Tetrachloroethylene

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. 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". 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: : 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. : 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. ==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, and the United States. Out of concern for carcinogenic effects in dry cleaning workers, Canadian regulations for tetrachloroethylene were implemented in 2003 to limit national use to 1,600 tonnes per year; by 2025, the dry cleaner industry reduced the amount to 600 tonnes per year. ==Environmental effects==

During typical industrial use, tetrachloroethylene may be released into the environment by evaporation and spills. In air, it undergoes degradation by reacting with hydroxyl radicals, producing phosgene, trichloroacetyl chloride, hydrogen chloride, carbon dioxide, and carbon monoxide in amounts of a few micrograms per cubic metre, existing in the atmosphere for about 100 days. Although tetrachloroethylene exists in air, surface water, and groundwater, the levels are typically low, likely causing little toxic exposure to the general public. Tetrachloroethylene exhibits high mobility in soil, and releases to soil can travel vertically and horizontally, affecting groundwater, surface water, soil gas, and indoor air. Factors like soil permeability and local climatology can enhance or inhibit tetrachloroethylene mobilization in soil; soils with low permeability, such as clay, have been demonstrated to slow mobility, while mobility is expected to be enhanced during storm events and in higher permeability soils, such as sand and gravel. A higher organic carbon content in soil may limit tetrachloroethylene's mobility due to its high soil sorption coefficient. When released to groundwater and surface water, a fraction of the release comingles with the water due to tetrachloroethylene's relatively low solubility. Tetrachloroethylene readily volatilizes from water and a release can present vapor intrusion concerns. Tetrachloroethylene has a greater density than water, and sufficient quantities released to groundwater and surface water can accumulate at the bottom of a water body or aquifer and result in the formation of dense non-aqueous phase liquid (DNAPL), making remediation a difficult and long process. ==Remediation and degradation==

; cis-DCE, cis-1,2-dichloroethylene; VC, vinyl chloride; ethene; RDase, reductase; H2, hydrogen gas; HCl, hydrochloric acid Groundwater impacted by tetrachloroethylene can be remediated through several methods, including in-situ chemical oxidation (ISCO) and/or thermal treatment, bioremediation, groundwater extraction and treatment, air sparging, and natural attenuation. 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 ==