Inchemistry,trihalomethanes (THMs) arechemical compounds in which three of the four hydrogen atoms ofmethane (CH4) are replaced byhalogen atoms. Trihalomethanes with all the same halogen atoms are calledhaloforms. Many trihalomethanes find uses in industry assolvents orrefrigerants. Some THMs are also environmentalpollutants, and a few are consideredcarcinogenic.
| Molecular formula | IUPAC name | CAS registry number | Common name | Other names | Molecule |
|---|---|---|---|---|---|
| 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 |  |
Only chloroform has significant applications of the haloforms. In the predominant application, chloroform is required for the production oftetrafluoroethylene (TFE), precursor toteflon.[1] Chloroform is fluorinated by reaction withhydrogen fluoride to producechlorodifluoromethane (R-22). Pyrolysis of chlorodifluoromethane (at 550–750 °C) yields TFE, withdifluorocarbene as an intermediate.
Trihalomethanes released to the environment break down faster thanchlorofluorocarbons (CFCs), thereby doing much less damage to theozone layer.Trifluoromethane andchlorodifluoromethane are both used asrefrigerants. Chlorodifluoromethane is a refrigerantHCFC, orhydrochlorofluorocarbon, while fluoroform is an HFC, orhydrofluorocarbon. Fluoroform is not ozone depleting.
Chloroform is a commonsolvent in organic chemistry.
The total global flux of chloroform through the environment is approximately660000 tonnes per year,[2] and about 90% of emissions are natural in origin. Many kinds ofseaweed produce chloroform, andfungi are believed to produce chloroform in soil.[3]
Most of thehaloforms — specifically,chloroform (CHCl3),bromoform (CHBr3), andiodoform(CHI3) — are easy to prepare through thehaloform reaction, although this method does not lend itself to bulk syntheses. (Fluoroform (CHF3) cannot be prepared in this manner.)
Chloroform is produced by heating mixtures ofmethane ormethyl chloride withchlorine. Dichloromethane is a coproduct.[4]
Bromochlorofluoromethane is one of the simplest possible stable chiral compounds, and is used for studies.
Trihalomethanes were the subject of the first drinking water regulations issued after passage of the U.S.Safe Drinking Water Act in 1974.[5]
TheEPA limits the total concentration of the four chief constituents (chloroform,bromoform,bromodichloromethane, anddibromochloromethane), referred to as total trihalomethanes (TTHM), to 80parts per billion in treated water.[6]
Traces ofchloroform are produced in swimming pools.[7][8][9][10]
