Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

Table of contents
16. Conjugated Systems

Orbital Diagram:6-atoms- 1,3,5-hexatriene

Conjugated polyenes (i.e. hexatriene) are famous for their ability to resonate. Here we will explain that resonance using molecular orbitals. 


Drawing MO Diagrams

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in this video, we're gonna learn how to draw the molecular orbital diagrams of six Adam conjugated systems usually hex a trying Let's go ahead and get started. So as we know, well, conjugated Pauline's are famous for the ability to resonate. And we can explain that residence using Molecular Orbital's. So the primary purpose of this page is to apply the rules of building molecular orbital to a six Adam system. It does get more complicated. We're going to think a little bit more about this, but we're doing it together, so it's all gonna be okay. So let's talk about the things that we do know, which is that there are six atomic orbital's six pie electrons and six molecular orbital's alright, and I've gone ahead and, as always, filled in the first one for you so that we know what we're dealing with. It's also the easiest one and then were responsible for the rest. Okay, so what do you think is the best place to start here? Let's go ahead and fill in the first orbital and the last orbital for all of these. So I'm gonna fill in first, first, first, 1st and 1st, then I'm going to start flipping the last one. Flip, flip, flip, flip and flip. Cool. And if you guys don't mind, I'm even going to do one more little cheat, which is I'm gonna fill in a size six already because we already know that size six is always just gonna be every single orbital facing different direction. So we can already feel that one in just kind of, by definition, we're used to this by now. Cool. So that means that all we really need to worry about is thes four in the middle for these for side two through five. Okay, let's also just remind ourselves how many nodes were supposed tohave. So it's supposed to start at zero, and then 1234 and five. We can also fill in the notes for five. We know that it's just gonna be between every single orbital. Cool. The hard part is the others. Okay, cool. So let's start off with side to I don't think this one's gonna be too challenging. Let's look at this together. We need to put in one node. Where do you think is the best place? Let me know what I'm getting closer. Yeah, right here, Right. So this is it. Let's put in our node and that's it. That's totally symmetrical. That's really want to do it. Okay, let's go to To What do you think we should do for two? It's site three, but for two notes, it's actually not that hard. This is pretty straightforward. It's just gonna be here and here, right? So just to to to that makes sense that that's not really that difficult. Okay, now we get to Sigh four, and psy four and five are tricky enough so that I think that it is appropriate for me to actually use on obscure rule that was in my molecular orbital rules. And remember that that rule said that if in doubt, if you don't know where to put your notes, what do you do? You draw fake orbital's at position zero and position end plus one, and then you try to draw a sine wave. I haven't done this yet. We've just been using kind of common sense and symmetry so far. But at this point, this is the thing about three. Um, you guys might be putting it in a lot of different places like, for example, someone might say that it should be here, here and here. I don't know. Maybe they wanna go down the nodes. Or or maybe someone thinks that they should go here, here and here, which doesn't look bad. Um, this is probably not a good pick. I would not pick that one, but But, for example, it might be difficult to determine. Is that appropriate? Right, So let's go ahead versus the other one. So let's go ahead and draw those orbital's and draw a sine wave and see what we get. So what that means is, by that rule, what I would do is I would actually draw a fake Adam over here and a fake Adam over here, and I would draw these fake basically fake a sine wave coming from the first one and the second. But I want to just add an orbital. Just that you guys know what I'm doing. In terms of that, this is just a pattern that we're gonna learn. Okay? Thes orbital's don't exist. It's just helping me to figure out where to put the notes. Okay, Well, once you draw that, and once you start to think use that as a guideline. What you start to realize is that if I were to draw a wave that were basically going back and forth, right, let's say that I were to do this. I'm terrible. Let's say we will do it to do it this way, where it's going through the orbital's. Okay, that kind of works. But notice that what I have is that the the peaks and the valleys of the sine wave aren't very symmetrical. Like what I have is that they're more compressed in this side and there longer over here. Okay, so this isn't the best way that I could do it. A better way would be to do it here through the middle, then here and through the middle, like this. If you do it this way, what you notice is that and I mean, it's hard because I'm drawing this really ugly. But it's actually a much better wave, like in terms of the symmetry of the wave. If this part at the end was toe connect to this part of the beginning, all of these ups and downs would look much more symmetrical. Okay, So what I'm trying to do is I'm trying to use this as a kind of a guide to say that if you were to put your note here and here, that wouldn't make the best sine wave overall. The better sine wave would be if you were to put it here and here. Okay, that's what I'm trying to show you guys. So just you guys know this is the answer. Just another word of caution. In case just in case you wind up getting confused this later. For an even numbered molecular orbital, you could never pass through an orbital. You would only pass through Orbital's on odd remembered Molecular Orbital's. So the the answer of going through these orbital's was never really a great option anyway. Okay, um, I'm just letting you know now that we've already drawn the sine wave. Okay, Cool. So then that ones that we already have our notes in place. Now we just have to go to side five. Let's do the same thing. Let's draw our fake Adams and our fake orbital's. And now we realize is that we need four notes. We need four notes, which, by the way, that means that every single riel orbital is going to get a node except for one. Okay, let's think about it this way. Notice that five notes. Means that every single lobes switch, switch, switch, switch, switch. Right. Notice that for four for four notes or science five, we're gonna have four notes that can't pass through any lobes. And two of them, two of them can't have a note between them, but the rest should. So how do we do that? Well, guys, what makes the most sense is if any two of them have to be together without a node, it should be the ones in the middle, because those are the only ones that you have a chance of making a sine wave with if you keep them together. And that means that there should be another note here. And another note here. Okay, Now, if we were to use R sine wave method, what we would see is that the sine wave method does make sense, because what I would have is this, then this, then this. Oops. And I have to go back here. Oh, God. Sorry, guys. My pen is Well, my handwriting is sucking cool. Awesome. So what? We get is a pretty good flow. It's not perfect because there's an asymmetry here, but it's pretty good. If you were to do something different, you would get uglier wave. So, for example, if you were to I mean, there really isn't even another way to do this. That would be symmetrical. Um, but if you were Thio, let's say move it over. You know, like, that's the whole point. I'm trying to use this as an illustration for how you can't have a symmetry. So if you were to try to move this over so that did this on this, right, Well, what you would then get is something really weird That looks like this boom, boom, boom, boom like that. And then you have a bunch of little ones together and then big ones on the sides. So we want to do is you want to draw our nodes in a way that preserves basically a science a sine wave, uh, look as much as possible. And once again, what that would be is keep the two ones in the middle, and then the two on the side. So then it would give you something like this. Okay? And that one. I drew it ugly again, but you can see that it's better than the other options. Okay, Cool. So now we have our nodes in place, and now we just need to actually draw the orbital's in. So here what we would have is one phase change in one phase. Change. Cool. Um, for two, we would have this this this this for three. We would have this this this groups wow, this and then for four notes, I'm going in order of notes. It would be this this this, this and this. And at this point, we can erase our fake orbital's because we don't need them anymore. I just want to point out guys that this explanation that I gave you is the non technical way to try to tell you give you instructions on how to draw molecular orbital's. There actually is math behind why this is correct and the other versions air wrong. But that math is so complicated that I don't want to overcome key things. You shouldn't have that much math and or go. I'm trying to give you a very basic version of how you can arrive to this without doing like insane quantum calculations. Okay, Andi, I wanna make another point, which is that I'm probably gonna get comments on this video saying, Hey, Johnny, I did it another way. And why isn't this right? Why isn't this other way? Right? And what I would just tell you guys is because it's not gonna make as nice of a sine function as the ones that I showed you. It's not gonna be as even or symmetrical. Any other combinations are gonna have a little bit more. I don't know. It's a little bit more asymmetry. Okay, so that's why this would be the most favored. Actually, the technical reason has to do in math, which we're not going to get to. But this is a nice, like kind of shortcut to get the right phases in case that you are asked to do this. Okay, cool. So we have all our faces in place, and now we just to fill in our emos, and that's easy. It's just gonna be sigh one site to and side three. Would we expect this to be a stable molecule? Yeah, because notice that all of the electrons can congregate in tow bonding, orbital so these air all bonding. So that's going to increase the level of stability, these air, all anti bonding. These don't have electrons. Thankfully, they should get stars because they are going to decrease the stability of the molecule. And there are no non bonding. Uh, orbital's here because the fact that there's nothing at the halfway point Finally we have to determine our Homo and Lou Mo orbital's. So we know that I'm just gonna use yellow to say that yellow is our homo And then I'm going to use green say that green is my loom. Oh, so side three is my home. Oh, and side four is my limo. Thanks for joining me, guys. I hope that this helped and let's move on to the next video.