11. Radical Reactions
Radical reactions require an initial first step to get going. We call this reagent the radical initiator.
Heterolytic vs. Homolytic Bond Cleavage .
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starting from the very beginning. Radicals are these very high energy intermediates that in the natural world exists for only very short periods of time. If you ever hear about free radicals in nature or in your body thes air things that last for tiny amounts of time and they're gone, they terminate. So we when we're dealing with reactions with radicals, the first thing we need to ask is, how do we even get these radicals in the first place? So that brings us to radical initiators. Okay, so every radical reaction that we talk about is always going to start off with a radical initiator. So first of all, let's just talk about how radicals break off differently than regular regular single bonds. And basically, it turns out that single bonds can be broken in two different ways. They could be broken, hetero, politically or home elliptically. So let me show you the difference between that. A normal hair Olynyk cleavage. That means that I'm breaking this bond and I'm gonna get different charges on both sides would mean that two electrons both of the elections from that bond are moving toe one atom. That means that one atom is gonna have a negative charge, and one's gonna have a positive. So let's look at this example. Bond. Right here. I have a carbon and some kind of halogen. How could we predict which of the species would get the negative charge or would get the lone pair on it? Do you guys know how to predict that? The way that we would predict is that we would say the one that's the most Electra negative is the one that gets the electrons when the bond breaks. So there's actually a pretty powerful die poll going towards the ex, depending on which halogen were using. So what we would say is that if we were to break this bond, the way we would break it is towards the ex. Okay, so notice that I'm using a full arrow, and that's showing that both electrons kind of pick up, packed their bags and moved to the X. Okay, What I wind up getting is ion, so I wind up getting a C plus and an ex negative. Okay, this is like I said, this would be hetero elliptic lipstick, cleavage. Okay, now the reason that we call it hair Olynyk is because hetero stands for the word different. Okay, so you're getting different amounts of electrons on both. Okay, Now, notice what this creates is ions. Okay? Your products are different ions a cat ion and I and I am okay, so that's one way to break bonds. Okay, But another way to break them is that I could break them. Just taking one electron from each side so I could take one electron and give it to that ex. Could take another electron. Give it to that ex. Okay, Now, notice that one thing that was different about this bond than the other one was that there was really no die poll. I couldn't tell which one was more electric, negative or not, because they both had the same election negativity. So it's actually gonna be important. Okay, What that's going to do is it's going to give me instead of a negative and a positive that's going to give me two of the same thing. Hence the name home Olynyk cleavage. Okay. In this case, home meaning same that you're getting basically the same electrons on both and noticed that our product here would be radicals. All right. Cool. So far. Awesome. So, um, basically, I want to show you guys the difference between the arrows that I just drew when we want to draw that two electrons are moving to an atom. We say that full curved arrows are used to indicate the movement of two electrons. That means it's a full headed arrow Has both sides of that arrowhead. Okay, when we wanna Onley show that one electron is moving, we would use a half headed arrow or what is sometimes called a fishhook arrow because it's only got half of the arrowhead on it, Okay, Like I used on the X is okay, So it turns out that home Olynyk association is usually much higher. The association energy is typically higher than corresponding hair Olynyk association energy. So what that means is that most of time when we're breaking bonds in organic chemistry, we're actually gonna be using the blue method, the one that's hair Olynyk. Okay. And you're going to see that whether you get into other types of reactions or just later on in the course, you're gonna see that we're gonna use a lot of heroic cleavage. Okay, Home Olynyk cleavage. is really reserved just for radical reactions. Okay? And these reactions that air favorable for just a small set of reasons. Um, and it always starts off with initiator.
Chemical bonds can be cleaved in two ways:Heterolytically (ionic cleavage) and homolytically (radical cleavage).
Homolytic dissociation energy is much higher than a corresponding heterolytic dissociation energy.
What are Radical Initiators?
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So let's go ahead and see how this works. Basically, it turns out that not that many. Like I said, it takes a lot of energy toe break the bond so that only one electron goes to each. Okay, So what that means is that in order to get a radical initiator, we should actually start off with molecules that have relatively weak bonds. Okay, if they have relatively weak bonds, it will be easier to split them off home elliptically. Okay, If it's a very, very strong bond, forget it. That's gonna be difficult to start off with. Making a radical. Okay, so it turns out that there's three really common re agents that have weak bonds that are able thio easily make these radicals. Okay. And what that is is diatonic halogen. Okay. Peroxide and something called NBS. Okay, I'm going to start off with the easiest one. That'll work our way down. Okay, So die Atomic halogen. This is actually depending on which halogen to use. There's actually one of the weaker bonds in organic chemistry. Okay. And what that means is that in the presence of heat or light, Okay, you need heat or light. Why because, like I said, it takes a lot of energy to break these bonds so that one at one electron goes to each atom. So you need some kind of energy source to make this happen. Okay, so we usually use, like, ultraviolet light or heat on. What that's going to do is it's going to excite the electrons in that bond in a, um, and enable them to jump home politically onto each Adam. Okay, so in the presence of heater light, what we're gonna wind up getting is to ex radicals. Okay, that's called a radical initiator, because now I have my first radical that I can use for the mechanism. We don't know the mechanism yet, but at least you know, this is how you can get a radical cool, right? By the way, the arrows would just look like the ones that I drew One electron from the two electrons in the bond. One electron jumps to each x. Okay. The same thing can be said of peroxide. Peroxide also have a relatively weak bond. That 00 bond is relatively weak, and peroxide are gonna be able to once again break off home politically and in this case, it would make to o r radical initiators. Okay, so once again, I'm using a relatively weak bond in the presence of heater light, and that's going to associate home politically. And what I would end up getting is radicals. Okay. Lastly, we have something called NBS. Now, you guys aren't really gonna understand why this is called and be almost Saxena might until or go to in order to, we actually will understand why. How to name that. But for right now, all I want you guys feel to do is recognize the full name, recognize the short name, which is the acronym MBS, which is the way that most professors refer to it. But you should also recognize the structure, just in case. Professor wants to pull a fast one on you and test your knowledge, okay? And it turns out that NBS has again a pretty weak bond between that nitrogen and that, bro. Mean, so it turns out that NBS is actually a source of bromine radicals. And in the presence of heat or light, you get one electron moving to each. The part that we care about is that you're going to get one b R radical. Okay. In fact, we're not even gonna draw the other side because the other side is not really involved in reactions. All I care about is that I'm getting this one br radical that can then react with other things. All right, so I hope that you guys are understanding from the very beginning. We always need an initiator. We need something that's gonna that's gonna make this improbable type of cleavage happen. And that's through these weak, bonded elements like this. Okay, And now that we understand this, we can move on through the mechanism. So let's let's keep going. Let me know if you have any questions.
Radical initiators have relatively weak bonds that can be more easily cleaved by hemolysis.