Exploring Benzene and Aromaticity

Okay folks, this is video 14-2. In this video we're going to be looking at some of the unusual characteristics of benzene that distinguish it from other double bonded compounds such as the alkenes. So let's do some general information first. Hydrogen deficiency. The formula is C6H6. We actually had looked at this a few weeks ago and that has a hydrogen deficiency of 8. So remember, if you ever calculate a hydrogen deficiency of 8 or higher, you will always have a benzene ring. Don't try, do not go trying to make all combinations of alkenes, alkynes, and rings. It's not worth your time. The hydrogen deficiency of 8 or higher means you've got a benzene ring. So if you have a hydrogen deficiency of 10, that means you have a benzene ring, which is 8. and you have two left over then you can talk about oh there's another double bond there's another ring you know things like that so in this ring we notice that there are double bonds to all the carbons meaning that all of the carbons in this ring are sp2 hybridized excuse me and because of that sp2 hybridization we can also understand the geometry Each of these carbons is planar. And since each of the six carbons is sp2 hybridized, that means that the entire ring is planar. So all compounds like benzene are going to be planar. They're going to be flat. And there is this unique property of benzene, which is going to help explain why its reactivity is so much different than alkene's. The typical reactions that we have seen for alkenes, such as hydrogenation and electrophilic addition, those reactions will not be for benzene. Benzene will not undergo hydrogenation or the typical electrophilic addition. And the main reason for this is this property of aromaticity. Benzene and many other compounds like it are referred to as being aromatic. Part of that is because of the black soot that you observe when you burn these compounds, but more so, it's the physical property of aromaticity. And this physical property, it helps to explain why benzene and other aromatic compounds are unusually stable, their unique reactions, and even the toxicity we see, and the persistence of benzene and other aromatic compounds in the environment. So let's take a look now what we mean by this concept of aromaticity. So we start off first with conjugation. Conjugation in the case of benzene means that we're referring to these alternating double and single bonds going around the ring. Okay, so we're going to see other ringed compounds. where the pi bonds are all inside the ring, and in order for aromaticity, they must be conjugated in some way. Again, that's alternating double and single bonds. Since we have conjugation, it means that we have a p orbital on each of these carbons. So we're going to look at this from a different perspective. So I'm going to make believe that we're sticking our head in the screen, and we're looking edge-on. at that particular bond on the right. So if we now turn and rotate the benzene ring 90 degrees, so here we are now. So we're looking, here's the bond we're looking at. Now it's coming right out of the screen at us. This bond is now facing backwards. So we're kind of looking at it from a planar perspective. So where are those p orbitals? Okay, those are those figure 8 shaped orbitals. And now we have one on every single one of the carbon atoms of the benzene ring. Now because of these p orbitals, we have something called delocalization. It's delocalization of the pi electrons. What this means is that the pi electrons can move freely from one p orbital to the next. And whenever you have electrons that can move freely along through a conjugated system, we say that they are delocalized. So the delocalization I am representing on the top of the ring by that red circle and the same thing on the bottom of the ring. These are often referred to as a ring current. And this is one of the very important properties of aromaticity because this ring current, this conjugation, this delocalization of the pi electrons is what leads to this unique stability. In order to break up this conjugation, it takes a lot of energy. Hence, benzene tends to be very stable. So another factor to consider is resonance. Since the pi electrons can move freely between alternating single and double bonds, we can draw resonance structures. So we're going to start off, just to illustrate this very simply, here's a simple planar benzene ring, and you will notice that I've highlighted one of the double bonds in bold, and what we're going to do is we're going to move that double bond, one bond to the next, all the way around the ring. So there it goes to the next position. So notice I'm going around clockwise. That position, another, another, another. And then finally, we get back to where we started from. So when we consider the hybrid resonance structure for this, what we are seeing is the pi electrons going around and around the ring, clockwise, counterclockwise, it doesn't matter. And we can represent the hybrid by this very simple structure of benzene with simply a circle in the middle. So this is a neat shorthand way of drawing benzene without drawing in the hydrogens and without drawing in the alternating double bonds. I would suggest you avoid this simple hybrid structure. I think it would be better if you actually draw in the alternating double bonds. And the reason is that I've already seen some of you in class putting five bonds to carbon. When you have the ring in the middle, you kind of forget the double bonds are actually there. So I would strongly suggest that you do draw in the double bonds. So there is some very interesting historical depictions of what researchers and chemists thought benzene looked like over history. And... A lot of this ended when a chemist named Kekule came up with the alternating single and double bonds and the hybrid form. And apparently this vision of the structure of benzene came to him when he was sleeping one night after a hard day of thinking about benzene and was dreaming about snakes. And the snakes grabbed, you know, one snake grabbed its own tail and went spinning around. And to him, that represented the resonance structures of benzene. Personally, I think he was doing research on some alkaloids and had been tasting too many of his products. In any event, as you can see, there are a lot of very interesting structures that have been derived for benzene. Okay, so how can we tell then if some compound other than benzene is aromatic, that it has aromaticity? Well, you must look for the alternating single and double bonds. There must be conjugation and resonance. But there's actually more than that. And the final test that we typically do is something called Huckel's Rule. And what Huckel's Rule is all about is the total number of pi electrons that actually exist. And the number of pi electrons must equal 4n plus 2. And what n is, n can be any whole number, 0, 1, 2, 3. So what we can do is we can come up with a series of Huckel numbers of pi electrons that will satisfy aromatic compounds. So again, if I just randomly pick four whole numbers, 0, 1, 2, 3, plug those in for n, I get the Huckel numbers of 2. 2 pi electrons, 6 pi electrons, 10 pi electrons, 14 pi electrons, and that's high enough. You know, obviously we can keep going. So for benzene, the first test is for conjugation. Double, single, double, single, double, single. That's all conjugated. When we count up the total number of pi electrons, we have 1, 2, 3, 4, 5, 6. 6 is a Huckel number. Hence, benzene is aromatic. In the middle, I believe this is anthracene, it's a carcinogen. Do we have conjugation? Well, let's start at any double bond. Let's start here. Double, single, double, single, double, single, double, single, double, single, double, single, double, and back around. So we definitely have conjugation. Do we have a Huckels number of pi electrons? Again, let's count them. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. Yep, that is a Huckel number. This molecule, this entire molecule, is aromatic. So on the far right, we have a five-membered ring, where one of the atoms is a nitrogen. It has a lone pair being prominently shown here. And do we have, first off, conjugation? Well, we certainly have double, single, double, but then it looks like we have single, single, meaning no conjugation. However, nitrogen... and other atoms similar to nitrogen have a lone electron pair. Lone electron pairs can contribute towards aromaticity if they are conjugated with other pi bonds. And they are here, because we can take this lone pair of electrons, stick them in this bond to make a double bond, take these electrons and put them here as a negative charge. In other words, as a pair of electrons. and we can move those around the ring. So lone electron pairs can be part of an aromatic system. What that means is we have a p orbital on every one of these carbons and also a p orbital on the nitrogen where these two lone pair electrons exist. So we have conjugation because of the lone electron pair. Do we have a Huckel number of electrons? 1, 2, 3, 4. 5, 6. That's a Huckel number. Hence, this molecule is aromatic. So, how does that contrast to these three compounds, which are non-aromatic? Well, on the far left, it's easy to see that we don't have conjugation, because we go double, single, single. Same thing here, double, single, single. But notice in this compound, we go double, single, double, single, double, single. We're all conjugated, so it seems pretty good. But do we have a Huckel number? 1, 2, 3, 4, 5, 6, 7, 8. 8 is not a Huckel number of pi electrons, hence this stop sign type compound is not aromatic. Finally, we can have ions that are aromatic. So, for example, The brominated compound we see here undergoes ionization very easily. In other words, that bromine will leave. As it leaves, we get a carbocation. That carbocation is extremely stable. The reason it's stable is because that makes that 3-carbon ring aromatic. Remember, a carbocation is sp2 hybridized, and that means it has a p-orbital. It's an empty p orbital, but a p orbital nonetheless. And of course, we have p orbitals for the double bond. And since each carbon of the three-membered ring has a p orbital, that means we have conjugation. Do we have a Huckel number of electrons? The double bond, that's two electrons. Two was a Huckel number. Hence, that is aromatic. That's why that carbocation is so stable. Here's another example where we have two hydrogens that are unusually acidic, pKa of about 15, that's pretty much as acidic as water. So we can remove one of those and come up with the conjugate base. Once again, we have a situation where in this case we have an anion that is contributing to the pi electrons of an aromatic system. So again, we put in the p orbitals. The two electrons in the negative charge contribute to a p orbital. Since we have p orbitals on every atom of the ring, this also is an aromatic ion. Is it a Huckel number? 1, 2, 3, 4, 5, 6. That is a Huckel number. So that is also an aromatic ion. Okay, that will do it for now, and that is the end of this recording.

The formula is C6H6. We actually had looked at this a few weeks ago and that has a hydrogen deficiency of 8. So remember, if you ever calculate a hydrogen deficiency of 8 or higher, you will always have a benzene ring. Don't try, do not go trying to make all combinations of alkenes, alkynes, and rings.

It's not worth your time. The hydrogen deficiency of 8 or higher means you've got a benzene ring. So if you have a hydrogen deficiency of 10, that means you have a benzene ring, which is 8. and you have two left over then you can talk about oh there's another double bond there's another ring you know things like that so in this ring we notice that there are double bonds to all the carbons meaning that all of the carbons in this ring are sp2 hybridized excuse me and because of that sp2 hybridization we can also understand the geometry Each of these carbons is planar.

And since each of the six carbons is sp2 hybridized, that means that the entire ring is planar. So all compounds like benzene are going to be planar. They're going to be flat. And there is this unique property of benzene, which is going to help explain why its reactivity is so much different than alkene's. The typical reactions that we have seen for alkenes, such as hydrogenation and electrophilic addition, those reactions will not be for benzene.

Benzene will not undergo hydrogenation or the typical electrophilic addition. And the main reason for this is this property of aromaticity. Benzene and many other compounds like it are referred to as being aromatic. Part of that is because of the black soot that you observe when you burn these compounds, but more so, it's the physical property of aromaticity. And this physical property, it helps to explain why benzene and other aromatic compounds are unusually stable, their unique reactions, and even the toxicity we see, and the persistence of benzene and other aromatic compounds in the environment.

So let's take a look now what we mean by this concept of aromaticity. So we start off first with conjugation. Conjugation in the case of benzene means that we're referring to these alternating double and single bonds going around the ring. Okay, so we're going to see other ringed compounds. where the pi bonds are all inside the ring, and in order for aromaticity, they must be conjugated in some way.

Again, that's alternating double and single bonds. Since we have conjugation, it means that we have a p orbital on each of these carbons. So we're going to look at this from a different perspective.

So I'm going to make believe that we're sticking our head in the screen, and we're looking edge-on. at that particular bond on the right. So if we now turn and rotate the benzene ring 90 degrees, so here we are now. So we're looking, here's the bond we're looking at.

Now it's coming right out of the screen at us. This bond is now facing backwards. So we're kind of looking at it from a planar perspective. So where are those p orbitals?

Okay, those are those figure 8 shaped orbitals. And now we have one on every single one of the carbon atoms of the benzene ring. Now because of these p orbitals, we have something called delocalization. It's delocalization of the pi electrons. What this means is that the pi electrons can move freely from one p orbital to the next.

And whenever you have electrons that can move freely along through a conjugated system, we say that they are delocalized. So the delocalization I am representing on the top of the ring by that red circle and the same thing on the bottom of the ring. These are often referred to as a ring current. And this is one of the very important properties of aromaticity because this ring current, this conjugation, this delocalization of the pi electrons is what leads to this unique stability. In order to break up this conjugation, it takes a lot of energy.

Hence, benzene tends to be very stable. So another factor to consider is resonance. Since the pi electrons can move freely between alternating single and double bonds, we can draw resonance structures.

So we're going to start off, just to illustrate this very simply, here's a simple planar benzene ring, and you will notice that I've highlighted one of the double bonds in bold, and what we're going to do is we're going to move that double bond, one bond to the next, all the way around the ring. So there it goes to the next position. So notice I'm going around clockwise. That position, another, another, another.

And then finally, we get back to where we started from. So when we consider the hybrid resonance structure for this, what we are seeing is the pi electrons going around and around the ring, clockwise, counterclockwise, it doesn't matter. And we can represent the hybrid by this very simple structure of benzene with simply a circle in the middle. So this is a neat shorthand way of drawing benzene without drawing in the hydrogens and without drawing in the alternating double bonds. I would suggest you avoid this simple hybrid structure.

I think it would be better if you actually draw in the alternating double bonds. And the reason is that I've already seen some of you in class putting five bonds to carbon. When you have the ring in the middle, you kind of forget the double bonds are actually there.

So I would strongly suggest that you do draw in the double bonds. So there is some very interesting historical depictions of what researchers and chemists thought benzene looked like over history. And...

A lot of this ended when a chemist named Kekule came up with the alternating single and double bonds and the hybrid form. And apparently this vision of the structure of benzene came to him when he was sleeping one night after a hard day of thinking about benzene and was dreaming about snakes. And the snakes grabbed, you know, one snake grabbed its own tail and went spinning around.

And to him, that represented the resonance structures of benzene. Personally, I think he was doing research on some alkaloids and had been tasting too many of his products. In any event, as you can see, there are a lot of very interesting structures that have been derived for benzene. Okay, so how can we tell then if some compound other than benzene is aromatic, that it has aromaticity?

Well, you must look for the alternating single and double bonds. There must be conjugation and resonance. But there's actually more than that. And the final test that we typically do is something called Huckel's Rule.

And what Huckel's Rule is all about is the total number of pi electrons that actually exist. And the number of pi electrons must equal 4n plus 2. And what n is, n can be any whole number, 0, 1, 2, 3. So what we can do is we can come up with a series of Huckel numbers of pi electrons that will satisfy aromatic compounds. So again, if I just randomly pick four whole numbers, 0, 1, 2, 3, plug those in for n, I get the Huckel numbers of 2. 2 pi electrons, 6 pi electrons, 10 pi electrons, 14 pi electrons, and that's high enough. You know, obviously we can keep going.

So for benzene, the first test is for conjugation. Double, single, double, single, double, single. That's all conjugated. When we count up the total number of pi electrons, we have 1, 2, 3, 4, 5, 6. 6 is a Huckel number. Hence, benzene is aromatic.

In the middle, I believe this is anthracene, it's a carcinogen. Do we have conjugation? Well, let's start at any double bond.

Let's start here. Double, single, double, single, double, single, double, single, double, single, double, single, double, and back around. So we definitely have conjugation.

Do we have a Huckels number of pi electrons? Again, let's count them. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. Yep, that is a Huckel number. This molecule, this entire molecule, is aromatic.

So on the far right, we have a five-membered ring, where one of the atoms is a nitrogen. It has a lone pair being prominently shown here. And do we have, first off, conjugation? Well, we certainly have double, single, double, but then it looks like we have single, single, meaning no conjugation. However, nitrogen...

and other atoms similar to nitrogen have a lone electron pair. Lone electron pairs can contribute towards aromaticity if they are conjugated with other pi bonds. And they are here, because we can take this lone pair of electrons, stick them in this bond to make a double bond, take these electrons and put them here as a negative charge.

In other words, as a pair of electrons. and we can move those around the ring. So lone electron pairs can be part of an aromatic system. What that means is we have a p orbital on every one of these carbons and also a p orbital on the nitrogen where these two lone pair electrons exist. So we have conjugation because of the lone electron pair.

Do we have a Huckel number of electrons? 1, 2, 3, 4. 5, 6. That's a Huckel number. Hence, this molecule is aromatic.

So, how does that contrast to these three compounds, which are non-aromatic? Well, on the far left, it's easy to see that we don't have conjugation, because we go double, single, single. Same thing here, double, single, single. But notice in this compound, we go double, single, double, single, double, single. We're all conjugated, so it seems pretty good.

But do we have a Huckel number? 1, 2, 3, 4, 5, 6, 7, 8. 8 is not a Huckel number of pi electrons, hence this stop sign type compound is not aromatic. Finally, we can have ions that are aromatic. So, for example, The brominated compound we see here undergoes ionization very easily.

In other words, that bromine will leave. As it leaves, we get a carbocation. That carbocation is extremely stable.

The reason it's stable is because that makes that 3-carbon ring aromatic. Remember, a carbocation is sp2 hybridized, and that means it has a p-orbital. It's an empty p orbital, but a p orbital nonetheless.

And of course, we have p orbitals for the double bond. And since each carbon of the three-membered ring has a p orbital, that means we have conjugation. Do we have a Huckel number of electrons? The double bond, that's two electrons. Two was a Huckel number.

Hence, that is aromatic. That's why that carbocation is so stable. Here's another example where we have two hydrogens that are unusually acidic, pKa of about 15, that's pretty much as acidic as water. So we can remove one of those and come up with the conjugate base. Once again, we have a situation where in this case we have an anion that is contributing to the pi electrons of an aromatic system.

So again, we put in the p orbitals. The two electrons in the negative charge contribute to a p orbital. Since we have p orbitals on every atom of the ring, this also is an aromatic ion. Is it a Huckel number? 1, 2, 3, 4, 5, 6. That is a Huckel number.

So that is also an aromatic ion. Okay, that will do it for now, and that is the end of this recording.

Transcript for:Exploring Benzene and Aromaticity

Transcript for:
Exploring Benzene and Aromaticity