CHE 230
University of Kentucky
How to draw a free-radical chain mechanism
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.
- 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 free radical produced in the last step of the initiation part is the first free radical in 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.
- Every stoichiometric product is produced in the propagation part, even if it is also produced in the initiation part.
- The free radical produced in the last step of the propagation part is the same as the first free radical in the propagation part.
- No piece of the initiator appears in the propagation part.
Exceptions:
- 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.
- 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:
- Separate initiation and propagation parts are not labeled clearly.
- The initiator, or a part thereof, appears in the propagation.
- 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 initiation part.
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