Annulene - Online Tutor, Practice Problems & Exam Prep

Annulenes, a type of polyolefin, consist of a single ring with alternating single and double bonds, similar to benzene, which is known as 6-annulene. Larger annulenes, like 8-annulene (cyclooctatetraene), can adopt non-planar shapes to avoid antiaromaticity. The aromatic stability is influenced by the number of π electrons, following Hückel's rule. For rings with 9 or fewer atoms, planarity is favored for aromaticity, while rings with 10 or more atoms may become non-aromatic due to strain. Understanding these concepts is crucial for predicting molecular behavior in organic chemistry.

[6]annulene vs. [8]annulene

Hey guys. Now we're going to focus on a specific type of ring called an annulene. So annulenes or polyolefins, as they're sometimes called, need to be only 1 ring and you need to have alternating single bonds and double bonds like you would find in benzene. Okay? Now due to their simple structure, due to the fact that you can always predict that it's going to be a single bond, double bond, and it's going to alternate, the names of these annulenes can be simplified to just the number of carbons in the ring and then putting it around a bracket and then annulene. Actually, if you think about it, a benzene is a type of annulene. Right? So benzene can also be simplified to the name 6-annulene which is pretty cool. So if you're just walking around campus and you see someone with a clutch shirt on and it has a benzene, you can be like, that's a mighty fine 6-annulene you have there. And they're from Clutch so they're going to know what you're talking about. They're going to give you a fist bump right in the middle of the student union. You guys are going to be awesome.

Anyway, the point being that these annulenes can be summarized by the number of carbons and as you see, I have two different annulenes here. I have 6-annulene. I have 8-annulene. Now here's the deal. Remember that rule I told you guys about planarity? And I said, you know what? You can pretty much just assume that every molecule is going to be planar unless it's drawn really weird. Well, that rule is still going to apply except not to annulenes. Annulenes are the one exception. Why? Because annulenes can get very, very large. Imagine just putting like a 12 or a 14 or a 20 in front of the annulene. You're going to get this massive ring. And the thing about large rings is that those bonds get wobbly. They can start to bend, they can start to twist and they can start to go in directions that will not form a planar ring. Okay? So this can present a little bit of a problem to us. We, college students that don't really know if something's going to be planar or not. Okay? We're going to have to memorize some specific trends to be able to predict if something's going to be planar or not.

Now just a note of caution here. This is not something that most professors are going to ask you to know. 9 out of 10 professors are going to just brush over this subject and say, you know, pretty much assume that it's planar unless I tell you. The reason being that in order to really tell if a molecule is going to be planar or not, you have to use x-ray crystallography to measure the bond lengths and that is not something a professor wants to go into during a college organic chemistry class. Alright. So I'm going to teach you these rules just to be comprehensive but I want you to keep in mind that you might not have to use this on the test at all.

Here we go. I just want to show you guys the difference between 6-annulene and 8-annulene. 6-annulene or benzene is too small to fold or anything, so it's just going to be planar. Whereas 8-annulene would normally be what type of aromaticity? Anti-aromatic. This is an anti-aromatic molecule if it's drawn planar, right? But what annulene actually does because it hates being anti-aromatic is it folds up. On Wikipedia, it calls it like a tube shape. I call it like a taco shape because I'm really hungry. It kind of looks like a taco and you can like put some mystery meat in there and stuff. And the problem with that or actually the benefit of that is that these orbitals end up not facing the same direction. And what did I tell you guys happens if your orbitals face different directions? Non-aromatic state. In a non-aromatic state. Isn't that crazy? What you're supposed to do? You're supposed to memorize that this molecule actually does not look like a planar structure. It looks non-planar or non-aromatic. Now what I'm going to do in the next video is I'm going to teach you exactly what those rules are. Again, remember that you may not even need to use this on your exam but I'm just going to teach you in case you're curious or in case your professor is really stressing this in class. So let's go on and learn those rules.

Rules for Predicting Planarity

I'm going to teach you guys this rule through a really interesting example that might actually come up in your homework. So 8 annulene is also called cyclooctatetraene or cot for short. That's what I like to call it. I like to call it cot, cyclooctatetraene. Okay? And remember that this is your taco molecule, right? This is the molecule that hates being anti-aromatic, so it folds on itself so that it becomes non-aromatic. Well, here's the crazy thing. What happens if I ionize it to give it a net charge of 2 negative? What if I add 2 negative charges to the cyclooctatetraene?

Well, it turns out, first of all, what's that called? That would be called an eight annulene dianion. As you can see here, it has 2 negative charges. What would be the number of pi electrons in this molecule now? Let's count them up. It would be 2, 4, 6 but now wait, anions count as 2 so that would be 8, 10, right? So you have 10 pi electrons. Is that a good number or a bad number? That's a Huckel's rule number. This has an aromatic number of electrons. But remember, to be aromatic, you need to be planar. What did we say about the taco? It's not planar.

But wait, here's the confusing part, guys. Because molecules love being aromatic, they're going to do whatever possible to be aromatic and they're also going to do anything possible to not be anti-aromatic. That's the theme of this pattern here. It turns out that if you have a 2 negative charge on your molecule, it's actually going to flatten out again to become planar. This molecule actually is aromatic. I know that sucks in terms of memorizing. But you have to think about it, maybe less in terms of memorizing and more in terms of motivation. These molecules are motivated to be aromatic. So they're going to do anything they can. And they're unmotivated to be anti-aromatic, so they're going to do anything they can to fold out of the plane so they don't have to deal with that.

What are these rules that I keep talking about? Here it is. This only pertains to all cis-annulenes by the way. What would be an all cis-annulene? It just means that all of your double bonds have single bonds that are facing towards the ring. Both of these would be examples of all cis. Really, if you want an example of not all cis, it would be something like molecule B or molecule D. As you can see, there are some trans bonds here. That's a trans, and that's a trans bond. These larger rings would not apply to my rule and these are just going to be special cases that I'm going to tell you about.

But my rule specifically applies to stuff like cyclooctatetraene or this big thing that I have in molecule A. So let's go ahead and take a look. If you have 4n+2 pi electrons, is that a good number or a bad number? It's a good number, right? These molecules are going to want to do anything possible to be aromatic.

It turns out if they have 9 carbons or less, they will become planar to be aromatic, so they will be planar. However, if they have 10 carbons or more and they're all cis, then they're going to be too strained to make a planar structure because those bond angles are going to wind up getting more and more shallow. They're not going to be able to meet the 120 degree bond angle and it's going to be too stretched out. This will be too strained, and it will actually become non-aromatic. It's kind of a sad situation. It wants to be aromatic but it can't meet the right bond angles. It's just too big to stay as a plane, and it's going to start to twist. That's if you have 4n+2. This is the good numbers, right?

Well, what if you have the bad numbers for n? Then think about the motivation here. It's trying its hardest to not be anti-aromatic, right? It's trying to avoid anti-aromaticity. It's going to do whatever it can possible to fold out of the plane. That's exactly what happens if you have 8 carbons or more. If you have 8 carbons or more like cyclooctatetraene, 8 annulene, you're going to be non-aromatic because you're going to fold. Just like we did, I'm just going to put here fold taco. I'm literally writing taco there, so you can remember it's going to fold like a taco on itself.

But what happens if you have 7 carbons or less? Well then you're in a tough situation. If it has 7 atoms or less in the ring, then it's too small to fold. So it's going to have to be anti-aromatic. I'm going to put here too small, twofold. That means it's going to be anti-aromatic. That's the rule.

Another note here, guys. This is just a note of guidance here. It turns out that I put together these rules over years of tutoring and year-end doing a lot of research online and trying to figure out what most professors consider to be anti-aromatic, non-aromatic. But this is actually controversial. I'm going to go ahead and write here in a little bracket. I'm going to put controversial because some professors, remember I told you this has to do with bond lengths and X-ray crystallography. Maybe your professor is like a professional in X-ray crystallography and they have their own idea of what some of these molecules. So obviously go with your own professor's judgment. If they decide that they want a 7-membered ring to be non-aromatic, then go with it. Just go with the flow. But these rules should apply definitely to your textbooks and should apply to most sources that you would find online or most professors what they think. Okay? But don't argue with your professor about this. In general, don't argue with your professor because they're the ones that give you a grade, not me.

For the following molecules, go ahead and use the rules that we talked about to figure out what they would be. As you can see, only compound A and C actually can use these rules. Meaning that B and D, I will address separately as different types of molecules. Go ahead and do A and then, you know, we'll just take these questions one at a time.

Determine annulene aromaticity

What did you put for ₁₀annulene? ₁₀annulene has the right number of electrons to be aromatic. Unfortunately, it has the wrong number of carbons because, as we said, if you have the right number of electrons but 10 or more atoms in your ring, those bond angles are going to be too strained. As you can see, these bond angles don't look anything like 120 degrees. These bond angles are way bigger. They're way, way bigger than 120 degrees. What that means is that this molecule is going to be too strained. These bonds are going to be too strained to be in a perfect planar circle, and they're going to wind up twisting out of shape. So, this is going to be nonplanar. If it's nonplanar, then what can we conclude about its stability or about its aromaticity? That means it has to be non-aromatic. Sucks for this guy, right? He wanted it so bad, but he just couldn't become aromatic. So, guys, I'm going to kind of go like a little bit off tradition here. I'm going to go to 'c' now because 'c' is the next one that you can apply the rule to, and then we'll do 'b' and 'd' after that. So go ahead and do 'c' next.

Determine annulene aromaticity

So what can we conclude about the 9-annelene anion? Well, does it have the right number of pi electrons? Actually, yes, it does. It has 10 pi electrons, which would put it in the aromatic category. But does it have the right number of carbons? Actually, yes. This one just got lucky because we said that if you have the right number of electrons but you have 9 or fewer carbons or atoms in your ring, then it actually, despite the bond strain and the angle strain, we're going to still be able to make it planar. So this will be planar, meaning that it will be aromatic. This one just made it. Cool, right? Just so you know, this is the same logic that applies to our dianion up here, how it's 8-membered ring, so 8 members is less than 9, so is 9 or less, so then it would also be aromatic. It's the same rule that makes this aromatic is the same one that makes this aromatic. Now I'm going to explain B, which by the way, I'll just explain and then I'll explain D as well.

Determine annulene aromaticity

Alright, guys. So compound B, we couldn't apply the rule to. So this is more just like an exception that I want to explain. So remember that we talked about our 10-annulene at the very first page of this lesson. And we talked about how usually the hydrogens would face in the same direction so this would be a non-planar molecule. This is actually our example of a molecule that wasn't planar. Well, it turns out that there is a way to fix that. If you add what's called a methylene bridge, you just put one single carbon that connects the two sides, what you do is you take away that hydrogen interaction that would have prevented it from being planar. So now you actually allow it to be planar. This actually would be planar, and it is aromatic. It's an exception, guys, that you might see, and that I want you to know. It's just kind of something you have to memorize. I'm sorry. But you should just memorize that this is a molecule that is aromatic because those hydrogen interactions were removed. So now there's no reason that it can't be planar. Okay. Awesome. So now I'm just going to discuss D, and then we'll be done.

Determine annulene aromaticity

This isn't an all cisannulene, so we can't use the rules that I provided earlier. But I can just kind of walk you through this molecule and give you a general sense of what these larger annulenes do. Well, in this case, this is 14 annulene. Is that a good number of pi electrons? Yeah, that's the perfect number. For these really large annulenes that have the right numbers, we're always just going to assume that they're planar. We're going to assume that they're planar if it's 4+2. This one, I have no reason not to believe it's planar. Everything's drawn on the same plane. It has the right number of electrons. This would be aromatic. I'm just going to draw it inside. It's planar and it's aromatic since I'm running out of space. In the same way, guys, so what would be another planar large annulene? 18 annulene. 18 annulene, also large, also aromatic. You could just keep going. Now what about 4n annulene? What about something like 16 annulene? What do you think? What would be your suspicion on 16 annulene? Well, guys, 16 annulene would actually be described by the rules that we were talking about earlier even if it's not all cis. 16 annulene, wrong number of pi electrons, right? It's got a bad number. It's going to do anything possible to twist out of the plane. Do you think a ring with 16 atoms in it will be able to twist out of the plane? Of course, it will. So something like 16 annulene should be non-aromatic because nothing is forcing it to be in the plane, okay? And if you were to do tests on it with your instruments, you would find that it's a non-aromatic molecule, okay? Awesome. So really that is definitely more detailed than you probably need for this class, but now you guys understand that planarity rule and you guys are more familiar with this whole topic of annulenes. So let's move on to the next video.