Compound formed when the hydrogen in a hydrocarbon molecule, such as methane, is replaced by any of the halogens (fluorine, chlorine, bromine and iodine). Their breakdown in the stratosphere releases chlorine and bromine which take part actively in the destruction of stratospheric ozone. The best known group of halogenated hydrocarbons are the chloro-fluorocarbons (CFCs). The brominated compounds are referred to as halons.