Halomethane compounds are derivatives of methane (CH4) with one or more of the hydrogen atoms replaced with halogen atoms (F, Cl, Br, or I). Halomethanes are both naturally occurring, especially in marine environments, and human-made, most notably as refrigerants, solvents, propellants, and fumigants. Many, including the chlorofluorocarbons, have attracted wide attention because they become active when exposed to ultraviolet light found at high altitudes and destroy the Earth's protective ozone layer.
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] This method is useful for the production of CH_{4−n}Cl_{n} (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.
Traces of halomethanes in the atmosphere arise through the introduction of other non-natural, industrial materials.
- Free radical chlorination of methane (under ultraviolet light):
- CH4 + Cl2 → CH3Cl + HCl
- Halogenation of methanol. This method is used for the production of the mono-chloride, -bromide, and -iodide.
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Halons are usually defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens.[4] They are referred to by a system of code numbers similar to (but simpler than) the system used for freons.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:
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.[2] It is widely used as a paint stripper and a degreaser. In the food industry, it was previously used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavorings.[11] Its volatility has led to its use as an aerosol spray propellant and as a blowing agent for polyurethane foams.
Propellants
One major use of CFCs has been as propellants of
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 carcinogenicity and liver damage (e.g., carbon tetrachloride). Under certain combustion conditions, chloromethanes convert to phosgene, which is highly toxic.
See also
- Halocarbon
- Haloalkane
- Halogenation
External links
- Gas conversion table
- Nomenclature FAQ
- History of Halon use by the US Navy
- 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
References
- Günter Siegemund, Werner Schwertfeger, Andrew Feiring, Bruce Smart, Fred Behr, Herward Vogel, Blaine McKusick “Fluorine Compounds, Organic” Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002.^
- Manfred Rossberg, Wilhelm Lendle, Gerhard Pfleiderer, Adolf Tögel, Eberhard-Ludwig Dreher, Ernst Langer, Heinz Rassaerts, Peter Kleinschmidt, Heinz Strack, Richard Cook, Uwe Beck, Karl-August Lipper, Theodore R. Torkelson, Eckhard Löser, Klaus K. Beutel, Trevor Mann “Chlorinated Hydrocarbons” in Ullmann's Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. .^
- Gordon W. Gribble. Naturally Occurring Organohalogen Compounds Acc. Chem. Res., 1998^