Allylic Halogenation: Study with Video Lessons, Practice Problems & Examples

Allylic halogenation involves the substitution of an allylic hydrogen with a halogen in the presence of a radical initiator, such as heat or N-bromosuccinimide (NBS). The mechanism includes initiation, where diatomic halogens split into radicals, followed by propagation, where the allylic radical can resonate, leading to multiple products. This results in a mixture of products, such as allylic chlorination and bromination, highlighting the importance of resonance structures in predicting product distribution. Understanding these reactions is crucial for mastering organic synthesis and regioselectivity in halogenation processes.

Allylic Halogentation - General Mechanism

Hey, everyone. So in this video, we're going to talk about allylic halogenation. Now, before we do that, we're first going to take a look at a typical halogenation reaction. Recall the reaction of diatomic halogen with a double bond; this reaction proceeds through a bridged ion intermediate. So, if we take a look here, we have our alkene, and it's going to react with this diatomic halogen. Now, the way we're going to represent this halogen is we have a single bond between the 2 halogens. Remember, the halogens themselves have 3 lone pairs apiece. The way it works is this pi bond here breaks and grabs onto this halogen. When it does that, that's going to cause this bond to split and the electrons to go to this halogen. At the same time, the halogen kind of attacks back, and we need to do this in order to form our bridged ion intermediate. So what we get here initially is this. Here goes our chain. The pi bond was used to connect to the halogen. The halogen used one of its lone pairs to make a bond back. The halogen still has 2 lone pairs it's not using, but it's making 2 bonds so its formal charge will be positive. Now, this represents our bridged ion intermediate. What happens next is the other halogen that left with the electrons comes back. It's negatively charged because it gained its electron as well as the other halogen when this bond was broken. And what it does is it comes in and hits either side in order to open up this ring. Hits this side, which causes this ring to pop open. Now, here, what's going to happen is we're going to create this as a product. So here we have a dashed line. We have our halogen with its lone pairs, and here we have a wedge bond with our other halogen. We're going to say here what we made is anti because one is wedged, one is dashed or anti to each other. Vicinity, that just means they're on neighboring carbons, dihalic. So this is what traditionally happens within a halogenation reaction. We have an alkene reacted with a diatomic halogen in order to create an anti vicinal dihalide. Now, what's the difference with allylic halogenation? Well, we're going to say here, however, in the presence of a radical initiator, radical intermediates will predominate, changing the site of reaction. If we take a look here, we have our alkene still, but now instead of just having our diatomic halogen x2, we have this triangle here on the bottom. This represents heat, which is one of 3 types of radical initiators we can use. Anytime we have one of these radical initiators, we'll do allylic halogenation instead of our traditional halogenation. Now, besides heat, who are other types what are other types of radical initiators? Well, here we can say that heat and UV light are kind of combined together. They can be radical initiators. We also can have peroxides as radical initiators. So that can be in the form of ROOR or maybe H₂O₂. These are different types of peroxides. And then finally, NBS and bromosuccinimide could be a third type of radical initiator. So if you see a halogen with one of these 3, we're going to do allylic halogenation instead. Now, remember, your vinylic carbons are the carbons that are double bonded. Your allylic position is the carbon next to one of these double-bonded carbons. Okay. So, here this carbon in red is our allylic carbon. Now, what happens here under allylic halogenation is that we are going to basically replace an allylic hydrogen with a halogen. So what we would get as a product would be something like this. But how does this happen? Well, because radicals are involved, the general mechanism will follow an initiation step, a propagation step, and finally, a termination step. Now, in the initiation step, we're going to have our diatomic halogen. And the radical initiator, in this case, heat, is going to be used to cause this to split evenly. The electrons within this single bond will split evenly under what we call a homolytic cleavage. So, basically, this halogen takes its one electron in the bond, and then here, this halogen takes its one electron in the bond. And as a result, we create 2 radical halogens. Remember, they're radicals because we have an unpaired electron. In the propagation step, what happens next is one of these radicals that I just created is going to react with my alkene. So then we're going to have one of these halogens come in. Alright. So here, we're trying to deal with the allylic position. What's going to happen here is this halogen decides it wants to connect with the hydrogen, and it does. And then this electron sits on this carbon. So we're going to make initially is this new radical. Okay. And this is an allylic radical. Now, here we're going to say this propagation step is not done until we regenerate our initial radical. So we need to recreate this halogen radical we just lost, because it's no longer around. It's now in the form of HX. It combined it combined with the hydrogen that was in the allylic position. So what's going to happen next is another diatomic halogen's going to come in. This bond will break evenly. And then here this will connect to it. So here goes the halogen. And this step stops because we recreated our halogen. Now, the final step here is termination. And termination has basically our radicals disappearing. What's going to most likely happen here is this. We have this allylic radical, and it's going to react with one of the halogen radicals. This comes here, and this comes here. They combine here to make our final product. Now there are chances of other radicals somehow combining and finding each other, but their probabilities of happening are so small that we don't we basically ignore them. This is the one that is essential. This is the one that's important in greatest abundance and most likely to occur, where we're going to make our allylic position being occupied by halogen. So just remember, when we have the presence of our diatomic halogen with one of our radical initiators, we don't do simple halogenation. Instead, we do allylic halogenation, where the allylic hydrogen is replaced with a halogen.

Specific Reactions - Allylic Chlorination

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. 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 it has to do with that's the temperature that's most often used to perform an allylic chlorination at good yield. 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 Cl₂, we have to analyze the fact that can this radical resonate? Absolutely, it can. So, we should actually draw with allylic chlorination, you should draw a resonance structure in the propagation step that looks like this to show that you're not just going to react with 1 radical, you're actually going to react with 2. 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 are 2 different products that we can yield. We can yield a chlorine on that 3rd carbon or on that first. This means that we're actually going to get a mixture of products in allylic Chlorination. 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. 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. For allylic chlorination, always draw that resonance structure in your propagation step. Let's move on to the next reaction.

Specific Reactions - Allylic Bromination

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.

Here’s what students ask on this topic:

What is allylic halogenation and how does it differ from typical halogenation?

Allylic halogenation involves the substitution of an allylic hydrogen (a hydrogen atom attached to a carbon adjacent to a double-bonded carbon) with a halogen in the presence of a radical initiator, such as heat or N-bromosuccinimide (NBS). This process differs from typical halogenation, which usually involves the addition of halogens across a double bond to form vicinal dihalides. In allylic halogenation, the reaction proceeds through a radical mechanism, including initiation, propagation, and termination steps, leading to the formation of allylic halides. The presence of radical initiators changes the site of reaction from the double bond to the allylic position.

What are the common radical initiators used in allylic halogenation?

Common radical initiators used in allylic halogenation include heat, UV light, and peroxides. Specifically, heat (often at 400°C for allylic chlorination) and UV light can initiate the formation of radicals. Peroxides, such as hydrogen peroxide (H₂O₂) and organic peroxides (R-O-O-R), are also effective radical initiators. Additionally, N-bromosuccinimide (NBS) is frequently used in allylic bromination because it provides a controlled source of bromine radicals, minimizing unwanted side reactions and ensuring selective halogenation at the allylic position.

Why is N-bromosuccinimide (NBS) preferred in allylic bromination?

N-bromosuccinimide (NBS) is preferred in allylic bromination because it provides a controlled source of bromine radicals, which minimizes the formation of unwanted addition products. When Br₂ is used, it can lead to excessive addition across the double bond, forming dihalides. NBS, on the other hand, generates fewer bromine radicals, reducing the likelihood of addition reactions and favoring substitution at the allylic position. This selectivity makes NBS an ideal reagent for achieving high yields of allylic bromides with minimal side products.

What is the mechanism of allylic halogenation?

The mechanism of allylic halogenation involves three main steps: initiation, propagation, and termination. In the initiation step, a radical initiator (e.g., heat, UV light, or NBS) causes the homolytic cleavage of a diatomic halogen (X₂), forming two halogen radicals. During propagation, one halogen radical abstracts an allylic hydrogen, creating an allylic radical. This allylic radical can resonate, leading to multiple possible products. Another diatomic halogen molecule then reacts with the allylic radical, forming the final allylic halide and regenerating the halogen radical. Termination occurs when two radicals combine, ending the reaction.

What are the products of allylic chlorination and bromination?

In allylic chlorination, the products are allylic chlorides, where a chlorine atom replaces an allylic hydrogen. Due to resonance, multiple products can form, with chlorine attaching to different positions on the allylic carbon. Similarly, in allylic bromination, the products are allylic bromides, where a bromine atom replaces an allylic hydrogen. Using N-bromosuccinimide (NBS) ensures selective bromination at the allylic position, and resonance can lead to a mixture of products with bromine at different allylic positions. Both reactions highlight the importance of resonance in determining product distribution.