Tetrachloroethylene, also known as perchloroethylene or under the systematic name tetrachloroethene, and abbreviations such as perc, and PCE, is a chlorocarbon with the formula Cl2C=CCl2. It is a volatile, non-flammable, stable, colorless and dense liquid widely used for dry cleaning of fabrics and as a metal degreasing solvent, formerly as an oral anthelmintic. It has a mild, sweet, sharp odor, detectable by most people at a concentration of 50 ppm. Tetrachloroethylene is produced industrially by the chlorination or oxychlorination of hydrocarbons. Approximately a million tons of tetrachloroethylene are produced every year and the production amount is increasing every year, especially to be used in the production of HCFC and HFC refrigerants. It was first obtained from the thermal decomposition of hexachloroethane in 1820 by the English chemist-physicist Michael Faraday. Tetrachloroethylene is more stable compared to other chlorinated solvents and similar compounds as it does not react easily, does not tend to polymerize and has lower toxicity. Inhalation of vapours may affect the central nervous system, causing drowsiness, numbness, hallucinations and loss of consciousness. Exposure to high concentrations may irritate the skin and respiratory tract. It has been suspected of causing cancer in humans, but the evidence is limited. It was classified as "group 2A - probably carcinogenic" by the International Agency for Research on Cancer in 1995 due to sufficient evidence in some experimental animals and limited evidence for humans. The majority of studies investigating people exposed to tetrachloroethylene in the workplace and the environment have not found relation between tetrachloroethylene and human cancers. It is considered an industrial organic pollutant and has caused groundwater pollution in the past due to improper disposal. Its use has been restricted in some regions due to its environmental impacts and possible effects on human health, and alternatives are being explored in sectors such as dry cleaning.
History and production The English chemist Michael Faraday first synthesized tetrachloroethylene in 1820 by thermal decomposition of hexachloroethane:
Cl3C−CCl3 → Cl2C=CCl2 + Cl2 Tetrachloroethylene can be made by passing chloroform vapour through a red-hot tube, the side products include hexachlorobenzene and hexachloroethane, as reported in 1886. Most tetrachloroethylene is produced by high-temperature chlorinolysis of light hydrocarbons. The method is related to Faraday's method 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 (752 °F; 673 K) with chlorine, tetrachloroethylene is produced:
ClCH2−CH2Cl + 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. Worldwide production was about 1 million metric tons (980,000 long tons; 1,100,000 short tons) in 1985. In the USA, annual production was 700 million pounds (310,000 long tons) by 1978. Although in very small amounts, tetrachloroethylene occurs naturally in volcanoes along with trichloroethylene.
Uses Tetrachloroethylene is a nonpolar solvent for organic materials. Additionally, it is volatile, relatively stable, and non-flammable. For these reasons, it became a leading solvent in dry cleaning operations worldwide beginning in the 1940s. The chemist Sylvia Stoesser (1901–1991) had suggested tetrachloroethylene to be used in dry cleaning as an alternative to highly flammable dry cleaning solvents such as naphtha. Tetrachloroethylene is the most common solvent in dry cleaning and has been considered the standard for cleaning performance. It is a highly effective cleaning solvent with a KB-value of 90. Used tetrachloroethylene is recycled by distillation at its boiling point (121 °C). Perchloroethylene can cause color bleeding/loss, especially at higher temperatures. In some cases it may damage special trims, buttons and beads on some garments. It is better for oil-based stains than more common water-soluble stains. Due to its high volatility, it does not leave permanent smell on dry-cleaned clothes. A dry cleaning machine running on perchloroethylene is called a "perc machine".
It is also used to degrease metal parts in the automotive and other metalworking industries, usually as a mixture with other chlorocarbons. It has also been used in consumer products including paint strippers, aerosol preparations, adhesives, spot removers, and handicrafts.
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. In 1925, American veterinarian Maurice Crowther Hall (1881–1938), working on anthelminthics, demonstrated the effectiveness of tetrachloroethylene in the treatment of ancylostomiasis caused by hookworm infestation in humans and animals. Before Hall tested tetrachloroethylene on himself, in 1921 he discovered the effectiveness of carbon tetrachloride on intestinal parasites and was nominated for the Nobel Prize in Physiology or Medicine, but a few years later he found tetrachloroethylene to be more effective and safer. Tetrachloroethylene treatment has played a vital role in eradicating hookworms in the United States and abroad. Hall's innovation was considered a breakthrough in medicine. It was given orally as a liquid or in capsules along with magnesium sulfate to treat the Necator americanus parasite in humans.
Chemical properties and reactions Tetrachloroethylene is a derivative of ethylene with all hydrogens replaced by chlorine. By weight, it consists of 14.5% carbon and 85.5% chlorine. It is the most stable compound among all chlorinated derivatives of ethane and ethylene. It is resistant to hydrolysis and less corrosive than other chlorinated solvents. Tetrachloroethylene does not tend to polymerise, unlike the fluorine analogue tetrafluoroethylene (C2F4). Tetrachloroethylene may react violently with alkali metals, alkaline earth metals, strong alkalis (sodium hydroxide and potassium hydroxide), nitric acid, beryllium, barium and aluminium.
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