Chain Termination Step
Definition and meaning of Chain Termination Step in chemistry.
A chain termination step is the elementary step in a radical chain mechanism in which two reactive intermediates, usually free radicals, combine to form a stable, closed-shell product, ending that chain without generating new radicals.
In more detail
Because termination consumes two radicals while producing none, it removes chain carriers from the system rather than sustaining the cycle, in contrast to initiation (which creates radicals) and propagation (which conserves their number). Termination steps are strongly exothermic, since they form a new covalent bond without breaking one, but they occur relatively infrequently because radical concentrations during a chain reaction are very low, making radical-radical collisions statistically rare compared to a radical reacting with an abundant closed-shell molecule. Termination does not stop the overall reaction; it only ends the particular chain in which it occurs, while other chains continue propagating elsewhere.
Key facts
| Field | Organic Chemistry |
|---|---|
| Mechanism stage | Third stage of a radical chain reaction (after initiation and propagation) |
| Net radical change | Two radicals consumed, zero radicals produced |
| Also called | Chain-breaking step |
In the radical chlorination of methane, chain termination occurs through radical-radical combinations such as Cl• + Cl• → Cl2, CH3• + CH3• → CH3CH3, and CH3• + Cl• → CH3Cl.
Frequently asked questions
Why are termination steps less frequent than propagation steps?
Radical intermediates are present in very low, steady-state concentration during a chain reaction, so a radical is far more likely to collide with an abundant closed-shell molecule (propagation) than with another radical (termination).
Does a termination step stop the whole reaction?
No. It only ends the individual chain in which the two radicals combine; the overall reaction proceeds as long as other chains continue to be initiated and propagated.