Acidity of Aromatic Hydrocarbons

Concept: Concept: Aromatic hydrocarbon acidity

Video Transcript

Generally speaking, aromatic molecules are not acidic at all. But the concept of aromaticity can cause certain molecules to become acidic. Let's go ahead and explore how that happens.
As I said, aromatic compounds are not naturally acidic. The pKa of benzene which is your most common aromatic molecule, is if you guys member, 44. Is that a good pKa? Is that a good acid? No. It’s actually one of the worst acids possible. That’s obviously not going to be a great proton donor. But what we do find is that if a hydrocarbon can become aromatic by giving away a proton, basically being an acid, donating a proton. If that makes it become aromatic because it's becoming a conjugate base, then it's going to be uniquely acidic. Our stereotypical example here is cyclopentadiene. Cyclopentadiene is a hydrocarbon. You would expect it to have pKas of 44, something around there. But it's got these two hydrogens up here.
What’s special about these hydrogens? This is right now not a fully conjugated molecule. This is what we would consider non-aromatic because it fails one of Huckel’s Rule criteria. It fails one of the four tests because it's not fully conjugated. But check this out. What if a base were to come along and pull off a proton? What kind of charge would that carbon now get? Make bond, break a bond. I would wind up getting a new compound that looks like this – double bond, double bond, negative charge.
What can you tell me about that molecule? What’s the aromaticity of that molecule? This is aromatic. We learned about the four tests. We learned about Huckel’s Rule. This would be an aromatic molecule. The definition of a good acid is that after donating a proton, it becomes a stable conjugate base. Would you say that this conjugate base is pretty stable? Hell yeah! It's aromatic. Aromatic molecules are awesome at being stable. That’s what they do the best.
It turns out that you guys might have not memorized this. Maybe you've never heard of this before but it's okay. Cyclopentadiene is actually one of the most acidic hydrocarbons there is. These hydrogens at the top have a pKa of 15. Isn’t that a huge difference? That means that cyclopentadiene is even more acidic than water and alcohol. It's a better proton donor than even those because it can become aromatic after it donates that proton. Makes sense so far?
The opposite idea can apply for molecules that become anti-aromatic when donating protons. Let's look at this molecule. If a hydrocarbon becomes anti-aromatic after donating a proton, then it’s uniquely nonacidic. Cycloheptatriene, same thing. We don't expect it to be very acidic. But what if a base were to come along and pull off this proton? What kind of charge would it now receive? It would become negatively charged. I’d have double bond, double bond, double bond, negative charge.
The original molecule was what? Nonaromatic just like my previous example because it’s not fully conjugated. But afterwards, what’s the aromaticity of this molecule? This one is forced to become anti-aromatic which sucks. This is like the worst acid-base reaction ever. It’s saying “Why do they do this?” If you can imagine, the pKa of a normal sp3 hybridized CH bond is the pKa is usually 50. It’s one of the worst acids known to man.
What do you think the pKa of this specific molecule will be? Do you think it will be greater than 50 or less than 50? Because of the fact that this is making an anti-aromatic molecule, not just a normal carbanion. Carbanions already suck but it's not only a carbanion, it’s an anti-aromatic carbanion. You can't get a worse conjugate base than this. This is the conjugate base from hell. Instead of being a pKa of 50, this is going to be a pKa of 60 to 70. It's going to be very, very high. Remember this is on a log scale. This is impossibly difficult to remove that proton because that proton would make it anti-aromatic, removing that proton.
As you can see, aromatic molecules are not great acids or bases but the concept of aromaticity can be what is behind the motivating factor behind some really common acids like cyclopentadiene.
We're going to do some examples. I want you to read through them first. Try to answer them and then I'll go ahead and explain them for you. Start on the first one.

Concept: Example 1: Determining hydrocarbon acitidy


Concept: Example 2: Comparing hydrocarbon acidity