Trichloroethylene
The
chemical compound trichloroethylene is a
chlorinated hydrocarbon commonly used as an industrial
solvent. It is a clear nonflammable liquid with a sweet smell.
Its
IUPAC name is
trichloroethene. In industry, it is informally referred to by the abbreviations
TCE and
tri, and it is sold under a variety of trade names.
Production
Prior to the early 1970's, 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
- Cl2CHCHCl2 + Ca(OH)2 → ClCH=CCl2 + CaCl2
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
\nTrichloroethylene is a good solvent for a variety of
organic materials. When it was first widely produced in the 1920's, 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 surpassed it in this role in the 1950's.
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 1970's.
For most of its history, trichloroethylene has been widely used as a degreaser for metal parts. In the late 1950's, 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.
Health effects
\nWhen inhaled, trichloroethylene depresses the central nervous system. It symptoms are similar to those of
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.
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.
