Hexachlorocyclopentadiene

Hexachlorocyclopentadiene
IUPAC name 1,2,3,4,5,5-hexachlorocyclopenta-1,3-diene
Other names graphlox, perchlorocyclopentadiene
Identifiers
CAS number	77-47-4 Y
PubChem	6478
ChemSpider	6233 Y
Jmol-3D images	Image 1
SMILES ClC1(Cl)C(/Cl)=C(/Cl)\C(\Cl)=C1\Cl
InChI InChI=1S/C5Cl6/c6-1-2(7)4(9)5(10,11)3(1)8 Y Key: VUNCWTMEJYMOOR-UHFFFAOYSA-N Y InChI=1/C5Cl6/c6-1-2(7)4(9)5(10,11)3(1)8 Key: VUNCWTMEJYMOOR-UHFFFAOYAM
Properties
Molecular formula	C5Cl6
Molar mass	272.77 g mol−1
Appearance	Pale-yellow to amber-colored liquid
Odor	Pungent[1]
Density	1.702 g/cm3
Melting point	−10 °C
Boiling point	239 °C
Vapor pressure	0.08 mmHg
Hazards
Main hazards	Teratogen
Flash point	100 °C; 212 °F; 373 K
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Hexachlorocyclopentadiene, also known as C-56, 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 270,000 tons were produced until 1976, and some are still manufactured by Velsicol in the US and by Jiangsu Anpon Electrochemicals Co. in China. ^[2]

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:^[3]

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.

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.^[2]

References

1. ↑ CDC - NIOSH Pocket Guide to Chemical Hazards
2. ↑ 2.0 2.1 Robert L. Metcalf “Insect Control” in Ullmann’s Encyclopedia of Industrial Chemistry” Wiley-VCH, Wienheim, 2002. doi:10.1002/14356007.a14_263
3. ↑ 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