Allylic Halogenation

Concept: Concept: Allylic Halogentation - General Mechanism

7m
Video Transcript

In this video, we’re going to discuss an allylic site reaction called allylic halogenation. So one of the biggest things that you need for an allylic halogenation to take place is a double bond and a diatomic halogen. We’ve only got one issue, which is that if you recall, double bonds love to react with diatomic halogen all by themselves at the actual site of the double bond.
Remember that there's a bridged-ion intermediate that forms to make an addition reaction.
I'm just going to walk you guys through this in case you guys might have forgotten. Maybe it's been a long time since you've seen this. But if you guys recall, double bonds are good nucleophiles and they will go ahead and attack one of the halogens on the diatomic halogen.
This reaction proceeds through a bridged-ion intermediate. What we would always see is that the halogen would actually then attack back and kick out one halogen as an anion. What we would wind up getting is a bridged-ion that looked something like this and a lone halogen anion or halide.
At this point, I'm not sure if you guys remember but at this point a concept called Markovnikov addition takes over where the negatively charged species would attack the most substituted side of the bridged-ion and kick out the other side to face the other direction.
This was an excellent way to make anti-vicinal dihalides. None of this really is supposed to be a teaching moment. This is not supposed to be a teaching moment. If you really want to learn about this, you should go back and watch my halogenation video. This reaction is called halogenation.
You can find it in your addition chapter. Addition is a big concept that has to do with adding things to double bonds. You should go back and review that in case you’re confused. But I'm not here to really talk about halogenation.
What I’m here to talk about is allylic halogenation. How can I differentiate halogenation from allylic halogenation? It turns out that you’re just going to add one simple thing to change the entire reaction and to change the reaction site. That is heat or a radical initiator.
Remember from our radical chapter that we learned that there's three things that can cause radicals to begin. They were heat and light, or with a diatomic halogen or with peroxides or with NBS. These are all things that like to initiate radicals.
Heat would be a perfect example of something that can start these high-energy radicals forming. As you can see, the reagents are actually exactly the same as halogenation except that we've added this one factor. That one factor is going to change everything. It's going to change the site of the reaction completely.
Let's see how this happens. Now that we’ve added heat to the presence of diatomic halogen, instead of my double bond attacking the diatomic halogen right away, what's going to happen is that the diatomic halogen is going to split off into halogen radicals.
This is going to happen before any reaction at the double bond takes place. We're going to generate these two radicals.
What then happens is the propagation phase. In the propagation phase, what we find is that the hydrogen on the allylic position is very susceptible to being extracted because of the fact that it can form an allylic radical.
What we're going to find is that we're going to get a typical radical chain reaction type mechanism where we get one electron going into space as I call it, just going to the middle of nowhere. One electron from the bond to hydrogen joining it to make a new sigma bond. Then we have one leftover electron that is dumped off at that primary carbon.
Typically, primary carbons aren't great for radicals but this one is allylic so it’s actually going to be a great destination. What we're going to wind up getting is a radical that looks like this plus HX.
Your propagation phase is not really complete until you’ve generated the same radicals that you started with.
In this next step, we would then react with another portion of diatomic halogen and we would really do the same mechanism over again. We would show that one electron goes into the middle of space. The other one goes to join it.
But we've got one extra radical left and that's going to be dumped onto the halogen regenerating the original radical. We're going to get a product that now looks like this plus my X radical.
That's my initiation step. That's my propagation step. As you can see, notice that this product above my head looks far different from a typical halogenation reaction.
Halogenation, I would expect the X to actually add to the double bond. But because I had the radical forming, I've gotten a completely different reaction here.
What happens at termination? There's really only one meaningful termination step that we're going to draw, which should be the termination step of one radical, my allyl radical terminating with the halogen radical.
There are other potential termination steps. We're not going to worry about them too much because they're going to occur at such low volumes that it's not even going to matter.
All I’m really going to care about is at the end of the day, I'm creating an allylic halide.
I did this with the same exact reagents as halogenation except I added a radical initiator.
One important thing is being able to draw the mechanism and predict the products. But just as important as that is being able to recognize when this reaction is taking place.
As I've said several times, it only happens in the presence of a radical initiator.
Alright. So that’s it for this topic. Let's move on to the next video.

Concept: Concept: Specific Reactions - Allylic Chlorination

3m
Video Transcript

Hey guys. Now, let's discuss the specific reagents mechanisms and products of some allylic site halogenations, so the first one I want to start off with is Allylic chlorination. Allylic chlorination happens when you react a double bond with diatomic chlorine but wait you have to have a radical initiator, right? Remember we talked about what our radical initiators are, well, one of them is heat and in place of heat, we're going to specifically use 400 degrees Celsius, exactly why has to do with lab techniques and has to do with that's the temperature, that's most often used to perform in allylic chlorination at good yield. So, we're not going to go through the entire 3-step initiation, propagation and termination mechanism because that was already covered in the general mechanism, it's the same exact thing, however I do want to make a note of the fact that in our propagation step, when we form that allylic radical and right before we're about to go ahead and attack that with, let's say my CL, my CL, CL, okay? We have to analyze the fact that can this radical resonate? Absolutely, it can. So, we should actually draw within a little chlorination you should draw a resonance structure in the propagation step, that looks like this, okay? To show that you're not just going to react with one radical, you're actually going to react with two, you're going to react with the radical that was originally created in the allylic position but you're also going to react with the radical that resonates through the allylic position to the other side, meaning that, when we go ahead and when we continue our propagation phase there's two different products that we can yield, we can yield a chlorine on that third carbon or on that first. So, this means that we're actually going to get a mixture of products in the allylic fluorination, this is going to give us a mixture of products that look something like this, we're going to get some Chlorine on that radical position but we're also going to get some chlorination happening on this position, okay? And for the purposes of this class, we're not going to make a distinction between one or the other, you're just going to draw both products when they're possible, okay? Now, I understand, you might be thinking to yourself, but Johnny isn't one of them more stable than the other or wouldn't there be a major and a minor product, again not for the purposes of this class, it's going to be pretty much, it's going to be even enough so that we don't have to make a distinction and we can just draw both of them as potential products, okay? So, for allylic chlorination always draw that resonance structure in your propagation step. Alright, so let's move on to the next reaction.

Concept: Concept: Specific Reactions - Allylic Bromination

4m
Video Transcript

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, okay? So, n-bromosuccinimide, the reason that we use it is because it is the source of trace bromine, it turns out that if we use BR 2 in this reaction you're going to get too much addition products, 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 day a high halide, we wouldn't have added a radical initiator, okay? But, we don't, we're adding that heat or that light so that means we're obviously trying to get an allylic site 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 allylic halogenation 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. So, you can see that already, we're getting less bromine radical using this than we are using BR 2 because remember, that BR 2, when it splits it makes two radicals, whereas NBS only makes one, 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, okay? So, 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 want 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'd 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 resonate is towards. So, once again it's really difficult to distinguish between two products and anytime resonance is possible you're going to go ahead and draw multiple products for these halogenation reactions, okay? Specifically for allylic halogenation. So, I hope that made sense, let me know, let's move on to the next set of videos.