Free-radical chain mechanisms follow their own rules. They consist of three parts: initiation, propagation, and termination. (I don't require you to draw termination steps.) Free-radical chain reactions, like all reactions, have stoichiometric starting materials and products, but free-radical chain reactions may also have separate initiators, which are often present in catalytic quantities.
Please note that memorizing these rules will not teach you how to draw free-radical chain mechanisms. The only way you will learn to draw free-radical chain mechanisms is by practice. However, if you follow these rules while you practice, you will quickly get the hang of drawing free-radical chain mechanisms.
A free-radical reaction contains stoichiometric starting materials, and it may also contain an initiator that is present in substoichiometric (catalytic) quantities.
A free-radical chain mechanism consist of three parts: initiation, propagation, and termination.
The initiation part converts a stoichiometric starting material into a free radical, a compound with a single unshared electron. (Stoichiometric starting materials exclude catalysts and initiators. However, in autoxidation reactions, O2 often acts as both an initiator and a stoichiometric starting material.) Common initiation steps:
σ-bond homolysis of a stoichiometric starting material such as Br2 or a compound with a C–I bond;
O2 abstracts H from an X–H bond in a stoichiometric starting material;
σ-bond homolysis of an initiator such as ROOR or RN=NR to give RO· or R·, respectively, followed by abstraction of H from an X–H bond in a stoichiometric starting material;
in free-radical polymerizations only, σ-bond homolysis of an initiator, followed by addition of the radical to a C=C bond of a stoichiometric starting material.
The last step of the initiation part produces the free radical in the first step of the propagation part, and the last step of the propagation part also produces the free radical in the first step of the propagation part. In other words, the propagation part is circular, where the last step of the propagation part produces the radical that is needed to begin another cycle of the propagation part.
Each step in the propagation part must have an odd number of unshared electrons (usually one, but sometimes three) on each side of the arrow. Two compounds containing one unshared electron each never, ever, ever combine in the propagation part.
Each stoichiometric starting material must appear as such in the propagation part, even if it also appears in the initiation part. Corollary: At least one stoichiometric starting material will appear twice in the mechanism: once in the initiation part, and once in the propagation part.
Every stoichiometric product is produced in the propagation part, even if it is also produced in the initiation part.
No piece of the initiator appears in the propagation part.
In free-radical polymerizations, a fragment of the initiator is retained at the terminus of the polymer chain.
In autoxidation reactions, the O2 often acts as both an initiator and a stoichiometric starting material.
Because the propagation part of a free-radical chain mechanism is circular, it is often possible to initiate a free-radical reaction in different ways, entering the propagation cycle at different points by generating different radicals of the propagation part in the initiation part. For example, in the propagation part of the reaction Br2 + R–H → R–Br + HBr, there are two free radicals, R· and Br·. The reaction could be initiated by the homolysis of Br2 to give Br·, or it could be initiated by the abstraction of H atom from R–H by O2 to give R·.
The termination part consists of several one-step reactions, in each of which two compounds containing one unshared electron each combine in either a radical–radical combination or a disproportionation. In a disproportionation, one radical abstracts H from the C atom next to the radical-bearing atom of the other radical.
Common errors in drawing free-radical chain mechanisms:
The initiator, or a part thereof, appears in the propagation part.
Two radicals are allowed to combine with one another in the propagation part.
One of the stoichiometric starting materials appears in the initiation part, but not in the propagation part.
More than one product of the initiation part appears in the propagation part.
The last step of the initiation part does not produce the radical that appears in the first step of the propagation part.