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Halomethane compounds are molecules of methane (CH₄) with one or more of the hydrogen atoms replaced with halogen atoms. The halogens are found in group 17 of the periodic table of the elements. Forming covalent bonds, these compounds are generally stable.

While these are all considered to be human-made compounds, they may appear inadvertently via natural processes in the environment through the introduction of other non-natural industrial materials. An example is the creation of carbon tetrachloride in small (yet slightly dangerous) quantities when carbon bearing materials are present in drinking water disinfected with chlorine gas. As carbon tetrachloride is believed to be carcinogenic (cancer causing), chlorine gas is being replaced with chloramine for this use.

Some of these compounds, while normally inert, become active when exposed to ultraviolet light found at high altitudes and can destroy the earth's protective ozone layer. The most damaging ones are being phased out of use as industrial materials.

While most of these compounds are considered inert under normal conditions, some are irritants. At high temperatures there may be toxic byproducts produced.

Combinations of halogens with short carbon chains form related compounds such as haloethanes, halopropanes, halobutalines, etc, typically useful as refrigerants, fire suppressing gases, solvents, and as starting points for stable and useful plastics such as Teflon. Some are in crystal form.

Freon is a trade name for a group of chlorofluorocarbons used primarily as refrigerants. The word Freon® is a registered trademark belonging to E.I. du Pont de Nemours & Company.

The naturally occurring halogen elements forming halomethanes are:

• Fluorine, being the most active element forms extremely stable compounds with carbon.
  • Fluoromethane (methyl fluoride, CH₃F)
  • Difluoromethane (CH₂F₂)
  • Trifluoromethane (fluoroform, CHF₃), Freon-23
  • Carbon tetrafluoride (CF₄), a low boiling point gas used in refrigeration. Known as Freon-14, refrigerant 14, R14 and other names

• Chlorine
  • Dichloromethane (DCM, methylene chloride, CH₂Cl₂) is a widely used organic solvent mainly used as a solvent for industrial processes, stripping paint, and degreasing parts. However, it has other uses as varied as decaffeinating coffee and preparing flavor extracts.
  • Chloroform (trichloromethane, CHCl₃) used to be used as an anesthetic but fell out of use due to its toxicity. It finds use today as an industrial solvent and in the production of dyes and pesticides.
  • Carbon tetrachloride (CCl₄) binds four chlorine to each carbon, forming a non-flammable liquid of low boiling point. Once used as a cleaning fluid and in fire extinguishers, it is now known to be extremely toxic and carcinogenic.

• Bromine
  • Carbon tetrabromide (CBr₄)
  • Bromoform (CHBr₃)

• Iodine
  • Carbon tetraiodide (CI₄)
  • Iodoform (CHI₃)

• Combinations - substitutions of fluorine, chlorine and bromine for 3 or 4 of the hydrogens yields various refrigerant or fire suppressing gases.
  • Chlorodifluoromethane (CHClF₂), Freon-22
  • Chlorotrifluoromethane (CClF₃), Freon-13
  • Trichlorofluoromethane (CCl₃F), Freon-11
  • Dibromodifluoromethane (CBr₂F₂)
  • Dichlorodifluoromethane (CCl₂F₂), Freon-12
  • Dichlorofluoromethane (CHCl₂F), Freon-21
  • Diflorodibromomethane (CF₂Br₂)

• http://www.calstand.com/GasConversionTable.html