Hexachlorobenzene

Hexachlorobenzene, or perchlorobenzene, is a chlorinated hydrocarbon with the molecular formula C₆Cl₆. It (and its initials "HCB") is also sometimes used as a trivial name for the pesticide lindane, but its usage in that context is incorrect. It is a fungicide formerly used as a seed treatment, especially on wheat. Hexachlorobenzene controls the fungal disease bunt. It may be used with or without other seed treatments.

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Physicial and chemical properties

HCB is a white crystalline solid that has negligible solubility in water. It is soluble in diethyl ether, benzene, ethanol and chloroform. Its vapour pressure is 1.09×10⁻⁵ mmHg (1.45 mPa) at 20 °C. Its flash point is 242 °C and it sublimes at 322 °C.

Safety

Hexachlorobenzene is an animal carcinogen and is considered to be a probable human carcinogen. After its introduction as a fungicide in 1945, for crop seeds, this toxic chemical was found in all food types. Hexachlorobenzene was banned from use in the United States in 1966.

This material has been classified by the International Agency for Research on Cancer (IARC) as a Group 2B carcinogen (the agent is possibly carcinogenic to humans). Animal carcinogenicity data for hexachlorobenzene show increased incidences of liver, kidney (renal tubular tumours) and thyroid cancers.^[1] 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. Toxic to aquatic organisms. May cause long term adverse effects in the aquatic environment. Risk of bioaccumulation in an aquatic species is high.

Toxicology

• Oral LD50 (rat): 10,000 mg/kg
• Oral LD50 (mice): 4,000 mg/kg
• Inhalation LC50 (rat): 3600 mg/m³

Material has relatively low acute toxicity but is toxic because of its persistent and cumulative nature in body tissues in rich lipid content.

References

1. ^ Hexachlorobenzene. The Carcinogenic Potency Database Project, University of Berkeley. Retrieved on 2007-12-12.

1. International Agency for Research on Cancer. In: IARC Monographs on the Evaluation of Carcinogenic Risk to Humans. World Health Organisation, Vol 79, 2001pp 493-567
2. Registry of Toxic Effects of Chemical Substances. Ed. D. Sweet, US Dept. of Health & Human Services: Cincinnati, 2005.
3. Environmental Health Criteria No 195; International Programme on Chemical Safety, World health Organization, Geneva, 1997.
4. Toxicological Profile for Hexachlorobenzene (Update), US Dept of Health & Human Services, Sept 2002.
5. Merck Index, 11th Edition, 4600