Tetrachloroethylene

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 Cl2C\dCCl2. It is a non-flammable, stable, colorless and heavy liquid widely used for dry cleaning of fabrics, hence it is sometimes called "dry-cleaning fluid". It also has its uses as an effective automotive brake cleaner. It has a mild sweet, sharp odor, detectable by most people at a concentration of 50 ppm.

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).
 * C2Cl6 → C2Cl4 + Cl2

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 70 C to 77 C degrees Celsius but mine did not begin to boil until 120 C ".

A few years after its discovery, in the 1840s, Tetrachloroethylene was named Chlorethose by Auguste Laurent. The -ose ending was explained as the fourfold replacement of the hydrogens in ethylene. If only one atom of hydrogen was replaced, the word would end with -ase. By Laurent's logic, vinyl chloride would be named Chlorethase.

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 by the chemical reaction:
 * ClCH2CH2Cl + 3 Cl2 → Cl2C\dCCl2 + 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 e6MT in 1985.

Although in very small amounts, tetrachloroethylene occurs naturally in volcanoes along with trichloroethylene.

Uses
Tetrachloroethylene is an excellent nonpolar solvent for organic materials. Additionally, it is volatile, highly stable (easily recycled) and nonflammable, and has low toxicity. For these reasons, it has been widely used in dry cleaning worldwide since the 1930s. The chemist Sylvia Stoesser (1901–1991) 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 appears in a few consumer products including paint strippers, aerosol preparations 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 powerful effect 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. The recommended dose of Tetrachloroethylene for adults was about 3 mL.

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, C2F4.

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:
 * 4 C2Cl4 + 3 O2 -> 2 CCl3COCl + 4 COCl2

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.

Reduction
Tetrachloroethylene can be partially or completely reduced in the gas phase in the presence of catalysts such as nickel, palladium etc.:
 * C2Cl4 + 2 H2 -> 2 C + 4 HCl

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:
 * C2Cl4 + Cl2 -> C2Cl6

CFC-113 is produced by the reaction of tetrachloroethylene with chlorine and HF in the presence of antimony pentafluoride:
 * C2Cl4 + 3 HF + Cl2 -> CClF2CCl2F + 3 HCl

Nitration
Tetrachlorodinitroethane can be obtained by nitration of tetrachloroethylene with fuming nitric acid (conc. HNO3 rich in nitrogen oxides) or nitrogen tetroxide:
 * Cl2CCCl2 + N2O4 -> NO2Cl2CCCl2NO2

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 much less toxic than other chlorinated solvents. The acute and chronic toxicity of tetrachloroethylene is moderate to low. 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 is minimal because most tetrachloroethylene is used in liquid phase and modern machinery has closed systems to avoid any vapour escape.

Despite the advantages of tetrachloroethylene, many have called for its replacement from widespread commercial use. It has been described as a possible "neurotoxicant, liver and kidney toxicant and reproductive and developmental toxicant (...) a 'potential occupational carcinogen'".

As an anthelmintic, tetrachloroethylene was given orally to approximately fifty thousand people between 1925 and 1943. The most severe side effects were nausea and vomiting due to the gastric tract irritation. Most reported poisonings were manifestations of its narcotic effects.

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.

Carcinogenicity
Tetrachloroethylene has been classified as "probably carcinogenic to humans" (Group 2A) by the International Agency for Research on Cancer (IARC). There is a possibility that it is carcinogenic to humans in long-term exposure, but the evidence is limited since most of the evaluated dry-cleaners had heavy smoking and drinking habits which are known to cause multiple types of cancer. For context, drinking hot beverages and consuming red meat are also classified as Group 2A by IARC.

Epidemiologic cancer research has been conducted in the dry-cleaning industry since 1960. The evidence demonstrates a positive association between exposure to dry-cleaning chemicals, bladder cancer, non-Hodgkin lymphoma, and multiple myeloma in adults. A review of 109 occupational studies estimated a mean exposure of 59 ppm in dry-cleaning employees. Exposure to tetrachloroethylene in a typical dry cleaning shop is considered far below the levels required to cause any risk.

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 Europe, 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 cometabolism by Pseudomonas sp. Products of biological reductive dechlorination include trichloroethene, cis-1,2-dichloroethene, vinyl chloride, ethene and chloride.