How are free radicals involved in the halogenation of organic molecules?

In the halogenation of organic molecules, such as the reaction between methane and chlorine, free radicals act as transient, highly reactive intermediates that sustain a chain reaction. These reactions typically require an external energy source, such as ultraviolet light or high temperatures, to initiate the process.123

The mechanism generally follows three stages:

• Chain-initiation: Energy causes the homolytic fission of a halogen molecule (e.g., $C{l}_2$), producing two highly reactive halogen atoms (radicals) with unpaired electrons.142
• Chain-propagation: A halogen radical collides with an organic molecule, abstracting a hydrogen atom to form a new organic radical (e.g., a methyl radical, $·C{H}_3$). This organic radical then reacts with another halogen molecule to produce the final halogenated product and regenerate a halogen radical, which continues the cycle.12
• Chain-termination: The process concludes when two radicals collide and combine, effectively removing the reactive species from the system (e.g., two methyl radicals forming ethane or a methyl radical combining with a chlorine radical).1

Because the halogen radical is regenerated during propagation, a single initiation event can lead to the production of many product molecules.12