The two free radical reactions in our list, items (1) and (2), are different from the other processes in that the bonding changes involve single electrons rather than electron pairs.

We signify this when writing the mechanisms by using 'fish hooks', singly barbed arrows, to show electron flow. Other distinct features can be examined with reference to the generic mechanism below:

In rare instances, an example of which was discovered here at UMaine, the substitution step involves attack on carbon, rather than hydrogen:

All free radical reactions have some kind of initiation step, which serves to introduce free radicals into the reacting system.

• Most commonly for radical substitution, this is a homolytic bond dissociation induced by heat or light.

• If one of the reactants does not so dissociate, some initiating substance may be added that can easily be decomposed by heat or light to introduce radicals.

Once the reaction is initiated, subsequent steps form a chain, with each step producing the radical that is required for the other step. We call these the propagation steps.

Your job in writing a specific mechanism for a specific reaction is to match the various compounds and intermediates to their roles in the general mechanism above.