Free Radical Substitution Reaction
Free radical substitution reactions are initiated by radicals in the gas phase or in non-polar solvents. For example, methane and chlorine react in the presence of sunlight or heat to give methyl chloride.
Light energy or heat causes homolytic fission of chlorine producing chlorine radicals which attack methane to form methyl chloride.
The mechanism is supported by the fact that no reaction occurs in the dark, and in the presence of tetraethyl lead (0.02%) the reaction takes place at 140°C. Tetraethyl lead decomposes at 140°C to ethyl radical which produces chlorine radical from chlorine for the propagation of the reaction as given above.
The reaction proceeds by the repetition of step:2 and step:3.
When the ratio of methane to chlorine is high, methyl chloride is formed predominantly and when chlorine is in excess all the hydrogens are replaced to give carbon tetrachloride.
Source: Reactions, Rearrangements and Reagents By S.N.Sanyal
