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Alcohol nomenclature

Created by Jay.

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Video transcript

In this video, we'll look at the classification and nomenclature of alcohols. So here I have my generic alcohol up at the top, and I have an alkyl group over here on the left, and I have an OH on the right, which is called a hydroxyl group. Let's look at the classification of our alcohol. So if I have a carbon bonded to an OH, and that carbon is bonded to one other carbon in this alkyl group here, that's said to be a primary alcohol. This carbon over here is bonded to two carbons in those two alkyl groups, so it is therefore a secondary alcohol. And then this carbon over here on the right is bonded to three other carbons, so therefore it is said to be a tertiary alcohol. Let's take a look at the nomenclature of alcohols. And we'll start with some real simple molecules here. So if I had a molecule that looked like that, and I wanted to name it using IUPAC nomenclature, I want to number my carbon chain to give that OH the lowest number possible. So therefore, this carbon would get a number 1, this carbon would get a number 2, and this carbon would get a number 3. Now, if that OH weren't there, then we'd have to have a three-carbon alkane, which we would call propane. But since we have our OH there, this is actually an alcohol, and alcohol is going to have the ol ending, so this is called propanol. So let's go ahead and write propanol here. And the OH group is coming off of carbon 1, so we're going to say that's 1-propanol, like that. How would we classify this alcohol? Well, the carbon right here that is bonded to the OH, that carbon is bonded to one other carbon right here. So this would be a primary alcohol. So 1-propanol is a primary alcohol in terms of its classification. Let's look at a similar-looking molecule, and it's still three carbons, but this time we put the OH on the carbon in the middle there. So once again, you're going to go ahead and number it. So this is carbon 1, this is carbon 2, this is carbon 3. It's a three-carbon alcohol, so it's also called propanol. The difference is the hydroxyl group is on a different carbon. It's now on carbon 2. So we're going to go ahead and write 2-propanol here, which is the IUPAC name. This is also called isopropanol, rubbing alcohol. It's all the same stuff, but 2-propanol would be the proper IUPAC nomenclature. How would you classify 2-propanol? So once again, we find the carbon attached to the OH, that's this one. How many carbons is that carbon attached to? It's attached to one and two other carbons, so therefore this is a secondary alcohol. So we have an example of a primary alcohol and an example of a secondary alcohol here. Let's do a little bit more complicated nomenclature question. So let's go ahead and draw out a larger molecule with more substituents. So let's put an OH here, and let's do something like that. And then let's go ahead and do that as well. So give the full IUPAC name for this molecule. So you want to find the largest carbon chain that includes the OH. So you have to find the longest carbon chain that includes the OH, and you want to give the OH the lowest number possible, so that's going to mean that you're going to start over here and make this carbon number 1 like that. So if that's carbon number 1, this must be carbon number 2, 3, 4, 5, 6, and 7. So we have a seven-carbon alcohol. So a seven-carbon alcohol would be heptanol, so we can go ahead and start naming this. I'll make sure to give us plenty of space here. So we have heptanol, and we know that the OH is coming off of carbon 2, so we can go ahead and write 2-heptanol like that. Let's look at the other substituents that we have. Well, what do we have right here coming off of our ring? That's a three-carbon alkyl group, so that would propyl, so we have 3-propyl. So go ahead and write 3-propyl in here like that. And what else do we have? At carbon 5, we have two substituents. So we have a chloro group right here, and we have a methyl group right here. And remember your alphabet, so C comes before M. So we can go ahead and put our methyl in there coming off of carbon 5, so it'd be 5-methyl, like that. And then also coming off 5 we have chloro, so 5-chloro right in here. And that should do it. Everything follows the alphabet rules. We have 5-chloro-5-methyl-3-propyl-2-heptanol for this molecule. What about a problem that includes some stereochemistry? So let's say they give us one where we have to worry about stereochemistry. So let's go ahead and draw another chain out here. So let's see something like that, and let's make an OH group going away from us. And then let's go ahead and make this one coming out at us like that. So give the full IUPAC name for this molecule, and you have to include stereochemistry. So once again, find your longest carbon chain that includes your OH group, and you want to give that OH the lowest number possible so it takes precedence over things like alkyl groups and halogens and double bonds. So we're going to start from the left, so 1, 2, 3, 4, 5, 6, 7, 8, 9, like that. So we have a nine-carbon alcohol, so that'd be nonanol, and the alcohol's coming off of carbon 3, the OH is coming off of carbon 3. So we have 3-nonanol like that. And let's see, what's our other substituents? Well, at carbon 6, we have a two-carbon substituent. So that would be an ethyl group, so let's go ahead and put in our 6-ethyl. So we have 6-ethyl, let's go ahead and put that dash in there. So so far, we have 6-ethyl-3-nonanol. And we have to worry about stereochemistry. So if they put in wedges and dashes on the problem, you need to think about stereochemistry. And you put those absolute configurations at the beginning of the IUPAC name. So let's figure out the stereochemistry at carbon 3, first of all. So the stereochemistry of carbon 3, so this is our chirality center right here, this carbon. And again, you go to the atoms that are directly connected to your chirality center, so that's carbon, carbon, oxygen. And then we also have coming off of our chirality center, we also have a hydrogen, which is coming at us like that. So that would be the lowest priority, so that would get a number 4 here. So the oxygen is the highest atomic number, so that's going to get a number 1. And then we have a longer chain over here for this carbon on the right, so that's going to get a number 2, then we have a number 3, and then the hydrogen would be a number 4. So there's a little trick that I covered in an earlier video. So if you ignore the hydrogen, it looks like you're going around this way, it looks like you're going around counterclockwise. So it looks like its S. But you have this lowest priority group is actually coming out at you, so remember the trick was, if it looks like it's S with those three, just reverse it. And so it must be R. It must have an absolute configuration of R at carbon 3. So we'll go ahead and put in here a 3R. And then we have to worry about the absolute configuration at carbon 6. So at carbon 6 here, so this is another chirality center, four different substituents attached to it. There's also a hydrogen attached to this carbon going away from us like that. And let's think about highest priority. Well, this chain over here on the left, this chain is going to get the highest priority. It has the most carbons, it has an oxygen over here, so it's definitely going to be highest priority. There are more carbons in this one over here on the right. So again when you assign priority, this is going to get highest one. And then this is going to get a third up here. So this time, you're going around 1-2-3, you're going around counterclockwise. But your lowest priority group, which is this hydrogen back here, is going away from you. So this actually is going to be S, so it's counterclockwise, so it's S. So 3R, 6S. And I went through those kind of fast, so you need to go back and watch some of the earlier videos on absolute configurations if that was a little bit too fast for you. All right. Let's look at cyclic alcohol, so ring systems. So let's look at an alcohol with six carbons in a ring, and then there's an OH coming off it like that. So six carbons without the OH, we would call that cyclohexane. And since this is an alcohol, we would just change that to cyclohexanol, so that's very simple nomenclature, cyclohexanol, like that. You don't really need a number, but you could write a 1 there. It's implied if you don't put it in. So that would be cyclohexanol. What about something that has-- how about a ring system with two hydroxyl groups? So let's say we'll put in some stereochemistry, too. So let's say we have an OH coming out at us, and then let's say we have an OH going away from us like that. So when you have a situation like this, when you have two alcohols in the same molecule, your prefix would be di. So this is actually a diol. And the nomenclature, it's based off the cyclohexane molecule, so you would write cyclohexane. And then right after the cyclohexane is where you put the diol, so "di" meaning two, two alcohols. You have to specify where those two alcohols are on the ring, so we need to go ahead and number our ring. Let's say this is carbon 1, that would make this carbon 2, carbon 3, and then carbon 4. So we have alcohols at the 1 and 4 positions, so we'll go ahead and write 1 and 4 here. And those two alcohols are trans to each other. One's coming out at you, and one is going away from you. So you could go ahead and write trans-1,4-cyclohexanediol. And that's a good IUPAC name. So other types of alcohols. Well, instead of a cyclohexane ring, we could have an alcohol based off of a benzene ring. So we put in our pi bonds like that. And if we have an OH here, it's a special type of alcohol called phenol. This molecule's called phenol. And also you'll see this hydroxybenzene portion of the molecule in lots of natural products. So this is actually a very important molecule to recognize, phenols. All right. Let's go ahead and put something on that ring. So let's go ahead, and we have phenol as our base here. And let's go ahead and let's put a bromine right here. So how would we name that molecule? Well, the base part of the molecule is the phenol molecule, so we'll go ahead and put phenol like that. And then we'll go ahead number it. So the OH, this must be carbon 1. And we want to give our substituent the lowest number possible, so we're going to give this carbon number 2. So we have a bromine coming off of carbon number 2. So it's very simple, all you have to do is write 2-bromo. So we have 2-bromophenol like that. So that's nomenclature for alcohols and a special type of alcohol called a phenol.