Concept: Concept: Allylic Halogentation - General Mechanism7m
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 Chlorination3m
Concept: Concept: Specific Reactions - Allylic Bromination4m