What is the term for the initial stage of a free radical chain reaction?

In a free radical chain reaction, the initial stage is referred to as the initiation phase. This stage is characterized by the formation of free radicals, which are highly reactive species containing unpaired electrons. The initiation step typically involves the breaking of a chemical bond (often a covalent bond), which can occur through various means such as heat, light, or chemical initiators.

During this stage, molecules such as diatomic halogens or peroxide compounds are often used, and their decomposition generates free radicals. These free radicals then serve as the key reactive intermediates that propagate the reaction in the subsequent stages. This sets the stage for the propagation step, where the generated radicals react with other molecules to produce additional radicals, thus continuing the chain reaction.

In contrast, the other terms relate to different stages of the reaction process. The terminating step involves the combination of free radicals to form stable products, while recombination refers to the process where two radicals combine to form a non-radical species. The propagation stage specifically deals with the series of reactions where radicals are regenerated and continue to react. Hence, the initiation phase is integral as it kicks off the entire chain reaction process.