Free Radical Halogenation

Back when I taught you guys about functional groups, I told you guys that Alkanes actually don't count as a functional group. OK? Even though they're super abundant, they're everywhere. And the reason that I said that is because it's true functional group that implies function that they actually do something and al canes really don't react with much at all. OK? Al canes just come from underground. You dig them up in petroleum, that's what oil is. It's al canes and you can't really react with it a whole much. They're super stable. All you can do is blow them up, OK? Um You can put them in your car and combust them, but you can't really react with them a whole lot. So they seem kind of worthless on first glance. But it turns out that there is one thing that they actually can undergo and that is that they can undergo a radical reaction. OK? Because radicals are very high energy. So they're gonna be able to react with something that's seemingly unreactive, which is al canes. So I wanna show you guys the mechanism by which they do that. So as I just said, Alkanes are the backbone of organic molecules, but they're almost completely unreactive. That's why they last for millions of years underground because they don't react with shit. OK? But there is one thing that they can do in the presence of radicals and they can add halogens. OK. So here I have an unreactive hydrocarbon and like I said, that's from the dinosaurs. It didn't do anything that whole time. Now, I bring it up to the, up to the science lab and I react it with a radical reaction and lo and behold, I get a halogen on that Alcaine. Now, what's cool about that is that now I can do a bunch of other types of reactions to that. This is now called a functional hydrocarbon. Why? Because now I have a function group, an alkyl hali. Once you have an oyl hali, that's the gateway towards organic synthesis. Because now guess what I can do a bunch of stuff so that I can do substitution reactions, elimination reactions, addition reactions, all kinds of stuff because I first added that halogen. OK. So what I'm gonna show you right now is really the first step of all organic synthesis. OK.

So let's just go ahead and talk about it. Turns out that radicals are so high energy that once they react to something, they're going to keep trying to give away that high energy intermediate. Okay, and what's happening is that it's like a game of hot potato where no one wants to have the hot potato so they keep passing it along and it forms. It's called a radical chain reaction. Okay, now the chain reaction, it actually does mean that it means that once you start it, it actually can't end until it's fully reacted. Until you fully reacted with all of your Al came. That is useful for us because, remember, Alcan's aren't that great to begin with. So if we can react them completely, that's a useful reaction as an organic chemist. So let's go ahead and see how this works. Our first step is gonna be the initiation step. The initiation step is where I get that first radical because I can't play a game of hot potato without the hot potato itself. So I have tow create that first radical Now you notice is that this mechanism is broken down into three different steps and we're actually gonna need to write all three of these steps. In fact, it's smart. They actually write the words if you do have to draw this mechanism for a test that you write these three words initiation propagation and termination. Okay, so let's look at the initiation step. And let's say that we're just using the easiest radical initiator, which is X two. Okay, let's use X to overheat. Okay, Now, what I taught you guys is that in the initiation step, what we're gonna wind up getting is electrons from two electrons, 11 on each side, jumping onto each X. So what I'm gonna wind up getting is X radical plus X radic. Okay, that's the end of my initiation step. Really? In the all I need for the bare minimum of my initiation Steptoe work all need is one radical in this case I have to. So I'm great. Okay. Now that I have that radical in place, that radical is free to react with other molecules. Okay? And it turns out that it happens to react really well with Al Keynes now, for the sake of a really simple mechanism, let's just use the simplest al cane possible, Which is methane? Okay, Methane just being a one carbon hydrocarbon. Okay, ch four. So now I've got ch four, and I'm reacting that with X radical. Okay, this X radical hates itself right now, it's super high energy, super unstable. It's saying, How can I get rid off this hot potato? Can I get rid of it? And then it sees all these electrons in the methane, and it's thinking, Hmm. Maybe I could take one of the electrons from one of those carbon hydrogen bonds, and that's exactly what it does. So it turns out that radicals are going to react with hydrogen in al canes. And the way we draw these arrows is just you know, radical reactions are always gonna have three arrows. So I'm gonna draw one fish hook into the middle of nowhere. Okay, then I'm going to draw another fishhook from C H. Bond meeting that one. Okay, what that's implying is that now there's gonna be a new bond between the H and the X that's gonna form from those two electrons, so that's looking great. But I still have one electron left over notice that the bond between the ch had two electrons. So where do you think that last electron goes? It goes on to the sea. Okay, It goes on to the carbon backbone. So what that's going to do is it's going to give me a structure that now looks like this. See, h h h radical. Now notice that I'm drawing the geometry different cause now this would be tribunal plainer, right? So you should draw it with, like, a triangle. And that would be plus h X making sense so far. So notice that the reason this is called a propagation step is because propagation means like I'm reproducing myself propagating and notice that the radical just reproduced itself. Now it kind of moved through my through my medium. And now I've got a radical on a new species. Okay, well, it turns out that your propagation step isn't done yet because you're not done with the propagation step until you fully reproduce yourself 100%. So what that means is not only do I need to have a radical at the end, I need to have the same exact radical that I started with. So if I start off with with an X X Radical. I need to end off with an ex radical. So what that means is, what could I were actually just in the middle of a propagation step right now. What could we react my see radical with to generate that original X radical that we had a beginning. Can you think of anything? It turns out the easiest thing to do is just to react it with another ex ex die atomic halogen You might be wondering. Well, Johnny, why isn't this already radicals? Because we just did that in the first step. We made it radicals. Well, it turns out that not all of the diatonic halogen is going to cleave at the same time. So some of it's gonna do the initiation step. But some of it isn't gonna be hit by enough light or enough heat to split up yet. So what that means is that the one interacting with here hasn't really hasn't cleaved yet. Okay, so this is one that's just waiting around for enough energy to finally do that home A little cleavage, but wait before the light can even get to it. Another radical just did so instead we're gonna propagate to the x X And the way we draw these arrows is once again three arrows. So I'm gonna take the radical. Always starts it. I'm gonna take the radical. I'm gonna put that one out into the middle of nowhere, okay? Between the sea and the X. So then I'm gonna take one electron from this bond and make it go there. This represents that. Now there's gonna be a new bond of two electrons between the sea and the X, but unfortunately, I got one electron left over. I'm gonna dump that one onto the X, so I'm gonna get it. The end of this step is now notice. See each h h. But now I've gotten X. Okay, what do we call that functional group when you have a carbon and a halogen attached to each other, I'll Kyohei light and notice that now I'm gonna have X radical. So that's the end of my propagation step. Because now I have the same exact radical that I started with. Okay, notice that my propagation step just created a very useful by product, which is that it made in alcohol. Hey, light. Okay. Which like I said Al Kyohei lied to do a whole lot more stuff. Then Al Keynes Dio. So this is a very useful reaction for me as an organic chemist. Okay, so now we're gonna do our termination step. Now, what is termination? Termination means that. Okay, you're producing all these radicals, these radicals, air spreading, radical, radical, radical. You're generating all of them. Okay, What happens in termination is that you finally have so many radicals that instead of propagating informing new radicals, they're actually extinguishing each other faster than they can make new radicals. Okay, because think about if you have to. Radicals coming together. If they hit each other, what do you think is gonna happen? Well, I've got one electron here. One electron here, they're gonna form a new bond. So the termination phase is what happens when there's so many radicals that they actually bump into each other. They collide into each other more often, they collide into unredacted species. Okay, so in my termination phase, what I do is I look at all the different possibilities off radicals that could have collided. So the easiest radicals to think of that could have collided or just X Plus X right. Because thes were being formed in high amounts, what happens if they hit each other? Well, I would just wind up getting the same thing all over again. I would do this, I would do that. And I would get X X case. That's one termination product, but there's actually some other termination products possible. Can you think of another one? Another one would be if I had an ex radical and a C H three radical. Right? Okay, That would be like the radical that I had appear in black. Where? Maybe instead of hitting an x X, that wasn't reacted yet. Maybe it reacts with a radical. Once again, I get my arrows. And what I get as a product here would be ch ch three x. Okay, so I get an alcohol. Hey, Light. Okay. Can you guys think of any more combinations? There actually is one more. And that last one would be Well, what if I got ch three radical plus ch three radical? Okay, now, this is tricky because now it looks like I'm gonna get a completely different products. And it's true. What I would get now is actually ethane ch three ch three. Crazy, right? So now what I just did is I just made a larger hydrocarbon than before before I just start off with methane and I'm getting ethane as a product. Okay, well, a few notes about these termination products. First of all, the first one is not going to really matter. Okay, so I'm just gonna say, I don't know. I'm trying to think of, like, a symbol that I can use, but just like, let's say I have termination product 12 and three, right. Number one isn't really gonna matter that much. And the reason is because after I make the X two, it can then just react with heater light again and make radicals again. So, really, this is a This is a termination phase that happens, and then it dissociates again. So you're not going to get a whole lot of X to forming because the X two keeps breaking up. Okay, so that's the first thing. So, actually, x two isn't gonna form a whole lot Now, your professor still probably wants to see that termination step. But just as your final product, you're not gonna get a whole lot of X to as a final product. Okay, so you can cancel out one another. One that we can cancel out is actually three. The reason is because the on leeway that I conform ethane here is if I have a ch three, um, ethyl radical on a metal radical colliding Well, the chances of having metal radicals is actually way lower than the chances of having a halogen radical. Why? Because you're forming the halogen radicals directly through my energy through my light or my heat. Whereas these metal radicals can Onley be formed after they've already reacted with halogen. So the chances of them colliding and making a bigger hydrocarbon are actually very small compared to the chances of them colliding with another halogen. So what I would say is that I'm going to get a very tiny amount of this, like scarce. Yes. Let me try that one more time. Scarce. Wow. I'm really bad at spelling, guys. Sorry. Scarce. Okay, I will get a tiny amount of this, but it's not gonna be a huge amount. Okay? It turns out that there's actually a way that we can even lower this even more. We could lower the amount of my larger al canes that we get. And the way we could do that is just by reacting with excess halogen. Okay, if I just react with a lot of halogen and a very low quantity of al cane than that even reduces the chances mawr that to Al Cane radicals were gonna collide. So depending on what my concentration is of Al Kane, I could basically take this down to zero. If I really wanted to, I could take this down to pretty much full zero. So we're gonna cross out one and three. That means that what's my riel main product? Well, the rial product is gonna be my alcohol. He lied. Okay? And that's the whole point of these. Let me just move out the way The whole point of this reaction is that I'm taking out cane and I'm making alcohol. Hey, lights out of it. Okay, Now this the reason I taught you guys about this is because your professor is gonna want to see if if you're asked to draw this mechanism, they want to see all three terminations. But that doesn't mean that you should draw three products. You should really only draw one major product, because that's gonna be, like, 99.99% of it. And that's the alcohol. Hey, light, all right?

Now I want you guys to practice the general mechanism for radical holiday nation all on your own. And I want you guys to notice that this Al cane that I'm reacting with has carbons of different stability is okay. I want you guys just to assume that we're going to react with the most stable carbon in this case. So you're gonna have to think back to what I talked about with radical stability to figure out which of those hydrogen is to pull off in the radical halogen nation. I think guys can get this, though. So I'm just gonna let you guys loose on your own, try to draw all three steps and then I'll give you guys the answer, so go for it.

we know we need to start with the initiation step here. But even before we get to that, I wanna ask you guys, which hydrogen did you react the radical with in my al cane? Um, basically, there's actually only one choice that made sense, and you should have reacted it with the H right here. The reason is because this age belongs to the Onley tertiary carbon on this molecule. Now, notice that there's no a Lilic sites here, So I don't worry about the residents thing. I just worry about which hydrogen has the most are groups around it. That would be that one right there. So if used any other hydrogen, unfortunately, didn't get the right answer. But hopefully this will be a learning experience for you. So now let's go ahead and draw the three steps. My first step is going to be initiation. I'll just draw it up here to make more room. Okay, so my initiation step is really easy. We're using X two again. So I'm just gonna draw like that and like that, I'm gonna do this and what I'm gonna wind up getting is to ex radicals. Cool. So now let's go into propagation. Now, this is the part where it actually matters which hydrogen I used and I just showed you is why we're going to use that hydrogen there. I'm going to redraw in my Al cane, and I'm gonna draw the hydrogen sticking off this way this time to still make it easier to pull it off. I'm gonna react that with X radical. And what that winds up giving me is three arrows. One here, one here and one there. Okay, So what I wanted getting is a radical that looks like this. Okay, that radical plus h x. Okay. Now what can that radical reacts with? Well, it in order to fully propagate and reproduce itself, it's gonna have to react with another X two. So I'm going to do this. That and that. And what that's going to give me is it's gonna give me and alcohol hail. I notice that this is now a tertiary alcohol. Hey, lied because erected at the tertiary position, and I'm going to get that final radical cool. So now we're just gonna end off with the termination step, okay? And with determination, step. We basically had three different possibilities. We had X terminating with X. That would be my first product. And that would give me basically, that would give me X two. Okay, then we had another possibility, which was now my al cane radical terminating with a halogen radical. And what that would give me is another equivalent of tertiary ocular. He lied. And then lastly, I had the third possibility, which would be I have basically to our groups colliding with each other. Okay? And that would give me a small amount of this kind of random looking thing, which is gonna be that's and then a single bond. And that single bond is attached to basically two more meth ALS and something like that. Okay, so anyway, no, that's ugly. Okay, But the whole point is that I can change the basically the concentration, the ingredients. Okay, I can change the reaction to yield a very small amount of this. So, really, in the end of the day, I'm not going to get a whole lot of one. I'm not gonna get a whole lot of three, but I am going to get a lot of two. So my final product would be this guy. And then Plus, I would get obviously a lot of h x as a byproduct. Okay, because that's gonna pretty much performing all the time, all right?