Halomethanes

Halomethanes are a class of organohalide compounds derived from methane (CH4) in which one or more hydrogen atoms are replaced by halogen atoms—fluorine, chlorine, bromine, or iodine. The general formula is CH4−nXn, where X is a halogen and n ranges from 1 to 4. Substituting different numbers and types of halogens yields a wide range of substances with varying physical properties, from gases to liquids at room temperature, and with diverse reactivities.

Representative members include fluoromethanes (CH3F, CH2F2, CHF3, and CF4), chloromethanes (CH3Cl, CH2Cl2, CHCl3, CCl4), bromomethanes (CH3Br, CH2Br2, CHBr3), and iodomethanes (CH3I, CH2I2, CHI3).

Many halomethanes are useful as solvents (such as dichloromethane and chloroform), reaction intermediates, or refrigerants; others have been used as fire suppressants (halons). Some subgroups are also significant greenhouse gases and ozone-depleting substances. Health and environmental concerns vary by compound. Several halomethanes are toxic or potentially carcinogenic; some have adverse effects on the liver or nervous system. Environmentally, chlorine- and bromine-containing halomethanes can contribute to ozone depletion and possess notable global warming potentials. Because of these risks, many halomethanes are subject to regulation and phaseouts under international agreements such as the Montreal Protocol, with transitions toward less persistent or more benign alternatives.