Free radical addition differs relatively little from free radical substitutions.

• Addition reactions are most often initiated by a peroxide or other substance easily decomposed homolytically, rather than by direct dissociation of one of the reactants.

• They also typically involve alkenes and alkynes, which are attacked by the electrophilic free radicals.

Here is the general scheme:

Notice how, after the initiation occurs, the overall transformation again involves a pair of propagation steps forming a chain. Thus, the key distinguishing factors of the two radical reactions are that:

• All bond making and breaking is homolytic;

• Product formation involves a chain reaction.

Some role-players for G-X are shown in the Table: