Trihalomethane

 ( Halogenated Solvents ) 

In chemistry, trihalomethanes ( THMs) are chemical compounds in which three of the four hydrogen atoms of methane () are replaced by halogen atoms. Trihalomethanes with all the same halogen atoms are called haloforms. Many trihalomethanes find uses in industry as or . Some THMs are also environmental , and a few are considered .

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Table of common trihalomethanes

+ Common trihalomethanes (ordered by molecular mass)
CHF3	trifluoromethane	75-46-7	fluoroform	Freon 23, R-23, HFC-23
CHClF2	chlorodifluoromethane	75-45-6	chlorodifluoromethane	R-22, HCFC-22
CHCl3	trichloromethane	67-66-3	chloroform	R-20, methyl trichloride
CHBrCl2	bromodichloromethane	75-27-4	bromodichloromethane	dichlorobromomethane, BDCM
CHBr2Cl	dibromochloromethane	124-48-1	dibromochloromethane	chlorodibromomethane, CDBM
CHBr3	tribromomethane	75-25-2	bromoform	methyl tribromide
CHI3	triiodomethane	75-47-8	iodoform	methyl triiodide

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Industrial uses Only chloroform has significant applications of the haloforms. In the predominant application, chloroform is required for the production of tetrafluoroethylene (TFE), 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

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Refrigerants and solvents Trihalomethanes released to the environment break down faster than haloalkane (CFCs), thereby doing much less damage to the ozone layer. Trifluoromethane and chlorodifluoromethane are both used as . 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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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 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. (Fluoroform () cannot be prepared in this manner.)

Chloroform is produced by heating mixtures of methane or methyl chloride with chlorine. Dichloromethane is a coproduct.

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

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Regulation Trihalomethanes were the subject of the first drinking water regulations issued after passage of the U.S. Safe Drinking Water Act in 1974.EPA Alumni Association: Senior EPA officials discuss early implementation of the Safe Drinking Water Act of 1974, Video, Transcript (see pages 12-13).

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 are produced in swimming pools.

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External links

• National Pollutant Inventory - Chloroform and trichloromethane
• How Ozone Technology Reduces Disinfection Byproducts
• EPA - Trihalomethanes in Drinking Water: Sampling, Analysis, Monitoring and Compliance (August 1983)

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