Organic Chemistry

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16. Conjugated Systems

Allylic Halogenation

In allylic halogenation, a radical initiator will be present which will change the reaction site of the molecule. 

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Allylic Halogentation - General Mechanism

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in this video we're going to discuss and a little excite reaction called a Lilic hallucination. So one of the biggest things that you need for a new little ecology nation to take place is a double bond and a die atomic halogen. We've only got one issue, which is that if you recall double bonds love to react with diatonic halogen all by themselves at the actual site of the double bond. Remember that there is a bridge I on intermediate that forms to make an addition. Reaction. I'm just gonna walk you guys through this, in case you guys might have for gotten. Maybe it's been a long time since you've seen this, but if you guys recall double bonds or good nuclear files and they will go ahead an attack one of the intelligence on the diatonic Teligent. Now this reaction proceeds through a bridge I on intermediate. So what we would always see is that the halogen would actually then attack back and kick out. One halogen has an anti on, so we would wind up getting is a bridge. I on that looked something like this and alone halogen. And I honor Hal, I okay, So at this point, I'm not sure if you guys remember, but at this point, a concept called Markov Nick Audition takes over where the negatively charged species would attack the most substituted side of the bridge ion and kick out the other side to face the other direction. And this was an excellent way to make anti vis inal die. Hey, Allied's anti vis inal die! Hey, Allied's okay, so none of this really is supposed to be a teaching moment. There's not supposed to be a teaching moment. If you really want to learn about this, you should go back and watch my hallucination. Video. This this reaction is called Halogen Nation. You can find it in your addition chapter. Okay? Addition is a big concept that has to do with adding things that sold Bonds. You should go back and review that in case you're confused. But I'm not here to really talk about Halladay Nation. What I'm here to talk about is a little ecology nation. So how can I differentiate halogen ation from a lil Ecology nation? Well, it turns out that you're just gonna add one simple thing to change the entire reaction and to change the reaction site and that is heat or in a radical initiator. Remember from our radical chapter that we learned that there's three things that can cause radicals to begin. And they were heat and light or, um, with a diatonic halogen or with peroxide or with NBS, these air, all things that like to initiate radicals. Okay. And he would be a perfect example of something that can start these high energy radicals forming. Well, as you can see, the re agents are actually exactly the same as Halogen Nation, except that we've added this one factor and 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 diatonic halogen instead of my double bond attacking the diatonic halogen right away. What's gonna happen is that the diatonic halogen is going to split off into halogen radicals. Okay, so this is gonna happen before any site at the double before any reaction of the Double one takes place, we're gonna generate these two radicals. Okay? What then happens is the propagation phase in the propagation phase. What we find is that the hydrogen on the Olympic position is very susceptible to being extracted because of the fact that it conform in a Lilic radical. So we're gonna find is that we're going to get a typical radical chain reaction type mechanism where we find we get one electron going into space, as I call it, just going into the middle of nowhere one electron from the bond to hydrogen joining it to make a new Sigma bond. And then we have one left over electron that is dumped off at that primary carbon. Now, typically, primary carbons aren't great for radicals, but this one is a Lilic, so it's actually gonna be a great destination. So we're gonna wind up getting is a radical that looks like this, plus h x. Okay, now you're propagation. Phase is not really complete until you've generated the same radical that you started with. So in this next step, we would then react with another portion of diatonic 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 gonna be dumped onto the halogen regenerating the original radical. So we're going to get a product that now looks like this, plus my ex radical. Okay, so that's my initiation step. That's my propagation step. As you can see, notice that this product in above my head looks far different from a typical halogen nation reaction hallucination. I would expect the XTO actually add to the double bond, but because I had the radical forming, I've gotten a completely different reaction here. Now, what happens? A termination. Well, there's really only one meaningful termination step that we're going to draw, which would be the termination step of one radical my alot radical, um, terminating with halogen radical. Okay, so there are other potential termination steps. We're not gonna worry about them too much because they're gonna Kurt such low volumes. And it's not even gonna matter. All I'm really going to care about is that the end of the day I'm creating an A Lilic. Hey, lied, Okay? And I did this with the same exact free agent's allegations, except I added a radical initiator. Okay, So one important thing is being able to draw the mechanism and predict the products. But Justus important is that is being able to recognize when this reaction is taking place. And as I've said several times, it on Lee happens in the presence of a radical initiator. All right, so that's it for this topic. Let's move on to the next video.
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Note: You may notice that propane, instead of butane was used to show the mechanism of radical halogenation. Just know that the reaction would still take place at the allylic position.

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Specific Reactions - Allylic Chlorination

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Hey, guys. Now let's discuss the specific re agents, mechanisms and products of some a Lilic site hallucinations. So the first one I want to start off with is a little bit chlorination. Ah, little chlorination happens when you react a double bond with di atomic chlorine. But wait, you have to have a radical initiator, right? Remember, we talked about what are radical Initiators are, well, one of them's 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 a little chlorination, a good yield. So we're not going to go through the entire three 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's a Lilic radical, and right before we're about to go ahead and attack that with, let's say, my C L my c l C l. 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 resonant 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 gonna react with two. You're gonna react with the radical that was originally created in the Olympic position. But you're also going to react with the radical that resonates through the little 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 gonna get a mixture of products in a little chlorination. This is going to give us ah, mixture of products that looks 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 gonna make a distinction between one or the other. You're just gonna 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 gonna be pretty much it's gonna be even enough so that we don't have to make a distinction and weaken draw, draw both of them as potential products. Okay, so for a little coronation, always draw that resonant structure in your propagation step. All right, so let's move on to the next reaction.
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Specific Reactions - Allylic Bromination

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another example of an a Lilic site. Apology Nation is a Lilic Brahman ation. Now, unlike a little chlorination, a Lilic domination actually performs best in N B s or a molecule called n bro Most Texana Meid, which I've included the structure of right behind me. Okay, So and Ramos Texana meid, the reason that we use it is because it is a source of Trace bro me in. It turns out that if we use BR to in this reaction, you're going to get too much addition, Product, you're gonna get too much of that die. Hey, lied that I told you. We're really not trying to get right now. If we wanted to die, Hey, lied. 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 in a little excite reaction, which means that NBS is gonna 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 intelligent nation. 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 gonna look just a little different. Your initiation step will actually include a radical being formed on the end, bro. Masic cinema and a radical being formed on the bro. Mean so you can see that already we're getting less roaming radical using this than we are using B R. Two. Because remember that br to when it splits it makes to radicals, whereas NBS Onley makes one. That's just one of the reasons why NBS because as less roaming, it's gonna 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 gonna 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, um with B r radical or let's say you wanted to propagate it through MBS. You would definitely have to draw the resonance structure. Before you could complete this reactions you'd want to draw radical here. And that means that once again, when you get a combination of products, we're going to get a product that has bro Ming in the original Ilic position. But then we're also going to get bro mean adding to the position that the radical resonated towards. So once again, it's really difficult to distinguish between the two products and anytime resonances possible, you're going to go ahead and draw multiple products for these hallucination reactions. Okay, specifically for a Lil Ecology nation. So I hope that made sense. Let me know. Let's move on to the next set of videos.
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