Aromaticity of Annulenes

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

5m
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

7m
Video Transcript

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 cyclooctatetrene or COT for short I like to call it COT cyclooctatetrene and remember that this is your taco molecule right this is the molecule that hates being anti aromatic so fold on itself so that it becomes non aromatic. Well here's the crazy thing what happens if I ionise it to give it a net charge of 2 negative OK so if I add 2 negative charges to the cyclooctatetrene. Well it turns out well first of all what's that called that would be called 8 annulene dianion. So 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 N ions count as 2 so that would be 8,10 so we have 10 pi electrons is that a good number or a bad number that's a huckel's rule number so this has an aromatic number of electrons but remember to be aromatic you need to be planer 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 because that's the theme of this pattern here so it turned out that if you have a negative a 2 negative charge on your molecule it's actually going to flatten out again to become planar, so this molecule actually is your aromatic.

I know that sucks in terms of memorising but you have to think about it maybe less in terms of memorising and more in terms of motivation. These molecules are motivated to be aromatic they're going to do anything they can and they're unmotivated to be antiaromatic so they're going to do anything they can to fold out of the plan so they don't have to deal with that. So what are these rules that I keep talking about well here it is this only pertains to all cis annulenes by the way so what do we mean by all cis annulenes it just means that all of your double bonds have similar ones that are facing in towards the ring so both of these would be examples of all cis really if you want an a example of not all cis it would be something like molecule B or molecule D, as you can see there are some transbonds here that's a trans and that's a transbond so 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 ok. But my rule specifically applies to stuff like cyclooctatetrene or this big thing that I have in molecule A, so let's go ahead and take a look. If you have 4N plus 2 pi electrons is that a good number or a bad number? It's a good number right so these molecules are going to want to do anything possible to be aromatic and it turns out if they have 9 carbons or less ok 9 carbons or less they will become planer to be aromatics so they will be planer.

However if they have 10 carbons or more and they're all cis then they're going to be too strained to make a planer structure because those bond angles are going to wind up getting more and more shallow so they're not going to be able to meet the 120 degree bonding angle and it's going to be too stretched out so this will be too strained and it will actually become non aromatic so it's kind of a sad situation it wants to be aromatic but it can't meet the right bond angle its just too big to stay as a plane it's going to start to twist. So that's if you have 4N plus 2 so this is the good numbers right well what if you have the bad numbers 4N then think about the motivation here it's trying its hardest to not be antiaromatic right its trying to avoid antiaromaticity so it's going to do whatever it can possible to fold out of the plane and that's exactly what happens if you have 8 carbons or more if you have 8 carbons or more like cyclooctatetrene you're going to be non aromatic because you're going to fold, just like we did I am just going to put here fold taco I'm literally writing taco there so you can member its 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 the 7 carbons or I should say I keep saying carbons but I just mean atoms 7 atoms or less in the ring so it's going to be too small to fold so it's going to have to be anti aromatic so I'm going to put here too small to fold and that means it's going to be anti aromatic. So 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 doing a lot of research online and trying to figure out what most professors consider to be antiaromatic non aromatic but this is actually controversial so 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 links and X ray crystallography maybe your professor is like a professional X ray crystallography and they have their own idea of what some of these molecules, so obviously go with your own professor's judgement 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 but don't argue with your professor about this one in general don't argue with your professor because they're the ones that give your grade not me alright so 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. So go ahead and do A and then you know we'll just take these questions one at a time.

Concept: Example 1: Determine annulene aromaticity

2m
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

2m
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

2m
Video Transcript

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 hydrogen's would face in the same direction so this would be a non planar molecule this is actually or 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 he 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, so this actually would be planar and it is aromatic and it's an exception guys that you might see and I want you to know. It's just kind of it's just something I had to memorise I'm sorry but you should just memorise that this is a molecule that is aromatic because those hydrogen interactions were removed so now there's no reason that can't be planar. Awesome so now I'm just going to discuss D and then be done.

Concept: Example 4: Determine annulene aromaticity

3m
Video Transcript

This isn't an all cis annulene 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 a perfect number so guys for these really large annulenes's have the right numbers we're always just going to assume that they're planar.

We're going to assume they're planar if it's 4N pulls 2 so this one I have no reason not to believe it's planar everything's you know drawn on the same plane, it has the number of electrons so 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 also large also aromatic you could just keep going. Now what about 4N annulene so 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're talking about earlier even if it's not all cis 16 annulene wrong number of tie 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 stay twisted 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 and if you were to do tests on it with your instruments you would find that it's a non aromatic molecule. Awesome so really that is definitely more detail than you probably need for this class but now you guys understand that plane area rule and you guys are more familiar with this whole topic of annulenes. So let's move on to the next video.