chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, 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 element ...

s in which three of the four hydrogen atoms of

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

() are replaced by

halogen The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...

atoms. Trihalomethanes with all the same halogen atoms are called haloforms. Many trihalomethanes find uses in industry as

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

s or

refrigerant A refrigerant is a working fluid used in the cooling, heating, or reverse cooling/heating cycles of air conditioning systems and heat pumps, where they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are ...

s. Some THMs are also environmental

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

s, and a few are considered

carcinogen A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

ic.

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 . It is a colorless gas. Its structure is . It is used primarily in the industrial preparation of fluoropolymers. It is the simplest perfluorinated alkene. It was first repor ...

(TFE), precursor to

teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off from ...

. Chloroform is fluorinated by reaction with

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

to produce chlorodifluoromethane (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

Trihalomethanes released to the environment break down faster than chlorofluorocarbons (CFCs), thereby doing much less damage to the

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

. Trifluoromethane and chlorodifluoromethane are both used as

s. 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). They are produced as volatile derivatives of methane, ethane, ...

, or

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

, while fluoroform is an HFC, or

hydrofluorocarbon Hydrofluorocarbons (HFCs) are synthetic organic compounds that contain fluorine and hydrogen atoms, and are the most common type of organofluorine compounds. Most are gases at room temperature and pressure. They are frequently used in air condit ...

. 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 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 as ...

produce chloroform, and

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

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

chloroform Chloroform, or trichloromethane (often abbreviated as TCM), is an organochloride with the formula and a common solvent. It is a volatile, colorless, sweet-smelling, dense liquid produced on a large scale as a precursor to refrigerants and po ...

(), bromoform (), and

iodoform Iodoform (also known as triiodomethane) is the organoiodine compound with the chemical formula . It is a pale yellow, crystalline, volatile substance, with a penetrating and distinctive odor (in older chemistry texts, the smell is sometimes refe ...

are easy to prepare through the haloform reaction, although this method does not lend itself to bulk syntheses. (

Fluoroform Fluoroform, or trifluoromethane, is the chemical compound with the formula . It is a hydrofluorocarbon as well as being a part of the haloforms, a class of compounds with the formula (X = halogen) with C3v symmetry. Fluoroform is used in divers ...

() cannot be prepared in this manner.) Chloroform is produced by heating mixtures of

methyl chloride Chloromethane, also called methyl chloride, Refrigerant-40, R-40 or HCC 40, is an organic compound with the chemical formula . One of the haloalkanes, it is a colorless, sweet-smelling, flammable gas. Methyl chloride is a crucial reagent in indus ...

with

chlorine Chlorine is a chemical element; it has Symbol (chemistry), 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 ...

. Dichloromethane is a coproduct. Bromochlorofluoromethane is one of the simplest possible stable chiral compounds, and is used for studies.

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 primary 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 wa ...

in 1974. The EPA limits the total concentration of the four chief constituents (

, bromoform,

bromodichloromethane Bromodichloromethane is a trihalomethane with formula . It is a colorless, nonflammable liquid which will dissolve in water, or evaporate in air. Most of the chemical is produced through the chlorine disinfection process, and as a result it can ...

, and dibromochloromethane), referred to as total trihalomethanes (TTHM), to 80

parts per billion In science and engineering, the parts-per notation is a set of pseudo-units to describe the small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measur ...

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)
{{Authority control Halogenated solvents Refrigerants