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. Several of these are easy to prepare through the haloform reaction.
Table of common trihalomethanes
|IUPAC name||CAS registry number||Common name||Other names||Molecule|
|CHF3||trifluoromethane||75-46-7||fluoroform||Freon 23, R-23, HFC-23|
|CHCl3||trichloromethane||67-66-3||chloroform||R-20, methyl trichloride|
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
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 solvent in organic chemistry.
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Trihalomethanes are formed as a by-product predominantly when chlorine is used to disinfect drinking water. They are generally referred to as disinfection by-products. They result from the reaction of chlorine or bromine with organic matter present in the water being treated. The THMs produced have been associated through epidemiological studies with some adverse health effects. Many governments set limits on the amount permissible in drinking water. However, trihalomethanes are only one group of many hundreds of possible disinfection by-products—the vast majority of which are not monitored—and it has not yet been clearly demonstrated which of these are the most plausible candidate for causation of these health effects. In the United States, the EPA limits the total concentration of the four chief constituents (chloroform, bromoform, bromodichloromethane, and dibromochloromethane), referred to as total trihalomethanes (TTHM), to 80 parts per billion in treated water.
Traces of chloroform is also formed in swimming pools that are disinfected with chlorine or hypochlorite in the haloform reaction with organic substances (e.g. urine, sweat, hair and skin particles). Although it possible to inhale THMs, the U.S. EPA, has determined that this exposure is minimal compared to that from consumption. In swimmers, uptake of THMs is greatest via the skin with dermal absorption accounting for 80% of THM uptake. Exercising in a chlorinated pool increases the toxicity of a "safe" chlorinated pool atmosphere with toxic effects of chlorine byproducts greater in young swimmers than older swimmers. Studies in adolescents have shown an inverse relationship between serum testosterone levels and the amount of time spent in public pools. Chlorination by-products have been linked as a probable cause.
- EPA Alumni Association: Senior EPA officials discuss early implementation of the Safe Drinking Water Act of 1974, Video, Transcript (see pages 12-13).
- Dae Jin Sung; Dong Ju Moon; Yong Jun Lee; Suk-In Hong (2004). "Catalytic Pyrolysis of Difluorochloromethane to Produce Tetrafluoroethylene". International Journal of Chemical Reactor Engineering. 2: A6. doi:10.2202/1542-6580.1065.
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- Drobnic, Franchek; Freixa, Assumpci??; Casan, Pere; Sanchis, Joaqu??N; Guardino, Xavier (1996). "Assessment of chlorine exposure in swimmers during training". Medicine & Science in Sports & Exercise. 28 (2): 271–274. doi:10.1097/00005768-199602000-00018. ISSN 0195-9131.
- Aiking, Harry; van Ackert, Manila B.; Schölten, Rob J.P.M.; Feenstra, Jan F.; Valkenburg, Hans A. (1994). "Swimming pool chlorination: a health hazard?". Toxicology Letters. 72 (1–3): 375–380. doi:10.1016/0378-4274(94)90051-5. ISSN 0378-4274.
- Nickmilder, M.; Bernard, A. (2011). "Associations between testicular hormones at adolescence and attendance at chlorinated swimming pools during childhood". International Journal of Andrology. 34 (5pt2): e446–e458. doi:10.1111/j.1365-2605.2011.01174.x. ISSN 0105-6263. PMC 3229674.