Tetrachloroethylene


Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, 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. Worldwide production was about in 1985.

Production

first synthesized tetrachloroethylene in 1821 by thermal decomposition of hexachloroethane.
Most tetrachloroethylene is produced by high temperature chlorinolysis of light hydrocarbons. The method is related to Faraday's discovery since hexachloroethane is generated and thermally decomposes. 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, tetrachloroethylene is produced by the chemical reaction:
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 a United States Environmental Protection Agency report of 1976, the quantity of tetrachloroethylene produced in the United States in 1973 totaled 320,000 metric tons. By 1993, the volume produced in the United States had dropped to 123,000 metric tons.

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. It is also used to degrease metal parts in the automotive and other metalworking industries, usually as a mixture with other chlorocarbons. It appears in a few consumer products including paint strippers and spot removers. It is also used in aerosol preparations.
It is used in neutrino detectors where a neutrino interacts with a neutron in the chlorine atom and converts it to a proton to form argon.

Historical applications

Tetrachloroethylene 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 of hookworm infestation.

Health and safety

The acute toxicity of tetrachloroethylene "is moderate to low". "Reports of human injury are uncommon despite its wide usage in dry cleaning and degreasing".
The International Agency for Research on Cancer has classified tetrachloroethylene as a Group 2A carcinogen, which means that it is probably carcinogenic to humans. Like many chlorinated hydrocarbons, tetrachloroethylene is a central nervous system depressant and can enter the body through respiratory or dermal exposure. Tetrachloroethylene dissolves fats from the skin, potentially resulting in skin irritation.
Owing to tetrachloroethylene's toxicity and cancer risks, California's Air Resources Board banned the substance from use in new dry-cleaning machines in 2007, with older PCE-using machines shut down by mid-2010 and the use of all such machines to be discontinued in California by 2023.
Animal studies and a study of 99 twins showed there is a "lot of circumstantial evidence" that exposure to tetrachloroethylene increases the risk of developing Parkinson's disease ninefold. Larger population studies are planned. Also, tetrachloroethylene has been shown to cause liver tumors in mice and kidney tumors in male rats.
At temperatures over, such as in welding, tetrachloroethylene can be oxidized into phosgene, an extremely poisonous gas.
The U.S. National Institute for Occupational Safety and Health has compiled extensive health and safety information for tetrachloroethylene, including recommendations for dry cleaning establishments.
Tetrachloroethylene exposure has been linked to pronounced acquired color vision deficiencies after chronic exposure.

Testing for exposure

Tetrachloroethylene 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, tetrachloroethylene can be detected in the breath for weeks following a heavy exposure. Tetrachloroethylene and trichloroacetic acid, a breakdown product of tetrachloroethylene, can be detected in the blood.
In Europe, the Scientific Committee on Occupational Exposure Limits recommends for tetrachloroethylene an occupational exposure limit of 20 ppm and a short-term exposure limit of 40 ppm.

Environmental contamination

Tetrachloroethylene 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 are released. Because of its mobility in groundwater, its toxicity at low levels, and its density, cleanup activities are more difficult than for oil spills: oil has a specific gravity less than 1. Recent research on soil and ground water pollution by tetrachloroethylene has focused on in-place remediation. 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. 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, water, soil, 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 to be 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 persistent under conditions. It is degraded by reductive dechlorination under anaerobic conditions, with degradation products such as trichloroethylene, dichloroethylene, vinyl chloride, ethylene, and ethane.. It has an ozone depletion potential of 0.005, where CFC-11 is 1.