Tetrachloroethylene

Tetrachloroethylene
IUPAC name Tetrachloroethene
Other names Perchloroethene; Perchloroethylene; Perc; PCE
Identifiers
CAS number	127-18-4 Y
ChemSpider	13837281 Y
UNII	TJ904HH8SN Y
EC number	204-825-9
UN number	1897
KEGG	C06789 Y
ChEBI	CHEBI:17300 N
ChEMBL	CHEMBL114062 Y
RTECS number	KX3850000
Jmol-3D images	Image 1
SMILES Cl/C(Cl)=C(/Cl)Cl
InChI InChI=1S/C2Cl4/c3-1(4)2(5)6 Y Key: CYTYCFOTNPOANT-UHFFFAOYSA-N Y InChI=1/C2Cl4/c3-1(4)2(5)6 Key: CYTYCFOTNPOANT-UHFFFAOYAO
Properties
Molecular formula	C2Cl4
Molar mass	165.83 g mol−1
Appearance	Clear, colorless liquid
Density	1.622 g/cm3
Melting point	−19 °C; −2 °F; 254 K
Boiling point	121.1 °C; 250.0 °F; 394.2 K
Solubility in water	0.015 g/100 mL (20 °C)
Viscosity	0.89 cP at 25 °C
Hazards
MSDS	External MSDS
R-phrases	R40 R51/53
S-phrases	S23 S36/37 S61
Main hazards	Harmful (Xn), Dangerous for the environment (N)
NFPA 704	0 2 0
Flash point	Not flammable
Related compounds
Related Related organohalides	Tetrabromoethylene Tetraiodoethylene
Related compounds	Trichloroethylene Dichloroethene Tetrachloroethane
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene ("perc" or "PERC"), and many other names, is a chlorocarbon with the formula Cl₂C=CCl₂. It is a colorless liquid widely used for dry cleaning of fabrics, hence it is sometimes called "dry-cleaning fluid." It has a sweet odor detectable by most people at a concentration of 1 part per million (1 ppm). Worldwide production was about one million metric tons in 1985.^[1]

Production

Michael Faraday first synthesized tetrachloroethylene in 1821 by thermal decomposition of hexachloroethane.

C₂Cl₆ → C₂Cl₄ + Cl₂

Most tetrachloroethene is produced by high temperature chlorinolysis of light hydrocarbons. The method is related to Faraday's discovery since hexachloroethane is generated and thermally decomposes.^[1] Side products include carbon tetrachloride, hydrogen chloride, and hexachlorobutadiene.

Several other methods have been developed. When 1,2-dichloroethane is heated to 400 °C with chlorine, tetrachloroethene is produced by the chemical reaction:

ClCH₂CH₂Cl + 3 Cl₂ → Cl₂C=CCl₂ + 4 HCl

This reaction can be catalyzed by a mixture of potassium chloride and aluminium chloride or by activated carbon. Trichloroethylene is a major byproduct, which is separated by distillation.

According to an United States Environmental Protection Agency (EPA) report of 1976, the quantity of tetrachloroethylene produced in the United States in the 1973 totaled 320,000 metric tons (706 million lb).^[2] By 1993, the volume produced in the United States had dropped to 123,000 metric tons (271 million lb).^[3]

Uses

Tetrachloroethylene is an excellent solvent for organic materials. Otherwise it is volatile, highly stable, and nonflammable. For these reasons, it is widely used in dry cleaning. Usually as a mixture with other chlorocarbons, it is also used to degrease metal parts in the automotive and other metalworking industries. It appears in a few consumer products including paint strippers and spot removers.

Historical applications

Tetrachloroethene was once extensively used as an intermediate in the manufacture of HFC-134a and related refrigerants. In the early 20th century, tetrachloroethene was used for the treatment for hookworm infestation.^[4]

Health and safety

The International Agency for Research on Cancer has classified tetrachloroethene as a Group 2A carcinogen, which means that it is probably carcinogenic to humans.^[5] Like many chlorinated hydrocarbons, tetrachloroethene is a central nervous system depressant and can enter the body through respiratory or dermal exposure.^[6] Tetrachloroethene dissolves fats from the skin, potentially resulting in skin irritation.

Animal studies and a study of 99 twins showed there is a "lot of circumstantial evidence" that exposure to tetrachloroethene increases the risk of developing Parkinson's disease ninefold. Larger population studies are planned.^[7]

At temperatures over 315 °C (599 °F), such as in welding, tetrachloroethylene can be oxidised into phosgene, an extremely poisonous gas.^[8][9] Tetrachloroethylene should not be used near welding operations, flames, or hot surfaces.^[10]

Testing for exposure

Tetrachloroethene exposure can be evaluated by a breath test, analogous to breath-alcohol measurements. Because it is stored in the body's fat and slowly released into the bloodstream, tetrachloroethene can be detected in the breath for weeks following a heavy exposure. Tetrachloroethylene and trichloroacetic acid (TCA), a breakdown product of tetrachloroethene, can be detected in the blood.

In Europe, the Scientific Committee on Occupational Exposure Limits (SCOEL) recommends for tetrachloroethylene an occupational exposure limit (8h time-weighted average) of 20 ppm and a short-term exposure limit (15 min) of 40 ppm.^[11]

Environmental contamination

Tetrachloroethene is a common soil contaminant. With a specific gravity greater than 1, tetrachloroethylene will be present as a dense nonaqueous phase liquid if sufficient quantities of liquid are spilled in the environment. Because of its mobility in groundwater, its toxicity at low levels, and its density (which causes it to sink below the water table), cleanup activities are more difficult than for oil spills. Recent research has focused on the in place remediation of soil and ground water pollution by tetrachloroethylene. Instead of excavation or extraction for above-ground treatment or disposal, tetrachloroethylene contamination has been successfully remediated by chemical treatment or bioremediation. Bioremediation has been successful under anaerobic conditions by reductive dechlorination by Dehalococcoides sp. and under aerobic conditions by cometabolism by Pseudomonas sp.^[12][13] Partial degradation daughter products include trichloroethylene, cis-1,2-dichloroethene and vinyl chloride; full degradation converts tetrachloroethylene to ethene and hydrogen chloride dissolved in water.

Estimates state that 85% of tetrachloroethylene produced is released into the atmosphere; while models from OECD assumed that 90% is released into the air and 10% to water. Based on these models, its distribution in the environment is estimated to be in the air (76.39% - 99.69%), water (0.23% - 23.2%), soil (0.06-7%), with the remainder in the sediment and biota. Estimates of lifetime in the atmosphere vary, but a 1987 survey estimated the lifetime in the air has been estimated at about 2 months in the Southern Hemisphere and 5–6 months in the Northern Hemisphere. Degradation products observed in a laboratory include phosgene, trichloroacetyl chloride, hydrogen chloride, carbon dioxide, and carbon monoxide. Tetrachloroethylene is degraded by hydrolysis, and is also persistent under aerobic conditions. This compound is degraded by reductive dechlorination with anaerobic conditions present, with the degradation products like trichloroethene, dichloroethene, vinyl chloride, ethene, and ethane.^[14]

References

Further reading

• "Toxicological Profile for Tetrachloroethene". Agency for Toxic Substances and Disease Registry. 1997. 

• Doherty, R.E. (2000). "A History of the Production and Use of Carbon Tetrachloride, Tetrachloroethylene, Trichloroethylene and 1,1,1-Trichloroethane in the United States: Part 1 - Historical Background; Carbon Tetrachloride and Tetrachloroethylene". Environmental Forensics 1 (2): 69–81. doi:10.1006/enfo.2000.0010. 

External links

• ATSDR Case Studies in Environmental Medicine: Tetrachloroethylene Toxicity U.S. Department of Health and Human Services
• Tetrachloroethylene (Perchloroethylene) U.S. Department of Health and Human Services
• Australian National Pollutant Inventory (NPI) page
• "Toxic Fumes May Have Made Gunman Snap", by Julian Kesner, New York Daily News, April 20, 2007.