Free-radical substitution reactions typically occur in alkanes or at the alkyl side chains of aromatic hydrocarbons (benzylic positions). These reactions are initiated by the homolytic cleavage of a bond (such as in a halogen molecule) in the presence of heat or ultraviolet light ($h\nu$) to generate free radicals . The reaction proceeds via a chain mechanism involving three main steps: initiation, propagation, and termination . While the benzene ring itself undergoes electrophilic substitution, the alkyl side chain of an alkylbenzene (like toluene) will undergo free-radical substitution upon halogenation in the presence of sunlight.

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