Chloroform

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Chloroform
Chloroform displayed.svgChloroform
General
Other names Trichloromethane
Methane trichloride
R-20
Molecular formula CHCl3
Molar mass 119.4 g/mol
Appearance colorless liquid
SMILES ClC(Cl)Cl
CAS number [67-66-3]
EINECS number 200-663-8
Properties
Density and phase 1.48 g/cm³, liquid
Solubility in water 0.8 g/100 ml at 20 °C
Melting point −63.5 °C
Boiling point 61.2 °C
Viscosity 0.542 cP at 25 °C
Structure
Molecular shape Tetrahedral
Dipole moment 1.08 D (gas)
Thermodynamic data
Standard enthalpy
of formation
ΔfH°liquid
−134.3 kJ/mol
Standard enthalpy
of formation
ΔfH°gas
−103.2 kJ/mol
Standard molar entropy
S°gas
295.6 J.K–1.mol–1
Safety data
EU classification Harmful
Irritant
Carc. Cat. 3
R-phrases R22, R38, R40
R48/20/22
S-phrases S2, S36/37
NFPA 704

NFPA 704.svg

0
2
0
 
PEL-TWA (OSHA) 50 ppm (240 mg/m3)
IDLH (NIOSH) approx. 500 ppm
Flash point nonflammable
RTECS number FS9100000
Supplementary data page
Structure & properties n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related Haloforms Fluoroform
Bromoform
Iodoform
Related Chloromethanes Chloromethane
Dichloromethane
Carbon tetrachloride
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)

Chloroform, also known as trichloromethane and methyl trichloride, is a chemical compound with the formula CHCl3. At room temperature, it is a colorless liquid that is volatile but nonflammable. It is a highly effective solvent for various organic compounds used in basic research and industrial processes. It is useful for extracting alkaloids (such as morphine) of pharmaceutical importance from plant materials (such as poppies). It is also a reagent for certain chemical reactions.

Contents

Chloroform should, however, be used with caution, as it is harmful to both human health and the environment. Inhaling its vapors depresses the central nervous system and can cause dizziness, fatigue, and headache. Chronic exposure may damage the liver and kidneys, and some people have an allergic reaction to it.

History

Chloroform was first prepared in July 1831, by the American physician Samuel Guthrie, when he mixed whiskey with chlorinated lime.[1] A few months later, it was independently produced by Eugène Soubeiran in France and Justus von Liebig in Germany.[2] Jean-Baptiste Dumas named the compound in 1834.

In 1847, the Edinburgh obstetrician James Young Simpson experimented with chloroform narcosis on himself, then began using it as an anesthetic to assist women during childbirth.[3] 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 twentieth century. It was, however, 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, but it, too, was later found to be carcinogenic.

Notable characteristics

As suggested by its chemical formula, each chloroform molecule has three chlorine (halogen) atoms attached to a central carbon atom. For this reason, it is placed within the group of compounds known as trihalomethanes.

Chloroform has a boiling point of 61.2°C, a melting point of −63.5°C, and a density of 1.48 g/cm³. It does not support combustion in the air, but it will burn when mixed with more flammable materials.

This chemical reacts with aqueous sodium hydroxide (usually in the presence of a phase transfer catalyst) to produce dichlorocarbene. This reagent effects ortho-formylation of activated aromatic rings such as phenols, producing aryl aldehydes in what is known as the Reimer-Tiemann reaction. Alternatively, the carbene can be trapped by an alkene to form a cyclopropane derivative.

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 + 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.

The first industrial process was 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. Some 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 can occur inadvertently even in domestic settings. For example, the mixture of sodium hypochlorite solution (bleach) and methyl ethyl ketone (nail-varnish remover) produces chloroform. So too does a mixture of pool chlorine and acetone.

Uses

Until recently, chloroform has been used mainly to produce the freon refrigerant R-22. This use, however, is declining, as the Montreal Protocol takes effect and 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. It is used as a solvent for research in academic chemistry laboratories. As a solvent it can be used to bond pieces of acrylic glass (known under the trade name 'Perspex'). Chloroform is a most effective solvent for alkaloids in their base form and thus is used to extract plant material for pharmaceutical processing. For example, it is commercially used to extract morphine from poppies, scopolamine from Datura plants.

It is often used (with phenol) to separate DNA from other cellular material, in the presence of an extraction buffer. The DNA goes into the supernatant, while the protein and insoluble cellular materials precipitate between the layers of buffer and chloroform.

Chloroform containing deuterium (heavy hydrogen), CDCl3, is a common solvent used in NMR spectroscopy.

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 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. Approximately 10 percent 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 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 percent diphenylamine, 5 percent dimethylaminobenzaldehyde, and then dried, turn yellow in phosgene vapor.

Notes

  1. Chloroform: The Molecular Lifesaver. Retrieved May 15, 2007.
  2. General Anaesthesia, Chloroform. Retrieved May 15, 2007.
  3. General Anaesthesia, History of chloroform anesthesia. Retrieved May 17, 2007.

References

  • McMurry, John. 2004. Organic Chemistry, 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052
  • Morrison, Robert T. and Robert N. Boyd. 1992. Organic Chemistry, 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-643669-2
  • Solomons, T.W. Graham and Fryhle, Craig B. 2004. Organic Chemistry, 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998
  • Sheffer, Marla, ed. 2004. Chloroform (Concise International Chemical Assessment Document, No. 58). Geneva: World Health Organization. ISBN 9241530588

External links

All links retrieved May 16, 2013.


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