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Studying for a test? Prepare with these 7 lessons on Aromatic compounds.
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Benzene was first discovered by the great Michael Faraday. And at that time, chemists could figure out the molecular formulas of compounds, but they didn't know the exact dot structures. And so chemists knew that benzene contained six carbons and six hydrogens, but they weren't sure exactly how those atoms were connected. And so there were several different proposals for structures for benzene. And the winning structure was proposed by August Kekule who said that benzene contains six carbons in a ring, and then you have alternating single and double bonds in that ring. And the story goes that Kekule came up with this dot structure when he had a dream and he saw some snakes bite each other's tails. And the snakes then whirled around in a circle. And that gave Kekule the idea for the ring. So this is one possible dot structure for benzene but I didn't have to draw my double bonds in this place. I could actually show a resonance structure for benzene. I could take these electrons, move them over here and push these electrons over here. And then these electrons would be over here. And so a resonance structure for benzene. I could have my pi electrons over here, over here, and over here. So either one of these Kekule structures is an acceptable dot structure for benzene. Now remember, in reality, since these are resonance structures, the actual molecule is more of a hybrid of these two molecules. And with that in mind, sometimes chemists will prefer to represent benzene with the six carbons in a ring and with a circle here in the center to represent the delocalization of those pi electrons. So maybe this is the lazy way to represent benzene. And this is called a Robinson circle after the great synthetic organic chemist, Sir Robert Robinson. And so sometimes the Robinson circle is a useful way to represent benzene. And sometimes a Kekule structure is over here on the left. And you would use a Kekule structure if you're trying to show the mechanism of reactions that benzene does. Let's look at how to name derivatives of benzene. So here we have benzene with a methyl group coming off of it. And so you could just call this molecule methyl benzene. So that's one possible name for it. But most people don't call it methyl benzene. It's called toluene. So it's such a common molecule in organic chemistry that toluene an acceptable IUPAC name. And in this example, we have an alkyl substituent that has only one carbon. This methyl group has only one carbon versus the six in the benzene ring. Well, what happened if your alkyl group had more carbons than your benzene ring, which is the situation over here on the right. So if we count up how many carbons we have, we have a total of seven carbons. And so in this case, we're actually going to mean this is an alkane and name the benzene ring as a substituent coming off of our alkane. So a seven carbon alkane would be called heptane. So I can go ahead and write heptane here. Then I have a benzene ring coming off of carbon 4. And when you're naming a benzene ring as a substituent, so it would be C6, H5 substituent coming off of my ring here, we call it a phenyl group. So I have a phenyl group coming off of carbon 4. So this would be 4-phenyl heptane as the IUPAC name for this molecule. Let's look at some other examples where we see benzene with one group on the ring. And these are all very famous monosubstituted benzenes. And because they are so famous, their common name is acceptable in IUPAC nomenclature. So a benzene ring with an OH group on it is called phenol. And we can use that when we are naming molecules. Benzaldehyde would be an aldehyde coming off of a benzene ring. And benzaldehyde is, of course, famous for having the smell of almonds. It's a really, really wonderful smell which is also why you'll hear some of these molecules referred to as aromatic compounds. Originally, it was because of the smell. And we're going to see in future videos what aromatic means in a chemistry sense. Over here, we have a benzoic acid, a carboxylic acid functional group coming off of our benzene ring here like that. And so, here I have seven of the most famous and most common monosubstituted benzene derivatives. And so these are molecules that most professors will have you memorize because you can use these names when you're trying to name more complicated benzene derivatives. So commit these to memory. Let's look at some disubstituted benzene derivatives. So this molecule over here on the left has two methyl groups coming off of the benzene ring. And so this first molecule has a methyl group coming off carbon 1, and coming off of carbon 2. So we could call this 1, 2 dimethylbenzene. That would be an acceptable IUPAC name. But whenever you have a benzene ring with two methyl groups on it, the common name for that is xylene. And so if we wanted to call this molecule xylene, technically, all three of these molecules will be xylenes. They're all benzene ring with two methyl groups coming off of it. And so we have to distinguish these xylenes from each other. And so when you have two groups that are right next to each other on a benzene ring, in this case my methyl groups, my methyl groups are right next to each other on my benzene ring, we say that that relationship is ortho. So I could call this molecule ortho xylene. And that would be another acceptable name. And sometimes you'll just see an O there. So you call it o-xylene. And that's fine too. We go over here to this molecule. This is also xylene. But we can see the methyl groups are in slightly different positions. Right now, we would have 1, 3 dimethylbenzene. So 1, 3 dimethylbenzene would be an acceptable name for this molecule. And when you have two groups that are a carbon away from each other, so this relationship is said to be meta in organic chemistry. So you could call this a meta, let me take off that m here as I am running out of room here. You can call this meta-xylene. So this is meta-xylene or just m-xylene. And then finally another xylene molecule. This time our two methyl groups would be at carbons 1 and 4. So you call this 1, 4 dimethylbenzene. Let me go ahead and write that in. 1, 4 dimethylbenzene would be one IUPAC name for it. But again, most people would name this as a xylene derivative. So it's a xylene derivative. This time my two groups are opposite each other. So they're opposite each other on the ring. And we call this relationship para in organic chemistry. So you could say it's para-xylene or also p-xylene. Let's look at some more examples of disubstituted benzene rings. And so here we go. We're actually going to use the monosubstituted derivatives that we talked about above. So if I look at this molecule over here on the left, I can see that this is the phenol portion of the molecule. So I can go ahead and say that this is phenol. And then in terms of identifying the bromine, I have two options here. I could use a number. I could say that bromine is at carbon 4. So I could call this 4-bromophenol. And that's an acceptable IUPAC name. Or I could use the OMP system that we talked about above. And when you have two groups that are opposite on the ring, we call that para. So I could also call this molecule para-bromophenol. And that's an acceptable name as well. Or I could even shorten it to p-bromophenol. Let's do this disubstituted benzene ring over here. And if I look at it, I can see that is benzoic acid. That's one of the ones that we memorized above. So I can go and write the parent name as being benzoic acid right here. And I now have to identify my substituent coming off of my benzene ring. So benzoic acid would make this carbon 1. And then my substituent is coming off of carbon 3, and my substituent is a nitro group. So I could call this 3-nitrobenzoic acid. Or I could say that the relationship between those two things coming off my benzene ring would be meta. So I could call this meta-nitrobenzoic acid. So let me see if I have room here. So meta-nitrobenzoic acid. Or I could just say m-nitrobenzoic acid. So all of those would be correct IUPAC names. Let's look at polysubstituted benzene derivatives now. And so these are actually two of the more famous examples that we could do. Once again, we're going to try to find a monosubstituted parent name here. And if I look, I can see that right here, this would be benzaldehyde. And so usually aldehydes have precedence over alcohols. And so that's why we're going to name this as a benzaldehyde derivative. So I'm going to go ahead and-- let's see, just to make sure I have enough room to start naming it by saying the parent name is benzaldehyde right here. And since the aldehyde gets precedence, this would get a carbon 1. We want to get the lowest number possible to our substituents coming off my ring. So I'm going to go this way. And I can see that I have a group coming off of carbon 3. And it's an ether group. So we talked about how to name ethers as substituents. This would be a methoxy substituent. So this would be coming off of carbon 3. So I'm going to write 3-methoxy benzaldehyde like that. And then I go over here to carbon 4. And I could say I have an OH group. And if I'm naming an alcohol as a substituent, I would call this a hydroxy or a hydroxyl group. And I'm going to say that this is a hydroxyl group at carbon 4. So 4-hydroxy-3-methoxy benzaldehyde would be the IUPAC name for this molecule. This molecule is better known as vanillin. So the smell of vanilla is probably my favorite smell. So I really enjoyed doing labs that involved the vanillin molecule. Let's do one more, another extremely famous example. I can see that I'm trying to find a monosubstituted benzene derivative, that would be toluene right here. So I'm going to go ahead and put toluene as my parent name. And that would make this carbon 1. The methyl group would become carbon 1. So I go ahead and number to get the lowest number possible. And I can see that I have three nitro substituents located at 2, 4, and 6. So to finish my IUPAC name, it would be 2, 4, 6. I have three nitro, so that would be tri- as my prefix, so trinitro toluene. And so this is also a very famous molecule. It's not normally called 2, 4, 6 trinitro toluene. Most people in the general public would, of course, know this as TNT. So the famous explosive. So this would be TNT. And we just named it. Of course, the name comes from the nomenclature for polysubstituted benzene derivatives. So hopefully this video just shows you an introduction as to how to name molecules with benzene in them.