Perchloroethylene

Tetrachloroethylene, also known as perchloroethylene or under the systematic name tetrachloroethene, and abbreviations such as perc (or PERC), and PCE, is a chlorocarbon with the formula . It is a non-flammable, stable, colorless and heavy liquid widely used for dry cleaning of fabrics and occasionally as a highly effective automotive brake cleaner. It has a mildly sweet, sharp odor, detectable by most people at a concentration of 50 ppm.

Tetrachloroethylene is regarded as a toxic substance, a human health hazard, and an environmental hazard. 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.

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. 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:
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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 in 1985. 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.

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, and spot removers.

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 anthelmintics, 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 get rid of the ''Necator americanus'' parasite in humans.

Chemical properties and reactions

Tetrachloroethylene is a derivative of ethylene with all hydrogens replaced by chlorine. 14.49% of the molecular weight of tetrachloroethylene consists of carbon and the remaining 85.5% is 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. It does not tend to polymerise like fluorine analogue tetrafluoroethylene, .

Tetrachloroethylene may react violently with alkali or alkaline earth metals, alkalis (sodium hydroxide and potassium hydroxide), nitric acid, beryllium, barium and aluminium.

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.

Chlorination

Hexachloroethane is formed when tetrachloroethylene reacts with chlorine at 50–80 °C in the presence of a small amount of iron(III) chloride (0.1%) as a catalyst:
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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.

Thermal decomposition

Tetrachloroethylene begins to thermally decompose at 400 °C, decomposition accelerates around 600 °C, and completely decomposes at 800 °C. Organic decomposition products identified were trichlorobutene, 1,3-dichloro-2-propanone, tetrachlorobutadiene, dichlorocyclopentane, dichloropentene, methyl trichloroacetate, tetrachloroacetone, tetrachloropropene, trichlorocyclopentane, trichloropentene, hexachloroethane, pentachloropropene, hexachloropropene, hexachlorobutadiene.

Health and safety

Tetrachloroethylene is considered to be a toxin. It is identified as a health hazard and environmental hazard. Exposure to tetrachloroethylene, especially long-term, may harm the nervous system, cause organ damage, and increase cancer risk. It may also have effects on pregnancy and the fetus.

Reports of human injury are uncommon despite its wide usage in dry cleaning and degreasing. Although limited by its low volatility, tetrachloroethylene has potent anaesthetic effects upon inhalation. The risk depends on whether exposure is over minutes or hours, or over years.

Despite the advantages of tetrachloroethylene, cancer research and government environmental agencies have called for its replacement from widespread commercial use. It is described as a possible neurotoxicant, liver and kidney toxicant and reproductive and developmental toxicant (...) a potential occupational carcinogen. On the other hand, dry cleaning industry emphasizes minimal risk because modern machinery use closed systems to avoid any vapour escape and to optimize recycling.

Metabolism

Tetrachloroethylene's biological half-life is approximately 3 days. About 98% of the inhaled tetrachloroethylene is exhaled unchanged and only about 1–3% is metabolised to tetrachloroethylene oxide which rapidly isomerises into trichloroacetyl chloride. Trichloroacetyl chloride hydrolyses to trichloroacetic acid.

Neurotoxicity

Tetrachloroethylene can harm the nervous system, cause developmental deficits in children, impair vision, and increase the risk of psychiatric diagnoses.

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.

Although tetrachloroethylene is suspected to be a human carcinogen, there is no convincing evidence to support this. According to IARC, there is no evidence that tetrachloroethylene is responsible for the cancer cases in dry cleaners, as most of the dry cleaners with cancer diagnoses were smokers and drinkers, and many other chemicals besides tetrachloroethylene are used in dry cleaning. Tetrachloroethylene is known not to cause tumour formation in humans. It also does not trigger uncontrolled DNA synthesis in animal cells, which plays a major role in cancer formation.

A study published in 1999, which examined cancer cases among aircraft workers in the United States over a 36-year period, found no difference in cancer rates among those who worked with tetrachloroethylene compared to those who had not used tetrachloroethylene. No significant increase was found in the incidence of suspected lymphoma, leukemia, or liver cancer.

A study published in Sweden in 2011, which investigated cancer rates among dry cleaners exposed to tetrachloroethylene for many years and laundry workers who did wet cleaning without using this chemical, based on a total of more than nine thousand people, found that there was no difference in the probability of cancer between the two groups: there was no significant increase in the incidence of esophageal, cervical, liver, kidney and bladder cancers, which are suspected to be caused by tetrachloroethylene, between the two groups.

Testing for exposure

Tetrachloroethylene exposure can be evaluated by a breath test, analogous to breath-alcohol measurements. Also, for acute exposures, tetrachloroethylene in expired air can be measured. Tetrachloroethylene can be detected in the breath for weeks following a heavy exposure. Tetrachloroethylene and its metabolite trichloroacetic acid, can be detected in the blood.

In the European Union, the Scientific Committee on Occupational Exposure Limits (SCOEL) recommends for tetrachloroethylene an occupational exposure limit (8-hour time-weighted average) of 20 ppm and a short-term exposure limit (15 min) of 40 ppm.

Remediation and degradation

In principle, tetrachloroethylene contamination can be remediated by chemical treatment. Chemical treatment involves reducing metals such as iron powder.

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

Further reading

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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
*Australia
National Pollutant Inventory (NPI)
page
Sustainable uses and Industry recommendations
European Chlorinated Solvents Association

{{Chlorides
Commodity chemicals
Dry cleaning
Halogenated solvents
IARC Group 2A carcinogens
Chloroalkenes
Perchlorocarbons
Anthelmintics