Hexachlorobenzene, or perchlorobenzene, is an aryl chloride and a six-substituted chlorobenzene with the molecular formula C6Cl6. It is a fungicide formerly used as a seed treatment, especially on wheat to control the fungal disease bunt. Its use has been banned globally under the Stockholm Convention on Persistent Organic Pollutants.
Physical and chemical properties Hexachlorobenzene is a stable, white, crystalline chlorinated hydrocarbon. It is sparingly soluble in organic solvents such as benzene, diethyl ether and alcohol, but practically insoluble in water with no reaction. It has a flash point of 468 °F and it is stable under normal temperatures and pressures. It is combustible but it does not ignite readily. When heated to decomposition, hexachlorobenzene emits highly toxic fumes of hydrochloric acid, other chlorinated compounds (such as phosgene), carbon monoxide, and carbon dioxide.
History Hexachlorobenzene was first known as "Julin's chloride of carbon" as it was discovered as a strange and unexpected product of impurities reacting in Johan Jacob von Julin's nitric acid factory. In 1864, Hugo Müller synthesised the compound by the reaction of benzene and antimony pentachloride, he then suggested that his compound was the same as Julin's chloride of carbon. Müller previously also believed it was the same compound as Michael Faraday's "perchloride of carbon" (Hexachloroethane), obtained a small sample of Julin's chloride of carbon to send to Richard Phillips and Faraday for investigation. In 1867, Henry Bassett proved that the compound produced from benzene and antimony was the same as Julin's carbon chloride and named it "hexachlorobenzene". Leopold Gmelin named it "dichloride of carbon" and claimed that the carbon was derived from cast iron and the chlorine was from crude saltpetre. Victor Regnault obtained hexachlorobenzene from the decomposition of chloroform and tetrachloroethylene vapours through a red-hot tube. The crystal structure of hexachlorobenzene was determined by Kathleen Lonsdale who showed the molecule to be planar and hexagonal.
Synthesis Large-scale manufacture for use as a fungicide was developed by using the residue remaining after purification of the mixture of isomers of hexachlorocyclohexane, from which the insecticide lindane (the γ-isomer) had been removed, leaving the unwanted α- and β- isomers. This mixture is produced when benzene is reacted with chlorine in the presence of ultraviolet light (e.g. from sunlight). However, manufacture is no longer practiced following the compound's ban. Hexachlorobenzene has been made on a laboratory scale since the 1890s, by the electrophilic aromatic substitution reaction of chlorine with benzene or chlorobenzenes. A typical catalyst is ferric chloride. Much milder reagents than chlorine (e.g. dichlorine monoxide, iodine in chlorosulfonic acid) also suffice, and the various hexachlorocyclohexanes can substitute for benzene as well.
Usage Hexachlorobenzene was used in agriculture to control the fungus tilletia caries (common bunt of wheat). It is also effective on tilletia controversa, dwarf bunt. The compound was introduced in 1947, normally formulated as a seed dressing but is now banned in many countries. A minor industrial phloroglucinol synthesis nucleophilically substitutes hexachlorobenzene with alkoxides, followed by acidic workup.
Environmental considerations HCB production peaked at around 100,000 tons per year in the late 1970's. Since then, usage has been declining steadily, with production of less than 90 tons per year by the mid 1990s. The half-life in soil is estimated to be 9 years. The mechanism of its toxicity and other adverse effects remain under study.
Safety Hexachlorobenzene can react violently with dimethylformamide, particularly in the presence of catalytic transition-metal salts.
Toxicology Oral LD50 (rat): 10,000 mg/kg Oral LD50 (mice): 4,000 mg/kg Inhalation LC50 (rat): 3600 mg/m3 Hexachlorobenzene has relatively low acute toxicity but is persistent and cumulative nature in body tissues in rich lipid content. Hexachlorobenzene has been classified by the International Agency for Research on Cancer (IARC) as a Group 2B carcinogen (possibly carcinogenic to humans). Animal carcinogenicity data for hexachlorobenzene show increased incidences of liver, kidney (renal tubular tumours) and thyroid cancers. Chronic oral exposure in humans has been shown to give rise to a liver disease (porphyria cutanea tarda), skin lesions with discoloration, ulceration, photosensitivity, thyroid effects, bone effects and loss of hair. Neurological changes have been reported in rodents exposed to hexachlorobenzene. Hexachlorobenzene may cause embryolethality and teratogenic effects. Human and animal studies have demonstrated that hexachlorobenzene crosses the placenta to accumulate in foetal tissues and is transferred in breast milk. HCB is very toxic to aquatic organisms. It may cause long term adverse effects in the aquatic environment. Therefore, release into waterways should be avoided. It is persistent in the environment. Ecological investigations have found that biomagnification up the food chain does occur. Hexachlorobenzene has a half-life in the soil of between 3 and 6 years. Risk of bioaccumulation in an aquatic species is high.
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