Before going into the details of allylic bromination, let’s quickly recall what the term allylic means. An allylic position is the carbon atom adjacent to a double bond. Here is a summary of the different positions and their corresponding carbocations that you will encounter often in organic chemistry:

Now, both allylic carbocations and allylic radicals are exceptionally stable because of resonance delocalization and the relative ease of breaking allylic C-H bonds. Here is how allylic radicals rank in the stability chart of radicals:

Another quick reminder that we use fishhook arrows to show the movement of single electrons in reactions and resonance structures of radicals.

One electron from the unpaired electron and one electron from the π bond combine to form a new π bond, while the other electron from the π bond becomes the new radical center.

So, what is Allylic bromination?

Allylic bromination is a selective free-radical substitution reaction that replaces an allylic hydrogen with a bromine atom while leaving the carbon-carbon double bond intact. So, in allylic bromination, the Br atom appears on the carbon next to the double bond.

It is most commonly carried out using N-bromosuccinimide (NBS) in the presence of light (hν), heat (Δ), or a radical initiator such as a peroxide, species, which are the catalyst for generating the radical:

Here is how allylic bromination compares to the anti-Markovnikov radical bromination of alkenes, where the Br atom ends up on the less substituted carbon of the double bond, whereas in allylic bromination, it appears on the carbon atom next to the double bond

They are both radical reactions, so the question is: Why is the regiochemistry of allylic bromination different from that of the free-radical bromination of alkenes? To answer this, we will compare the mechanisms of both reactions step by step and see how the different reaction pathways lead to different regioselectivities.

The Mechanism of Allylic Bromination

Step 1: The first step of allylic bromination is the homolytic cleavage of the N-Br bond (initiation) of the N-bromosuccinimide (NBS):

Notice that the imide group can stabilize the radical by two additional resonance structures, which help to initiate the homolysis of the N-Br bond:

Step 2: After this, the Br radical abstracts an allylic H, forming the corresponding allylic radical:

Step 3: The HBr produced in this step then reacts with NBS, producing Br₂ in low concentration.

Step 4: In the next step, the Br₂ is then quickly captured by the allylic radical, thus keeping the concentration of HBr and Br₂ at a minimum, suppressing the competing electrophilic addition to the double bond.

The process repeats until the termination and consumption of the reactant(s).

Looking at the last step, there is one question we didn’t address here: where is the Br₂ coming from?

The source of Br₂ is the NBS, which, besides producing the Br radical, generates a low concentration of Br₂.

Allylic Bromination vs Addition to the Double Bond

Now, let’s go back and figure out this question: why does the Br radical generated by NBS not add to the double bond like in the anti-Markovnikov bromination?

The answer is it does! However, because the concentration of HBr is low (remember, HBr is needed to supply the hydrogen and convert the radical into alkyl bromide), the addition reaction reverses and proceeds by allylic bromination:

The regiochemistry of Allylic Bromination

The example we discussed above was based on the simplest alkene with an allylic position (propene). As a result, only one product could be obtained since the two resonance structures are superimposable mirror images:

However, if the radical stabilization does not result in identical allylic radicals, a mixture of allylic bromides is obtained:

Notice that both allylic resonance structures contribute to the formation of the two constitutional isomers. So, whenever you are asked to determine the products of allylic bromination, draw both resonance forms and place the Br atoms accordingly.

For example, predict the products of allylic bromination of the following alkene:

Step 1: Draw the allylic radical:

Step 2: Draw the resonance structures of the radical:

Step 3: Add the Br to the allylic radical of each resonance structure:

Stereochemistry of Allylic Bromination

There is no stereochemical control on the allylic bromination. Just like any radical (or carbocation) reaction, whenever possible, both R and S configurations of the radical carbon are formed.

For example, 1-butene, which we discussed earlier, forms three products in total:

One of the radicals forms a mixture of enantiomers, while the other one can only form one product. This product is a constitutional isomer of the two enantiomers.