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 elemen ...

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

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 Tetrafluoroethylene (TFE) is a fluorocarbon with the chemical formula C2 F4. It is the simplest perfluorinated alkene. This gaseous species is used primarily in the industrial preparation of fluoropolymers. Properties Tetrafluoroethylene is a ...

, precursor to teflon. Chloroform is fluorinated by reaction with hydrogen fluoride to produce

chlorodifluoromethane Chlorodifluoromethane or difluoromonochloromethane is a hydrochlorofluorocarbon (HCFC). This colorless gas is better known as HCFC-22, or R-22, or . It was commonly used as a propellant and refrigerant. These applications were phased out under ...

(R-22). Pyrolysis of chlorodifluoromethane (at 550-750 °C) yields TFE, with

difluorocarbene Difluorocarbene is the chemical compound with formula CF2. It has a short half-life, 0.5 and 20 ms, in solution and in the gas phase, respectively.Douglas A Jean Osteraas "Difluorocarbene Modification of Polymer and Fiber Surfaces," ''Journal ...

as an intermediate. :CHCl3 + 2 HF -> CHClF2 + 2 HCl :2 CHClF2 -> C2F4 + 2 HCl

Refrigerants and solvents

Trifluoromethane Trifluoromethane or fluoroform is the chemical compound with the formula CHF3. It is one of the " haloforms", a class of compounds with the formula CHX3 (X = halogen) with C3v symmetry. Fluoroform is used in diverse applications in organic sy ...

and

are both used as refrigerants. Trihalomethanes released to the environment break down faster than

chlorofluorocarbons Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and propan ...

(CFCs), thereby doing much less damage to the

ozone layer The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relat ...

. Chlorodifluoromethane is a refrigerant

HCFC Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and prop ...

, or

hydrochlorofluorocarbon Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and propa ...

, while fluoroform is an HFC, or hydrofluorocarbon. Fluoroform is not ozone depleting. Chloroform is a common solvent 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 Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of ''Rhodophyta'' (red), '' Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such a ...

produce chloroform, and fungi are believed to produce chloroform in soil. Most of the specifically,

chloroform Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various ref ...

(),

bromoform Bromoform (CHBr3) is a brominated organic solvent, colorless liquid at room temperature, with a high refractive index, very high density, and sweet odor is similar to that of chloroform. It is one of the four haloforms, the others being fluoroform, ...

(), and iodoform are easy to prepare through the

haloform reaction In chemistry, the haloform reaction is a chemical reaction in which a haloform (, where X is a halogen) is produced by the exhaustive halogenation of an acetyl group (, where R can be either a hydrogen atom, an alkyl or an aryl group), in the ...

, 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 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 (

bromodichloromethane Bromodichloromethane is a trihalomethane with formula C H Br Cl2. Bromodichloromethane has formerly been used as a flame retardant, and a solvent for fat In nutrition, biology, and chemistry, fat usually means any ester of fatty aci ...

, and

dibromochloromethane Dibromochloromethane is a colorless to yellow, heavy and nonflammable compound with formula . It is a trihalomethane In chemistry, trihalomethanes (THMs) are chemical compounds in which three of the four hydrogen atoms of methane () are replaced ...

), referred to as total trihalomethanes (TTHM), to 80 parts per billion in treated water. Traces of

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