Editing Free-radical addition (section)

== Mechanism and regiochemistry ==
[[File:Peroxide_Free-radical-addition.png|thumb|619x619px|{{Visible anchor|Radical hydrobromination}} of an alkene]]
The basic steps in any free-radical process (the radical chain [[Reaction mechanism|mechanism]]) divide into:<ref name=Wade />
* [[radical initiator|Radical initiation]]: A radical is created from a non-radical precursor.
* [[Chain propagation]]: A radical reacts with a non-radical to produce a new radical species
* [[Chain termination]]: Two radicals react with each other to create a non-radical species

In a free-radical addition, there are two chain propagation steps.  In one, the adding radical attaches to a [[Multiple bond|multiply-bonded]] precursor to give a radical with lesser bond order.  In the other, the newly-formed radical product abstracts another substituent from the adding reagent to regenerate the adding radical.<ref name="March" />{{Rp|pages=743–744}}

In general, the adding radical attacks the alkene at the [[Steric effects|most sterically accessible]] (typically, least substituted) carbon; the radical then stabilizes on the [[Markovnikov's rule|more substituted]] carbon.<ref name="March" />{{Rp|pages=188,751}}  The result is typically anti-[[Markovnikov's rule|Markovnikov]] addition, a phenomenon [[Morris Kharasch]] called the "peroxide effect".<ref name="Kharasch" /> Reaction is slower with alkynes than alkenes.<ref name="March" />{{Rp|page=750}}

In [[Free-radical addition#Radical hydrobromination|the paradigmatic example]], [[hydrogen bromide]] radicalyzes to monatomic bromine.  These bromine atoms add to an alkene at the most accessible site, to give a bromoalkyl radical, with the radical on the more substituted carbon.  That radical then abstracts a hydrogen atom from another HBr molecule to regenerate the monatomic bromine and continue the reaction.<ref name="March" />{{Rp|page=758}}