Radical Substitution: Halogenation of Alkanes and Alkyl Substituents

Radical halogenation proceeds through initiation, propagation, and termination. During initiation, the halogen molecule undergoes homolytic cleavage under light or heat to form reactive radicals. Propagation involves two steps where radicals abstract atoms and regenerate themselves. Termination occurs when two radicals couple, depleting reactive intermediates and stopping the reaction.

In the initiation step, a chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. These radicals then enter the propagation phase where they abstract hydrogen atoms from alkane molecules, starting the chain reaction that produces halogenated products.

Propagation occurs in two steps. First, a chlorine radical abstracts hydrogen from methane, producing a methyl radical and hydrogen chloride. Second, the methyl radical abstracts a chlorine atom from another chlorine molecule, forming methyl chloride and regenerating the chlorine radical. This cycle repeats, sustaining the chain reaction.

Excess alkane ensures monochlorination because the high concentration of unreacted alkane molecules makes it more likely that radicals will encounter and react with alkane rather than the more reactive halogenated product. When chlorine is limited and alkane is abundant, the primary product methyl chloride undergoes fewer subsequent chlorination reactions.

Only fluorine, chlorine, and bromine react with alkanes via radical halogenation. Fluorine is the most reactive, chlorine is moderately reactive, and bromine is less reactive than chlorine. Iodine is generally unreactive towards alkanes through this mechanism, making it unsuitable for radical halogenation reactions.

Chlorination of higher alkanes produces a mixture of isomeric mono-chlorinated alkanes and polychlorinated products. Because chlorine is unselective, it attacks multiple positions on the alkane and can substitute multiple hydrogens. This lack of selectivity makes radical halogenation unsuitable as a synthetic method for preparing specific halogenated products.

Termination occurs when any two radicals couple together, forming a stable bond and destroying the reactive intermediates. This coupling depletes the radical species needed to propagate the chain reaction, causing the halogenation to stop. Common termination products include chlorine-chlorine or radical-radical combinations that are no longer reactive.