Tetrachloroethylene, also known as perchloroethylene or under the systematic name tetrachloroethene, and abbreviations such as perc, and PCE, is a chlorocarbon with the formula . 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 1839 by the French chemist Victor Regnault.

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

French chemist Henri Victor Regnault first synthesized tetrachloroethylene in 1839 by thermal decomposition of hexachloroethane following Michael Faraday's 1820 synthesis of protochloride of carbon (carbon tetrachloride).

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Faraday was previously falsely credited for the synthesis of tetrachloroethylene, which in reality, was carbon tetrachloride. While trying to make Faraday's "protochloride of carbon", Regnault found that his compound was different from Faraday's. Victor Regnault stated "According to Faraday, the chloride of carbon boiled around to degrees Celsius but mine did not begin to boil until ".

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. In the USA, annual production was 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 volatiliy, it does not leave permanent smell on dry-cleaned clothes. A dry cleaning machine running on perchloroethylene is called a "perc machine".

thumb|left|upright|A 1990s dry cleaning machine designed for use with tetrachloroethylene, Germany.

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. 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.

Oxidation

Oxidation of tetrachloroethylene by ultraviolet radiation in air produces trichloroacetyl chloride and phosgene:

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This reaction can be halted by using amines and phenols (usually N-methylpyrrole and N-methylmorpholine) as stabilisers. But the reaction can be done intentionally to produce trichloroacetyl chloride.

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CFC-113 is produced by the reaction of tetrachloroethylene with chlorine and HF in the presence of antimony pentafluoride:

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Nitration

Tetrachlorodinitroethane can be obtained by nitration of tetrachloroethylene with fuming nitric acid (conc. rich in nitrogen oxides) or nitrogen tetroxide:

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The preparation of this crystalline solid compound from tetrachloroethylene and nitrogen tetroxide was first described by Hermann Kolbe in 1869.

Health and safety

The main routes of exposure to tetrachloroethylene are by inhalation, and potentially by ingestion or exposure to eyes and the skin. Systemic effects of exposure may include depression of brain function, although with substantial acute exposure, there is risk of depressed breathing, coma or death. In 2020, the United States Environmental Protection Agency stated that "tetrachloroethylene exposure may harm the nervous system, liver, kidneys, and reproductive system, and may be harmful to unborn children", and reported that numerous toxicology agencies regard it as a carcinogen, including the UK Health Security Agency. Although limited by its low volatility, tetrachloroethylene has potent anaesthetic effects upon inhalation. The risk depends on whether exposure is over minutes, hours or years. Attempts to reduce exposure and health risks have been adopted in the dry cleaning and laundry industries by introducing closed machinery systems to minimize vapor escape and optimize recycling.

Carcinogenicity

Tetrachloroethylene has been classified as "Group 2A: Probably Carcinogenic" by the International Agency for Research on Cancer (IARC) due to sufficient evidence in experimental animals and limited evidence in humans for non-Hodgkin lymphoma, urinary bladder cancers, and cancers of the esophagus and cervix. In the United States, the EPA considers tetrachloroethylene as "likely to be carcinogenic to humans by all routes of exposure" based on suggestive evidence from human epidemiology, and certain evidence from animal toxicology studies, while the US National Toxicology Program considers tetrachloroethylene as "reasonably anticipated to be a human carcinogen."

International advisories and compliance

The World Health Organization published a 2010 advisory on tetrachloroethylene as a possible contaminant of indoor air and drinking water, with concern for its potential carcinogenicity. Out of suspicion that tetrachloroethylene is carcinogenic, the European Union REACH program regards tetrachloroethylene as a hazardous compound requiring a warning that it may cause serious eye irritation, skin irritation, produce an allergic skin reaction, or cause drowsiness or dizziness.

Similar advisories and regulatory mandates for tetrachlorethylene in the workplace and public exist in Australia, Canada, the United Kingdom, In-situ thermal treatment in conjunction with ISCO may also accelerate treatment.

Bioremediation usually entails reductive dechlorination under anaerobic conditions by Dehalococcoides spp. Under aerobic conditions, degradation may occur via co-metabolism by Pseudomonas sp. Products of biological reductive dechlorination include trichloroethylene, cis-1,2-dichloroethylene, vinyl chloride, ethylene and chloride.

Explanatory notes

References

  • Australian National Pollutant Inventory (NPI) page
  • Sustainable uses and Industry recommendations, European Chlorinated Solvents Association