Tetrachloroethylene

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 volatility, it does not leave permanent smell on dry-cleaned clothes. A dry cleaning machine running on perchloroethylene is called a "perc machine".

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, spot removers, and handicrafts.

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 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. Tetrachloroethylene does not tend to polymerise, unlike the fluorine analogue tetrafluoroethylene (C₂F₄).

Tetrachloroethylene may react violently with alkali metals, alkaline earth metals, strong 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.

Chlorination

Hexachloroethane is formed when tetrachloroethylene reacts with chlorine at 50–80 °C (122–176 °F) in the presence of a small amount of iron(III) chloride (0.1%) as a catalyst:

Cl₂C=CCl₂ + Cl₂ → Cl₃C−CCl₃

CFC-113 is produced by the reaction of tetrachloroethylene with chlorine and HF in the presence of antimony pentafluoride:

Cl₂C=CCl₂ + 3 HF + Cl₂ → ClF₂C−CCl₂F + 3 HCl

Nitration

Tetrachlorodinitroethane can be obtained by nitration of tetrachloroethylene with fuming nitric acid (conc. HNO₃ rich in nitrogen oxides) or nitrogen tetroxide:

Cl₂C=CCl₂ + N₂O₄ → NO₂Cl₂C−CCl₂NO₂

The preparation of this crystalline solid compound from tetrachloroethylene and nitrogen tetroxide was first described by Hermann Kolbe in 1869.

Thermal decomposition

Thermal decomposition of tetrachloroethylene begins at 400 °C (752 °F), accelerates around 600 °C (1,112 °F), and is complete at 800 °C (1,470 °F). A large number of decomposition products are produced.

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.

The largest industrial groups exposed to tetrachloroethylene include laundry and dry cleaning occupations, metalworking, metal degreasing or forging workers.

Tetrachloroethylene is generally classified as a toxin, 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, including the UK Health Security Agency.

Reports of human injury are not well-documented, despite its wide usage in dry cleaning and degreasing, and because rigorous research of exposure conditions and the associated risks is limited. 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.

Despite its advantages for dry cleaning and metal degreasing, cancer research and government environmental agencies have called for replacement of tetrachloroethylene from widespread commercial use. 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.

Metabolism

The biological half-life of tetrachloroethylene is approximately 3 days, with about 98% of the inhaled tetrachloroethylene exhaled unchanged and only about 1–3% 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. 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."

The carcinogenic potential of tetrachloroethylene remains under-studied with only limited evidence in humans, although there is sufficient proof in experimental animals that tetrachloroethylene is carcinogenic. Its high lipophilicity indicates that several organ systems, including the brain, kidneys, and liver, could be affected in tetrachloroethylene toxicity and carcinogenicity.

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 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.

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

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. ISCO agents commonly used to remediate tetrachloroethylene-contaminated groundwater include zerovalent iron, permanganates, and peroxides. 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.