Hexachlorobenzene

Chembox new
Name = Hexachlorobenzene
ImageFileL1 = Hexachlorobenzene.png ImageSizeL1 = 100px
ImageNameL1 = Hexachlorobenzene
ImageFileR1 = Hexachlorobenzene-3D-vdW.png ImageSize1 = 120px
ImageName1 = Hexachlorobenzene
IUPACName = Hexachlorobenzene
OtherNames = Perchlorobenzene
HCB
Section1 = Chembox Identifiers
CASNo = 118-74-1
SMILES = Clc1=c(Cl)c(Cl)=c(Cl)c(Cl)=c1Cl
Section2 = Chembox Properties
Formula = C₆Cl_{6
MolarMass = 284.80 g/mol
Density = 2.04 g/cm³
MeltingPt = 231 °C
BoilingPt = 323-326 °C}

Hexachlorobenzene, or perchlorobenzene, is a chlorinated hydrocarbon with the molecular formula C₆Cl₆. It is a fungicide formerly used as a seed treatment, especially on wheat to control the fungal disease bunt. It has been banned globally under the Stockholm Convention on persistent organic pollutants. [ [http://www.pops.int/documents/pops/default.htm Stockholm Convention on Persistent Organic Pollutants website] ] "Not to be confused with benzene hexachloride, another name for lindane."

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.

afety

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 (possibly carcinogenic to humans). Animal carcinogenicity data for hexachlorobenzene show increased incidences of liver, kidney (renal tubular tumours) and thyroid cancers.cite web | title=Hexachlorobenzene | publisher=The Carcinogenic Potency Database Project | url=http://potency.berkeley.edu/chempages/HEXACHLOROBENZENE.html | accessdate=2007-12-12] 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/m³Material has relatively low acute toxicity but is toxic because of its persistent and cumulative nature in body tissues in rich lipid content.

Unique Exposure Incident

In Anatolia, Turkey between 1955 and 1959, during a period when bread wheat was unavailable, 500 people were fatally poisoned and more than 4,000 people fell ill by eating bread made with HCB-treated seed that was intended for agriculture use. Most of the sick were affected with a liver condition called "porphyria cutanea tarda", which disturbs the metabolism of hemoglobin and results in skin lesions. Almost all breastfeeding children under the age of two, whose mothers had eaten tainted bread, died from a condition called "pembe yara" or "pink sore," most likely from high doses of HCB in the breast milk. [Natural Resources Defense Council (2001). Healthy Milk, Healthy Baby: Chemical Pollution and Mother's Milk. Retrieved September 13, 2008 from the World Wide Web: http://www.nrdc.org/breastmilk/chem4.asp] In one mother's breast milk the HCB level was found to be 20 parts per million in lipid, approximately 2,000 times the average levels of contamination found in breast-milk samples around the world. [Jensen, A.A. and S.A. Slorach. Chemical Contaminants in Human Milk, 1991, Boca Raton Ann Arbor Boston: CRC Press, Inc.] [Courtney, K.D. Hexachlorobenzene (HCB): A Review, Environmental Research 1979; 20: p. 225-266.] Follow-up studies 20 to 30 years after the poisoning found average HCB levels in breast milk were still more than seven times the average for unexposed women in that part of the world (56 specimens of human milk obtained from mothers with porphyria, average value was 0.51 ppm in HCB-exposed patients compared to 0.07 ppm in unexposed controls),Gocmen, A., et al. Hexachlorobenzene Episode in Turkey, Biomedical Environmental Science 1989; 2(1): p. 36-43.] [Cripps, D.J., et al. Porphyria Turcica Due to Hexachlorobenzene: A 20 to 30 Year Follow-up Study on 204 Patients, British Journal of Dermatology 1984; 111(4): p. 413-422.] and 150 times the level allowed in cow's milk. [Peters, H.A., et al. Epidemiology of Hexachlorobenzene-induced Porphyria in Turkey: Clinical and Laboratory Follow-up After 25 Years, Archives of Neurology 1982; 39(12): p. 744-749.]

In the same follow-up study of 252 patients (162 males and 90 females, avg. current age of 35.7 years), 20-30 years postexposure, many subjects had dermatologic, neurologic, and orthopedic symptoms and signs. The observed clinical findings include scarring of the face and hands (83.7%), hyperpigmentation (65%), hypertrichosis (44.8%), pinched faces (40.1%), painless arthritis (70.2%), small hands (66.6%), sensory shading (60.6%), myotonia (37.9%), cogwheeling (41.9%), enlarged thyroid (34.9%), and enlarged liver (4.8%). Urine and stool porphyrin levels were determined in all patients, and 17 have at least one of the porphyrins elevated. Offspring of mothers with three decades of HCB-induced porphyria appear normal.

References

Cited works

Additional references
*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
*"Registry of Toxic Effects of Chemical Substances". Ed. D. Sweet, US Dept. of Health & Human Services: Cincinnati, 2005.
*Environmental Health Criteria No 195; International Programme on Chemical Safety, World health Organization, Geneva, 1997.
*Toxicological Profile for Hexachlorobenzene (Update), US Dept of Health & Human Services, Sept 2002.
*"Merck Index", 11th Edition, 4600