Chemistry 3719 - Dr. Peter Norris
Free Radical Halogenation of Alkanes
A conceptually different type of mechanism to those encountered so far is the free radical halogenation of unactivated alkanes to produce haloalkane products. This type of reaction requires that free radicals be generated which then react via a chain mechanism to afford products. We will use the single headed arrow notation here to describe homolytic bond cleavage processes. Keep in mind that radical chlorination of alkanes is less selective than the corresponding bromination since chlorine radicals are much more reactive (and therefore much less selective). The following overview will go through the major steps involved in the free radical bromination of a simple alkane.
The radical bromination is initiated by cleavage of a relatively weak Br-Br bond using either heat or UV light as the energy source. Only a few Br2 molecules need to be cleaved to get this reaction going.


The highly reactive bromine radicals then go in search of an electron to regain their stable octet configuration. When the Br radical encounters the alkane it will "strip off" (or "abstract") a hydrogen atom along with one of the C-H bond electrons.


This hydrogen abstraction results in the formation of H-Br and a new radical.  Because a new radical is generated, we term this a propagation step since the new radical will then react further, thus propagating the chain. Notice how the bromine radical selectively abstracts the hydrogen which leads to the most stable (3o) radical, even though there are nine 1o hydrogens present in the substrate.  Chlorine radicals would be less selective and remove the first hydrogen they encounter.


The carbon radical is now in need of finding an extra electron, and will abstract a bromine atom when it encounters a bromine molecule.  This is also a propagation step since a new radical (the bromine radical) is formed).


The new bromine radical can go on to react with another molecule of the alkane, and thus the chain continues. The major organic product is the haloalkane.


The following animation puts these events in sequence:



The main reason that this type of reaction is not self-perpetuating is that radicals can come together to form covalent bonds in what are known as termination steps.  Since no new radical is formed, the chain cannot be propagated and the reaction eventually comes to a halt.  The following would be termination steps in the above reaction:



Peter Norris, Ph.D.