A Halogenation reaction occurs when one or more fluorine, chlorine, bromine or iodine atoms replace hydrogen atoms in organic compound. The order of reactivity is fluorine > chlorine > bromine > iodine. Fluorine is especially aggressive and can react violently with organic materials. It also tends to make the most stable of the organohalogens and it is difficult to remove a fluorine atom once added. Conversely, iodine is more difficult to add to an organic molecule but once an iodoorganic forms, the iodine atom is easily removed. Thus the electronegativity of the halogen atom is a driving force for halogenation reactions. The reactions also depend on the nature of the substrate molecule that is being halogenated.
Halogenations occur by several different processes depending on the substrate: saturated hydrocarbons halogenate via a free radical process; unsaturated organics halogenate via an addition reaction; aromatics halogenate via electrophilic substitution.