CHLOROFORM

'Chloroform', also known as 'trichloromethane' and 'methyl trichloride', is a chemical compound with formula CHCl₃. It does not undergo combustion in air, although it will burn when mixed with more flammable substances. It is a member of a group of compounds known as trihalomethanes. Chloroform has myriad uses as a reagent and a solvent. It is also considered an environmental hazard.

Contents

History

Production

Inadvertent synthesis of chloroform

Uses

As a solvent

As a reagent in organic synthesis

Safety

References

See also

External links

History

In 1847, the Edinburgh obstetrician James Young Simpson first used chloroform for general anesthesia during childbirth. The use of chloroform during surgery expanded rapidly thereafter in Europe. In the United States, chloroform began to replace ether as an anesthetic at the beginning of the 20th century; however, it was quickly abandoned in favor of ether upon discovery of its toxicity, especially its tendency to cause fatal cardiac arrhythmia analogous to what is now termed "sudden sniffer's death". Ether is still the preferred anesthetic in some developing nations due to its high therapeutic index and low price. Trichloroethylene, a halogenated aliphatic hydrocarbon related to chloroform, was proposed 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. At 400-500 °C, a free radical halogenation occurs, converting the methane or chloromethane to progressively more chlorinated compounds.
:CH₄ + Cl₂ → CH₃Cl + HCl
:CH₃Cl + Cl₂ → CH₂Cl₂ + HCl
:CH₂Cl₂ + Cl₂ → CHCl₃ + HCl
Chloroform undergoes further chlorination to give CCl₄:
:CHCl₃ + Cl₂ → CCl₄ + HCl
The output of this process is a mixture of the four chloromethanes: chloromethane, dichloromethane, chloroform (trichloromethane), and carbon tetrachloride, which are then separated by distillation.
Chloroform was first produced industrially by the reaction of acetone (or ethanol) with sodium hypochlorite or calcium hypochlorite, known as the haloform reaction. The chloroform can be removed from the attendant acetate salts (or formate salts if ethanol is the starting material) by distillation. This reaction is still used for the production of bromoform and iodoform. The haloform process is obsolete for the production of ordinary chloroform. It is, however, used to produce deuterated material industrially. Deuterochloroform is prepared by the reaction of sodium deuteroxide with chloral hydrate. ALL of the aldehyde hydrogen is retained in the product, though, and samples of higher isotopic purity are obtained from trichloroacetophenone as starting material.

Inadvertent synthesis of chloroform

Haloform-like reactions also occur inadvertently even in domestic settings. Sodium hypochlorite solution (chlorine bleach) and methyl ethyl ketone (nail-varnish remover) produces chloroform.
Pool chlorine and acetone will also produce chloroform.

Uses

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. In addition, it is used under research conditions to anesthetize mosquitoes for experiments, most frequently for the study of malaria.

As a solvent

Chloroform is a common solvent because it is relatively unreactive, miscible with most organic liquids, and conveniently volatile. Small amounts of chloroform are used as a solvent in the pharmaceutical industry and for producing dyes and pesticides. Chloroform is an effective solvent for alkaloids in their base form and thus plant material is commonly extracted with chloroform for pharmaceutical processing. For example, it is commercially used to extract morphine from poppies, scopolamine from ''Datura'' plants. Chloroform containing deuterium (heavy hydrogen), CDCl₃, is a common solvent used in NMR spectroscopy. It can be used to bond pieces of acrylic glass (which is also known under the trade name 'Perspex').

As a reagent in organic synthesis

As a reagent, chloroform serves as a source of the dichlorocarbene CCl₂ group.^[1] It reacts with aqueous sodium hydroxide (usually in the presence of a phase transfer catalyst) to produce dichlorocarbene, CCl₂.^[2][3] This reagent effects ortho-formylation of activated aromatic rings such as phenols, producing aryl aldehydes in a reaction known as the Reimer-Tiemann reaction. Alternatively the carbene can be trapped by an alkene to form a cyclopropane derivative.

Safety

As might be expected from its use as an anesthetic, inhaling chloroform vapors depresses the central nervous system. IDLH according to NIOSH is approximately 500 ppm. 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 damage to the liver (where chloroform is metabolized to phosgene) and to the kidneys, and some people develop sores when the skin is immersed in chloroform.
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 eleventh report on carcinogens 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.
During prolonged storage hazardous amounts of phosgene can accumulate in the presence of oxygen and ultraviolet light. To prevent accidents commercial material is stabilized with ethanol or amylene, but samples that have been recovered or dried no longer contain any stabilizer and caution must be taken with those. Suspicious bottles should be tested for phosgene. Filter paper strips, wetted with 5% diphenylamine, 5% dimethylaminobenzaldehyde, and then dried, turn yellow in phosgene vapor.
Commonly used in DNA extractions and generally in conjunction with phenol to form a biolayer with extraction buffer (tris etc). DNA will form in the supernatant while protein and non soluble cell materials will precipitate between the buffer chloroform layers.

References

1. Srebnik, M.; Laloë, E. "Chloroform" Encyclopedia of Reagents for Organic Synthesis" 2001 John Wiley. DOI: 10.1002/047084289X.rc105
2. Vogel, E.; Klug, W.; Breuer. A. "1,6-Methano[10]annulne" Organic Syntheses, Collected Volume 6, p.731 (1988). CV6P0731.pdf
3. Gokel, G. W.; Widera, R. P.; Weber, W. P. Phase-Transfer Hofmann Carbylamine Reaction: tert-Butyl Isocyanide" Organic Syntheses, Collected Volume 6, p.232 (1988).

See also

★ Haloalkane

★ Halomethane

★ Chloromethane

★ Dichloromethane

★ Carbon tetrachloride (Tetrachloromethane)

★ Fluoroform

★ Bromoform

★ Iodoform

★ Fictional applications of real materials

External links

★ Chloroform "The Molecular Lifesaver" An article at Oxford University providing interesting facts about chloroform.

★ Concise International Chemical Assessment Document 58

★ History of chloroform anesthesia

★ IARC Summaries & Evaluations: Vol. 1 (1972), Vol. 20 (1979), Suppl. 7 (1987), Vol. 73 (1999)

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★ National Pollutant Inventory - Chloroform and trichloromethane

★ NIST Standard Reference Database

★ Story on Chloroform from BBC's The Material World (28 July 2005)

★ Sudden Sniffer's Death Syndrome article at Carolina Poison Center