In chemistry, trihalomethanes (THMs) are

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

s in which three of the four hydrogen atoms of methane () are replaced by halogen atoms. Many trihalomethanes find uses in industry as

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

s or refrigerants. THMs are also environmental

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...

s, and many are considered

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subst ...

ic. Trihalomethanes with all the same halogen atoms are called haloforms.

Table of common trihalomethanes

Industrial uses

Only chloroform has significant applications of the haloforms. In the predominant application, chloroform is required for the production of tetrafluoroethylene, precursor to teflon. Chloroform is fluorinated by reaction with hydrogen fluoride to produce chlorodifluoromethane (R-22). Pyrolysis of chlorodifluoromethane (at 550-750 °C) yields TFE, with difluorocarbene as an intermediate. :CHCl3 + 2 HF -> CHClF2 + 2 HCl :2 CHClF2 -> C2F4 + 2 HCl

Refrigerants and solvents

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

in organic chemistry.

Occurrence and production

The total global flux of chloroform through the environment is approximately tonnes per year, and about 90% of emissions are natural in origin. Many kinds of seaweed produce chloroform, and

fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

are believed to produce chloroform in soil. Most of the specifically, chloroform (), bromoform (), and iodoform are easy to prepare through the haloform reaction, although this method does not lend itself to bulk syntheses. Note that fluoroform () cannot be prepared in this manner. Chloroform is produced by heating mixtures of methane or methyl chloride with

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

. Dichloromethane is a coproduct.

Regulation

Trihalomethanes were the subject of the first drinking water regulations issued after passage of the U.S.

Safe Drinking Water Act The Safe Drinking Water Act (SDWA) is the principal federal law in the United States intended to ensure safe drinking water for the public. Pursuant to the act, the Environmental Protection Agency (EPA) is required to set standards for drinking w ...

in 1974. The

EPA The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...

limits the total concentration of the four chief constituents ( chloroform, bromoform,

bromodichloromethane Bromodichloromethane is a trihalomethane with formula C H Br Cl2. Bromodichloromethane has formerly been used as a flame retardant, and a solvent for fats and waxes and because of its high density for mineral separation. Now it is only used as a ...

, and dibromochloromethane), referred to as total trihalomethanes (TTHM), to 80 parts per billion in treated water. Traces of chloroform are produced in swimming pools.

References

External links

National Pollutant Inventory - Chloroform and trichloromethane

EPA - Trihalomethanes in Drinking Water: Sampling, Analysis, Monitoring and Compliance (August 1983)
{{Halomethanes Halomethanes Halogenated solvents Refrigerants