Another example of an allylic site halogenation is allylic bromination. Now unlike allylic chlorination, allylic bromination actually performs best in NBS or a molecule called N-bromosuccinimide, which I've included the structure of right behind me. N-bromosuccinimide, the reason that we use it is because it is a source of trace bromine. It turns out that if we use Br2 in this reaction, you're going to get too much addition product. You're going to get too much of that dihalide that I told you we're really not trying to get right now. If we wanted a dihalide, we wouldn't have added a radical initiator. But we don't. We're adding that heat or that light, so that means we're obviously trying to get an allylic reaction which means that NBS is going to be our best bet.
Now once again, I'm not going to draw the full mechanism for this reaction since it closely mirrors the general mechanism that I already told you for a little chlorination. But a few things to keep in mind, in the initiation step, there is kind of an extra thing you have to think about which is since we're starting with NBS, your initiation step is going to look just a little different. Your initiation step will actually include a radical being formed on the N-bromosuccinimide and a radical being formed on the bromine. You can see that already we're getting less bromine radical using this than we are using Br2 because remember that Br2 when it splits, it makes 2 radicals whereas NBS only makes 1. That's just one of the reasons why NBS because it has less bromine, it's going to be less reactive towards that double bond and have less addition cross product.
Another thing to keep in mind is that in the propagation step, we're not going to draw the whole thing, but once again you are going to be required to draw a resonance structure because once again, you've made a radical that can resonate. So before you can attack that radical, you can, let's say, you want to terminate it with Br radical or let's say you wanted to propagate it through NBS, you would definitely have to draw the resonance structure before you could complete this reaction. So you'd want to draw a radical here, and that means that once again we would get a combination of products. We're going to get a product that has bromine in the original allylic position. But then we're also going to get bromine adding to the position that the radical resonated towards. Once again, it's really difficult to distinguish between the 2 products, and anytime resonance is possible, you're going to go ahead and draw multiple products for these halogenation reactions, specifically for allylic halogenation. I hope that made sense. Let me know. Let's move on to the next set of videos.