Tetrachloroethylene

Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and abbreviations such as "perc" (or "PERC"), and "PCE", is a chlorocarbon with the formula Cl2C=CCl2. It is a colorless 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 odor, similar to the smell of chloroform, detectable by most people at a concentration of 1 part per million (1 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 to degrees Celsius but mine did not begin to boil until
".
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=CCl2 + 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 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. Otherwise, it is volatile, highly stable 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.
Health and safety
The acute toxicity of tetrachloroethylene is moderate to low. Reports of human injury are uncommon despite its wide usage in dry cleaning and degreasing.
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'"..
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.
References
Further reading
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
Australian National Pollutant Inventory (NPI) page
Sustainable uses and Industry recommendations
Commodity chemicals
Dry cleaning
Halogenated solvents
Hazardous air pollutants
IARC Group 2A carcinogens
Chloroalkenes
Perchlorocarbons