Organic Chemistry

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16. Conjugated Systems

Thermal Cycloaddition Reactions

Thermal Cycloaddition reactions are pericyclic reactions in which 2 pi bonds are destroyed after a heat-activated cyclic mechanism.

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concept

MO Theory of Thermal Cycloadditions

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Hey, guys, in this video, we're gonna dive deep into a type of Paris I click reaction called a thermal cyclo edition. So thermal cycle auditions are Perry cyclic reactions in which two PI bonds are destroyed. By the way, we know that it's, too, because the cyclo addition, they always destroyed two pi bonds after a heat activated cyclic mechanism. So we know that all Perry cyclic mechanisms air cyclic. But this happens to be a thermal controlled one, not a photo chemically controlled Perry cyclic reaction. So it turns out that there's already a very popular example of this type of reaction in your organic chemistry textbook, and that's called a deal's elder reaction. Now you might have not studied the deals all the reaction yet, or maybe you just got finished studying it. But this reaction is an example of a thermal cyclo edition. So here's just the general kind of run down of what you would get. You would have ah, three pi Bonds reacting under heat toe form, a new cyclic product that only has one pie pond. Okay, that's the basics of how it works now. Very quickly. I do want to show you guys. The mechanism the mechanism would have to would be a cyclic, um, concerted reaction where all of your double bonds are forming new bonds within the system to make a new ring. So, for example, my, uh, this'll double bond right here would form a single bond in between the two. Then that's that's basically going to form a new bond between the dying and the Al Cane. Then this double bond would come down and form a new double bond here. And then finally, this double bond would come around and attack this one. Now, actually, even though I showed that it's making a bond, typically this first mechanism arrow is actually drawn to that carbon just so people understand that it's actually going to form a new link to that carbon. Okay, And what you would get as a result is a new cyclic adducts where now you have these two molecules that came together and made a new ring. Okay, now that's the basics of the mechanism. But to really understand what's driving a cyclo edition of thermal cycle edition, you we need to go back to frontier molecular orbital theory. We need to understand what frontier orbital's are. We need to understand how Homo and Lou Mo are interacting for these two molecules, and it turns out that it's a very kind of simple rule, which is that in a cyclo edition, the homo from one molecule which I'm calling Homo a must fill the loom. Oh, from molecule B. So that means that the homo from one is going to kick up electrons to fill the loom o of the other molecule. And that is how they're going to join together and make a new ring. Okay, now, according to a frontier molecular orbital theory, this bonding interaction is the strongest when symmetry and energy levels between your molecular orbital's match closely. So what that means is, what do we talk? What are you talking about? With symmetry, symmetry has to do with the Lopes. Remember that you have. Whenever you have molecular orbital's, you have lobes facing in different directions, right? So what you want to do is you want to make sure that the terminal lobes of your homo match the terminal lobes of your loom. Oh, so that there could be a good bonding overlap between them. That's what we call symmetry, and I'm gonna show you more an example. But right now, I just know that it has to do with what direction the lobes are facing. You want them facing similar directions, Okay. And then what? We in my energy, the energy also has to match because you want your homo and your limo to be close and energy and not far in energy. If they're very far and energy one is much higher than the other. It is difficult to make a strong bond. So what you're trying to do is you're trying to find the homo that's closest to the others. Loom. Oh, and that's where you're gonna find a really strong interaction. Okay, so just to kind of summarize what I just said in order for a cycle addition to take place, you're looking at two things. One, the reaction must be what's called symmetry allowed. Symmetry allowed has to do with the orbital's matching up properly. Okay, symmetry allowed, not disallowed. Disallowed is also called just, you know, forbidden. This is another term. Your textbook may use symmetry forbidden or just forbidden. Um, it needs to be some symmetry allowed. Okay. And the second one is that you want to try toe, minimize your home A limo gap, which is what I was just talking about. In terms of energy levels, the gap gets bigger the further apart, those homo no mo orbital's are in energy. So you want to make sure that you're trying to minimize that gap as much as possible. Cool. Awesome, guys. So now it's time to actually get into the molecular orbital theory of a cyclo edition. I hope you're excited. So here's our dying. Let's start off with our dying. Um, you should know how to draw the molecular orbital's for a dying. At this point, we've We've gone over that in other videos and what we know about a dying is that it has four pi electrons. So that means that sy one should get two electrons. Side two should get two electrons and I've already gone ahead and labeled are Homo and our Lubo limo Orbital's for the dying. Okay, once again, A is just to signify that we're dealing with this molecule a I'm calling it a for this reaction. Okay. Now, in heat, we're going to be exposing that dying to another, um, conjugated molecule in this sense. In this case, it's the simplest conjugated molecule, which is an AL Keen. But it doesn't just have to be in Al Qaim. It could be another dying. That would be fine. Just something else that's conjugated that has home AlumAl orbital's. Okay, so in this case, we should know how to fill in the orbital's for this. How maney pilot trains are there too. So that means that I should fill in one and to and I've already also filled in that this is the home for molecule B. And on top, we have the loom. Oh, for molecule B. So what did we say about what we're trying to accomplish in a cycle audition or trying to do is we're trying to take the electrons from Homo A and use them toe fill the orbital of Lou Moby. Okay, so that's what the heat energy is going to do, you might say. Well, Johnny, why would these electrons raise an energy? Well, that's why you need to use heat. He is going to be able to allow those electrons to jump into a higher energy state, which is the loom. Oh, for the other molecules Okay. So just, you know, in order to predict that this is gonna be either symmetry allowed or symmetry disallowed, we are gonna have to draw what the molecular orbital's look like. Okay, now there are some shortcuts, and I'm able to show you later, but right now, we definitely want to draw those molecular orbital's. So if you don't mind, let's go ahead and draw them right next to the ones that are important, which are We should draw what this one looks like. We should draw what this one looks like. And then we should draw these two over here. Right. Cool. Awesome. So what would the orbital's look like for the first dying? Right? Remember that? The way I always like to do it is I always put my shadings on the bottom to start off. So I would do this. Fill in, fill in, fill in, fill in, Cool. And for the next one, we know that. But if you guys have been practicing drawing like a little orbital's, you should know that there's gonna be that this one's not gonna move. This one's gonna go up. And then we're gonna have one note in the middle, which means that this gets shaded down and this gets shaded up. Cool. So this is what my homo A looks like, and this is what we're gonna be looking at to figure out symmetry. Okay, Now, let's look at home O B. And Lou, Moby home. Obi would just be this and Lou Moby would be this. So the two orbital's I'm gonna be looking at to determine symmetry are gonna be which ones I'm gonna be looking at this orbital here, this molecular orbital here. And I'm also going to be looking at this molecular orbital here. Okay? And as I go into the bottom part here where we're gonna be analyzing a little bit more closely these air the orbital's I'm gonna be bringing down. Okay, So let's go ahead and look at this little box that I've created to help us think about how distractions going to take place. So we know that the homo a comes from side to of my dying, and what it looks like is above Let's go ahead and bring that down and fill it in. So it looks like this Cool. We know that Lou Lou Moby from my double bond from Al Cane. Looks like this. Let's go ahead and bring that down here. Cool. Awesome, guys. So now we have to figure out we have to determine if this is gonna be symmetry allowed or if it's disallowed. If it's allowed, we give it a check. Mark, if it's disallowed toe, give it an X. Okay, so how can we tell? Okay. Well, guys, just so you know, a new single bond can Onley form when you have two of the same phase come together when you have two of the same phase lobes overlapping. That's when you conform a new single bond. Okay, so could we form a new single bond the way this is drawn? Yes, we could, because notice that the terminal ends of both of my home and limo they match notice that this terminal end looks the same as this one. Right? And notice that this terminal end looks the same as this one. Okay, this is what we call orbital symmetry where there's gonna be a possibility for a strong overlap between them because the fact that they can match up. So what that means is that you could form a new single bond here, and you could form a new single bond here. Cool. So this would be symmetry allowed because of the way they can overlap. Now, I do want to make a note for you guys later when we actually get to drawing reactions when you get to drawing the mechanism and maybe drawing stereo chemistry. Usually we don't draw it in this way, with the homo on top and the loom O on the bottom. Actually, most textbooks and professors do it the other way, with the homo on the top and the loom o on the bottom. But this doesn't have to do with it. Doesn't change your answer. It just it's just a different way of looking at it. Okay, later on. It actually is more convenient to do it that way, because then you can look at stereo chemistry better. But the reason I'm doing it here in this case is because I'm trying to show how the homo is, how the limo is higher and energy, and the homo is lower and energy on. I want to compare that, but later on, once we get into, like actually drawing thermal cycle editions, we'll probably start putting the home on the top and the loom o on the bottom so that we can draw the stair chemistry better. Okay, but just letting you know that for right now this works just great, since we're just talking with a theory right now. Cool. So then the question is, Is this the smallest home? A limo gap possible? And you might say, Johnny, what are you talking about? Well, because this is the thing. Notice that there are actually two Ho Mose and to Lou most on this molecule. I mean, in this reaction, there's Homo a, which is reacting with blue Moby, right? But there's also the possibility that Homo be could react with Lou Mo a. Right there's there's no reason. There's like there's no principle that saying that that can't happen. It's not impossible. It is possible that homo be could kick electrons in tow loom. Okay, okay. But the question is which one of the gaps is smaller is that is the gap for eight for a to be smaller or is be to a smaller and it turns out that if you were to measure it out, this is actually the smallest home. A limo gap. So then this would be the preferred the preferred mechanism that would take place. Okay, so, guys, So now we've just shown that this psycho edition is possible due to symmetry and due to similar energy levels between your homo on your limo in the next video, we're going to try to see how favored Homo be and limo A would be to react with each other.
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example

Determining Favorability of HOMOb to LUMOa

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Now, let's look at the favorability of going the other way around. Trying toe make a bonding interaction between Homo B and Lou Mo A. Okay, so this also means that we're gonna have to redraw our molecular orbital because they have changed. Homo B is pretty easy. We already drew this notice that it is this guy right here. So I'm just gonna bring that down home. A B would look like this quote now loom. Oh, hey, We didn't draw because limo is actually side three of the other molecule, so that means we have to draw it from scratch. Let's go ahead and draw it right here. One to three for so much room for improvement. That's not the best. So how do we draw this? Remember that for side three, this would continue to face down so it would be dark at the bottom. And remember that this gives my last orbital two chances to flip so it would have been down then on side to it flipped up and then on side three, it flips back down and then finally remember that I need how many nodes? Two notes because I need one increasing note with every M o. So that means that I would have a note here and a node here, meaning that I should have Orbital's the phases facing up in the middle. And that is my m o diagram for side three. This is what is interacting with homo be, and I'm gonna bring this down. Let's bring that down into this area. So what I have is down. Up, up, down. Awesome. Okay, so now we're ready to determine and to make some decisions about if this is gonna be favorable or non so. First of all, let's look at the symmetry of this reaction. Is this a symmetry allowed or a symmetry disallowed process. And guys, what I see is that actually, this is symmetry allowed again. Because what I have is that this one matches this one, right? So there could be a bond there, and then this one. Oops. I wanna use a different color. This terminal end matches this terminal end. Right? So, once again, the this is symmetry allowed bonds can form here. What I could do is I could form a bond here and a bond there. Cool. So we know that bonds could forms. This is symmetry allowed. Let's go to the next one. Is this the smallest Homo Lamo Gap possible? And what we see here is that I actually went ahead and I brought down the same exact distances from my original L CEO diagram into here. So notice that this is purposeful. The reason that the Homo and the loom Oh, look a little bit further apart in this one than they do on the first one is because they are further apart. If you actually look at them, they are drawn much further apart. And that's the way that it works. Usually there's gonna be one set that's closer. And once that that's further away. The set that's further away isn't gonna work as well. So this would get a big X in smallest home, a limo gap. Is it possible to make this one work? Yes, but then you would need to try to decrease, try to do different things to the molecules to try to decrease the gap between them so that the home, a limo or the bonding interaction would be favorable. So several of you if I wouldn't have gone through this exercise many of you would have probably thought, Well, Johnny, why couldn't just go the other way? And this is why? Because it's not just about being symmetry allowed. It's also about having the smallest Homo Luneau gap possible. So, guys, now you know how to find the favorable reaction of a cyclo edition. Remember that this is true for any conjugated molecules and be not just for a four member chain and a two member chain. It could be go up too many, many, uh, Adams in the conjugated system, and now you have the tools to figure out if it's favored or not. Okay, so we're done with this video. Let's move onto the next one.

Who's ready for some practice? See if you can predict the correct answer.

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Problem

Use FMOT to predict the product of following cycloaddition reaction.

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Problem

Use FMOT to predict the product of following cycloaddition reaction.

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