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CAS number 201-029-3
Molecular formula C5Cl6
Molar mass 272.77 g mol−1
Density 1.702 g/cm3
Melting point

−10 °C

Boiling point

239 °C

Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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 pollutants. Collectively, the pesticides derived from hexachlorocyclopentadiene are called the cyclodienes. An estimated 2.7 x 105 tons were produced until 1976, and some are still manufactured.[1]


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:[2]

C5H6 + 6 Cl2 → C5H2Cl8 + 4 HCl
C5H2Cl8 → C5Cl6 + 2 HCl

Hexachlorocyclopentadiene readily undergoes the Diels-Alder reaction to give a variety of adducts 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 SOCl2

Additionally hexachlorocyclopentadiene is the precursor to the pesticides mirex and kepone, although these are not classified as cyclodienes.


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.[1]


  1. ^ a b Robert L. Metcalf “Insect Control” in Ullmann’s Encyclopedia of Industrial Chemistry” Wiley-VCH, Wienheim, 2002. doi:10.1002/14356007.a14_263
  2. ^ 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


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