For a closer look at 4-atom pi conjugated systems, we will use the structure of 1,3-butadiene.
Drawing MO Diagram for Dienes
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Hey, guys, In this video, we're gonna learn how to draw the molecular orbital diagrams for four Adam pie conjugated systems. So, guys, for Adam pie conjugated systems are usually in the form of dying. So two double bonds next to each other and just, you know, a dying can also be more generalized to be called a Pauline. So I'm gonna be referring anything that's conjugated by with four or more atomic orbital's as being some form of Pauline because that's usually the way that it works. Okay, and we know that Pauline's can resonate. OK, but what we're gonna do here is we're gonna learn how to draw the molecular orbital's for a dying for for Adam conjugated system using the rules of molecular orbital's that I've already showed you guys before. So nothing is gonna change. This is just a nap. Lick ation. Okay, um, so let's go ahead and do this example. Okay, So it says predict the L CEO model of 13 b two dying. Identify the homo and limo. Orbital's very cool. So let's go ahead and just start off with the basics here, which is that we have four atomic, orbital's and We have four a four electrons in those orbital's because we have two pi bonds that makes sense, right? And we know that, according to AFP, about principle, that means that two electrons are going to sit in Sai one and two electrons are going to sit inside to okay. What we might not remember is how to actually draw the molecular orbital. So let's go ahead and do that now. So let's what would be the next thing that we do with all of our orbital's already drawn for us, We just have to draw in the face is what's the next thing to do? Let's go ahead and shade the first orbital's so the first orbital should not change. I should do this This and this. Cool! Awesome! So that's our first step. The next step is let's go to the last orbital insert flipping it because you know that has to flip every time. So this one has to go up and then down and then up again. Cool, Awesome! And now we need to increase our nodes. Okay, so now we have to increase our nodes. So we have. We're starting off with zero notes here then it needs to be 12 and three. So the way we're going to do this is tryto add these notes in the most symmetrical way possible. So that means that I'm gonna put one note in the middle here, a note here, a note here for two and then finally, for three, it's just a node in between every single orbital cool. And that means that now I'm ready to shade the other orbital's in. And what that means is that I would get one phase change here, right? Because that's just the only note. Then I would get to face changes for this one. So I get the first phase change in the second phase change and then the last one I would get just get all of them are changing. Very cool. Awesome. So now we know what our basically what are molecular orbital's look like? We know how many electrons there are, and now we just have to indicate which ones are the Homo and blue mo orbital's okay, and what we would have first of all, what I have for this section here is just how maney pie electrons we have in each one So then this one would be, uh, too pie Electrons, two pi electrons, zero pie electrons and zero pie electrons. Okay, now, you don't have to do this every time. But since we're new at looking at home own loom Oh, I just want to make it very clear which of these have electrons won't really is don't. And we know that according to the ordering, it would only be up to side, too, right? So that means that my homo orbital or the highest occupied one, must be site, too. And that means that my loom Oh, orbital must be side three because that is the lowest energy one that doesn't have any electrons in it. Once again, these air collectively known as your frontier orbital's. You may not know what they do yet, but it's important that you're able to identify them. Cool. So that's it for this video. In the next video, I want to talk about a specific type of notation that's used for Dying's
Alternative MO Notation for Dienes
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So, guys. It turns out that specifically for four Adam conjugated systems for Dying's ah lot of textbooks and some professors will actually describe molecular Orbital's as the sum of pie orbital's so they'll think about it. As you know, you have four molecular, your molecular orbital's with four. Loeb with four orbital's and those orbital's are actually the sum of different combinations of two orbital's. Okay, this is Onley true for four orbital systems. In case you see it, I want you to know, First of all, that is not very important. So this type of notation doesn't supercede what I already taught you. You already know how to do for orbital systems or for atomic orbital systems. But I just wanted to go ahead and do an example with you so that in case you see it, you know what you're looking at. So this is the way that it works. So, basically, you know how usually we start with our dark lobes facing down on our first orbital. That's just the way that I like to do it. So that is usually, um if that's your starting point, then that would be called your positive pie. Because just the way that you started, Okay? If you were to then draw an anti bonding orbital from there, remember that the first one is to stay the same, but the second one is to flip right? So that would be called the Positive pie Star. Because it's like you started with a positive pie. But then you made it anti bonding. Okay, well, just just as correct, uh, of a starting point would be the negative pie. Negative pie just means that you flipped it the other way. That's totally find some people want to use the negative pies. They're starting point all the time. I just like to put the dark Loeb down. But if you're using the this one, this would be called negative pie. And then if you wanted to make the anti bonding version of it, the antibody would be negative. Pie star got it So far. Now we know that, according this is, this is just basically looking at two different methane molecules. We know that what would happen is that inside one, you would have two electrons here and two electrons here. Okay, so that's that's kind of like what I'm showing you so far. Now, when it comes to your size, let's go up and look again at this. What we want to do is figure out how can we describe these four molecular orbital's in terms of some of two pi molecular orbital's. Okay, So for example, notice that for Sy one, I have four Lopes all facing down, right? So that means that if I wanted to use this type of notation, what would be the right summation of these orbital's to make that thing And I'm gonna drop for you right now and hopefully you'll understand better. I will just draw it right here. So we have a little bit of room, sy. One should be equal to what it should be equal to positive pie. Plus, I should put in brackets positive pie, positive pi plus positive pie. Why? Because the way that I drew it had both lobes at the bottom and it was just two of those. So it was a positive pie, plus another one of those. So they're next to each other. That would be Sai Wan. Cool. Let's go ahead and do side to now. What is side to gonna be? Well, let's go ahead and look at it again for reference. So side to it. Looks like the first two didn't change. But the second one did. So then what this one would be if we look at it. If you look at all the different possibilities, it's actually gonna be positive pie again. Plus negative pipe. Right? Because in this case, what we're adding the second one that we're adding is the opposite configuration or the opposite phase. Okay, the opposite phase of my bonding orbital would be this one. So that means that what side to is is Actually, it's you're adding a positive plot pi plus a negative pie. Cool. Let's keep going side three. So side three notice that it appears to be to well, it appears to be to anti bonding, right, because notice that the first one has the face is different. The second one also has the faces different notes that the first one is starting off with the lobe down the first load down. The second one is starting off with the first Loeb up. So let's see what it would be. That means that it would be positive pie star right, because that's the first one plus negative pie star right, because if you add those together, don't you get signed? Three. It's just basically adding the top one there and the top one there, and you would get side three. And then finally, site four would be what it would be that everything's changing. It actually looks like it's the same thing twice, right? It's your anti bonding, um, one twice. So what that means is that it would just be Sigh Star Plus sighs, star, Why? I kept saying side I'm a pie. Positive? Yeah, I should have put a positive around it, so it would be positive. Positive, positive, positive pie star. Does this make sense? Cool. So don't be saying, Johnny, why are you showing me this like, why is this important? The truth of the matter is, it's not important at all. This just This is just a specific type of notation that certain textbooks and certain professors like to use to represent a four Adam conjugated system. Sometimes they might say, um, discuss Cy three in terms of pie, and then you'd have to actually do this. Um, but for the purposes of actually understanding molecular orbital theory, this doesn't provide any extra insight. It's just showing you all the different combinations off smaller molecular orbital's that could be used to make a bigger one. Okay, cool, guys. So I hope that this made sense. Unless you want to the next video.