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Friday, April 10, 2009

Carbon tetrachloride

Carbon tetrachloride, also known by many other names (see Table) is the organic compound with the formula CCl4. It is a reagent in synthetic chemistry and was formerly widely used in fire extinguishers, as a precursor to refrigerants, and a cleaning agent. It is a colourless liquid with a "sweet" smell that can be detected at low levels.

Both carbon tetrachloride and tetrachloromethane are acceptable names under IUPAC nomenclature. Colloquially, it is called "carbon tet".

History and synthesis

The production of carbon tetrachloride has steeply declined since the 1980s due to environmental concerns and the decreased demand for CFCs, which were derived from carbon tetrachloride. In 1992, production in the U.S.-Europe-Japan was estimated at 720,000 tonnes.

Carbon tetrachloride was originally synthesised in 1839 by reaction of chloroform with chlorine, from the French chemist Henri Victor Regnault, but now it is mainly synthesized from methane:

CH4 + 4 Cl2 → CCl4 + 4HCl

The production often utilizes by-products of other chlorination reactions, such as the syntheses of dichloromethane and chloroform. Higher chlorocarbons are also subjected to "chlorinolysis:"

C2Cl6 + Cl2 → 2 CCl4

Prior to the 1950s, carbon tetrachloride was manufactured by the chlorination of carbon disulfide at 105 to 130 °C:

CS2 + 3Cl2 → CCl4 + S2Cl2

Properties

In the carbon tetrachloride molecule, four chlorine atoms are positioned symmetrically as corners in a tetrahedral configuration joined to a central carbon atom by single covalent bonds. Because of this symmetrical geometry, the molecule has no net dipole moment; that is, CCl4 is non-polar. Methane gas has the same structure, making carbon tetrachloride a halomethane. As a solvent, it is well suited to dissolving other non-polar compounds, fats and oils. It can also dissolve iodine. It is somewhat volatile, giving off vapors having a smell characteristic of other chlorinated solvents, somewhat similar to the tetrachloroethylene smell reminiscent of dry cleaners' shops.

Solid tetrachloromethane has 2 polymorphs: crystalline II below -47.5 °C (225.6 K) and crystalline I above -47.5 °C.

At -47.3 °C it has monoclinic crystal structure with space group C2/c and lattice constants a = 20.3, b = 11.6, c = 19.9 (.10-1 nm), β = 111°.

Uses

In the early 20th century, carbon tetrachloride was widely used as a dry cleaning solvent, as a refrigerant, and in lava lamps. Small, carbon tetrachloride fire extinguishers were widely used and took the form of a brass bottle with a hand pump to expel the liquid.

However, once it became apparent that carbon tetrachloride exposure had severe adverse health effects, safer alternatives such as tetrachloroethylene were found for these applications, and its use in these roles declined from about 1940 onward. Carbon tetrachloride persisted as a pesticide to kill insects in stored grain, but in 1970, it was banned in consumer products in the United States.

One specialty use of "carbon tet" was by stamp collectors to reveal watermarks on the backs of postage stamps. A small amount of the liquid was placed on the back of a stamp sitting in a black glass or obsidian tray. The letters or design of the watermark could then be clearly detected.

Prior to the Montreal Protocol, large quantities of carbon tetrachloride were used to produce the freon refrigerants R-11 (trichlorofluoromethane) and R-12 (dichlorodifluoromethane). However, these refrigerants are now believed to play a role in ozone depletion and have been phased out. Carbon tetrachloride is still used to manufacture less destructive refrigerants.

Carbon tetrachloride has also been used in the detection of neutrinos. Carbon tetrachloride is one of the most potent hepatotoxins (toxic to the liver), and is widely used in scientific research to evaluate hepatoprotective agents 7,8

Reactivity

Carbon tetrachloride has practically no flammability at lower temperatures. Under high temperatures in air, it forms poisonous phosgene.

Because it has no C-H bonds, carbon tetrachloride does not easily undergo free-radical reactions. Hence it is a useful solvent for halogenations either by the elemental halogen, or by a halogenation reagent such as N-bromosuccinimide.

In organic chemistry, carbon tetrachloride serves as a source of chlorine in the Appel reaction.

Solvent

It is used as a solvent in synthetic chemistry research, but because of its adverse health effects, it is no longer commonly used, and chemists generally try to replace it with other solvents.[citation needed] It is sometimes useful as a solvent for infrared spectroscopy because there are no significant absorption bands > 1600 cm-1. Because carbon tetrachloride does not have any hydrogen atoms, it was historically used in proton NMR spectroscopy. However, carbon tetrachloride is toxic, and its dissolving power is low. Its use has been largely superseded by deuterated solvents, which offer superior solvating properties and allow for deuterium lock by the spectrometer. Use of carbon tetrachloride in determination of oil has been replaced by various other solvents.

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Deuterated Organic Compounds