Reactions of Alkanes
Reactions of Alkenes
Reactions of Conjugated Species
|Halogenation (chlorination or bromination) of alkanes occurs by means of a free radical chain mechanism. As we discussed before, an important consideration of regioselectivity in this reaction is that the halide radical will preferentially attack to yield the most substituted alkyl radical. More highly substituted radicals are more stable. This is also true for free radical halogenation of alkenes and aromatic compounds with aliphatic side chains. Free radical halogenation 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, stabilizing it.|
An important consideration to remember with free radical halogenation of alkenes at the allylic position is that this reaction may be competing with electrophilic addition of halogen directly to the double bond. Halogen molecules will preferentially react by electrophilic addition, except at high temperatures, where the free radical halogenation occurs with substitution for an allylic hydrogen.