Carbon tetrachloride

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

Both carbon tetrachloride and tetrachloromethane are acceptable names under IUPAC nomenclature, depending on whether it is seen as an inorganic or an organic compound. Colloquially, it is called "carbon tet".

Production
Carbon tetrachloride was originally synthesised in 1839 by reaction of chloroform with chlorine, but the most common synthesis is now chlorination of carbon disulfide at 105 to 130 °C:


 * CS2 + 3Cl2 → CCl4 + S2Cl2

CCl4 can also be prepared from methane via the reaction:
 * CH4 +  4 Cl2  →  CCl4  +  4HCl

It is also a byproduct in the synthesis of dichloromethane and chloroform.

Properties
In the carbon tetrachloride molecule, four chlorine atoms are positioned symmetrically as corners in a tetrahedral configuration joined to a carbon atom, in the center, by single covalent bonds. Because of this symmetrical geometry, the molecule has no net dipole moment; that is, CCl4 is non-polar. As a solvent, it is well suited to dissolving other non-polar compounds, fats and oils. 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 allotropes: crystaline II below -47.5 °C (225.6 K) and crystaline 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 fire extinguishers. 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.

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.

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.

As a 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 substitute it with other solvents. 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.

Safety
Exposure to high concentrations of carbon tetrachloride (including vapor) can affect the central nervous system, degenerate the liver and kidneys and may result (after prolonged exposure) in coma and even death. Chronic exposure to carbon tetrachloride can cause liver and kidney damage and could result in cancer More information can be found in Material safety data sheets.

Carbon tetrachloride is also both ozone-depleting and a greenhouse gas. However, since 1992 its atmospheric concentrations have been in decline for the reasons described above (see also the atmospheric time-series figure).