hexachlorocyclopentadiene

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.

== 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