View source for Hexachlorocyclopentadiene

{{chembox
|   Name = Hexachlorocyclopentadiene
|   ImageFile = Hexachlorocyclopentadiene-2D-skeletal.png
|   ImageSize = 
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| Section1 = {{Chembox Identifiers
|   CASNo = 77-47-4
|   MW = 272.77
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| Section2 = {{Chembox Properties
|C=5|Cl=6
|   MeltingPt = −10 °C
|   BoilingPt = 239 °C
|   Density = 1.702 g/cm3
|   VaporPressure = 
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| Section7 = {{Chembox Hazards
|   NFPA-H = 
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'''Hexachlorocyclopentadiene''' is an [[organochlorine compound]] that is a precursor to several pesticides.  This colourless liquid is an inexpensive reactive [[diene]].  Many of its derivatives proved to be highly controversial, as studies showed them to be [[persistent organic pollutant]]s. Collectively, the pesticides derived from hexachlorocyclopentadiene are called the cyclodienes.  An estimated 270,000 tons were produced until 1976, and some are still manufactured.<ref name=Ullmann>Robert L. Metcalf “Insect Control” in Ullmann’s Encyclopedia of Industrial Chemistry” Wiley-VCH, Wienheim, 2002.  {{DOI|10.1002/14356007.a14_263}}</ref> 

==Synthesis and applications==
Hexachlorocyclopentadiene is prepared by chlorination of [[cyclopentadiene]] to give 1,1,2,3,4,5-octachlorocyclopentane, which in a second step undergoes dehydrochlorination:<ref>Dieter Hönicke, Ringo Födisch, Peter Claus, Michael Olson “Cyclopentadiene and Cyclopentene” Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a08_227}}</ref>
:C<sub>5</sub>H<sub>6</sub>  +  6 Cl<sub>2</sub>   →   C<sub>5</sub>H<sub>2</sub>Cl<sub>8</sub>  +  4 HCl
:C<sub>5</sub>H<sub>2</sub>Cl<sub>8</sub>   →   C<sub>5</sub>Cl<sub>6</sub>  +  2 HCl

Hexachlorocyclopentadiene readily undergoes the [[Diels-Alder reaction]] to give a variety of [[adduct]]s that were commercialized as pesticides.  The main derivatives are:
*[[aldrin]] from norbornadiene (the related [[Dieldrin]] is a metabolite of aldrin)
*bromodan from  allyl bromide
*[[chlordane]] from cyclopentadiene, followed by chlorination
*[[endrin]] from acetylene, followed by cyclopentadiene, followed by epoxidation
*[[heptachlor]] from cyclopentadiene, followed by monochlorination
*isobenzan from dihydrofuran followed by chlorination
*[[endosulfan]] from 1,4-dihydroxy-2-butene, followed by esterification with SOCl<sub>2</sub>
Additionally hexachlorocyclopentadiene is the precursor to the pesticides [[mirex]] and [[kepone]], although these are not classified as cyclodienes.

==Regulation==
Almost all derivatives have been banned or are under consideration for banning, according to the deliberations of the [[Stockholm Convention on Persistent Organic Pollutants]].

==Insect resistance==
In addition to regulatory pressures, these pesticides became less effective owing to genetic mutations of the targeted insects.  The number of insects resistant to cyclodienes and [[lindane]] approached 300 by 1989.<ref name=Ullmann/>
==References==
<references/>

[[Category:Organochlorides]]
[[Category:Dienes]]

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[[Category:2Fix]]