Side-Chain Halogenation

Concept: Concept: Side-Chain Halogenations

7m
Video Transcript

Now that we're pretty much professionals at adding things to benzene using EAS mechanisms, we're actually going to have to learn how R-groups that are on the benzene uniquely like to react with other reagents. These are in general called side chain reactions.
It turns out that the alkyl group that is directly attached to a benzene is known as an alkyl side chain. These are groups, even though they might seem like it's just like a normal R-group, they're actually special because they contain what we call a benzylic position. What's the benzylic position? The benzylic position is the position next to the benzene. This position is uniquely stable. It's uniquely stable due to conjugation. If you can put a reactive intermediate on that benzylic position, it's going to be more stable than normal because of conjugation because of resonance.
Recall that benzylic radicals in particular are actually the most stable radicals out there. A benzylic radical loves to form because when it can form, it can resonate throughout the whole ring. Let's actually investigate that further. Let's draw all the resonance structures of a benzylic radical. Let's say I form my radical through some kind of radical reaction. What's going to happen is that that radical isn't just going to stay there on the primary carbon. Heck, no. It's going to resonate throughout that whole ring. Let's go ahead and draw this.
Remember that whenever we resonate a radical, we use the three half-headed arrows. We’d get something like this. We would do half-headed arrow here, half-headed arrow here, one electron left over. What we would get is now a double bond here, a double bond here, a double bond here and a radical. Everyone cool with that? If you’re not, let’s do it again. Let's do another one.
That radical isn't stuck there. It can still keep going. This radical could come here and try to make a double bond. But it needs one more radical so it's going to steal one. It’s going to steal one electron from the other bond. But now we've got one radical left, so it's going to come here. Now I'm going to get a resonance structure that looks like this. But we're not done. We know that this radical can keep moving along. Then we're going to do make a double bond, bring it over from that double bond and then dump the extra radical here. Then finally, you know how this ends. This ends the same way it began with this radical making a double bond, this one coming to join it and then I get one extra radical left over. As you guys can see, this radical is like in heaven right now. That radical is the most stable radical ever because it's right next to a benzene that can form all these resonance structures. It's awesome.
Looking back to our conjugation chapter, remember that whenever a conjugation is present, special reactions could take place at those conjugated sites. That's exactly what happens here. In fact, these reactions are the identical reactions to allylic chlorination and the allylic bromination. The only thing is that we're using a benzylic position instead. You can think of these as simply same as allylic reactions.
If you're wondering, “Johnny, it's been a long time. I don't remember the allylic reactions.” You can always type in the search bar. We've got a search bar. You can go allylic chlorination and the video will show up. But this comes from your conjugation section of your text. But it's the same thing.
Benzylic coronation and benzylic combination. Benzylic chlorination would be a diatomic chlorine with heat or light as a radical initiator. This is going to be a radical reaction. It means it's going to have an initiation step, a propagation step and a termination step. You're going to wind up getting Cl radicals that wind up making, remember there's an H here. That wind up making a radical here. Once you form the radical there, you propagate, you chlorinate and you wind up getting this. You wind up getting this product where you get basically a benzylic chlorination.
If you're looking for the mechanism of this which really is not the emphasis of this chapter. This was discussed in your conjugation section so just look up the mechanism in allylic chlorination because it's the same exact reaction, except that now you're using a benzene instead of a double bond. But it's the same thing where you're going to get initiation, propagation and termination. All the same as your allylic reactions.
Benzylic bromination. Benzylic bromination, remember you need trace amounts of bromine because you don't want any cross reactions. That's what NBS is. NBS is a source of trace bromine and that's why we use it. NBS with heat or light is going to make Br radicals and those Br radicals are going to wind up planting a radical on the benzylic position. Eventually they terminate or they propagate through Br2. They wind up getting a benzylic bromine.
These are what's called your side chain reactions because this actually has nothing to do with benzene. This has nothing to do with EAS. This is just a reaction that can occur at the benzylic position because it’s so stable. I hope that made sense. I hope I didn't throw you off. I got a little bit scared there for a second. But anyway, I hope this makes sense. It's really the same thing that we would have learned in the conjugation section of your text. Let's move on to the next reaction.