Heptachlor
Encyclopedia
Heptachlor is an organochlorine compound that was used as an insecticide
. Usually sold as a white or tan powder, heptachlor is one of the cyclodiene insecticides. In 1962, Rachel Carson
's Silent Spring
questioned the safety of heptachlor and other chlorinated insecticides. Due to its highly stable structure, heptachlor can persist in the environment for decades. The US EPA
has limited the sale of heptachlor products to the specific application of fire ant control in underground transformers. The amount that can be present in different foods is regulated.
of hexachlorocyclopentadiene
and cyclopentadiene. The resulting adduct
is brominated followed by treatment with hydrogen chloride
in nitromethane
in the presence of aluminum trichloride or with iodine monochloride
.
Compared to chlordane
, it is about 3 - 5 times more active as an insecticide, but more inert chemically, being resistant to water and caustic alkalies.
(POP). It has a half life of ~1.3-4.2 days (air),~0.03-0.11 years (water),~0.11-0.34 years (soil). One study described its half life to be 2 years and claimed that its residues could be found in soil 14 years after its initial application. Like other POPs, heptachlor is lipophilic
and poorly soluble in water (0.056 mg/L at 25 °C), thus it tends to accumulate in the body fat of humans and animals.
Heptachlor epoxide is more likely to be found in the environment than its parent compound. The epoxide also dissolves more easily in water than its parent compound and is more persistent. Heptachlor and its epoxide absorb to soil particles and evaporate.
The International Agency for Research on Cancer (IARC) and the EPA have classified the compound as a possible human carcinogen. Animals exposed to Heptachlor epoxide during gestation and infancy are found to have changes in nervous system and immune function. Higher doses of Heptachlor when exposed to newborn animals caused decrease in body weight and death.
The U.S. EPA MCL for drinking water is 0.0004 mg/L (Heptachlor) and 0.0002 mg/L (Heptachlor epoxide). The U.S. FDA limit on food crops is 0.01 ppm, in milk is 0.1 ppm, and on edible seafoods is 0.3 ppm. The Occupational Safety and Health Administration (OSHA) has limit of 0.5 mg/m3 (cubic meter of workplace air) for 8-hour shifts and 40-hour work weeks.
The compound was designated as a Class 2B or "possible human carcinogen" by The International Agency for Research on Cancer (IARC). ATSDR report in 1993 found no studies with respect to death in humans after oral exposure to heptachlor or heptachlor epoxide.
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...
. Usually sold as a white or tan powder, heptachlor is one of the cyclodiene insecticides. In 1962, Rachel Carson
Rachel Carson
Rachel Louise Carson was an American marine biologist and conservationist whose writings are credited with advancing the global environmental movement....
's Silent Spring
Silent Spring
Silent Spring is a book written by Rachel Carson and published by Houghton Mifflin on 27 September 1962. The book is widely credited with helping launch the environmental movement....
questioned the safety of heptachlor and other chlorinated insecticides. Due to its highly stable structure, heptachlor can persist in the environment for decades. The US EPA
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
has limited the sale of heptachlor products to the specific application of fire ant control in underground transformers. The amount that can be present in different foods is regulated.
Synthesis
Analogous to the synthesis of other cyclodienes, heptachlor is produced via the Diels-Alder reactionDiels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
of hexachlorocyclopentadiene
Hexachlorocyclopentadiene
Hexachlorocyclopentadiene 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...
and cyclopentadiene. The resulting adduct
Adduct
An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
is brominated followed by treatment with hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
in nitromethane
Nitromethane
Nitromethane is an organic compound with the chemical formula . It is the simplest organic nitro compound. It is a slightly viscous, highly polar liquid commonly used as a solvent in a variety of industrial applications such as in extractions, as a reaction medium, and as a cleaning solvent...
in the presence of aluminum trichloride or with iodine monochloride
Iodine monochloride
Iodine monochloride is an interhalogen compound with the formula ICl. It is a red-brown compound that melts near room temperature. Because of the difference in the electronegativity of iodine and chlorine, ICl is highly polar and behaves as a source of I+....
.
Compared to chlordane
Chlordane
Chlordane, or chlordan, is an organochlorine compound that was used as a pesticide. This white solid was sold in the U.S. until 1983 as an insecticide for crops like corn and citrus and on lawns and domestic gardens.-Production and uses:...
, it is about 3 - 5 times more active as an insecticide, but more inert chemically, being resistant to water and caustic alkalies.
Metabolism
Soil microorganisms transform heptachlor by epoxidation, hydrolysis, and reduction. When the compound was incubated with a mixed culture of organisms, chlordene (hexachlorocyclopentadine, its precursor) formed, which was further metabolized to chlordene epoxide. Other metabolites include 1-hydroxychlordene, 1-hydroxy-2,3-epoxychlordene, and heptachlor epoxide. Soil microorganisms hydrolyze heptachlor to give ketochlordene. Rats metabolize heptachlor to the epoxide 1-exo-1-hydroxyheptachlor epoxide and 1,2-dihydrooxydihydrochlordene. When heptachlor epoxide was incubated with microsomal preparations form liver of pigs and from houseflies, the products found were diol and 1-hydroxy-2,3-epoxychlordene. Metabolic scheme in rats shows two pathways with the same metabolite. The first involves following scheme: heptachlor → heptachlor epoxide → dehydrogenated derivative of 1-exo-hydroxy-2,3-exo-epoxychlordene → 1,2-dihydrooxydihydrochlordene. The second involves: Heptachlor → 1-exo-hydroxychlordene → 1-exo-hydroxy, 2,3-exo-epoxychlordene → 1,2-dihydrooxydihydrochlordene.Environmental impact
Heptachlor is persistent organic pollutantPersistent organic pollutant
thumb|right|275px|State parties to the Stockholm Convention on Persistent Organic PollutantsPersistent organic pollutants are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes...
(POP). It has a half life of ~1.3-4.2 days (air),~0.03-0.11 years (water),~0.11-0.34 years (soil). One study described its half life to be 2 years and claimed that its residues could be found in soil 14 years after its initial application. Like other POPs, heptachlor is lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...
and poorly soluble in water (0.056 mg/L at 25 °C), thus it tends to accumulate in the body fat of humans and animals.
Heptachlor epoxide is more likely to be found in the environment than its parent compound. The epoxide also dissolves more easily in water than its parent compound and is more persistent. Heptachlor and its epoxide absorb to soil particles and evaporate.
Toxicity of heptachlor and related derivatives
The range of oral rat values are 40 mg/kg to 162 mg/kg. Daily oral doses of heptachlor at 50 and 100 mg/kg were found to be lethal to rats after 10 days. For heptachlor epoxide, the oral LD50 values ranging from 46.5 to 60 mg/kg. With rat oral of LD5047mg/kg, heptachlor epoxide is more toxic. A product of hydrogenation of heptachlor, β-dihydroheptachlor, has high insecticidal activity and low mammalian toxicity, rat oral LD50>5,000mg/kg.Human impact
Humans are exposed to heptachlor through drinking water and foods, including breast milk. Heptachlor epoxide is derived from a pesticide that was banned in the U.S. in the 1980s. It is still found in soil and water supplies and can turn up in food and be passed along in breast milk. High levels of it seemed to increase type 2 diabetes risk to about 7 percent (Harmon 2010).The International Agency for Research on Cancer (IARC) and the EPA have classified the compound as a possible human carcinogen. Animals exposed to Heptachlor epoxide during gestation and infancy are found to have changes in nervous system and immune function. Higher doses of Heptachlor when exposed to newborn animals caused decrease in body weight and death.
The U.S. EPA MCL for drinking water is 0.0004 mg/L (Heptachlor) and 0.0002 mg/L (Heptachlor epoxide). The U.S. FDA limit on food crops is 0.01 ppm, in milk is 0.1 ppm, and on edible seafoods is 0.3 ppm. The Occupational Safety and Health Administration (OSHA) has limit of 0.5 mg/m3 (cubic meter of workplace air) for 8-hour shifts and 40-hour work weeks.
The compound was designated as a Class 2B or "possible human carcinogen" by The International Agency for Research on Cancer (IARC). ATSDR report in 1993 found no studies with respect to death in humans after oral exposure to heptachlor or heptachlor epoxide.