Hexachlorobutadiene

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ImageFile = Hexachloro.png ImageSize = 200px
IUPACName = Hexachloro-1,3-butadiene
OtherNames = Perchlorobutadiene, perchloro-1,3-butadiene
Section1 = Chembox Identifiers
CASNo = 87-68-3
PubChem = 6901
SMILES =C(=C(Cl)Cl)(C(=C(Cl)Cl)Cl)Cl
Section2 = Chembox Properties
Formula = C₄Cl₆
MolarMass = 260.76 g/mol
Appearance = Colorless liquid
Density = 1.665 g/mL at 25 ℃
MeltingPt = -22 - -19 ℃
BoilingPt = 210-220 ℃
Solubility =
Section3 = Chembox Hazards
ExternalMSDS = [http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/112194 Sigma Aldrich]
MainHazards =
FlashPt =
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Hexachlorobutadiene, Cl₂C=C(Cl)C(Cl)=CCl₂, is a colorless liquid at room temperature that has an odor similar to that of turpentine. It is a chlorinated aliphatic diene with niche applications but is most commonly used as a solvent for other chlorine-containing compounds.Manfred Rossberg et al. “Chlorinated Hydrocarbons,” Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co, 2006, DOI: 10.1002/14356007.a06_233.pub2] Kenric A. Marshall, “Chlorocarbons and Chlorohydrocarbons, Survey,” Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons Inc, 2003, DOI: 10.1002/0471238961.1021182218050504.a01.pub2]

ynthesis

Hexachlorobutadiene, or HCBD, is primarily produced in chlorinolysis plants as a by-product in the production of carbon tetrachloride and tetrachloroethene. Chlorinolysis is a radical chain reaction that occurs when hydrocarbons are exposed to chlorine gas under pyrolytic conditions. The hydrocarbon is chlorinated and the resulting chlorocarbons are broken down. This process is analogous to combustion, but with chlorine instead of oxygen.Peter Schmittinger et al. “Chlorine,” Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co, 2006, DOI: 10.1002/14356007.a06_399.pub2]

Hexachlorobutadiene occurs as a by-product during the chlorinolysis of butane derivatives in the production of both carbon tetrachloride and tetrachloroethene. These two commodities are manufactured on such a large scale, that enough HCBD can generally be obtained to meet the industrial demand. Alternatively, hexachlorobutadiene can be directly synthesized via the chlorination of butane or butadiene.

Reactivity

The products of chlorinolysis reactions heavily depend upon both the temperature and pressure under which the reaction occurs. Thus, by adjusting these reaction conditions in the presence of chlorine gas, hexachlorobutadiene can be even further chlorinated to give tetrachloroethylene, hexachloroethane, octachlorobutene and even decachlorobutane. In general, increasing the number of chlorine substituents on a compound increases its toxicity but decreases its combustibility. Chlorination via carbon skeleton cleavage is thermodynamically preferred, whereas chlorinated C₄ products are favored at lower temperatures and pressures. The three chlorinolysis products of hexachlorobutadiene are shown in the reactions below.

Applications

One of the primary applications of hexachlorobutadiene is as a solvent for chlorine, a good illustration of the common aphorism “like dissolves like.” The molar solubility of chlorine in HCBD at 0 °C is around 34% (2.17 mol/L). The solubility of another chlorine solvent, carbon tetrachloride, at 0 °C is about 30% (3.11 mol/L). One mole of C₄Cl₆ can dissolve more chlorine than one mole of CCl₄, but the molecular weight difference between the two solvents is such that per liter of solvent, more chlorine can be dissolved in carbon tetrachloride. Shown below is the molar solubility of hexachlorobutadiene compared to carbon tetrachloride at various temperatures.

Just like chlorine, many other chlorine-containing compounds can be readily dissolved in a solution of hexachlorobutadiene. As a solvent, it is unreactive toward common acids and select non-nucleophilic bases. An illustrative application HCBD as a solvent is the FeCl₃-catalyzed chlorination of toluene to give pentachloromethylbenzene. Hexachlorobutadiene is used exclusively over carbon tetrachloride in this reaction because ferric chloride is insoluble in CCl₄. [Pravin Khandare and Ron Spohn, “Toluenes, Ring-Chlorinated,” Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons Inc, 2001, DOI: 10.1002/0471238961.18091407120914.a01.pub2] [Michael T. Holbrook, “Carbon Tetrachloride,” Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons Inc, 1993, DOI: 10.1002/0471238961.0301180208151202.a01]

Given its affinity for chlorinated compounds, liquid HCBD is used as a scrubber in order to remove chlorine containing contaminants from gas streams. An example of this application is its use in the production of HCl gas as the primary contaminants, especially Cl₂, are more soluble in hexachlorobutadiene than the gaseous hydrogen chloride.

In IR spectroscopy, hexachlorobutadiene is occasionally used as a mull in order to analyze the stretching frequencies of C-H stretching bands. The usual mulling agent, Nujol, is a hydrocarbon and thus exhibits C-H stretching bands that can interfere with the signal from the sample. Since HCBD contains no C-H bonds, it can be used instead to obtain this portion of the IR spectrum. Unfortunately, some organometallic compounds react with HCBD, and therefore, care must be taken when selecting it as a mulling agent so as not to destroy the sample. [Gregory S Girolami, Thomas B. Rauchfuss, and Robert J. Angelici, “Synthesis and Technique in Inorganic Chemistry,” University Science Books, 1999, ISBN: 0935702482.]

Hexachlorobutadiene has yet another, albeit somewhat dated, application as an algicide in industrial cooling systems. Although HCBD is a potent herbicide, in recent years, this particular application has been discouraged due to the high toxicity of the compound at low concentrations. [Wolfgang Dekant and Spiridon Vamvakas, “Toxicology,” Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co, 2004, DOI: 10.1002/14356007.b07_155.pub2] [Nancy R. Passow, “Regulatory Agencies, Chemical Process Industry,” Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons Inc, 2003, DOI: 10.1002/0471238961.0308051316011919.a01.pub2]

Toxicity

Hexachlorobutadiene has been observed to produce systemic toxicity following exposure via oral, inhalation, and dermal routes. Effects may included fatty liver degeneration, epithelial necrotizing nephritis, central nervous system depression and cyanosis. [ATSDR (Agency for Toxic Substances and Disease Registry). [1994] . Toxicological Profile for Hexachlorobutadiene. U.S. Department of Health and Human Services. Public Health Service.]

The carcinogenicity of Hexachlorobutadiene has been classified by the United States Environmental Protection Agency [U.S. EPA (United States Environmental Protection Agency) [1991] . Hexachlorobutadiene. Integrated Risk Information System. Washington, DC: United States Environmental Protection Agency. Available at: http://cfpub.epa.gov/ncea/iris/index.cfm. Accessed 9/05/2008.] has classified hexachlorobutadiene as a group C Possible Human Carcinogen. The American Conference of Governmental and Industrial Hygienists has classified Hexachlorobutadiene as an A3 Confirmed Animal Carcinogen with Unknown Relevance to Humans. [ACGIH (American Conference of Governmental Industrial Hygienists) [2001] . Hexachlorobutadiene Documentation of Threshold Limit Values and Biological Exposure Indices. 7th Ed. Cincinnati, OH.]

References