Trihalomethane

Trihalomethanes (THMs) are chemical compounds in which three of the four hydrogen atoms of methane (CH₄) are replaced by halogen atoms. Many trihalomethanes find uses in industry as solvents or refrigerants. THMs are also environmental pollutants, and many are considered carcinogenic. Trihalomethanes with all the same halogen atoms are called haloforms.

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Table of common trihalomethanes

Industrial uses

Refrigerants

Trifluoromethane and chlorodifluoromethane are both used as refrigerants in some applications. Trihalomethanes released to the environment break down faster than chlorofluorocarbons (CFCs), thereby doing much less damage to the ozone layer (if they contain chlorine). Chlorodifluoromethane is a refrigerant HCFC, or hydrochlorofluorocarbon, while fluoroform is an HFC, or hydrofluorocarbon. Fluoroform is not ozone depleting.

Unfortunately, the breakdown of trihalomethane HCFCs does still result in the creation of some free chlorine radicals in the upper atmosphere and subsequent ozone destruction. Ideally, HCFCs will be phased out entirely in favour of entirely nonchlorinated refrigerants.

Solvents

Chloroform (trichloromethane) is a very common solvent used in organic chemistry. It is a significantly less polar solvent than water, well-suited to dissolving many organic compounds.

Although still toxic and potentially carcinogenic, chloroform is significantly less harmful than carbon tetrachloride. Because of the health and regulatory issues associated with the use of carbon tetrachloride, in modern chemistry laboratories chloroform is used as a cheaper, cleaner alternative wherever possible.

Water pollutants

Trihalomethanes are formed as a byproduct when chlorine or bromine are used to disinfect water for drinking. They result from the reaction of chlorine and/or bromine with organic matter in the water being treated. The THMs produced may have adverse health effects at high concentrations, and many governments set limits on the amount permissible in drinking water. In the United States, the EPA limits the total concentration of chloroform, bromoform, bromodichloromethane, and dibromochloromethane to 80 parts per billion in treated water. This number is called "total trihalomethanes" (TTHM).

Chloroform is also formed in swimming pools which are disinfected with chlorine or hypochlorite in the haloform reaction with organic substances (urine, sweat and skin particles). The reaction to phosgene under the influence of UV is also possible.