Organic Chemistry

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17. Aromaticity


Four tests

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in this video, we're going to discuss the four distinct tests that let us know if a molecule is aromatic or not. So for molecules to be considered aromatic, it has to do more than just smell nice. These molecules actually did get their original names from their pleasant aromas. The smells of cinnamon and almonds actually come from aromatic molecules. But these days we need more than just a sniff. Tests to tell something's aromatic. We're gonna wanna use science, right? So what we're going to go through is the four distinct tests that help us defying these aromatic molecules. Really? Precisely. Okay. Now, as a side note, the's molecules, if they are aromatic, are gonna be called Hucles Rule compounds for reasons that will discuss later. So let's start off with the easiest rule to apply. Which is that for molecules to be aromatic, it has to be cyclic. So here I'm gonna be comparing and contrast ing and aromatic molecule toe one that doesn't meet the specific rule. So you guys can kind of see the difference. Here we have benzene, which you guys already know is an aromatic molecule that's on the left on the right notice that I have a chain. So I have a chain versus a ring. Okay. Now, the fact that venting is a ring doesn't by itself make it aromatic. But it does make it fulfill the first test. So this one passes the first test, and this one fails the first test, right? Because it's not a ranked perfect. That's easy. So let's move on to the second one. The second one is fully conjugated. Okay, guys. So this idea of fully conjugated actually comes from prior lessons Now are clutch videos cover the concept of conjugation. So if you want to brush up in this idea of conjugation, feel free to go into your search bar and type conjugation and the videos should pop up, all right. They could also be located in the textbook that the video textbook that you're using. Okay. So, fully conjugated. All it means is that all Adams on? This is a big word. Perimeter of ring must resonate. Okay. You didn't realize that fully conjugated meant so much stuff. Okay, so let's break this down. Okay? All atoms on the perimeter of the ring now, what's the perimeter? The perimeter will be the outside. So in this case, in the case of benzene, it would just be all six atoms. But in the case of larger rings, ones that have maybe Adams within the ring, you only need the very outside circle to be able to resonate. You don't need the atoms in the middle to resonate. That's why the word perimeter is important and they must be able to resonate. And for you to know what type of Adam can resonate, go back to the residents or conjugation area. But just as a quick reminder, it's pi bonds. It's orbital's that can participate in residents like cat ions and ions. Lone pairs. Okay, so as we can see, we have two rings now. They both passed the ring, the cyclic test. But we have an issue, whereas my benzene has three double ones back to back, so every single Adam here can resonate. OK, every single atom can participate in residence. But what's wrong with the dying guys? This dying is what we call in isolated dying. Do you remember what isolated dying means? It specifically means that it can't resonates. OK, there's a dying that does not get stabilized through conjugation Why? Because it has s p three hybridized carbons in the middle that do not have any available orbital's to resonate. So this that will bond is stuck. This double bonus stuck. There is no congregation here. This would not be an aromatic molecule because it fails the conjugation test. Okay, so it's not just being a ring. You also need to be a fully conjugated ring. Bueno. Good. So far, let's move on to the third test. It takes more than those two. We need some other stuff. So plainer guys, plainer just means it needs to be flat, right? Okay, Now, plainer is it's own topic. We're going to be discussing Maurin depth on how to figure out all of these different types of parameters. Right? But for right now, you can assume that any ring will be plainer unless shown otherwise. Okay, so here notice that now, in this example, I have two molecules that are both cyclic and they're both fully conjugated. See, they both passed the first two tests, but we've got an issue. The first one we're gonna assume to be plainer because I have no reason to believe otherwise. Whereas on the second one. This one is gonna have some trouble being planner. Why? Because notice that these two hydrogen are both faced in the same exact direction. That means that these hydrogen, they're gonna have interactions as they hit each other on their gonna bend the ring downwards. Okay, They're gonna make it kind of convex. So this one would fail the plane or test. Now, you might be wondering, Johnny, how do I know that a ring is plain or not? They're actually, There are ways to know that will go in later into later. But for right now, we're gonna assume that a ring is plainer unless we're given reason to believe it's not. For example, this one was drawn in a peculiar way. So I would think those hydrogen zehr gonna interact with each other and make it not plainer. Okay, Now, every wondering why is this important? Because in order for all of the orbital's to resonate, they have toe all be lined up correctly. So I'm just gonna draw kind of a three d version of benzene on. What we see is that Ben's ing, because it is plainer. All of these orbital's can nicely arranged and electrons can easily flow throughout that basically that what we call a pie electron conjugated framework. Okay, but we notice is that if we take a molecule that is bending out of shape, let's say I'm not going to draw this perfectly. But let's say it with something like this, you have orbital's facing in different directions. Okay, Thes orbital's that all faced, um, in wrong directions will not be able to congregate with each other. They're not gonna be able to resonate appropriately. So that's why you would find that this one is not aromatic. All right, so now we're finally at the fourth rule. The fourth rule is the most technical of the three. And that is Hucles Rule or four end plus two. Number of pie electrons. Okay, so, guys, this is a skill of itself, and we're gonna address this Ah, whole concept in a different video. I'm gonna teach you how to count pie electrons. I'm gonna teach you a little bit more about what Hucles rule means. I'm even gonna teach you why this is important. But for right now, just take my word for it. That this molecule does have four end plus two, right? And this molecule, even though this cycle beauty, beauty, dying, that's what it's called even though it is cyclic, even though it is fully conjugated. And even though it is plainer, I have no reason to believe it's not plainer. Just take my word for it that this is not foreign plus two. And what means what does that mean? That means that even though this guy was so excited to be aromatic, guess what? He is not gonna be aromatic. He's actually gonna be pretty surprised, Pretty unhappy when he finds out what he is. Feel sorry for him. But anyway, these are the four tests of air Metis City. Never forget them. These air. This is the foundation of the entire concept of their mentis city, which is gonna be very important this semester. So now let's talk about what it means to pass the tests and what it means to fail them. If you pass all the tests okay, then you're considered aromatic. That's what I just told you guys at the beginning of this page. But how about if you fail one or more of the tests? Okay, so what does it mean to fail the test. Failing a test could be I'm not plainer. It could be. I'm not sick like Johnny. It could be Johnny. I am not fully conjugated. Those would all be failures of Hucles rule or of these four tests. So we would consider them non aromatic. Okay, now we've got a special case, Any compound that meets all four of these conditions. But it has four n pi electrons instead of four and plus two. I'll teach you how to count that later. That's gonna be considered anti aromatic. That one really sucks. Okay, so the only way I could be anti aromatic is if I pass all of the tests. But I have four N instead of foreign plus two number of pie electrons. So if something is not cyclic, can it be anti aromatic? No. We need to pass all the tests in order to be anti dramatic. We just need to have the wrong number of pie electrons. Okay, now something. As a side note, guys, anti aromatic compounds are said to instead of be Hucles rule compounds Member Hucles rule was aromatic. Bresse lows Rule is the rule that we use for anti aromatic So if you ever see the term breast lows rule, that means that it's following the four tests for anti Aromatic, which would be cyclic, fully conjugated, plainer everything the same but for and number of pie electrons instead of foreign plus two. That would be breast lows rule. Alright, guys. So hopefully that makes sense. Now we're gonna dive more into what it means to count a pie electron, because that's really important. So let's go ahead and take a look at some next videos.