radical chain mechanism
Imagine a line of dominoes where each falling tile, as it lands, sets up a fresh tile that then topples too. One small push at the start keeps the whole line going. A radical chain mechanism works like this: a single radical, created once, sets off a self-renewing cycle that converts a huge amount of starting material, regenerating a radical with every turn.
The mechanism has three stages. Initiation makes the first radical, usually by homolysis of a weak bond under heat or light (for example Cl2 splitting into two chlorine radicals). Propagation is the engine: a radical reacts with a stable molecule to give a product plus a new radical, and that new radical does it again. Crucially, each propagation step consumes one radical and makes one radical, so the number of radicals stays constant and the cycle is self-sustaining; only a few propagation steps repeat thousands of times. Termination ends the chain when two radicals happen to meet and pair their lonely electrons into a normal bond, removing radicals from the pool. Because radicals are rare, termination is the exception, not the rule.
This three-part pattern (initiate, propagate, terminate) is the backbone of radical halogenation, autoxidation, combustion, and radical polymerization. It explains why a tiny amount of initiator, or a single spark, can transform an entire flask or start a fire: the work is done not by the initiator but by the chain it lights. It also explains why inhibitors (antioxidants) work by quietly mopping up the chain-carrying radicals so propagation stops.
In the chlorination of methane: initiation Cl2 to 2 Cl-dot (light); propagation step 1 Cl-dot + CH4 to HCl + CH3-dot, propagation step 2 CH3-dot + Cl2 to CH3Cl + Cl-dot; termination 2 Cl-dot to Cl2. The two propagation steps loop over and over.
Each propagation turn uses one radical and makes another, so the chain feeds itself.
A common slip is to write propagation steps that destroy a radical without making a new one; if no radical is regenerated, the chain dies. In a valid propagation step the radical count must stay the same: one in, one out.