Why do combustion reactions often involve free-radical intermediates?

Combustion reactions often involve free-radical intermediates because they function as chain reactions, which are highly efficient, self-sustaining sequences of chemical steps.12

In these processes, free radicals—highly reactive atoms or molecular fragments with unpaired electrons—act as "chain carriers". The mechanism typically follows three stages:312
* Initiation: Heat or other energy sources break chemical bonds to form the initial reactive intermediates.12
* Propagation: These radicals react with original fuel molecules to produce stable products while simultaneously generating new radicals. This cycle allows a single radical to trigger the production of many product molecules.12
* Branching: In many combustion scenarios, the reaction involves "branched-chain" steps where the number of radicals increases with each propagation cycle. This leads to rapid auto-acceleration, which is often observed as a flame or, in extreme cases, an explosion.41

Because these radicals are transient and highly reactive, they facilitate the rapid pace characteristic of combustion, allowing the reaction to proceed until the fuel is consumed or the chain is terminated by recombination.526