkylating agent]]s because chloride is a [[leaving group]].<ref name="Ullmann">M. Rossberg et al. "Chlorinated Hydrocarbons" in ''Ullmann's Encyclopedia of Industrial Chemistry'' 2006, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a06_233.pub2}}</ref>

[[Alkanes]] and [[aryl]] alkanes may be chlorinated under [[free-radical]] conditions, with UV light. However, the extent of chlorination is difficult to control: the reaction is not [[regioselectivity|regioselective]] and often results in a mixture of various isomers with different degrees of chlorination, though this may be permissible if the products are easily separated. Aryl chlorides may be prepared by the [[Friedel-Crafts halogenation]], using chlorine and a [[Lewis acid]] catalyst.<ref name="Ullmann" /> The [[haloform reaction]], using chlorine and [[sodium hydroxide]], is also able to generate alkyl halides from methyl ketones, and related compounds. Chlorine adds to the multiple bonds on [[alkene]]s and [[alkyne]]s as well, giving di- or tetrachloro compounds. However, due to the expense and reactivity of chlorine, organochlorine compounds are more commonly produced by using hydrogen chloride, or with chlorinating agents such as [[phosphorus pentachloride]] (PCl<sub>5</sub>) or [[thionyl chloride]] (SOCl<sub>2</sub>). The last is very convenient in the laboratory because all side products are gaseous and do not have to be distilled out.<ref name="Ullmann" />

Many organochlorine compounds have been isolated from natural sources ranging from bacteria to humans.<ref name="Gribble">{{cite journal | title = Naturally Occurring Organohalogen Compounds | author = Gordon W. Gribble | journal = [[Acc. Chem. Res.]] | volume = 31 | issue = 3 | pages = 141–52 | year = 1998 | doi = 10.1021/ar9701777}}</ref><ref name="Gribble99">{{cite journal | title = The diversity of naturally occurring organobromine compounds | author = Gordon W. Gribble | journal = [[Chemical Society Reviews]] | volume = 28 | issue = 5 | pages = 335–46| year = 1999 | doi = 10.1039/a900201d}}</ref> Chlorinated organic compounds are found in nearly every class of biomolecules including [[alkaloid]]s, [[terpene]]s, [[amino acid]]s, [[flavonoid]]s, [[steroid]]s, and [[fatty acid]]s.<ref name="Gribble" /><ref>{{cite journal | author = Kjeld C. Engvild | title = Chlorine-Containing Natural Compounds in Higher Plants | journal = [[Phytochemistry (journal)|Phytochemistry]] | volume = 25 | issue = 4 | pages = 7891–91 | year = 1986 | doi=10.1016/0031-9422(86)80002-4| bibcode = 1986PChem..25..781E }}</ref> Organochlorides, including [[Polychlorinated dibenzodioxins|dioxins]], are produced in the high temperature environment of forest fires, and dioxins have been found in the preserved ashes of lightning-ignited fires that predate synthetic dioxins.<ref>{{cite journal|author=Gribble, G. W.|year=1994|title=The Natural production of chlorinated compounds|journal=Environmental Science and Technology|volume=28|pages=310A–319A|doi=10.1021/es00056a712|issue=7|pmid=22662801|bibcode=1994EnST...28..310G}}</ref> In addition, a variety of simple chlorinated hydrocarbons including dichloromethane, chloroform, and [[carbon tetrachloride]] have been isolated from marine algae.<ref>{{cite journal | doi = 10.1021/np50088a001 | author = Gribble, G. W. | title = Naturally occurring organohalogen compounds – A comprehensive survey | journal = Progress in the Chemistry of Organic Natural Products | year = 1996 | volume = 68 | pages = 1–423 | pmid = 8795309 | issue = 10}}</ref> A majority of the [[chloromethane]] in the environment is produced naturally by biological decomposition, forest fires, and volcanoes.<ref>[http://www.atsdr.cdc.gov/toxprofiles/tp106-c1-b.pdf Public Health Statement – Chloromethane] {{webarchive|url=https://web.archive.org/web/20070927203426/http://www.atsdr.cdc.gov/toxprofiles/tp106-c1-b.pdf |date=2007-09-27 }}, [[Centers for Disease Control]], Agency for Toxic Substances and Disease Registry</ref>

Some types of organochlorides, though not all, have significant toxicity to plants or animals, including humans. Dioxins, produced when organic matter is burned in the presence of chlorine, and some [[insecticide]]s, such as [[DDT]], are [[persistent organic pollutant]]s which pose dangers when they are released into the environment. For example, DDT, which was widely used to control insects in the mid 20th century, also accumulates in food chains, and causes reproductive problems (e.g., eggshell thinning) in certain bird species.<ref>{{cite book | title=Introduction to Ecotoxicology | publisher=Blackwell Science | year=1999 | isbn=978-0-632-03852-7 | page=68 | author=Connell, D.|display-authors=etal}}</ref> Due to the ready homolytic fission of the C–Cl bond to create chlorine radicals in the upper atmosphere, [[chlorofluorocarbon]]s have been phased out due to the harm they do to the ozone layer.<ref name="Greenwood844" />