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Chloroform is the organic compound with formula CHCl3. This colorless, sweet-smelling, dense liquid is a trihalomethane. It is also considered somewhat hazardous. Several million tons were produced annually as a precursor to teflon and refrigerants, but the latter application is being phased out.

Production

In industry, 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 + Cl2CH2Cl2 + 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.

Deuterochloroform

An archaic industrial route to chloroform involved the reaction of acetone (or ethanol) with sodium hypochlorite or calcium hypochlorite, known as the haloform reaction. The chloroform can be removed from the coproducts 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 The haloform process is obsolete for the production of ordinary chloroform.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 CDCl3. Deuterochloroform can also be prepared by the reaction of sodium deuteroxide with chloral hydrate, or from ordinary chloroform.

Inadvertent synthesis of chloroform

The haloform reaction can also occur inadvertently in domestic settings. Sodium hypochlorite solution (chlorine bleach) mixed with common household liquids such as acetone, butanone, ethanol, or isopropyl alcohol may produce some chloroform, in addition to other compounds such as chloroacetone, or dichloroacetone.

Uses

The major use of chloroform today is in the production of the chlorodifluoromethane (R-22), a major precursor to tetrafluoroethylene:
CHCl3 + 2 HF → CHClF2 + 2 HCl
The reaction is conducted in the presence of a catalytic amount of antimony pentafluoride.Chlorodifluoromethane is then converted into tetrafluoroethylene, the main precursor to Teflon. Before the Montreal Protocol, chlorodifluoromethane (R22) was also popular refrigerant.

As a solvent

Chloroform is a common solvent in the laboratory because it is relatively unreactive, miscible with most organic liquids, and conveniently volatile. Chloroform is 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 used in commerce to extract morphine from poppies and 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 (also known under the trade names Perspex and Plexiglas). A solvent of phenol:chloroform:isoamyl alcohol 25:24:1 is used to dissolve non-nucleic acid biomolecules in DNA and RNA extractions.

As a reagent in organic synthesis

As a reagent, chloroform serves as a source of the dichlorocarbene CCl2 group. It reacts with aqueous sodium hydroxide usually in the presence of a phase transfer catalyst to produce dichlorocarbene, CCl2. 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.

History

Chloroform was discovered in July, 1831 by the American physician Samuel Guthrie, and independently a few months later by the French chemist Eugène Soubeiran and Justus von Liebig in Germany, all of them using variations of the haloform reaction. Soubeiran produced chloroform through the action of chlorine bleach powder (calcium hypochlorite) on acetone (2-propanone) as well as ethanol. Chloroform was named and chemically characterised in 1834 by Jean-Baptiste Dumas.
Chloroform in its liquid state shown in a test tube
Chloroform was developed on 4 November 1847 by James Young Simpson, head of midwifery at Edinburgh Hospital/University, and was mainly used as an anesthetic. Inhaling chloroform vapors depresses the central nervous system of a patient, causing dizziness, fatigue and unconsciousness , allowing a doctor to perform simple surgery or various, otherwise painful, operations. In 1847, the Edinburghmarker 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 favour 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 (~1.5-2.2) and low price.

One possible mechanism of action for chloroform is that it increases movement of potassium ions through certain types of potassium channels in nerve cells.

Safety

As might be expected for an anesthetic, inhaling chloroform vapors depresses the central nervous system. It is immediately dangerous to life and health at approximately 500 ppm, according to the United States National Institute for Occupational Safety and Health. Breathing about 900 ppm 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.

Though normally minute and diluted amounts are exposed to humans, chloroform can be harmful. Chloroform can be exposed via inhalation, ingestion of drinking water and foods made with chlorinated water, as well as dermal contact. It is readily excreted through exhalation, and minuscule amounts are excreted through urination and feces. The Environmental Protection Agency reports, a fatal oral dose of chloroform may be as low as 10 mL (14.8 g), with death due to respiratory or cardiac arrest .

Animal studies have shown that miscarriages occur in rats and mice that have breathed air containing 30 to 300 ppm of 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 as a consumer product in the United Statesmarker since 1976.

The National Toxicology Program's eleventh report on carcinogens implicates it as reasonably anticipated to be a human carcinogen, a designation equivalent to International Agency for Research on Cancer 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 chloroform is stabilized with ethanol or amylene, but samples that have been recovered or dried no longer contain any stabilizer, and caution must be taken. Suspicious bottles should be tested for phosgene. Filter-paper strips, moistened with 5% diphenylamine, 5% dimethylaminobenzaldehyde, and then dried, turn yellow in phosgene vapor.

References

  1. M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.
  2. Canadian Patent 1085423
  3. Srebnik, M.; Laloë, E. "Chloroform" Encyclopedia of Reagents for Organic Synthesis" 2001 John Wiley.
  4. Calderone, F.A. J. Pharmacology Experimental Therapeutics, 1935, 55(1), 24-39, http://jpet.aspetjournals.org/cgi/reprint/55/1/24.pdf
  5. http://www.epa.gov/ttn/atw/hlthef/chlorofo.html


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