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Tetrachloroethylene

Tetrachloroethylene

Overview
Tetrachloroethylene, also known under its systematic name tetrachloroethene and many other names, is a chlorocarbon with the formula Cl2C=CCl2. It is a colourless liquid widely used for dry cleaning
Dry cleaning
Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically tetrachloroethylene , abbreviated "perc" in the industry and "dry-cleaning fluid" by the public...

 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 1 megatonne in 1985.

Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....

 first synthesized tetrachloroethene in 1821 by thermal decomposition of hexachloroethane
Hexachloroethane
Hexachloroethane, also known as perchloroethanes , C2Cl6, is a colorless solid at room temperature which is used by the US Military in smoke compositions, e.g...

.
C2Cl6 → C2Cl4 + Cl2


Most tetrachloroethene is produced by high temperature chlorinolysis of light hydrocarbons.
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Encyclopedia
Tetrachloroethylene, also known under its systematic name tetrachloroethene and many other names, is a chlorocarbon with the formula Cl2C=CCl2. It is a colourless liquid widely used for dry cleaning
Dry cleaning
Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically tetrachloroethylene , abbreviated "perc" in the industry and "dry-cleaning fluid" by the public...

 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 1 megatonne in 1985.

Production


Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....

 first synthesized tetrachloroethene in 1821 by thermal decomposition of hexachloroethane
Hexachloroethane
Hexachloroethane, also known as perchloroethanes , C2Cl6, is a colorless solid at room temperature which is used by the US Military in smoke compositions, e.g...

.
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. Side products include carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...

, hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

, and hexachlorobutadiene
Hexachlorobutadiene
Hexachlorobutadiene, Cl2C=CC=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.-Synthesis:Hexachlorobutadiene, or...

.

Several other methods have been developed. When 1,2-dichloroethane
1,2-Dichloroethane
The chemical compound 1,2-dichloroethane, commonly known by its old name of ethylene dichloride , is a chlorinated hydrocarbon, mainly used to produce vinyl chloride monomer , the major precursor for PVC production. It is a colourless liquid with a chloroform-like odour...

 is heated to 400 °C with chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

, tetrachloroethene is produced by the chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

:
ClCH2CH2Cl + 3 Cl2 → Cl2C=CCl2 + 4 HCl
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...


This reaction can be catalyzed by a mixture of potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...

 and aluminium chloride
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...

 or by activated carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

. Trichloroethylene
Trichloroethylene
The chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. It is a clear non-flammable liquid with a sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, which is commonly known as chlorothene.The IUPAC name is...

 is a major byproduct, which is separated by distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

.

According to an EPA report of 1976, the quantity of Tetrachloroethylene (also known as perchloroethylene or PCE) produced in the United States in just one year 1973, totaled 706 million pounds (320,000 metric tons). Diamond Shamrock, Dow Chemical Company, E.I DuPont and Vulcan Materials Company (Chemical Division) were among the top eight producers nationwide.

Uses


Tetrachloroethylene is an excellent solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 for organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 materials. Otherwise it is volatile, highly stable, and nonflammable. For these reasons, it is widely used in dry cleaning
Dry cleaning
Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically tetrachloroethylene , abbreviated "perc" in the industry and "dry-cleaning fluid" by the public...

. Usually as a mixture with other chlorocarbons, it is also used to degrease metal parts in the automotive
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 and other metalworking industries. It appears in a few consumer products including paint stripper
Paint stripper
Paint stripper, or paint remover, is the generic name given to solvent mixtures designed to remove paint and other finishes and also to clean the underlying surface. The principal active ingredient is usually dichloromethane...

s and spot removers.

Historical applications


Tetrachloroethene was once extensively used as an intermediate in the manufacture of HFC-134a
1,1,1,2-Tetrafluoroethane
1,1,1,2-Tetrafluoroethane, R-134a, Genetron 134a, Suva 134a or HFC-134a, is a haloalkane refrigerant with thermodynamic properties similar to R-12 , but with less ozone depletion potential...

 and related refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...

s. In the early 20th century, tetrachloroethene was used for the treatment for hookworm
Hookworm
The hookworm is a parasitic nematode that lives in the small intestine of its host, which may be a mammal such as a dog, cat, or human. Two species of hookworms commonly infect humans, Ancylostoma duodenale and Necator americanus. A. duodenale predominates in the Middle East, North Africa, India...

 infestation.

Health and safety


The International Agency for Research on Cancer
International Agency for Research on Cancer
The International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organisation of the United Nations....

 has classified tetrachloroethene as a Group 2A carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

, which means that it is probably carcinogenic to humans. Like many chlorinated hydrocarbons, tetrachloroethene is a central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 depressant and can enter the body through respiratory or dermal exposure. Tetrachloroethene dissolves fats from the skin, potentially resulting in skin irritation.

Animal studies and a study of 99 twins by Dr. Samuel Goldman and researchers at the Parkinson's Institute in Sunnyvale, California determined there is a "lot of circumstantial evidence" that exposure to Tetrachloroethlene increases the risk of developing Parkinson's disease ninefold. Larger population studies are planned.

At temperatures over 600 °F (315.6 °C), such as in welding, tetrachloroethylene can decompose into phosgene
Phosgene
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles...

, an extremely poisonous gas. Tetrachloroethylene should not be used near welding operations, flames, or hot surfaces.

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
Trichloroacetic acid
Trichloroacetic acid is an analogue of acetic acid in which the three hydrogen atoms of the methyl group have all been replaced by chlorine atoms....

 (TCA), a breakdown product of tetrachloroethene, can be detected in the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

.

In Europe, the Scientific Committee on Occupational Exposure Limits (SCOEL) recommends for tetrachloroethylene an occupational exposure limit
Occupational exposure limit
An occupational exposure limit is an upper limit on the acceptable concentration of a hazardous substance in workplace air for a particular material or class of materials. It is typically set by competent national authorities and enforced by legislation to protect occupational safety and health...

 (8h time-weighted average) of 20 ppm and a short-term exposure limit (15 min) of 40 ppm.

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
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 at low levels, and its density (which causes it to sink below the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

), 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. Partial degradation daughter products include trichloroethylene
Trichloroethylene
The chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. It is a clear non-flammable liquid with a sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, which is commonly known as chlorothene.The IUPAC name is...

, cis-1,2-dichloroethene
1,2-Dichloroethene
1,2-Dichloroethene, commonly called 1,2-dichloroethylene or 1,2-DCE, is an organochloride with the molecular formula C2H2Cl2. It is a highly flammable, colorless liquid with a sharp, harsh odor. It can exist as either of two geometric isomers, cis-1,2-dichloroethene or trans-1,2-dichloroethene,...

 and vinyl chloride
Vinyl chloride
Vinyl chloride is the organochloride with the formula H2C:CHCl. It is also called vinyl chloride monomer, VCM or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride . At ambient pressure and temperature, vinyl chloride...

; full degradation converts tetrachloroethylene to ethene and 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
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

, 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.

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