20.16 : 自由基取代：烷烃和烷基取代基的卤化

Alkanes react with molecular halogens in the presence of light or heat to form halogenated alkanes.

Such substitution reactions are called radical halogenations, and proceed via initiation, propagation, and termination steps.

Consider the chlorination of methane to produce methyl chloride.

During the initiation step, the chlorine molecule undergoes homolytic cleavage to form two highly reactive chlorine radicals.

Next, in the first propagation step, the chlorine radical abstracts hydrogen from a methane molecule to produce a methyl radical and a hydrogen chloride molecule.

In the second propagation step, the methyl radical abstracts a chlorine atom from a chlorine molecule, forming methyl chloride and regenerating the chlorine radical.

Finally, coupling of any two radicals depletes the reactive intermediates and terminates the reaction.

Alkane chlorination often results in multiple substitutions or polychlorination. An excess of alkane ensures monochlorination.

Interestingly, chlorination of higher alkanes produces a mixture of isomeric mono-chlorinated alkanes and polychlorinated products.

Notably, only fluorine, chlorine, and bromine react with alkanes via radical halogenation; iodine is unreactive.