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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:3-atoms- Allylic Ions

Let's see what makes 3-atom conjugated systems unique from a molecular orbital perspective. 


Orbital Diagram

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Hey, guys, in this video, I'm gonna show you how to draw molecular orbital diagrams for three Adam conjugated systems. Let's get started. So, guys, because ah, three Adam conjugated system is an odd number of atoms. What we're usually gonna end up dealing with is in a Lilic position. Remember that the a little position, or in a little position means it's the position next to a double bond. And that's usually the case when you have an odd number of atoms, because many times it's that there's a dull bond. And then there's something next to the dull bond that's like the third Adam. Okay, now what we know about a little positions from previous lessons is that a little positions air able to resonate. Remember that the Olympic position can resonate from one side of the Olympic of the Dole bond to the other. Okay, so it turns out that that whole idea of resonating in a little position can actually be explained through Molecular Orbital's. You already know how to explain it. Using resonance, you should be able to draw resonance arrows and be able to show how the lily position can move from one side to the next. But it actually turns out that the the fundamental explanation off until Ilic position reaction can actually be explained through molecular orbital theories. That's what we're gonna do right here. I'm going to show you guys how molecular orbital's explain the reactive positions of analytic ion. So here we go. It says simplified l CEO model of a pro Pinel ion. And he would have drawn is a General Prepon ill ion. Basically, we've got our three Adam, um, conjugated system. So we have three atomic orbital's let's just say a B and C right. And what we see is that we have a double bond that we know that the double bond is gonna contribute one pie electron for each atom. So we have 12 and then we have this unknown. I honor this unknown non bonding orbital in position, see, and that I've labeled with a question mark, and the reason is because what I'm about to explain to you applies to any eye on, no matter what. The identity of that question mark is whether it's empty, whether it's an empty orbital or whether it's a radical or whether it's a lone pair it doesn't matter. What I'm about to teach you applies in all those situations. And what the molecular orbital theory would say about this molecule is that how would you? How would you structure the molecular orbital's Well? It says that you would have, first of all, a molecular orbital with zero nodes, one note in two notes, and that we would put those pie electrons in order of softball principle. So we would put the two electrons from that first double bond into sy one right, which is the most stable, the bonding orbital. And then what we would do is we would put whatever is left over whatever is here, whether it's zero electrons, one electron or two electrons they would go into side to. Does that make sense? So far, no matter what the question mark is, we know what's going into side to now what is unique about side, too. Look a Tsai to notice that side to actually has a node at Adam B. If this is Adam a Adam B and Adam see like we had before, we had a B and C right now is that there's a noted Adam B. What does that mean? What that means is that regardless of the identity of the ion, it cannot react at position. Be it can Onley react at either position a or a position See. But because no electrons ever pass through, I don't be. You will never find the little position reacting there. So what this does is it explains the theory behind the resonance structures that we've learned how to draw for so long. Remember that in a Lilic position, ion can resonate to the other side of the bond, but it can never resonate to the middle. You've never seen that. For example, this. Let's say this is a positive charge. You can't move it to the middle. You can Onley switch places with the Taliban, but you can't move it to the middle. And the reason is because the molecular orbital theory states that there's no way that I am can react at the middle carbon because that orbital doesn't even have any electrons. It can Onley react at the orbital's with electrons, which would either be orbital see in this case or are brittle A in this case, but never be. Isn't that so cool? So we're going to do next is an example

Explaining Resonance through MO Theory

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use both resonance theory and m o theory to predict the reactive sites of the following Radical. So, guys, first of all, let's do the resonance one, because that's the one that we should know the best. You should already know this from prior lessons. So how does a radical resonate, remember? Does it resonate with one arrow, two arrows or three arrows? 33 of the fishhook arrows. So we would expect is that you have a resident structure. Looks like this fishhook, fishhook, fishhook Does that ring a bell? So let's go ahead and draw our resident structure. And what we would expect is that now I would get a new double bond here, and a new radical here makes sense. Cool. We would put this in brackets and then using resonance theory, which would be the reactive sites of that radical. Where would be the place is that the radical could react. It would be here and here. Okay, those are the two positions that can react as a radical. So if we were to call this Adam a Adam B and Adam C of the pi conjugated system, right of the conjugated system, what we would say is that resonance theory? I'll just say resonance predicts, um, radical character at Positions A and C. Right? Resonance predicts, uh, there's an s there. If you can't see it, predicts radical character at positions and see those are the positions that could react as radicals. Now let's go ahead and actually draw out the M o diagram and see if the M O diagram also predicts it. So, first off, how many atomic Orbital's do we have? Three. Right, Because we have three atoms that air conjugated. That means that I'm going to draw three energy states. So this is gonna be one to And three. Let me try toe. Make it a little more even. There we go. Okay, so we know this is gonna be side one site, too. Side three. And how maney, How many atoms? I'm sorry. How many electrons total are there? There are three. There's one electron from the radical A. And then there's one pie electron from B, and there's one pie electron from C. So one of these is a radical because it's not bound. It's not, Um, it's not resonating with another loan electrons. So it's called a radical but the other ones are called pie electrons because they're inside of a pi bon cool. Awesome. So we have those three single electrons. Now we have to draw our energy states. So I'm gonna try to do this quickly. And some of you may already have these memorized by now because we've done this a few times. But if not, that's fine. We're going to go through the rules again. So how do we predict where our lobes are? The phases? So remember the first one. Let's just draw them all with the dark facing down. Remember that your next rule says that you would then draw your first one the same in every single energy state. Remember that. The next rule says that you would flip the other one every time, flip, flip, and then we would increase the number of nodes. So we have zero nodes on the first one, and then one note on the second meaning that I should put a note right down the middle. And then for the last one, I should put a note here and a note here, which would be to lastly, we have one note that needs to be deleted. I'm sorry. One orbital is deleted because a node passes through it And there you go. Those are That's our M O diagram. Now we just fill it in with electrons. And what we would do is according to AFP, about principle. Start with Sy one and then Paul Exclusion says you can only put too. So we're gonna have to put another one up here. So what does this M o diagram tell us? What it tells us is that the radical is on Lee going to react in two places and not three. It's on Lee going to react at Orbital A and it orbital, see, But it cannot react orbital be because orbital B is a node. So by definition, mathematically, no electrons can react at that position because it can't have any electrons. Isn't that cool? So let's just go ahead and talk about M o theory. Actually, I'm gonna take myself out of the screen so that we have a little more space to right. So then what? We're gonna say m o theory. Theo re predicts that the radical can on Lee react at positions A and C, but not B. I'm just being extra clear there because that is a unique. That's like a unique learning that you see once you draw the molecular orbital diagram and really guys, when it comes down to it. Molecular orbital theory is the underlying theory that that provides us with the knowledge of reactivity, of lots of reactions. Resonance was one way to, um, to kind of it's a shorthand to draw it, but really, the truth theory lies in the M 03 So it's really cool that I'm getting to show this and getting to share this with you guys and hopefully help you understand it. Cool. So that's it for this video. Let's we want to the next one.

Consider the MO's of following allyl cation. Which of the following are HOMO and LUMO?