Free Radical Halogenation occurs when either bromine or chlorine is added to an alkane in the presence of light (hv.) Free radical halogenation with flourine or iodine is rarely used because flourine is so reactive that it is not very selective, and the addition of Iodine is so endothermic to be effective as a synthesis of alkyl halides.

The reaction begins with an initiation step: the separation of the halide into its radicals by the addition of ultra-violet light. Note the use of a single headed arrow to show the movement of only one electron.

Initiation Step:

This step is immediately followed by the Propogation Steps: First the formation of the Tertiary Radical:
Propogation Steps:

And then the reaction of the radical with another molecule of Cl2:

Termination steps for free radical reactions are when the radicals collide. This stops the chain reaction because it does not reform the halide radical.

Termination Steps:




In this reaction, the halide radical will attack primarily in the place that will yield the most substituted alkyl radical. This is because more highly substituted radicals are more stable. It also prefers the allylic position, the position next to a double bond, and the benzyllic position, the first carbon in a side-chain off a benzene ring because both allow resonance of the radical over one or more carbons, effectively stabilizing it. Free-radical bromination takes place by the exact same mechanism; however, bromine chooses highly substituted carbons more selectively.

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