Four Tests of Aromaticity

Concept: Concept: Four tests

11m
Video Transcript

In this video, we're going to discuss the four distinct tests that let us know if a molecule is aromatic or not. For a molecule 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 test to tell if something is aromatic. We're going to want to use science.
What we're going to go through is the four distinct tests that help us to find these aromatic molecules really precisely. As a side note, these molecules if they are aromatic are going to be called Huckel’s Rule compounds for reasons that we’ll discuss later.
Let's start off with the easiest rule to apply which is that for a molecule to be aromatic, it has to be cyclic. Here I’m going to be comparing and contrasting an aromatic molecule to 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. I have a chain versus a ring. The fact that benzene is a ring doesn’t by itself make it aromatic. But it does make it fulfill the first test. This one passes the first test and this one fails the first test because it's not a ring. Perfect. That's easy.
Let’s move on to the second one. The second one is fully conjugated. This idea of fully conjugated actually comes from prior lessons. Our Clutch videos cover the concepts of conjugation. 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. They can also be located in the video textbook that you're using.
Fully conjugated, all it means is that all atoms on perimeter of ring must resonate. You didn't realize that fully conjugated meant so much stuff. Let's break this down. All atoms on the perimeter of the ring. What’s the perimeter? The perimeter will be the outside. 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 atoms 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. They must be able to resonate. For you to know what type of atom can resonate, go back to the resonance or conjugation area. But just as a quick reminder, it’s pi bonds. It’s orbitals that can participate in resonance like cations, anions, lone pairs.
As we can see, we have two rings now. They both passed the cyclic test, but we have an issue. Whereas my benzene has three double bonds back to back, so every single atom here can resonate. Every single atom can participate in resonance. But what's wrong with the diene? This diene is what we call an isolated diene.
Do you remember what isolated diene means? It specifically means that it can’t resonate. These are diene that does not get stabilized through conjugation. Why? Because it has sp3 hybridized carbons in the middle that do not have any available orbitals to resonate. This double bond is stuck. This double bond is stuck. There is no conjugation here. This would not be an aromatic molecule because it fails the conjugation test. 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.
Planar. Planar just means it needs to be flat. Planar is its own topic. We're going to be discussing more in depth on how to figure out all of these different types of parameters. But for right now, you can assume that any ring will be planar unless shown otherwise. Here, notice that now in this example, I have two molecules that are both cyclic and they're both fully conjugated. They both passed the first two tests.
But we've got an issue. The first one we're going to assume to be planar because I have no reason to believe otherwise. Whereas on the second one, this one is going to have some trouble being planar. Why? Because notice that these two hydrogens are both faced in the same exact direction. That means that these hydrogens are going to have interactions as they hit each other and they're going to bend the ring downwards. They're going to make it kind of convex. This one would fail the planar test.
You might be wondering, “Johnny, how do I know that a ring is planar or not?” There are ways to know that we’ll go in later. But for right now, we're going to assume that a ring is planar unless we’re given reason to believe it's not. For example, this one was drawn in a peculiar way. I would think those hydrogens are going to interact with each other and make it not planar.
If you’re wondering why is this important, because in order for all of the orbitals to resonate, they have to all be lined up correctly. I’m just going to draw a 3D version of benzene. What we see is that benzene because it is planar, all of these orbitals can nicely arrange and electrons can easily flow throughout that basically what we call a pi electron conjugated framework.
But what we notice is that if we take a molecule that is bending out of shape, I'm not going to draw this perfectly but let's say it was something like this. You have orbitals facing in different directions. These orbitals that are all faced in wrong directions will not be able to conjugate with each other. They're not going to be able to resonate appropriately. That's why you would find that this one is not aromatic.
Now we're finally at the fourth rule. The fourth rule is the most technical of the three. That is Huckel’s Rule or 4n+2 number of pi electrons. This is a skill of itself. We're going to address this whole concept in a different video. I'm going to teach you how to count pi electrons. I’m going to teach you more about what Huckel’s Rule means. I'm even going to teach you why this is important.
But for right now, just take my word for it that this molecule does have 4n+2. This molecule even though this cyclobutadine, that's what it's called, even though it is cyclic, even though it is fully conjugated, and even though it is planar. I have no reason to believe it’s not planar. Just take my word for it that this is not 4n+2.
What does that mean? That means that even though this guy was so excited to be aromatic, he is not going to be aromatic. He's actually going to be pretty surprised, pretty unhappy when he finds out what he is. I feel sorry for him. But anyway, these are the four tests of aromaticity. Never forget them. This is the foundation of the entire contents of aromaticity which is going to be very important this semester.
Now let’s talk about what it means to pass the test and what it means to fail them. If you passed all the tests, 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? What does it mean to fail a test? Failing a test could be ‘I'm not planar’. It could be ‘I'm not cyclic, Johnny.’ It could be ‘Johnny, I am not fully conjugated.’ Those would all be failures of Huckel’s Rule or of these four tests. We would consider them non-aromatic.
Now we've got a special case. Any compound that meets all four these conditions but it has 4n pi electrons instead of 4n+2. I’ll teach you how to count that later. That's going to be considered anti-aromatic. That one really sucks. The only way I can be anti-aromatic is if I pass all of the tests but I have 4n instead of 4n+2 number of pi electrons.
If something is not cyclic, can it be anti-aromatic? No. We need to pass all the tests in order to be anti-aromatic. We just need to have the wrong number of pi electrons.
As a side note, anti-aromatic compounds are said to instead of be Huckel’s Rule compounds, remember Huckel’s Rule was aromatic. Breslow’s Rule is the rule that we use for anti-aromatic. If you ever see the term Breslow’s Rule, that means that it’s following the four tests for anti-aromatic which would be cyclic, fully conjugated, planar, everything the same but 4n number of pi electrons instead of 4n+2. That would be Breslow’s Rule.
Alright guys. So hopefully that makes sense. We’re going to dive more into what it means to count a pi electron because that's really important. Let's go ahead and take a look at some next videos.