Halomethane
Halomethane compounds are derivatives of
Structure and properties
Like methane itself, halomethanes are tetrahedral molecules. The halogen atoms differ greatly in size and charge from hydrogen and from each other. Consequently, most halomethanes deviate from the perfect tetrahedral symmetry of methane.[1]
The physical properties of halomethanes depend on the number and identity of the halogen atoms in the compound. In general, halomethanes are volatile but less so than methane because of the polarizability of the halides. The polarizability of the halides and the polarity of the molecules makes them useful as solvents. The halomethanes are far less flammable than methane. Broadly speaking, reactivity of the compounds is greatest for the iodides and lowest for the fluorides.
Production
Industrial routes
The halomethanes are produced on an industrial scale from abundant precursors such as natural gas or methanol, and from halogens or halides. They are usually prepared by one of three methods.[2]
- Free radical chlorination of methane (under ultraviolet light):
- CH4 + Cl2 → CH3Cl + HCl
This method is useful for the production of CH4−nCln (n = 1, 2, 3, or 4). The main problems with this method are that it cogenerates HCl and it produces mixtures of different products. Using CH4 in large excess generates primarily CH3Cl and using Cl2 in large excess generates primarily CCl4, but mixtures of other products will still be present.
- Halogenation of methanol. This method is used for the production of the mono-chloride, -bromide, and -iodide.
- CH3OH + HCl → CH3Cl + H2O
- 4 CH3OH + 3 Br2 + S → 4 CH3Br + H2SO4 + 2 HBr
- 3 CH3OH + 3 I2 + P → 3 CH3I + HPO(OH)2 + 3 HI
- Halogen exchange. The method is mainly used to produce fluorinated derivatives from the chlorides.
- CH3Cl + HF → CH3F + HCl
- CH2Cl2 + HF → CH2FCl + HCl
- CH2Cl2 + 2 HF → CH2F2 + 2 HCl
- CH2Cl2 + F2 → CH2F2 + Cl2
- CHCl3 + HF → CHFCl2 + HCl
- CHCl3 + 2 HF → CHF2Cl + 2 HCl
- CHCl3 + F2 → CHF2Cl + Cl2
- CHCl3 + 3 HF → CHF3 + 3 HCl
- CHCl3 + F2 + HF → CHF3 + Cl2 + HCl
- CCl4 + HF → CFCl3 + HCl
- CCl4 + 2 HF → CF2Cl2 + 2 HCl
- CCl4 + F2 → CF2Cl2 + Cl2
- CCl4 + 3 HF → CF3Cl + 3 HCl
- CCl4 + F2 + HF → CF3Cl + Cl2 + HCl
- CCl4 + 4 HF → CF4 + 4 HCl
- CCl4 + F2 + 2 HF → CF4 + Cl2 + 2 HCl
- CCl4 + 2 F2 → CF4 + 2 Cl2
- Reaction of methane with hypochlorous acid, producing water.
- CH4 + HOCl → CH3Cl + H2O
- Reaction of methanol with hypochlorous acid, producing hydrogen peroxide.
- CH3OH + HOCl → CH3Cl + H2O2
Traces of halomethanes in the atmosphere arise through the introduction of other non-natural, industrial materials.
In nature
Many marine organisms
- 2 CH4 + 2 Cl− + O2 → 2 CH3Cl + 2 OH−
Classes of compounds
Halons are usually defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens.[4] They are referred to by a system of code numbers similar to (but simpler than) the system used for freons. The first digit specifies the number of carbon atoms in the molecule, the second is the number of fluorine atoms, the third is the chlorine atoms, and the fourth is the number of bromine atoms. If the number includes a fifth digit, the fifth number indicates the number of iodine atoms (though iodine in halon is rare). Any bonds not taken up by halogen atoms are then allocated to hydrogen atoms.
For example, consider Halon 1211. This halon has number 1211 in its name, which tells it has 1 carbon atom, 2 fluorine atoms, 1 chlorine atom, and 1 bromine atom. A single carbon only has four bonds, all of which are taken by the halogen atoms, so there is no hydrogen. Thus its formula is CF2ClBr, hence its IUPAC name is bromochlorodifluoromethane.
ANSI/ASHRAE Standard 34-1992
The refrigerant naming system is mainly used for fluorinated and chlorinated short alkanes used as refrigerants. In the United States, the standard is specified in ANSI/ASHRAE Standard 34–1992, with additional annual supplements.[5] The specified ANSI/ASHRAE prefixes were FC (fluorocarbon) or R (refrigerant), but today most are prefixed by a more specific classification:
- CFC—list of chlorofluorocarbons
- HCFC—list of hydrochlorofluorocarbons
- HFC—list of hydrofluorocarbons
- FC—list of fluorocarbons
- PFC—list of perfluorocarbons(completely fluorinated)
The decoding system for CFC-01234a is:
- 0 = Number of double bonds (omitted if zero)
- 1 = Carbon atoms -1 (omitted if zero)
- 2 = Hydrogen atoms +1
- 3 = Fluorine atoms
- 4 = Replaced by Bromine ("B" prefix added)
- a = Letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal.
Other coding systems are in use as well.
Hydrofluoro compounds (HFC)
Hydrofluorocarbons (HFCs) contain no chlorine. They are composed entirely of carbon, hydrogen, and fluorine. They have no known effects on the ozone layer; fluorine itself is not ozone-toxic.
Overview of principal halomethanes
Most permutations of hydrogen, fluorine, chlorine, bromine, and iodine on one carbon atom have been evaluated experimentally.
Systematic name | Common/trivial name(s) |
Code | Use | Chemical formula |
---|---|---|---|---|
Tetrachloromethane | Carbon tetrachloride, Freon 10
(Freon is a trade name for a group of chlorofluorocarbons used primarily as a refrigerant. The main chemical used under this trademark is dichlorodifluoromethane. The word Freon is a registered trademark belonging to DuPont.) |
CFC-10 | Formerly in fire extinguishers | CCl4 |
Tetrabromomethane
|
Carbon tetrabromide | CBr4 | ||
Tetraiodomethane | Carbon tetraiodide | CI4 | ||
Tetrafluoromethane
|
Carbon tetrafluoride, Freon 14 | PFC-14 (CFC-14 and HF-14 also used, although formally incorrect) |
CF4 | |
Chloromethane | Methyl chloride | Methylation agent; e.g., for methyl trichlorosilane | CH3Cl | |
Dichloromethane | Methylene chloride | Solvent | CH2Cl2 | |
Trichloromethane | Chloroform | Solvent | CHCl3 | |
Trichlorofluoromethane | Freon-11, R-11 | CFC-11 | CCl3F | |
Dichlorodifluoromethane | Freon-12, R-12 | CFC-12 | CCl2F2 | |
Chlorotrifluoromethane | CFC-13 | CClF3 | ||
Chlorodifluoromethane | R-22 | HCFC-22 | CHClF2 | |
Trifluoromethane | Fluoroform | HFC-23 | In semiconductor industry, refrigerant | CHF3 |
Chlorofluoromethane | Freon 31 | Refrigerant (phased out) | CH2ClF | |
Difluoromethane | HFC-32 | Refrigerant with zero ozone depletion potential | CH2F2 | |
Fluoromethane | Methyl fluoride | HFC-41 | Semiconductor manufacture | CH3F |
Bromomethane | Methyl bromide | Soil sterilant and fumigant, currently being phased out. It strongly depletes the ozone layer and is highly toxic. | CH3Br | |
Dibromomethane | Methylene bromide | Solvent and chemical intermediate. | CH2Br2 | |
Tribromomethane | Bromoform | For separation of heavy minerals | CHBr3 | |
Bromochloromethane | Halon 1011 | Formerly in fire extinguishers | CH2BrCl | |
Bromochlorodifluoromethane | BCF, Halon 1211 BCF, or Freon 12B1 | Halon 1211 | CBrClF2 | |
Bromotrifluoromethane | BTM, Halon 1301 BTM, or Freon 13BI | Halon 1301 | CBrF3 | |
Trifluoroiodomethane | Trifluoromethyl iodide | Freon 13T1 | Organic synthesis | CF3I |
Iodomethane | Methyl iodide | Organic synthesis | CH3I |
Applications
Because they have many applications and are easily prepared, halomethanes have been of intense commercial interest.
Solvents
Dichloromethane is the most important halomethane-based solvent. Its volatility, low flammability, and ability to dissolve a wide range of organic compounds makes this colorless liquid a useful solvent..
Propellants
One major use of CFCs has been as propellants of
Fire extinguishing
At high temperatures, halons decompose to release
However, Halon 1301 fire suppression is not completely non-toxic; very high temperature flame, or contact with red-hot metal, can cause decomposition of Halon 1301 to toxic byproducts. The presence of such byproducts is readily detected because they include hydrobromic acid and hydrofluoric acid, which are intensely irritating. Halons are very effective on Class A (organic solids), B (flammable liquids and gases), and C (electrical) fires, but they are unsuitable for Class D (metal) fires, as they will not only produce toxic gas and fail to halt the fire, but in some cases pose a risk of explosion. Halons can be used on Class K (kitchen oils and greases) fires, but offer no advantages over specialised foams.
Flooding systems may be manually operated or automatically triggered by a VESDA or other automatic detection system. In the latter case, a warning siren and strobe lamp will first be activated for a few seconds to warn personnel to evacuate the area. The rapid discharge of halon and consequent rapid cooling fills the air with fog, and is accompanied by a loud, disorienting noise.
Halon 1301 is also used in the F-16 fighter to prevent the fuel vapors in the fuel tanks from becoming explosive; when the aircraft enters an area with the possibility of attack, Halon 1301 is injected into the fuel tanks for one-time use. Due to ozone depletion, trifluoroiodomethane (CF3I) is being considered as an alternative.[13]
Chemical building blocks
Chloromethane and bromomethane are used to introduce
Safety
Haloalkanes are diverse in their properties, making generalizations difficult. Few are acutely toxic, but many pose risks from prolonged exposure. Some problematic aspects include
See also
References
- ^
- ^ .
- .
- ISBN 978-0-19-920463-2.
- ^ "ASHRAE Bookstore". Archived from the original on 2006-06-15. Retrieved 2009-10-07.
- ^ "Ozone Layer Protection". US EPA. October 14, 2020.
- .
- ^ Lerner & K. Lee Lerner, Brenda Wilmoth (2006). "Environmental issues : essential primary sources". Thomson Gale. Retrieved 2006-09-11.
- ^ US EPA, OAR (February 15, 2013). "Ozone Layer Protection". US EPA.
- ^ All Things Considered, NPR News, 5:24 p.m., December 11, 2007.
- ^ Office of Environmental Health Hazard Assessment (September 2000). "Dichloromethane" (PDF). Public Health Goals for Chemicals in Drinking Water. California Environmental Protection Agency. Archived from the original (PDF) on 2009-10-09.
- ^ 3-III-2 HALON 1301 REPLACEMENTS Archived 2008-04-19 at the Wayback Machine
- ^ "Credit Card Freedom | Red Smoothie Detox & Dessert Photographs". www.afrlhorizons.com. Archived from the original on July 11, 2007.
External links
- Gas conversion table
- Nomenclature FAQ
- History of Halon use by the US Navy Archived 2000-08-19 at the Wayback Machine
- Ozone Loss: The Chemical Culprits
- EIA Environmental Investigation Agency
- Environment Investigation Agency – EIA Global Environmental Investigation Agency in the USA
- Alphabetical list of all halomethanes