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How does light initiate the reaction between methane and chlorine?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

The reaction between methane and chlorine is a photochemical reaction initiated by the absorption of ultraviolet radiation.

The process begins with a chain-initiation step where light energy causes the endothermic dissociation of a chlorine molecule ( $C l_{2}$ ) into two highly reactive chlorine atoms ( $\cdot C l$ ). Once formed, these chlorine atoms collide with methane molecules, triggering a two-step chain reaction:

Propagation: A chlorine atom reacts with methane to produce a methyl radical ( $\cdot C H_{3}$ ) and hydrogen chloride. This methyl radical then reacts with another chlorine molecule to form the final product, chloromethane ( $C H_{3} C l$ ), while regenerating a chlorine atom to continue the cycle.12

Because the chlorine atom is regenerated in the second propagation step, a single photon can initiate the production of many molecules of chloromethane before the chain is eventually terminated by the combination of radicals.12