Trichloroethylene
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General |
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| Name | Trichloroethylene |
| Chemical formula | ClCH=CCl2 |
| Appearance | Colorless liquid |
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Physical |
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| Formula weight | 131.4 g/mol |
| Melting point | 200 K (-73 °C) |
| Boiling point | 360 K (87 °C) |
| Density | 1460 kg/m3 (liquid) |
| Solubility | insoluble in water |
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Thermochemistry |
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| ΔfH0gas | -7.78 kJ/mol |
| ΔfH0liquid | -42.3 kJ/mol |
| ΔfH0solid | ? kJ/mol |
| S0gas, 100 kPa | ? J/(mol·K) |
| S0liquid, 100 kPa | ? J/(mol·K) |
| S0solid | ? J/(mol·K) |
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Safety | |
| Ingestion | May cause nausea, stomach irritation. Inhaling vapors from stomach into lungs causes symptoms like those of inhalation. |
| Inhalation | Can cause dizziness, drowsiness, confusion, unconsciousness, and cardiac failure. May irritate mucous membranes. |
| Skin | May cause skin irritation. Prolonged exposure may lead to chronic irritation. |
| Eyes | May cause burning sensation, watering. |
| More info | Hazardous Chemical Database (http://ull.chemistry.uakron.edu/erd/chemicals/8/7197.html) |
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SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references </font> | |
| Contents |
Production
Prior to the early 1970s, most trichloroethylene was produced in a two-step process from acetylene. First, acetylene was treated with chlorine using a ferric chloride catalyst at 90 °C to produce 1,1,2,2-tetrachloroethane according to the chemical equation
- HC≡CH + 2 Cl2 → Cl2CHCHCl2
The 1,1,2,2-tetrachloroethane is then dehydrochlorinated to give trichloroethylene. This can either be accomplished with an aqueous solution of calcium hydroxide
or in the vapor phase by heating it to 300-500°C on a barium chloride or calcium chloride catalyst
- Cl2CHCHCl2 → ClCH=CCl2 + HCl
Today, however, most trichloroethylene is produced from ethylene. First, ethylene is chlorinated over a ferric chloride catalyst to produce 1,2-dichloroethane.
- CH2=CH2 + Cl2 → ClCH2CH2Cl
When heated to around 400 °C with additional chlorine, 1,2-dichloroethane is converted to trichloroethylene
- ClCH2CH2Cl + 2 Cl2 → ClCH=CCl2 + 3 HCl
This reaction can be catalyzed by a variety of substances. The most commonly used catalyst is a mixture of potassium chloride and aluminum chloride. However, various forms of porous carbon can also be used. This reaction produces tetrachloroethylene as a byproduct, and depending on the amount of chlorine fed to the reaction, tetrachloroethylene can even be the major product. Typically, trichloroethylene and tetrachloroethylene are collected together and then separated by distillation.
Uses
Trichloroethylene is a good solvent for a variety of organic materials. When it was first widely produced in the 1920s, its major use was to extract vegetable oils from plant materials such as soy, coconut, and palm. Other uses in the food industry included coffee decaffeination and the preparation of flavoring extracts from hops and spices. It was also used as a dry cleaning solvent, although tetrachloroethylene (also known as perchloroethylene) surpassed it in this role in the 1950s.
Due to concerns about its toxicity, the use of trichloroethylene in the food and pharmaceutical industries has been banned in much of the world since the 1970s.
For most of its history, trichloroethylene has been widely used as a degreaser for metal parts. In the late 1950s, the demand for trichloroethylene as a degreaser began to decline in favor of the less toxic 1,1,1-trichloroethane. However, 1,1,1-trichloroethane production has been phased out in most of the world under the terms of the Montreal Protocol, and as a result trichloroethylene has experienced a resurgence in use. It has also been used for drying out the last bit of water for production of 100% ethanol.
Supplanting chloroform and ether for a significant period of time, trichloroethylene demonstrated superior efficacy in induction times and cost-effectiveness. Pioneered by ICI in Britain, its development was hailed as a revolution: lacking the great hepatotoxic liability of chloroform and the unpleasant pungency and inflammability of ether, it nonetheless had several pitfalls, including the sensitization of the myocardium to epinephrine, potentially acting in an arrhythmogenic manner. Its low volatility demanded the employment of carefully regulated heat in its vaporization. Research demonstrating its transient elevation of LFTs (Liver Function Tests) raised concerns regarding its hepatoxic potential; several deaths occurred as a result, though the incidence was comparable to that of halothane hepatitis. Incompatibility with soda lime (the CO2 adsorbent utilized in closed-circuit, low-flow rebreathing systems) presented dangers: it was readily decomposed into 1,2-dichloroacetylene, a neurotoxic compound potentially responsible for its hepatoxic potential, though its metabolite trichloroacetic acid is more probably the etiological source of the latter. Halothane usurped a great portion of its market in 1956, with its total abandonment not achieved until the 1980s, when its use as an analgesic in obstetrics was implicated in foetal death. Concerns of its carcinogenic potential were raised simultaneously.
The active metabolite of trichloroethylene is trichloroethanol, identical to that of chloral hydrate. Therefore, concerns of the carcinogenicity of the latter have been raised, and is subject to on-going debate.
Health effects
When inhaled, trichloroethylene depresses the central nervous system. Its symptoms are similar to those of alcohol intoxication, beginning with headache, dizziness, and confusion and progressing with increasing exposure to unconsciousness and death. Caution should be exercised anywhere a high concentration of trichloroethylene vapors may be present, because it quickly desensitizes the nose to its scent, and it is possible to unknowingly inhale harmful or even lethal amounts of the vapor -- that is, it has poor warning properties.
The long-term effects of trichloroethylene on human beings is unknown. In animal studies, chronic trichloroethylene exposure has produced liver cancer in mice, but not in rats. Studies on its effects on reproduction in animals have been similarly inconsistent, and so no conclusive statements about its ability to cause birth defects in humans can be made.
Recent research has focussed on aerobic degradation pathways in order to reduce environmental pollution through the use of genetically modified bacteria. Limited success has been attained thus far; the intended application is for treatment and detoxification of industrial wastewater. Organochlorine compounds present a serious environmental liability particularly given their great resistance to natural degradation and their high marine toxicity.
The weak association between trichloroethylene and cancer are responsible for its questionable status: until recent years, the US Agency for Toxic Substances and Disease Registry (ATSDR) contended that it was had little-to-no carcinogenic potential, and was probably a co-carcinogen—that is, it acted in concert with other substances to promote the formation of tumors. More recent analyses indicate low-level evidence of a mutagenic or teratogenic effect; thus, it is known that it promotes the formation of tumors, though the exact pathway is not well-understood. Its long-term safe use as a surgical anesthetic did not lead to an increased incidence of cancer as compared to background levels, indicating that any such effect is most probably extremely low-level. It is current categorized as IARC 2A, analogous to trichloromethane—reasonably anticipated to be a human carcinogen. More information on the carcinogenic potential of organochlorine compounds may be gleaned from the [report on carcinogens (http://ehp.niehs.nih.gov/roc/toc10.html)].
In recent times, there has been a substantial reduction in the production output of trichloroethylene; alternatives for use in metal degreasing abound, chlorinated aliphatic hydrocarbons being phased out in a large majority of industries due to the potential for irreversible health effects and the legal liability that ensues as a result.
External Links
- Tenth Report on Carcinogens: Trichloroethylene Monograph (http://ehp.niehs.nih.gov/roc/tenth/profiles/s180tce.pdf)