Halomethane
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 man-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.
Like methane itself, halomethanes are tetrahedral molecules. The halogen atoms differ greatly in size and charge from hydrogen and from each other. Consequently, the various halomethanes deviate from the perfect tetrahedral symmetry of methane.
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.
The halomethanes are produced on a massive scale from abundant precursors, i.e. natural gas or methanol, and from halogens or halides. They are usually prepared by one of three methods.
This method is useful for the production of CH4−xClx (x = 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.
Many marine organisms biosynthesize halomethanes, especially bromine-containing compounds. Small amounts of chloromethanes arise from the interaction of chlorine sources with various carbon compounds. The biosyntheses of these halomethanes are catalyzed by the chloroperoxidase and bromoperoxidase enzymes, respectively. An idealized equation is:
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