Aromatic Hydrocarbons - Online Tutor, Practice Problems & Exam Prep

To determine if a molecule is aromatic, it must meet four criteria: it must be cyclic, fully conjugated, planar, and contain a specific number of pi electrons following Hückel's rule, which states that the number of pi electrons must equal 4n + 2 (where n is a non-negative integer). Molecules with 4n pi electrons are antiaromatic and unstable, while those failing any of the tests are nonaromatic. Odd numbers of pi electrons also disqualify a molecule from being aromatic, often arising from radicals.

Aromaticity of Hydrocarbons

Hey guys. So now that we know those 4 tests of aromaticity and now that we're experts on counting up pi electrons, it's time to put that information together to figure out if a molecule is aromatic or not. Remember what those four tests of aromaticity were. We had the whole "it has to be a ring thing", cyclic. We had fully conjugated. We had that it has to be planar. But remember, there was that last rule that was a little confusing, the Huckel's rule rule. Now we know how to count up pi electrons but n=4n+2 is still kind of confusing. What does that mean? The whole reason that we have this idea of n=4+2 is because someone realized that 4+2 would be an easy shorthand to memorize these magical numbers that make a molecule extra stable and therefore aromatic. The way it works is that n is equal to any integer, any whole number. Integer. And when you make n to equal any integer, then what you do is you wind up getting these numbers that are the super stable numbers. That would be if n=0, then that would be 2. If n=1, then that would be 4 times 1 plus 2 which would be equal 6. If n=2, then that would be 2 times 4 plus 2 which equals 10 and so on and so forth. We get these numbers that go 2, 6, 10, 14, etc. Forever. These numbers are the Huckel's rule numbers and some of my students just prefer to memorize the numbers instead of 4+2 because they think it's easier that way. I'm going to leave that to you. If you want to just memorize 2, 6, 10, 14 instead of 4+2, if that's easier for you, go for it. All I care about is that you use the right numbers on your exam.

Now you might be wondering why are these numbers so great? Why are they so stable? They look like normal numbers to me. That is a topic for a different video. In another video, we're going to discuss why these numbers actually contribute to stability and why they make the molecules so badass. But for right now, just memorize it. Then remember that we had this other category which was let's say that you meet the first three tests but we get a n=4n number of pi electrons. Well, these are different numbers, right? These are going to be the multiples of 4. These are going to be numbers like 4, 8, 12, 16. Guess what? These are magical numbers as well but they're magical in a bad way. They suck. They make the molecules super unstable. In fact, it's really hard to even synthesize these molecules in the lab because they are so unstable. So these molecules are going to be what we call antiaromatic. Remember these are molecules that are much less stable than normal. Remember that these are called said to follow Breslow's rule because Breslow's rule said that you have the first three tests met. You're still cyclic. You're still fully conjugated. You're still planar but you have the wrong number of pi electrons. In fact, you have a n=4n number of pi electrons.

Now let's talk about this third category of nonaromatic. Recall that I stated that nonaromatic molecules are simply molecules that fail 1 or more of the tests. So if you're not a ring, you're automatically nonaromatic. Now one thing I want to point out is that some pi electrons can actually count towards failing the rule. If you have an odd number of pi electrons, so that would literally be any odd number. For example, the number, for the number 7, the number 7 doesn't fall into any of these types of electrons. It's not 4n+2. It's not 4n. It's simply left out. We never discussed it. If you have an odd number of pi electrons, you're also said to fail Huckel's rule. Pi electrons. Keep that in mind that you can even fail, the test of aromaticity by the number of pi electrons that you have. You might be wondering when would you get an odd number of pi electrons? When you have radicals because radicals count as only 1.

Now I have a ton of practice for you guys. We're just going to do 1 at a time. Go ahead and look at the first one. I know it's a little bit too easy. But let me know if you think that first molecule is aromatic or not based on the 4 tests of aromaticity and your ability to count up 4n+2pi electrons. Go.

Determine the aromaticity

Is this molecule aromatic? Of course it is. But let's obviously go through the 4 tests to verify it. First, is it a ring? Yes. Second, is it fully conjugated? Yep. Third, is it planar? You bet. Fourth, does it have a n=4n+2 number of π electrons? It has 6π electrons, so that would be one of those magical Huckel's rule numbers. This is aromatic. Awesome. Now we know how to prove something that you just vaguely knew before. Now move on to the second compound and tell me what type of aromaticity it has.

Determine the aromaticity

Is this compound aromatic? Actually, no, it's not. In fact, it's antiaromatic. Let's go ahead and find out why.

• Is it cyclic? Yes.
• Is it fully conjugated? Yes, it is. Positive charges can participate in resonance, so the entire every single atom can resonate.
• Is it planar? Yep.
• Does it have 4+2 number of pi electrons? No, it actually has 2,4. It only has 4 pi electrons which, if you recall, is one of the really bad numbers. This is a 4n number, so it's got 4n number of pi electrons. That means this is going to be antiaromatic. Not so bad, right?

Let's move on to the next question.

Determine the aromaticity

What was your conclusion here? Hopefully, everyone said non-aromatic because we have a problem right from the start. It is not cyclic. So immediately, I don't have to keep going. I don't have to test anything else out. If it fails one or more of my tests, it's automatically non-aromatic, which simply means that it's in a category that would have been counted by all the other chapters of Organic Chemistry. It does not belong in this chapter of Organic Chemistry. We can just move on to the next compound.

Determine the aromaticity

Hey, everyone. Let's continue our discussion on aromaticity by taking a look at option d here. Here we have a 7 carbon ring with a negative charge in the center. Now, what does that really mean? Well, what it means is that 6 out of my 7 carbons, so almost all the carbons except for 1, are double bonded, and the one carbon that's not double bonded is the one that possesses this negative charge. When would carbon have a negative charge? Well, that happens when it has a lone pair. So this is the way we'd actually depict this cyclic structure. Now, let's go through our checklist to see if it is aromatic, anti aromatic, or non aromatic. First, we would say that this is a cyclic compound. It's pretty clear to see that. So that's off the list. Secondly, we can say that it is fully conjugated because remember, we have alternating double and single bonds. Now, this carbon up here with its lone pair negative charge is also part of this full conjugation because we could resonate this lone pair here to make a double bond, which would resonate this double bond here as a lone pair. And it could do it throughout the structure. So it is indeed fully conjugated. So, it follows the second rule, we can say that it is planar for the third rule. And then finally, if we look, we would say that this thing has 4n π electrons. And if we counted the number of electrons, we'd have, what? 2, 4, 6, 8 π electrons. So we follow the first three rules and we have 4n π electrons, which gives us 8 π electrons. We're following Breslow's rule. So we're gonna say that this is antiaromatic. Now, make sure you verify with your professor and your manuals because this here is an antiaromatic structure following our checklist. There could be, exceptions where we're talking about, oh, some advanced technology can say that this isn't quite planar, and because of that, it might be nonaromatic. But when we're following the rules that we've set forth, we know that it's fully conjugated and through resonance, all the carbons could technically be double bonded at some point. And because of that that's what gives it its planarity or its planar structure, making it follow rule 3. So based on this alone and based on our checklist, we're saying that this is antiaromatic. But again, please verify with your textbook and your professor as well because this one is quite a tricky example. Alright. So, again, now that we've done option d, continue on to the next video to take a look at another structure and determine if it's aromatic, antiaromatic, or nonaromatic as well.

Determine the aromaticity

This next compound looks a little bizarre, but all the same rules apply to polycyclic molecules that apply to monocyclic molecules. So it still is cyclic, right? So we would go ahead and check that off. Second, it's fully conjugated. There's no atom here that can't resonate. Third, it's planar. I haven't been given a reason to believe it's not planar. And then fourth, how many electrons does it have? You got it, guys. This is again πe=8 electrons. This is a bad number. We're on a bad streak here; we've got a bunch of unstable compounds. This is antiaromatic. Alright. Cool. So, let's move on to the next compound.

Determine the aromaticity

All right, guys. So I'm hoping you didn't put aromatic here because there's a big problem with this molecule. Let's go step by step. The first one is, is this cyclic? Of course, it's a ring. 2, is it fully conjugated? No, it's not. This is an example of a molecule that is not fully conjugated. Remember, the entire perimeter of the ring needs to be able to resonate. What I see here is that this double bond can resonate. This orbital can resonate. But here I have an sp³ hybridized carbon that does not have any available orbitals to resonate. In fact, to put electrons, to move electrons into that carbon, I would need to somehow break a bond to hydrogen. Remember that you can't move atoms in a resonance structure, so this absolutely is not fully conjugated. It fails the fully conjugated test which means I don't need to look any further. This is automatically non-aromatic. Make sense? Yeah, it does. Move on to the next one.

Determine the aromaticity

These are getting harder, so what do you guys say about this one? So this is a very common molecule that you could see on your exam, a little bit higher in the difficulty level. This is an aromatic molecule. This is actually a very famous polycyclic aromatic molecule called azulene, which we will discuss later on in more depth. But for right now, how could we prove that azulene is an aromatic molecule? Because it's cyclic, it's polycyclic, so for sure. Secondly, it's fully conjugated. Thirdly, it's planar. And then fourthly, I'm running out of space here. It has how many electrons? It has 10π electrons which happens to be one of my magical Huckel's rule numbers. So I'm going to say n4+2. This is aromatic. Azulene displays highly unusual stability for its level of unsaturation. So, the definition of aromaticity as you lean meets it very well. Let's move on to the next compound.

Determine the aromaticity

What do you put for letter h? Guys, this compound is non-aromatic. Why? Because it's cyclic, obviously. Secondly, it is fully conjugated because remember that radicals can participate in resonance. In fact, they do all the time. Thirdly, it's planar. And then fourthly, what number of pi electrons does it have? Well, it has 2+1 because remember that radicals only contribute one electron to the pi conjugated system. That means we've got a total of 3 pi electrons. What did I tell you guys about the significance of odd-numbered pi electrons? What does that mean? What test does it mean? Remember that if you have an odd number of pi electrons, you actually fail the Huckel's rule test. If you fail a test, you're just like the rest of the non-aromatic compounds. You do not belong in this chapter. This would be an odd number. It fails the rule. We're just going to say it's non-aromatic. It does not possess any unique stability. It does not possess any unique instability. It's just a normal molecule just like the rest of the molecules in organic chemistry. Awesome. So, one more question and then we move on to the next topic.

Determine the aromaticity

So guys, everything on this page was fairly easy and now I just threw in a really hard question at the end. I'm just envisioning a few of you arguing with your friends on the computer saying, no it's this. No it's that. It's really this. And you're like about to punch your friend and then you're like, let's just play the video and see what it is. Well, I'm sorry to tell you that you might both be wrong because this is such a hard problem. I'm going to take myself out of the screens so we can talk about it. Guys, sadly this molecule is actually drumroll aromatic. How in the world is this molecule aromatic? I know that was the last thing you were thinking. How could that be? Okay. Well, let's go through the first step. The first one is that obviously it's cyclic.

2, is it fully conjugated? Well, there's an area here that is missing conjugation. If you'll notice, this carbon here has 2 H's, correct? This carbon is not fully conjugated. That means that in order for my pi conjugation system to exist, it can't exist on this ring because this part of the ring is not fully conjugated. Remember that I told you guys, I'll just bring myself back into the camera really quick. Remember that I told you guys that in order to be fully conjugated, you just need it to be conjugated around the perimeter of the molecule or around one perimeter. You just need to have one loop of pi electrons that make sense. Now typically for the monocyclic or for the polycyclic molecules we've been looking at, we go around both rings. But notice that on this molecule, we've got a problem. This carbon right here is not fully conjugated. There's no way that I can go all the way around. If I go all the way around, I'm going to get stuck here. So that means that this ring can't work which means that my pi conjugated system is actually limited to only this ring. This is the perimeter that I care about. This is the ring that is fully conjugated. What that means is my loop or my loop of conjugation is only around this ring and not around this ring. It is fully conjugated but only with one of the rings. Meaning that 3, is it planar? Sure. 4, how many pi electrons are in that ring? Well since let's say this is ring A and this is ring B, right? The pi electrons in ring B don't count because they're not part of the pi conjugated system. That means I only count the electrons that are in the ring that's actually fully conjugated which would make 2, 4, 6 electrons. This has 6 pi electrons which makes it aromatic. Actually this is similar to a benzene ring just with carbons coming off of it. But it has pretty much the same stability that you would find in a benzene ring. So, I'm sorry to burst your bubble. I know that you were doing great with this page. In fact, you were probably like in snooze mode and all of a sudden, you're painfully woken up and you realize that you could be thrown a question like this on your exam and get it wrong. So I just want to let you guys know that it's not always easy. You have to think. You have to use what's here. Can't turn it off just yet. But if you apply the four rules, you should be fine. Let's move on to the next topic.

Here’s what students ask on this topic:

What are the four criteria for a molecule to be considered aromatic?

For a molecule to be considered aromatic, it must meet four criteria: it must be cyclic, fully conjugated, planar, and contain a specific number of pi electrons following Hückel's rule. Hückel's rule states that the number of pi electrons must equal $4^n+\; 2$ (where n is a non-negative integer). These criteria ensure that the molecule has a stable electronic structure, making it aromatic.

What is Hückel's rule and how does it determine aromaticity?

Hückel's rule is a criterion used to determine if a molecule is aromatic. It states that a molecule is aromatic if it has a cyclic, planar structure with a fully conjugated system of pi electrons, and the number of pi electrons must be equal to $4^n+\; 2$ (where n is a non-negative integer). This results in specific numbers of pi electrons (2, 6, 10, 14, etc.) that confer extra stability to the molecule.

What is the difference between aromatic, antiaromatic, and nonaromatic compounds?

Aromatic compounds meet all four criteria: cyclic, fully conjugated, planar, and having $4^n+\; 2$ pi electrons, making them stable. Antiaromatic compounds meet the first three criteria but have $4^n$ pi electrons, making them highly unstable. Nonaromatic compounds fail one or more of the criteria, making them neither particularly stable nor unstable.

Why are molecules with 4n pi electrons considered antiaromatic?

Molecules with $4^n$ pi electrons are considered antiaromatic because they meet the first three criteria of aromaticity (cyclic, fully conjugated, and planar) but have a number of pi electrons that leads to significant electronic instability. This instability arises because the electron configuration does not allow for the delocalization that stabilizes aromatic compounds, making antiaromatic molecules highly reactive and difficult to synthesize.

How do radicals affect the aromaticity of a molecule?

Radicals affect the aromaticity of a molecule by introducing an odd number of pi electrons. Since Hückel's rule requires a specific even number of pi electrons ($4^n+\; 2$), the presence of an odd number of pi electrons disqualifies the molecule from being aromatic. This often results in the molecule being classified as nonaromatic, as it fails to meet the required criteria for aromaticity.