Organic Chemistry

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16. Conjugated Systems

Thermal Electrocyclic Reactions

Thermal Electrocyclic reactions are pericyclic reactions in which 1 pi bond is destroyed after a heat-catalyzed cyclic mechanism.



MO Theory of Thermal Electrocyclics

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Now it's time to learn about a new type of Paris cyclic reaction called thermal electricity. Click reactions. So, guys, thermal electric cyclic reactions are just gonna be Perry cyclic reactions in which one pie bond is destroyed. And that's because any electricity click reaction is going to involve one pipe on changing from the reacting to the products. And that happens through a heat activated cyclic mechanism because we know that it's thermal, so it's gonna be he activated. Cool. So what do we need to know about this reaction? Well, one thing that you should keep in mind is that this reaction is always intra molecular, so it's not gonna involve two different molecules. It's just gonna be one molecule reacting with itself in a cyclic mechanism to make a new ring. Okay, it's usually a ring forming reaction and notice that what would happen here is that we would start off in this specific example. We're starting off with three pipe bonds in the reacted, and then after heat is applied, we end up with two pi bonds. So this just goes ahead and it verifies that this is electricity because remember that in electricity, click reaction you're always going to lose one pie bond, okay? Or one pilot is going to change from the reaction to the products. Cool. Also noticed that here we're applying heat. Now, I do want to quickly go over the mechanism really quick and just show you guys what would happen. So what would happen is that it's concerted and cyclic, so it's all gonna happen at the same time. You don't really need to know where to start or end from. But we do need to know is that we need to make that new single bond, right? So what? The way I would start it is I would take the electrons from this double bond and make a new single bond. And then that means that this needs to break and put its electrons there. Which means that this one needs to break and push its electrons there. And there you have it. You would have your new ring. And now we're missing one pipe on. Okay. So, guys, it turns out that all conjugated Pauline's are capable of doing these intro molecular electricity click reactions. So it's not like a specific type that's possible. This could happen with any. Um, Pauline, however many pi bonds long, it could happen. However, the the stereo chemistry is variable, meaning that you're not always going to get the same exact product. Many times you can immediately predict what ring you're going to get, but to know where the substitutes are gonna be, we need to think one level deeper, and we're gonna have to think about frontier molecular orbital theory, right? We're gonna need to think about Homo and limo Orbital's and figure out how that plays into this. So how do you determine the stereo chemistry of the product? The way you do it is by looking at the homo orbital of the molecule. Okay. And it turns out that the Homo orbital is capable of cycle izing in one of two ways, either in a con road to Tory direction. Okay. Ah, Conroe auditory direction or in a diss roto Torrey direction. Okay, so when we look at electricity, click reactions, we're gonna be looking at intra molecular reactions, and we're gonna try to figure out Did this molecule did the molecular orbital have to rotate, uh, Conroe territory or disrobe hitori in order to form a new signal bond. And I know I just kind of did some hand motions, but here, I'm going to show you exactly what I mean. I have some examples drawn out for you already. So to make a new single bond, we're gonna need two of the same phase lobes. Toe overlap. Okay, So what that means is that if we're trying to make a new bond, let's say with this, too. With this four pi system, this four pi conjugated system here that I have on the left, what I would need is, I would need like, for example, the positive here toe overlap with the positive here. Okay, Notice that they're on opposite sides, though. So the only way that can happen is if both orbital's are rotating Conroe territory. So what that means is that this one's rotating to the right, and this one is also rotating to the right. Okay, because if they both rotate the same direction, what's gonna end up happening is that the positives are gonna overlap. Does that make sense? That's what Conroe Totori means that you're It's kind of like they're rotating the same direction. But what's actually happening is that you're bringing to opposites together. Okay, so orbital's rotate the same direction. That's Conroe Torrey. But what about a six pie system or three double bond system? If we were to try to make a ring out of this one, which is actually the example that we had at the top that would actually be an example of disrobed territory? Because in that case, when you draw the Homo orbital, what you wind up finding is that your orbital's are perfectly symmetrical on both sides of the terminal ends. So if you rotate Conroe territory, they're actually gonna get the opposites interacting. So disrobe Natori rotation happens when they rotate in opposite directions. One is clockwise, and one is counter clockwise to create that same type of overlap that would lead to a single bond. Okay, now, why is this important? It sounds like no matter what, we can get an electricity click reaction toe happen thermal. No matter what, it's either going to rotate Convent Torrey or destroyed Torrey. But what matters is the substitue INTs, because if you have any substitue, it's located on the terminal ends. Let's say here or here, let's say that I put in alcohol here right. How do you know if that alcohol is gonna face up or down after the new mechanism has taken place? Well, that's why you have toe look at the rotation type. The only way you can predict it is to know the rotation type. So the focus of our electricity click problems is not actually gonna be on forming the ring because that's the easy part. We're always going to assume that the rink and form the focus is going to be on the stereo chemistry. Because if you can predict the stereo chemistry accurately, that means you understand the molecular orbital, um, rotation that's occurring. Okay, so in the next video, we're going to do an example, including stereo chemistry.
  • Always intramolecular


Predicting Electrocyclic Products

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predict the product in the following electricity click reaction label the reaction as either Conroe, auditory or disrobe. Natori. So, guys, I just want to, um, emphasize here that we know a reaction is going to take place, no matter what. We know that we're going to get a mechanism. It looks something like this where Let's say this double bond makes a single bond here, and then this double bond goes here. So we're going to expect to get is some kind of square, right? With a double bond here and with methyl groups here. Does that make sense so far? Because we're making a new single bond here. But what is the stereo chemistry of these metal groups? Are they sis? Are they Trans? You can't just draw this and get the right answer. We have to actually provide the stereo chemistry, the whole the whole product, not just ignoring stereo chemistry. So to figure out if these were facing up or down, we're gonna have to look at the molecular orbital's. So where do you guys think is a good place to start? We're starting from scratch here. What do you think we should do first? So probably a good place to start is let's start drawing our molecular orbital's of a dying. So that means I have to draw four of four, right? A grid of 44 So one, 23 for it's not bad. I'm gonna use my copy paste feature, and you guys can always pause the video if you need to. Cool. Awesome. So now we can go ahead and draw in our molecular orbital names so we know this is side one side, too. Site three and side four and then we can shade these in. You guys should be pretty good at shading in Dying's by now. Why don't we just do this very quickly? It would be that all the bottom is filled. It would be that there's one note on the second one. So then this is what it looks like because there's one note in the last one has to flip for three and four. The first one was in change for three and four. The back when keeps flipping xanthan, this would go down and then this would go up cool. And then finally we need to notes and three notes. That means that these two go up here and that. Then this one goes up and down. Cool. So those are emos. We should then fill in with the number of pilot Trump's we have, which is for cool. Awesome. So now what do we do? What's the next step? This was important. We did need to do this. But what's the next step, guys? We need to analyze the homo. So remember that we learned in frontier molecular orbital theory. We learned about homo and limo. When you learn how to identify both, well, it turns out that we don't need to worry about the loom. Oh, for electricity, electricity. Onley focuses on the Homo because it's just the homo interacting with itself. So that's the good thing that the in terms of the frontier orbital's, there's less to worry about. It's just the homo. So the homo happens to be this orbital here, and this is the one we have to figure out if it's gonna be Conroe Totori or destroy auditory. Okay, Now, the way that we tend that we typically do this is to actually try to do it in three D so we can figure out what it's gonna look at, like at the end. So what I'm going to try to do is I'm going to try to draw it like this with the orbital's facing up so we can see how they interact with each other. Okay, something like that. So let's go ahead and draw in our orbital's. We're gonna draw one to three for. Does that make sense? So what I'm trying to do is I'm trying to actually keep it similar to that one. But now I'm going to be involving what we know about these Orbital's here. Let's go ahead and shade them in. Let's say that this side right here is the beginning of my M o on the other side. So that means that what it would look like is shade the bottom shade the bottom, shade the top and shape the top. Cool. Awesome. Now, don't forget we have our substitue ints that we need to include. So where are our substitue? It's going well, they should go on the inside. So what that means is that I'm gonna draw these as metals. I should draw a substitute going in here and a substitute going in here. Does that make sense? so far, this has to do with these metal groups right here. And this is how we're going to determine the stereo chemistry. Awesome guys. So now we have to determine if it's gonna be convert a Torrey or Disrobe Hitori. What do you guys think? So it should actually be Conroe Totori, right? Because notice that my lobes are on opposite ends. So what that means is they both have to rotate the same direction. That means this one. Let's say it has to rotate clockwise. This one also needs to rotate clockwise. Or if you picked counterclockwise, they should both rotate counterclockwise. So what that means for my product is that what it's actually gonna look like is like this? So I have my let's try to throw this in three D still in three dimensions. So I have my new square, my new cyclo Boutin. I know that I'm gonna for mental bond here, and then we have to look at with this rotation. Where would these groups go? Where would the metal groups go? So what I would see is that let's start off with the first one. The first one at the bottom. It was it's going into the page, but after I rotate it, it's gonna rotate down, right? It's gonna go. It's gonna go down because of the fact that, um, that thing is rotating clockwise the way that I drew it, it's rotating clockwise, so I mean, that's gonna rotate down. So I would expect this muscle group to go down like this. I hope that's making sense so far. Now the other one, that Z facing this way that's actually coming out of the page. In fact, one way to right. This could be that you would write this one into the page and this one out of the page because one is going into the page was going out of the page, right? So then that one, when it rotates, it's actually gonna face up right, because it's roading, rotating clockwise. Then this metal group would go up, which means that my product is actually gonna be a trans dime Ethel where this one faces up and this one faces down. Now, guys, it turns out that this is actually in an anti more. This is not a miso compound, so we should actually draw the other an anti Maura's well, meaning that we actually get to products for this reaction, we get the two different trans products that are possible, which is this one and this one. And you might be saying, Well, Johnny, how would you get the other one? That would be if you had rotated counter clockwise, then you would get the other one. Cool. Awesome, guys. So I guess now we just have toe label it. And the rotation turned out to be, um, con wrote territory. Cool. So, guys, we did it. Great job. We got our products. Just, you know, it's not always going to be two products. If they had been facing the same direction, that would then be a miso compound because as a plane of symmetry, and then you just get one product. But since there was a symmetry here, we had to draw both products as our answer. So remember, guys, when you're drawing electricity, click, you can't just draw them on the plane. You need to include stare chemistry, because that's really what your professor, what your homework is going to care about making the ring is the easy part. The hard part is using Homo loom Oh, frontier Orbital's to figure out the stereo chemistry. Great job lets him once the next video