Heptachlor is an chlorinated dicyclopentadiene insecticide and termiticide, first isolated from technical chlordane in 1946. It is a constituent of technicalgrade chlordane, approximately 10% by weight. In the US, the commercial production started in 1953 by the Velsicol Corporation, after registration for use. During the 1960s and 1970s, it was used primarily by farmers to kill termites, ants, and soil insects in seed grains and on crops, as well as by exterminators and home owners to kill termites. It acts as a nonsystemic stomach and contact insecticide. About 20% of heptachlor breaks down within hours into heptachlor epoxide in the environment and living organisms.
In 1962 Rachel Carson published Silent Spring where she questioned the safety of heptachlor. The environmental agencies and governments decided to phase out the compound from the environment because of its potential cancer risk and its persistence and bioaccumulation throughout the food chain. The use of heptachlor has been banned or severely restricted in 38 countries and 93 other do not allow the import of heptachlor.
In recent years, however, heptachlor (and the closely related chlordane) has been used for major road building projects in Africa. For protection of residential structures in Northeastern Australia and Asia, and for crop protection in South America. In September 2005, heptachlor was still approved by EPA for killing fire ants in power transformers in the US.
Heptachlor is produced commercially by the freeradical chlorination of chlordene in benzene containing from 0.5 to 5.0% of fuller's earth. The reaction is run for up to 8 hours. The chlordene starting material is prepared by the DielsAlder condensation of hexachlorocyclopentadiene with cyclopentadiene.
Technicalgrade heptachlor usually consists of 72% heptachlor and 28% impurities such as transchlordane, cischlordane, and nonachlor.