Aromaticity of Annulenes

Concept: Concept: [6]annulene vs. [8]annulene

Video Transcript

Hey guys! Now we’re going to focus on a specific type of ring called an annulene. Annulenes or polyolefin as they’re sometimes called are monocyclic hydrocarbons, one ring, that are fully conjugated. That means that to be an annulene, you need to be only one ring and you need to have alternating single bonds and double bonds like you would find in benzene.
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 put it around a bracket and then annulene.
Actually if you think about it, a benzene is a type of annulene. Benzene can also be simplified to the name [6]annulene which is pretty cool. If you're just walking around campus and you see someone with a Clutch shirt on, that has a benzene, you can be like “That's a mighty fine [6]annulene you have there.” They’re from Clutch so they're going to know what you're talking about and 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, 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.
Here's the deal. Remember that rule I told the 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.” 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 a 12 or a 14 or a 20 in front of the annulene. You’re going to get this massive ring. The thing about large rings is that those bonds getting wobbly so they can start to bend. They can start to twist. They can start to go in the directions that will not form a planar ring.
This can preset a little bit of a problem to us, we college students that don't really know if some of these are going to be planar or not. We're going to have to memorize some specific trends to be able to predict if something is going to be planar or not.
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. That is not something a professor wants to go into during a college organic chemistry class.
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 a 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 planar. Whereas [8]annulene would normally be what type of aromaticity? Anti-aromatic. This is an anti-aromatic molecule if it’s drawn planar.
But what [8]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 because I’m really hungry. It kind of looks like a taco and you can out some mystery meat in there and stuff.
The benefit of that is that these orbitals end up not facing the same direction. What did I tell you guys happens if your orbitals face different directions? They can’t conjugate. If they can’t conjugate with each other, you don't have anti-aromaticity. That means that [8]annulene actually exists in a non-aromatic state. Isn’t that crazy?
What are you supposed to do? You’re supposed to memorize that this molecule actually does not look like a planar structure. It looks nonplanar 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. Let’s go on and learn those rules.

Concept: Concept: Rules for Predicting Planarity


Concept: Example 1: Determine annulene aromaticity

Video Transcript

What did you put for A) [10]annulene? [10]annulene 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. They're going
to wind up twisting out of shape. 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. I’m going to kind of go 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. Then we'll do B and D after that. Go ahead and do C next.

Concept: Example 2: Determine annulene aromaticity

Video Transcript

What can we conclude about the all-cis [9]annulene anion? 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 less carbons or atoms in your ring, then actually despite the bond strain and the angle strain, we're going to still be able to make it planar. This will be planar, meaning
that it will be aromatic. This one just made it.
Just so you know, this is the same logic that applies to our dianion up here, how its 8-membered ring. 8 members is less than 9. It’s 9 or less so then it would also be able to be aromatic. It's the same rule
that makes this aromatic. It’s the same one that makes this aromatic.
Now I'm going to explain B, which by the way I'll just explain it and then I'll explain D as well. 

Concept: Example 3: Determine annulene aromaticity


Concept: Example 4: Determine annulene aromaticity