First let’s quickly review free radical halogenation:
CH3CH2CH3 + Br2 (in the presence of heat or light) –> CH3CHBrCH3
Depicted here is free radical bromonation (the halogen here is Br). However, this can also be accomplished with Cl or I
There are three steps to the mechanism in this reaction: Initiation, Propogation, Termination.
Free radical stability follows carbocation stability allylic > tertiary > secondary > primary > methyl. Therefore this reaction will want to form the most stable free radical during the propogation step and thus the halogen will appear in the more substituted position. For example, in this reaction, the Br appears on the most substituted carbon in the product.
Free radical bromonation is a slower reaction and depends on the stability of the free radical intermediate.
Free radical chloronation is a more rapid reaction and depends not on the stability of the free radical intermediate, but rather on the number of hydrogens present. Therefore, an unreliable mixture of products tends to be formed with free radical chloronation.