It is possible to use more than one halogen to form alkyl halide compounds. If the multiple halogens are the same type, the prefix “di” is used for two, “tri” for three, and “tetra” for four. Some chemicals, as has been previously mentioned, have trade names. There are also trade names for some of the alkyl halides. For example, a one-carbon radical with three chlorine atoms attached is called trichloro methane, or methyl trichloride; however, the trade name for the compound is chloroform. A methyl radical with four chlorines attached is named tetrachloro methane, or methyl tetrachloride, The trade name for the compound is carbon tetrachloride, a material that was used as a fire-extinguishing agent. It is no longer approved as an extinguishing agent because when it contacts a hot surface, it decomposes to phosgene gas. Shown in Figure 8.8 are the names, molecular formulas, and structures for some alkyl halides with multiple numders and combinations of halogens in the com pounds.