Acidity of Aromatic Hydrocarbons - Online Tutor, Practice Problems & Exam Prep

Aromatic compounds, like benzene, typically have high pKa values, indicating low acidity. However, certain hydrocarbons, such as cyclopentadiene, can become acidic by donating protons, leading to stable aromatic conjugate bases with pKa values around 15, making them more acidic than water. Conversely, cycloheptatriene, upon proton donation, forms an antiaromatic carbanion, resulting in extremely high pKa values (60-70), indicating poor acidity. Understanding aromaticity's role in acidity is crucial for grasping acid-base behavior in organic chemistry.

Aromatic hydrocarbon acidity

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 kind of your most common aromatic molecule, is, if you guys remember, 44. Is that a good pKa? Is that a good acid? No. It's actually one of the worst acids possible. It'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, right? 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 really typical example here is cyclopentadiene. Cyclopentadiene is a hydrocarbon. You would expect it to have pKas around 44, something around there. But it's got these two hydrogens up here. Now what's special about these hydrogens? Well, this is, right now, not a fully conjugated molecule. This is what we would consider non-aromatic. Correct? 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? Well, make a bond, break a bond. I would end up getting a new compound that looks like this. Double bond, bond, double bond, negative charge. What can you tell me about that molecule? What's the aromaticity of that molecule? Guys, this is aromatic. We learned about the four tests. We learned about Huckel's rule. This would be an aromatic molecule. Now, 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 freaking 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 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 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. Make sense so far?

Now, the opposite idea can apply for molecules that become aromatic after donating a proton. Then it's uniquely non-aromatic after donating a proton, then it's uniquely non-acidic. 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? Non-aromatic just like my previous example because it's not fully conjugated. But afterward, 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 did I do this? If you can imagine the pKa of a normal sp^3 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? Guys, 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. So, this is like 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 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. So start on the first one.

Determining hydrocarbon acitidy

What do you guys think about this acid? Would it be especially good? Would it be especially bad? Okay. In order to answer this question, we have to draw the conjugate base. We have to see what it would look like after it donates a proton. We can just use the letter B and a negative charge to stand for the base. After it extracts this proton, what does this molecule look like? The conjugate base now looks like this. So what's the stability of that molecule? Guys, this would be anti-aromatic, right? So the answer here would be that, and I mean this is, I guess, a free response answer, so it's going to be essay style. You do not have to write these exact words. I'm just saying the molecule would be uniquely nonacidic due to an anti-aromatic conjugate base. That's it. Just explaining how this is not a good acid. It's actually going to be a terrible acid. Okay? So now here's another one. This one requires a little more thinking, a little more thought. Would the following two hydrocarbons be expected to have similar acidities? Explain your reasoning. Go for that one and then I'll answer it.

Comparing hydrocarbon acidity

All right, guys. So the only way to solve this is just to draw the conjugate bases of both and see what they look like. At this point, we've done this a few times. I'm just going to draw the negative charge because we know that after you donate a proton, you get an electron pair. You would get a negative charge here. Hope that's fair. Then notice that on this molecule, we actually have 2 different sites that are not fully conjugated. It could actually have come the negative charge could be on the top or on the bottom. Either one you choose is fine. But no matter what, that's where you would want to draw the negative charge to compare the sites. Now what I would ask is, are the stabilities of these molecules similar at this point? What do you think? The stability of the first one could be categorized as let's say this is molecule A. It's what kind of molecule? It's aromatic. And then molecule B is what kind of molecule? What do you think? Is it also aromatic? No guys. This can't be aromatic because one of the carbons is still sp³ hybridized with 2 hydrogens with 4 bonds, 4 sigma bonds. So this is still going to be non-aromatic. Okay? So even though that negative charge might have helped with Huckel's rule, we're still missing a fully conjugated ring. This is still non-aromatic. The answer is no. No. Right? They would not be expected to have similar acidities. No. Molecule A is more acidic due to having an aromatic conjugate base. I don't have to mention the conjugate base for molecule B because it's assumed that it's not going to be aromatic. Molecule A is I'll take myself off the screen for a second. Molecule A is the one that has an aromatic conjugate base. Molecule B doesn't have that so it must not be as stable. Okay? So anyway guys, I hope that helps to clarify the rule of aromaticity and acidity. So let's move on to the next topic.

Here’s what students ask on this topic:

Why are aromatic hydrocarbons generally not acidic?

Aromatic hydrocarbons, like benzene, typically have high pKa values, around 44, indicating low acidity. This is because they are not good proton donors. The stability of the aromatic ring structure makes it energetically unfavorable to lose a proton, as this would disrupt the aromaticity. Therefore, aromatic hydrocarbons are generally not acidic.

What makes cyclopentadiene more acidic than benzene?

Cyclopentadiene is more acidic than benzene because it can become aromatic upon losing a proton. When cyclopentadiene donates a proton, it forms a stable aromatic conjugate base with a pKa of around 15. This stability is due to the aromaticity of the resulting conjugate base, making cyclopentadiene a better proton donor compared to benzene.

How does the concept of aromaticity affect the acidity of cycloheptatriene?

Cycloheptatriene becomes extremely non-acidic upon losing a proton because the resulting conjugate base is antiaromatic. The antiaromatic carbanion formed has a very high pKa value (60-70), indicating poor acidity. The instability of the antiaromatic conjugate base makes it very difficult to remove a proton from cycloheptatriene.

What is the pKa value of benzene, and what does it indicate about its acidity?

The pKa value of benzene is around 44, indicating that it is a very weak acid. This high pKa value means that benzene is not a good proton donor and is one of the least acidic hydrocarbons. The stability of its aromatic ring structure makes it energetically unfavorable to lose a proton.

Why is the conjugate base of cyclopentadiene considered stable?

The conjugate base of cyclopentadiene is considered stable because it is aromatic. Upon losing a proton, cyclopentadiene forms a conjugate base that follows Huckel's rule, making it aromatic. Aromatic molecules are highly stable due to the delocalization of π-electrons, which lowers the overall energy of the molecule.