Organic Chemistry

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11. Radical Reactions

Anti Markovnikov Addition of Br

The presence of radicals in some familiar looking addition reactions can completely change the product. 

Remember our friendly addition reaction hydrohalogenation? Notice that you achieve Markovnikov alkyl halide in this reaction.  

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Overview of Hydrohalogention.

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So now I want to show you guys how just having a radical initiator present in a reaction can completely change the expected product. And the reaction I want to talk about is called hydro hallucination. So in case you don't know a lot about this reaction, this is considered in addition reaction. Okay? And the mechanism of this reaction was that the double bond would grab the H, which is Elektra Filic. And then it would kick out the BR. So what I would wind up getting is an H on one side and a Carvel Catalan on the other. Now, there was a rule to figure out where the carb okada and went, and that was called Markov Niqabs rule Markov niqabs rules so that the carbon cotton will form in the most stable location or the one of the most are groups. That means the h went right there. Okay, then in the next step might be are negative, attacked the positive charge and what I want up getting is what we would call a Markov Nankov alcohol. Hey, lied. Okay, so this whole reaction was a Carvel cat and inter mediated reaction and we said that this would be a Markov Nankov addition of bro me Okay. To the double bond. Alright,

Now we see this reaction. Note that the only difference is the presence of a radical initiator. 

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How Radical Hydrohalogenation is different from typical Hydrohalogenation.

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Well, now what I want to show you is that just having a radical initiator present can completely change this reaction. And the example I want to use is the same reaction. Same re agents double bond with HBR. But now notice that there's peroxide present. Now, remember that peroxide was a form of radical initiator. So what we want to do in this first step is instead of doing a Carvel caddy, immediate reaction. This is actually gonna be a radical mediated reaction, which means that we're gonna have to use the three steps of initiation propagation and termination to figure out what this is going to dio. So let's go ahead and draw the first step, which is initiation. Okay. Now, for this initiation step, there's gonna be a little bit more complicated than usual just because I'm starting off with peroxide and this is actually not the radical that I want to use for my reaction. So my first step is going to be to generate my peroxide so o r. Two equivalents of our radical. Okay, but then one of those who are radicals is going to react with HBR. Okay, So what? That's going to do is that's going to make a radical that I can actually use in my reaction, that would be basically I would get r o H, which is alcohol, because I just got the but oh, are attaching to the h and I would get br radical. Okay, so I know that was a little bit longer than you used to for the initiation step, but you can consider that the initiation step isn't over until you get your target radical. Okay, in this case, that now I have my br radical, which is my target radical. Now, what I wanna do is I want to react that with my double bond. Oops. I forgot to say propagation. Let's do it. Propagation. Okay, so for the propagation step, what we're gonna see is we're gonna have a double bond, and we're gonna have that radical. Okay, now, typically, um, in a most in a regular radical reaction, I would expect this to react with one of the hydrogen on the AL came, but it turns out that double bonds are also very good sources of electrons. So instead of pulling off a h, it could just react to the double bond directly. Okay, so what I'm gonna expect is that I'm going to get this electron moving into the space in between one electron from my double bond also giving up, you know, also moving towards the BR to make a new bond. Okay, But now I have to figure out, OK, which Adam does the br attached to? Does it attach to the red carbon or does it add to the blue carbon? Both of these are attached to the double bonds. So which one do I pick? And the answer is that we're gonna pick the one that allows us to have the most stable intermediate. So basically noticed that this double bond had two electrons in it to begin with. Now, one of them just went out to meet the BR. Where's the other one gonna go? And that's gonna answer a question. It turns out that the last one would want to go to the place that's gonna make it the most stable. What should be the tertiary location? If the radicals moving to the tertiary location like this, then what that means is that the BR must be attaching to the less substituted position. Okay, So I'm gonna tell you guys what the significance of that is in a second. So then what happens is that we have to generate the original radical right? This radical here winds up reacting with HBR. Okay, so then what I went up getting is that that and that and I finish off my product on what my product looks like is now a bro mean here plus b r radical. Okay, I know. What my head was a little bit in the way for that. You guys can hopefully see it. Okay, so that's our propagation phase. Notice that I did get an alcohol. Hey, lied. But its attentional weird spot. Okay. And this is the part that's interesting, This reaction. I haven't drawn the termination step yet, but let's just go ahead and fill in these blanks. What kind of intermediate are we dealing with here? We're dealing with a radical intermediates. This is no longer Carvel Cat Ion. And because we're dealing with a radical intermediate, what that means is that this is going to be an anti Markov Nankov addition of bro me. Okay? The reason it's ante Markov Mikovits because notice that my bro mean attached to the least substituted spot. Okay, so this direction is very important because it's gonna be one of only two reactions we learn in organic chemistry. One that are anti Markov. Niqab. Okay, just you guys know it's two reactions. One is called. This is the radical edition of HBR, and another one is hydro generation oxidation. Okay, if you guys just remember those two, you're gonna be set, because later on, you're gonna need to know that. Okay, But this is a big deal, because now I know how to add. I know how to add intelligence, Markov Nankov through a normal Carvel cat ion mechanism. But now I also know how to add elegance in an antimicrobial called fashion. And that would just be the ad radicals. Okay, let's go ahead and finish up this termination step. Determination, step for this part. I'm not gonna be picky. Um, a lot of times, professors don't really want to slice a terminal termination, okay? Professors don't want to see, like, all of the termination products for this. They just want to see that you know what you're doing. Because there was a lot of radicals at the beginning So all I would do is I would terminate br with br. Okay, That's definitely a possibility. Um, and I mean, really, honestly, the two are groups coming together is gonna happen even less than before, so I wouldn't even put the two are groups together. Um, that would be one termination on, and that would really be the main termination. Okay, so basically what we're gonna be doing, I mean, another termination would just be, Yeah, another very important termination. I'm sorry. It would be this radical. Okay. On, Just like reacting with a eight radical whatever, that would be another one. Okay, these are the ones that are favored. But other than that, the other one's really aren't favored very often, so you wouldn't have to draw all the different possibilities. Okay, so I hope that this makes sense. Guys, you should be able to know how to draw the mechanism. But even more than that, you should be able to recognize when a direction is gonna be ante markovic off because it's using radicals. And this only happens when you're doing an addition of HBR using radicals. Okay, cool. So I hope that made sense. Let's move on

However, this one added reagent will lead to the formation of an anti-Markovnikov alkyl halide. Here’s the full mechanism:

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Provide the complete mechanism for the following radical hydrohalogenation. 

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Provide the complete mechanism for the following radical hydrohalogenation.

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So my first step is the initiation step. And the whole purpose of this is to make my br radical. So I'm gonna go ahead and do that. But I have to start off with the peroxide because that's my radical generator at the beginning. Or, my initiator. So now I've got to o r radicals and one of those is going to grab the HBR. So do this. That that and what I'm gonna wind up getting is r o H plus b r radical. Okay, so now I've got my bro mean radical so I could move to the propagation step, okay. And in the propagation step, I'm going to react with the double bond. I'm not going to react with the tertiary position because in this case, I'm adding it. First of all, this isn't a diatonic collagen. This is HBR. So it's its own thing. There is in addition, reaction. So I've got my br radical and I want to know which side to put the br on. And the answer to that question is pretty easy. It's just gonna be whatever side isn't the most stable for the radical. So in this case, my radical worshiped form should inform on the secondary or on the primary. The radical should form on the secondary right there. Which means that the BR should add on the primary. It's not going to do this. And I'm gonna wind up getting something. Looks like this radical here. Br here and then that is just going to propagate to HPR snow. I've got this. That that and that's going to give me my alcohol. Hey, lied. That looks like that. Okay, sorry about my head being in the way again, but I'll keep it there for you. Okay? So let's really my main product. Now, let's just do the termination step, Okay? So in my terminations that we said there were a lot that were possible because this is kind of a more complicated reaction, but one of them, that is possible would just be like br br. Okay, that's the most common one that you're gonna see. Okay. And that would give us br to all right. And obviously another one that could form would be determination, step that has my radical here and an h radical. Okay. And this one not as much as the other one. Cool guys, So I hope that made sense, just a mechanism that you are expected to know. But it's not that tricky. So I hope that you guys were able to recognize that. Let's move on to the next topic.
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