Chlorendic acid is a white crystalline material with chemical formula C9H4Cl6O4. It is also called HET acid, hexachloroendomethylenetetrahydrophthalic acid, 1,4,5,6,7,7-hexachloro-5-norbornene-2,3-dicarboxylic acid, and 1,4,5,6,7,7 hexachlorobicyclo[2.2.1]-5-heptene-2,3-dicarboxylic acid.[1]
It is produced both as acid and as its anhydride. The anhydride has CAS number 115-27-5.
Chlorendic acid is slightly soluble in water and nonpolar organic solvents (e.g. benzene, hexane, carbon tetrachloride). It is easily soluble in slightly polar organic solvents (ethanol, methanol, acetone). When heated, it loses water at 200 °C, forming an anhydride with melting point of 230–235 °C. When subject to pyrolysis, it decomposes to hydrochloric acid and various chlorinated compounds. It is resistant to hydrolytic dechlorination. It readily forms salts with metals, and esters.
When used in polymers, whether as a curing agent or as a flame retardant, it bonds covalently to the polymer matrix, which reduces its leaching to the environment. It may, however, be released when such materials are subjected to hydrolysis, and it can be formed by oxidation of chlorinated cyclodiene insecticides (e.g. endosulfan, chlordane, heptachlor, aldrin, dieldrin, endrin, and isodrin). Its half-life in soil is 140–280 days.
In Europe, 80% of chlorendic acid is used in the production of flame-resistant composites for building and transportation, while the rest is used in materials for corrosion-resistant fluid-storage equipment. In the USA, Latin America, and Asia, 20-30% is used in flame-retardant applications and the rest is used in corrosion-resistant plastics.
