In a chain reaction, what role does a chain carrier play?

• A. It initiates the reaction only
• B. It terminates the chain steps
• C. It acts as solvent
• D. It propagates chain steps by generating reactive intermediates and sustaining the reaction

Answer: (D)

In a chain reaction, a chain carrier is the reactive species that keeps the process going by propagating the steps and sustaining the reaction. After initiation creates these radicals, the chain carrier reacts with stable molecules to form new reactive intermediates and, in doing so, regenerates itself. This propagation cycle continues, allowing the reaction to proceed without constant external input until termination steps remove the carriers. For example, in radical halogenation, a chlorine radical abstracts a hydrogen from a substrate to form HCl and a new radical, which then continues by reacting with another molecule to keep producing radicals. This explains why the chain carrier’s role is to propagate and sustain, rather than simply initiating or terminating the process, or acting as a solvent.