Chloroform (also known as trichloromethane and methyl trichloride) is a chemical compound with formula CHCl3. It is a colorless liquid with a pleasant, nonirritating odor and a slightly sweet taste. It does not support combustion in air, although it will burn when mixed with more flammable substances. It is a member of a subset of environmental pollutants known as trihalomethanes, a by-product of chlorination of drinking water and a long-standing health concern.
History
Chloroform was first produced independently and simultaneously in 1831 by Justus von Liebig and the French chemist Eugene Soubeiran , who produced chloroform through the action of chlorine bleach powder (sodium hypochlorite) upon acetone (2-propanone) or ethanol (an application of the generic process known as Reimer-Tiemann formylation).
In 1847, the Edinburgh obstetrician James Young Simpson first used chloroform to effect general anesthesia during childbirth. The use of chloroform during surgery expanded rapidly thereafter, especially in Europe. In the United States, chloroform did not replace ether as an anesthetic until the beginning of the 20th century; however, its use was readily abandoned in favor of the latter upon discovery of its undesirable toxicological properties and its propensity to cause sudden, fatal cardiac arrhythmia in a manner analogous to what is now termed sudden sniffer's death. Ether is still of limited use in developing nations to due its high therapeutic index, economic viability and relative safety, possessing no untoward effects with the exception of its pungent, unpleasant odor and tendency to cause vomiting. Trichloroethylene, a related halogenated aliphatic hydrocarbon, later supplanted chloroform as a safer alternative, though it, too, was later found to be carcinogenic.
Production
Industrially, chloroform is produced by heating a mixture of chlorine and either chloromethane or methane to 400-500°C. At this temperature, a series of chemical reactions occur, converting the methane or chloromethane to progressively more chlorinated compounds.
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CH4 + Cl2 → CH3Cl + HCl
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CH3Cl + Cl2 → CH2Cl2 + HCl
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CH2Cl2 +Cl2 → CHCl3 + HCl
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CHCl3 + Cl2 → CCl4 + HCl
The output of this process is a mixture of the four chloromethanes, chloromethane, dichloromethane, chloroform (trichloromethane), and tetrachloromethane, which are then separated by distillation.
Uses
In the late 19th and early 20th centuries, chloroform was used as an inhaled anesthetic during surgery. However, safer, more flexible drugs have entirely replaced it in this role. The major use of chloroform today is in the production of the freon refrigerant R-22. However, as the Montreal Protocol takes effect, this use can be expected to decline as R-22 is replaced by refrigerants that are less liable to result in ozone depletion.
Smaller amounts of chloroform are used as a solvent in the pharmaceutical industry, and for producing dyes and pesticides.
Safety
As might be expected from its use as an anesthetic, inhaling chloroform vapors depresses the central nervous system. Breathing about 900 parts of chloroform per million parts air (900 parts per million) for a short time can cause dizziness, fatigue, and headache. Chronic chloroform exposure may cause liver and kidney damage, and some people develop sores when the skin is immersed in chloroform. Approximately 10% of the population has an allergic reaction to chloroform that produces a fever of around 40°C (104°F) upon exposure.
Animal studies have shown that miscarriages occur in rats and mice that have breathed air containing 30 to 300 ppm chloroform during pregnancy and also in rats that have ingested chloroform during pregnancy. Offspring of rats and mice that breathed chloroform during pregnancy have a higher incidence of birth defects, and abnormal sperm have been found in male mice that have breathed air containing 400 ppm chloroform for a few days. The effect of chloroform on reproduction in humans is unknown.
Chloroform once appeared in toothpastes, cough syrups, ointments, and other pharmaceuticals, but it has been banned in consumer products in the United States since 1976.
The NTP's tenth report on carcinogens http://ehp.niehs.nih.gov/roc/toc10.html implicates it as reasonably anticipated to be a human carcinogen, a designation equivalent to IARC class 2A. It has been most readily associated with hepatocellular carcinoma. Caution is mandated during its handling in order to minimize unnecessary exposure; safer alternatives, such as dichloromethane, have resulted in a substantial reduction of its use as a solvent.
External links
- Synthesis of chloroform http://www.rhodium.ws/chemistry/chloroform.html
- 10th ROC monograph http://ehp.niehs.nih.gov/roc/tenth/profiles/s038chlo.pdf
Last updated: 02-07-2005 04:28:17
Last updated: 04-25-2005 03:06:01