Chemical compound in very wide use
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Chemical compound
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. It also has its uses as an 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).
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:
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.
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:
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:
CFC-113 is produced by the reaction of tetrachloroethylene with chlorine and HF in the presence of antimony pentafluoride:
Nitration.
Tetrachlorodinitroethane can be obtained by nitration of tetrachloroethylene with fuming nitric acid (conc. rich in nitrogen oxides) or nitrogen tetroxide:
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 over a long term, may harm the nervous system, other organs, and increase the risk of getting cancer. 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.
Evidence from cohort and case-controlled epidemiologic studies demonstrates a positive association between cumulative exposures to tetrachloroethylene and the prevalence of bladder cancer, non-Hodgkin lymphoma, and multiple myeloma in adults. Some limited evidence of increased prevalence of kidney, lung, liver, and breast cancers with exposure to tetrachloroethylene has been found in epidemiologic research, but data quality limitations have produced variable results across studies.
Several modes of action are hypothesized for the carcinogenicity of tetrachloroethylene in humans, though existing data is insufficient for adequate characterization. Markers of oxidative metabolism of tetrachloroethylene and increased prevalence of abnormal hepatic sonographs have been observed in dry-cleaners and laundry workers exposed to tetrachloroethylene, which suggests a potential for hepatocellular damage through the formation of reactive intermediates from glutathione conjugates during metabolization. Although most genotoxicity assays of tetrachloroethylene produced negative findings for genotoxicity and mutagenicity, modest genotoxic effects and mutagenic effects have been identified under certain metabolic activation conditions, and several of tetrachloroethylene's metabolites have been shown to be mutagenic.
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.
References.
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