Hexachlorocyclopentadiene (HCCPD), also known as C-56, Graphlox, and HRS 1655, is an organochlorine compound with the formula C5Cl6. It is a precursor to pesticides, flame retardants, and dyes. It is a colourless liquid, although commercial samples appear lemon-yellow liquid sometimes with a bluish vapour. Many of its derivatives proved to be highly controversial, as studies showed them to be persistent organic pollutants. An estimated 270,000 tons were produced until 1976, and smaller amounts continue to be produced today. Two prominent manufacturers are Velsicol Chemical Corporation in the US and by Jiangsu Anpon Electrochemicals Co. in China.
Synthesis and production
Hexachlorocyclopentadiene is prepared by chlorination of cyclopentadiene to give 1,1,2,3,4,5-octachlorocyclopentane, which in a second step undergoes dehydrochlorination: The first procedure uses alkaline hypochlorite and after fractional distillation has a yield of about 75%, the other 25% consists of lower chlorinated cyclopentadienes. The second process uses thermal dechlorination, which occurs at 470-480 °C and gives a yield higher than 90%. Therefore, the first process is easier to perform, but the second gives a more pure product.
C5H6 + 6 Cl2 → C5H2Cl8 + 4 HCl
C5H2Cl8 → C5Cl6 + 2 HCl
Besides manufacturers that produce the chemical for scientific synthesis and reference, there are two companies that produce HCCPD for industrial use: Velsicol Chemical LLC in the United States and Jiangsu Anpon Electrochemicals Co. in China. The first produces the chemical on a large scale to be used for producing rubber adherents, flame retardants and pesticides. Velsicol knows the dangers of handling HCCPD and therefore requires its buyers to go through a strict review and educational program on the storage, use and disposal of the chemical. The company also provides safety data sheets and a handling guide on its website, and delivers the chemical to purchasers all over the world. Of the Chinese company less is known. It is said to be a company specialized in chloro-alkali and agrochemicals and operating as a subsidiary of China National Agrochemical Corporation.
In addition, 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) 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 dienochlor
Reactions
HCCP is electrophilic. It degrades in base. Alcoholysis affords ketals C5Cl4(OR)2.
HCCP readily undergoes Diels-Alder reactions with alkenes. This reaction is used to produce pesticides such as Aldrin (named after the reaction) and Isodrin. Most of these pesticides are no longer commercially available and banned by the Stockholm convention on persistent organic pollutants due to their toxicity to humans and animals.
Biodegradation
In surface water, photolysis is the most common reaction route, with a degradation half-life of 2 to 4 minutes. Deeper under water where less light penetrates hydrolysis and biodegradation are prominent pathways. The figure below gives an indication of possible degradation pathways when HCCPD is released into the environment. Light, water and oxygen can cause a double bond to oxidize to give a ketone structure, which can then undergo a ring-opening and form pentachlorodienoic acid, which forms two butadiene species with the release of carbon dioxide. However, this is only a minor pathway.
Toxicokinetics
HCCPD absorption in the body occurs mostly through the lungs, gastrointestinal tract and skin. Overall the levels of HCCPD in blood were lower when administered through food compared to when inhalation was used. This may indicate a poor absorption in the gastrointestinal tract due to binding to the gastrointestinal contents. When HCCPD is absorbed, it is distributed to the liver, kidney and lungs. The organ with the highest concentration differs when comparing rats and mice. The highest concentration in rats is found in the kidney versus in the liver of mice.