20.18: Radical Substitution: Halogenation of Alkanes and Alkyl Substituents
In the presence of heat or light, alkanes react with molecular halogens to form alkyl halides by a substitution reaction called radical halogenation. This reaction has three steps: initiation, propagation, and termination, as seen in the radical chlorination of methane to produce methyl chloride.
In the initiation step of the reaction, the chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. Propagation occurs in two steps. In the first propagation step, the chlorine radical abstracts hydrogen from a methane molecule to form a methyl radical and a hydrogen chloride molecule. In the second propagation step, the methyl radical abstracts a chlorine atom from a second chlorine molecule, producing methyl chloride and regenerating the chlorine radical. In the final termination step, coupling between two radicals destroys the reactive intermediate, resulting in the termination of the reaction.
The first product in this reaction, methyl chloride, is more reactive than methane and undergoes further chlorination to form methylene chloride, and eventually carbon tetrachloride. Methyl chloride is obtained as the major product when excess methane is reacted with a small amount of chlorine. Because chlorine is unselective, the chlorination of higher alkanes produces a mixture of isomeric mono-chlorinated alkanes and polychlorinated products, making this reaction unsuitable as a synthetic method. While bromine is less reactive than chlorine, fluorine is even more reactive; iodine is generally unreactive towards alkanes via radical halogenation.
