Introduction to Aromaticity

Concept: Concept: Intro to Aromaticity

9m
Video Transcript

Hey everyone! Now we’re going to talk about a brand new idea that's actually a huge theme of Organic Chemistry 2. That's called aromaticity.
Aromatic compounds are compounds that have a really high level of stability relative to their high electron density. Now it turns out that aromatic compounds have their own set of rules that will help us to determine if something is aromatic or not. But for right now we can generalize to just say that aromatic compounds are molecules that have a lot of double bonds within rings. Actually if you look at my shirt or if you look on top at the Clutch logo, you'll see that the Clutch logo actually is an aromatic molecule. These must be pretty badass molecules.
Let's explore a little further what makes them so cool. Their high-level of unsaturation, if you guys remember what the word unsaturation means, it means that everything is not a single bond. It means that you have double bonds, you have rings – stuff that’s making hydrogens less prominent on the molecule.
Whenever you have such a high level of unsaturation, you would predict that there would be a very high reactivity of that molecule. However, what we find is that molecules that are aromatic are actually extremely unreactive. Here it says ‘however they are difficult to react with’. We find that aromatic molecules actually buck the trend and are very difficult to make anything happen to.
What I want to do is I want to show you guys an aromatic molecule compared to just a regular double bond to show you just how unusual these molecules are.
Just so you guys know, you may or may not know these three reactions. My point here isn't to bust your you know what. It's just to give you an example of the way that benzene and aromatic compounds do not react the same way that double bonds do.
This first reaction here, I'm not sure if you guys remember, was called hydrohalogenation. You can find more information about all of these reactions in the addition chapter of your textbook. Hydrohalogenation was a reaction that added a hydrogen and a halogen across the double bond. What we would expect is that you would get an H and an X added to the double bond.
Guess what happens when you react HX with a benzene? Back in the old days, in the 1800s, scientists predicted that we would wind up getting 3 halogens reacting because you have three double bonds. But it turns out that’s not what happens. In fact, the answer would be no reaction. Why is that? That’s peculiar.
Let's look at the next one. This next reaction is called halogenation. This is another very, very common addition reaction. In halogenation, what we got was dihalides. We would actually get an anti-dihalide. If you look at a benzene ring, which by the way I already said a few times that this is called a benzene ring. If you look at a benzene ring, it has three double bonds. You might think that you’re going to get Xs everywhere on every single carbon. But in fact, the answer again remains no reaction.
Why is that? These double bonds look like they should be reacting. Why are they not reacting? It's so confusing. Let’s look at another example. This is a bit more tricky. You might not remember it. What happens when you have KMnO4 at 0 degrees’ temperature? This is a cold reaction of KmnO4. You got it. This would be a 1,2-syn diols reaction. I know you said it. I got you. You would wind up getting hydroxyl groups that are syn to each other.
You might think that if we react this reaction with benzene, surely you're going to get alcohols everywhere. You guys guessed it. No reaction. Why is that? It's so weird. That is exactly the question that tons of scientists were competing to answer back in the 1830s to 1850s. They are so confused why is this happening. They finally realized it's because of a phenomenon called aromaticity. Before we can really learn what the criteria of aromaticity are, we might need to learn about the categories of aromaticity because it turns out that there's different categories of aromatics. Let’s go over these really quick.
First of all is just the standard idea of aromaticity which I already explained. Aromaticity says that these compounds are going to display an unusually high level of stability. This is just what I was just describing in the above reactions.
Now there's another concept called non-aromatic. We have aromatic and we have non-aromatic. Non-aromatic compounds are compounds that don’t display any unique level of stability or instability. Basically, non-aromatic compounds are normal molecules, the kinds of molecules that we’ve discussed in every chapter previous to this one. If you think about any molecule that we’ve reacted with in the past, that would be non-aromatic. It would just mean that it's normal.
But then we’ve got this third category that you've definitely never interacted with before and that's called anti-aromatic. You might not understand why something is anti-aromatic. But for right now, I want you to know that it's going to be a compound that displays an unusually low level of stability. These molecules are actually very reactive. In fact, they're almost impossible to make. Scientists spend years making these anti-aromatic molecules because they’re so reactive that they’re unstable and they decompose on their own. We’re talking about crazy level of instability here.
Here's an example guys. What I’ve done here is I’ve given you 3 trienes. These are three molecules that have 3 double bonds apiece. What we find is that their structures aren’t that different. One of them has a ring, actually two of them have rings. One of them is a straight chain. But actually their stabilities are off-the-wall different because anti-aromatic, we said… Is that stable or unstable? Unstable. Anti-aromatic is going to be super difficult to synthesize. It barely even lasts at room temperature.
Then we have non-aromatic. Non-aromatic is just normal. That would just be a normal molecule that we would have discussed at a previous chapter, normal stability. Finally, we have aromatic which is just this badass idea that we're going to introduce and that we're going to continue to talk about in the next coming videos. These are very stable, very cool molecules.
I'm going ahead and putting greater than signs to show that stability increases as you move up this spectrum. You guys understand kind of the differences between the types of aromaticity. But we're going to need to learn a lot more and really dive into this idea before we can understand what separates an aromatic molecule from a non-aromatic molecule to an anti-aromatic molecule.
Let's go ahead and move on to the next video so we can get a better idea of what is it exactly that separates these molecules and makes them so different.