Illustrated Glossary of Organic Chemistry

Chain initiation (initiation): The first step in a free radical reaction mechanism, in which there are more product radicals than reactant radicals. Usually a radical chain initiation step has no radical reactant(s) and one or more radical product(s). Molecular oxygen can be thought of as a diradical, so any radical chain reaction beginning with O₂ has a radical in its initiation step.

Reaction Mechanism for Free Radical Bromination of Methane

	Photolysis of molecular bromine is a chain initiation step. There are no radical reactants, and two radical products.
	Hydrogen atom abstraction by bromine radical is a chain propagation step.
	Bromine atom abstraction is a chain propagation step.
	Several chain termination events are also occurring.