:Trihalomethane
{{short description|Methane (CH4) derivative with 3 halogen substituents}}
In chemistry, trihalomethanes (THMs) are chemical compounds in which three of the four hydrogen atoms of methane ({{chem2|CH4}}) are replaced by halogen atoms. Trihalomethanes with all the same halogen atoms are called haloforms. Many trihalomethanes find uses in industry as solvents or refrigerants. Some THMs are also environmental pollutants, and a few are considered carcinogenic.
Table of common trihalomethanes
class="wikitable"
|+Common trihalomethanes (ordered by molecular weight) |
Molecular
formula !IUPAC name !Common name !Other names !Molecule |
---|
CHF3
|trifluoromethane |75-46-7 |Freon 23, R-23, HFC-23 |align="center"|File:Fluoroform-3D-vdW.png |
CHClF2
|chlorodifluoromethane |75-45-6 |align="center"|chlorodifluoromethane |R-22, HCFC-22 |align="center"|File:Chlorodifluoromethane-3D-vdW.png |
CHCl3
|trichloromethane |67-66-3 |R-20, methyl trichloride |align="center"|File:Chloroform-3D-vdW.png |
CHBrCl2
|bromodichloromethane |75-27-4 |dichlorobromomethane, BDCM |align="center"|File:Bromodichloromethane-3D-vdW.png |
CHBr2Cl
|dibromochloromethane |124-48-1 |chlorodibromomethane, CDBM |align="center"|File:Dibromochloromethane-3D-vdW.png |
CHBr3
|tribromomethane |75-25-2 |methyl tribromide |align="center"|File:Bromoform-3D-vdW.png |
CHI3
|triiodomethane |75-47-8 |methyl triiodide |align="center"|File:Iodoform-3D-vdW.png |
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.{{cite journal |author1=Dae Jin Sung |author2=Dong Ju Moon |author3=Yong Jun Lee |author4=Suk-In Hong |title=Catalytic Pyrolysis of Difluorochloromethane to Produce Tetrafluoroethylene |journal=International Journal of Chemical Reactor Engineering |year=2004 |volume=2 |page=A6 |doi=10.2202/1542-6580.1065 |s2cid=97895482}} 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.
:
:
=Refrigerants and solvents=
Trihalomethanes released to the environment break down faster than chlorofluorocarbons (CFCs), thereby doing much less damage to the ozone layer. Trifluoromethane and chlorodifluoromethane are both used as refrigerants. 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.
Occurrence and production
The total global flux of chloroform through the environment is approximately {{val|660000}} tonnes per year,{{cite journal |last1=Gribble |first1=Gordon W. |year=2004 |title=Natural Organohalogens: A New Frontier for Medicinal Agents? |journal=Journal of Chemical Education |volume=81 |issue=10 |page=1441 |doi=10.1021/ed081p1441 |bibcode=2004JChEd..81.1441G}} and about 90% of emissions are natural in origin. Many kinds of seaweed produce chloroform, and fungi are believed to produce chloroform in soil.{{cite journal |last1=Cappelletti |first1=M. |year=2012 |title=Microbial degradation of chloroform |journal=Applied Microbiology and Biotechnology |volume=96 |issue=6 |pages=1395–409 |doi=10.1007/s00253-012-4494-1 |pmid=23093177 |s2cid=12429523}}
Most of the {{nowrap|haloforms{{tsp}}{{mdash}}{{tsp}}}}specifically, chloroform ({{chem2|CHCl3}}), bromoform ({{chem2|CHBr3}}), and iodoform {{nowrap|({{chem2|CHI3}}){{tsp}}{{mdash}}{{tsp}}}}are easy to prepare through the haloform reaction, although this method does not lend itself to bulk syntheses. (Fluoroform ({{chem2|CHF3}}) cannot be prepared in this manner.)
Chloroform is produced by heating mixtures of methane or methyl chloride with chlorine. Dichloromethane is a coproduct.{{cite book |doi=10.1002/14356007.a06_233.pub2 |chapter=Chlorinated Hydrocarbons |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2006 |last1=Rossberg |first1=Manfred |last2=Lendle |first2=Wilhelm |last3=Pfleiderer |first3=Gerhard |last4=Tögel |first4=Adolf |last5=Dreher |first5=Eberhard-Ludwig |last6=Langer |first6=Ernst |last7=Rassaerts |first7=Heinz |last8=Kleinschmidt |first8=Peter |last9=Strack |first9=Heinz |last10=Cook |first10=Richard |last11=Beck |first11=Uwe |last12=Lipper |first12=Karl-August |last13=Torkelson |first13=Theodore R. |last14=Löser |first14=Eckhard |last15=Beutel |first15=Klaus K. |last16=Mann |first16=Trevor |isbn=3527306730}}
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 in 1974.EPA Alumni Association: Senior EPA officials discuss early implementation of the Safe Drinking Water Act of 1974, [https://web.archive.org/web/20161105133921/http://www.epaalumni.org/history/video/interview.cfm?id=13 Video], [https://www.epaalumni.org/userdata/pdf/6014106B36AE81EB.pdf#page=12 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.{{cite web |title=EPA {{!}} Envirofacts {{!}} ICR {{!}} Regulations|url=https://archive.epa.gov/enviro/html/icr/web/html/regulations.html|access-date=2021-10-11|website=archive.epa.gov}}
Traces of chloroform are produced in swimming pools.{{cite journal |last1=Lindstrom |first1=A B |last2=Pleil |first2=J.D. |last3=Berkoff |first3=D.C. |year=1997 |title=Alveolar breath sampling and analysis to assess trihalomethane exposures during competitive swimming training |journal=Environmental Health Perspectives |volume=105 |issue=6 |pages=636–642 |issn=0091-6765 |doi=10.1289/ehp.97105636 |pmid=9288498 |pmc=1470079}}{{cite journal |last1=Drobnic |first1=Franchek |last2=Freixa |first2=Assumpci?? |last3=Casan |first3=Pere |last4=Sanchis |first4=Joaqu??N |last5=Guardino |first5=Xavier |year=1996 |title=Assessment of chlorine exposure in swimmers during training |journal=Medicine & Science in Sports & Exercise |volume=28 |issue=2 |pages=271–274 |issn=0195-9131 |doi=10.1097/00005768-199602000-00018 |pmid=8775165 |doi-access=free}}{{cite journal |last1=Aiking |first1=Harry |last2=van Ackert |first2=Manila B. |last3=Schölten |first3=Rob J.P.M. |last4=Feenstra |first4=Jan F. |last5=Valkenburg |first5=Hans A. |year=1994 |title=Swimming pool chlorination: a health hazard? |journal=Toxicology Letters |volume=72 |issue=1–3 |pages=375–380 |issn=0378-4274 |doi=10.1016/0378-4274(94)90051-5 |pmid=7911264 |url=https://www.researchgate.net/publication/222136311}}{{cite journal |last1=Nickmilder |first1=M. |last2=Bernard |first2=A. |year=2011 |title=Associations between testicular hormones at adolescence and attendance at chlorinated swimming pools during childhood |journal=International Journal of Andrology |volume=34 |issue=5pt2 |pages=e446–e458 |issn=0105-6263 |doi=10.1111/j.1365-2605.2011.01174.x |pmid=21631527 |pmc=3229674}}
References
{{Reflist}}
External links
- [https://web.archive.org/web/20060302162424/http://www.npi.gov.au/database/substance-info/profiles/23.html National Pollutant Inventory - Chloroform and trichloromethane]
- [https://web.archive.org/web/20060206224406/http://www.biozone.com:80/trihalomethanes.html How Ozone Technology Reduces Disinfection Byproducts]
- [https://nepis.epa.gov/Exe/ZyPDF.cgi/9100OQFI.PDF?Dockey=9100OQFI.PDF EPA - Trihalomethanes in Drinking Water: Sampling, Analysis, Monitoring and Compliance (August 1983)]
{{Halomethanes}}
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