We already know how to do radical halogenation, which is useful for halogenating alkanes.
And we also know that when specifically brominating, it is very easy to predict
where on the alkane the bromine atom will end up. There is another context in which we can brominate
at a particular location on a molecule other than an alkane. This can be referred to as bromination
at the allylic or benzylic position. Let’s take a look at what these positions are,
how the reaction works, and precisely what type of reagent will promote this reaction.
First let’s clarify what we mean by the allylic or benzylic position. When we say allylic position,
we are referring to the carbon adjacent to a double bond. So given this alkene,
this would be the allylic position. When we say benzylic position, we are referring to the carbon
adjacent to a benzene ring. So given this molecule here, this would be the benzylic position.
The kind of bromination we will be performing will involve placing a bromine atom specifically
at these locations. The reagent we will be using to achieve this is called N-bromo succinimide,
or NBS for short. Structurally speaking, succinimide looks like this, and N-bromo
refers to the presence of a bromo group on the nitrogen atom, so this is N-bromo succinimide.
The reason that we would use this reagent rather than molecular bromine
is that if we are trying to brominate a molecule with one or more pi bonds, molecular bromine tends
to do dibromination with pi bonds, something that is not of concern when the substrate is an alkane,
so if addition reactions are to be avoided, we will want to use another reagent, and that will be
NBS. The mechanism works as follows. In the presence of hydrobromic acid catalyst,
small amounts of molecular bromine will be produced.
This will be in small enough amounts that there is enough time for bromine to undergo homolysis
before having a chance to interact with a pi bond, so there will bromine radicals in solution.
Now let’s allow this to interact with an allylic substrate. The bromine radical can interact with
the hydrogen at the allylic position, generating HBr and producing this allylic radical.
This is a favorable position for radical formation because it is resonance stabilized by this pi
bond, just the way an allylic cation would be resonance stabilized, as the electron deficiency
can be delocalized. Then when the allylic radical encounters a bromine molecule,
one of the bromine atoms can interact with the radical to form a new carbon-bromine bond, and
we will propagate another bromine radical. We have successfully brominated at the allylic position,
and this will continue until all the NBS has been consumed.
So the major key to this reaction is keeping species that would react with the pi bond
to a minimal concentration, like hydrobromic acid and molecular bromine.
This way, there is ample time for homolysis to occur and radical chemistry to ensue.