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Organic chemistry naming examples involving branching and rings.  Created by Sal Khan.
Video transcript
I've pre-drawn some molecules here. Let's see if we can use what we know to name them. So what do we have? This first molecule right here, I have a bunch of rings. This is a one, two, three, four, five, six carbon ring. These are each four carbon rings: one, two, three, four, one, two, three, four, so the largest ring is essentially going to be our backbone. It's going to be this six-carbon ring right here: one, two, three, four, five, six carbons. That is a cyclohexane, all double bonds. That's where we get the -ane, six carbons hex-, it's in a cycle, cyclo-, and then we have two of these four-carbon rings. So four carbons, we're dealing with the prefix but-, but- for four. We've got two of them, so both of these are butyl groups. But they're in a cycle, so they're actually cyclobutyl groups. We have two things attached to this ring right there. If we only had one thing attached to it, you wouldn't have to number it. But when you have two things, you start numbering at one of them. Let's say we start numbering here, and you go in the direction so that the next group has the lowest number. So in this case, you want to go in the counterclockwise direction. If we went this way, it would be one, two, three, four, five. This guy would be a five. If we go in the counterclockwise, it would be one, two, three. This guy will only be a three. So this right here, we have two cyclobutyls, so it's dicyclobutyl. We have two of them and they are at the one and three positions. So at the one and three position, I have two cyclobutyls on my cyclohexane main ring. Let's try this one right here. I have a five-carbon ring, one, two, three, four, five right there, and then I have a one, two, three, four, five, six-carbon ring right there, so this is going to be the main ring. That is a cyclohexane. It has six carbons on them in a cycle, all single bonds. Attached to that, I have a cyclopentyl group, -yl for the group. This is a cyclopentyl group on it. We don't have to number it because it's only one group attached to the main ring. If there was another group, we would have to number it like we did up here. This is cyclopentyl. That's this part right here. Cyclopentyl attached to cyclohexane: cyclopentylcyclohexane. Let's try this one over here. The first thing we want to do, there's no cycles here, but we have to identify the longest chain. To do that, let's just count it out. It could be one, two, three, four, five, six, seven. That's not the longest chain. Maybe it's one, two, three, four, five, six, seven, eight. That looks like the longest chain, so let's make that the longest chain. Let's make that the longest chain right over there. We want to start numbering in the direction so we encounter the first attached groups first. We do want to start numbering down here, because we have groups attached right on the two carbon. If we started over here, we'd have to go pretty far until something's attached. So we go one, two, three, four, five, six. seven. eight. We know we're dealing with an octane, all single bonds. It's not a cyclooctane. It's not in a cycle, so we know we're dealing with an octane. Now we just have to add the groups to it. So what do we have here? This right here is just one carbon attached to the main chain. This is another carbon attached to the main chain. Both of these right here are methyl groups. Meth- is for one carbon. Those are methyl groups. If you look at all of them, these are the only methyl groups. These two up here aren't methyl groups, so we have two methyl groups on our entire chain, so it's going to be dimethyl. Both of the methyl groups our at our two position, so this is going to be 2,2-dimethyl. This part right here is 2,2. That right there is 2,2-dimethyl. We're going to decide what order to write it after we figure out what these are called because it has to be in alphabetical order. This is 2,2-dimethyl. The whole chain is an octane. What are these over here? How many carbons do we have here? One, two, three. They actually look the same. We have one, two, three here. We have one, two, three there, so these are both propyl groups. If we deal with common names, this is kind of that Y shape. You could call it sec-propyl because this carbon right here that's attached to the main chain is attached to two other carbons. But the more common one, because it forms this Y shape, is isopropyl. We have two isopropyl groups. These are both isopropyl. We would have diisopropyl. They're occurring at the four and five positions, two isopropyls at four and five. This is 4,5-diisopropyl. That's that group and that group right there are accounted for with this. Now we have to just figure out the order that we write it in. You ignore the di- or the tri- out front and you just look at them in alphabetical order. We have an I for isopropyl. We have an M for methyl. Let's write the isopropyl first. I've actually seen some people want to go for the p, but the main thing I ignore is just the di- or the tri- in front of the isopropyl. You shouldn't involve that. But everything after that, you do involve. So I'll write the isopropyl first. I comes before M, so this is going to be-- if we were going to write the whole thing, this is going to be 4,5-diisopropyl, 2,2-dimethyl. Actually, this should be a comma here. 4,5-diisopropyl- 2,2-dimethyloctane. And we're done. But this was just the common name. You might remember that when we deal with iso- or sec- or tert- butyl or propyl, that's the common name. If we want the systematic name, we can start at where we are attached to the main chain and view that as one and then make the longest chain with that as one. And so you could say that we have a chain there and this would be both of these cases. So this is one, two carbons. Two carbons, we're dealing with an ethyl. And on the first carbon, you have a methyl attached to it. So you could also call each of these groups a 1-methylethyl instead of an isopropyl. So you can either say isopropyl for each of these groups or you could call each of them a methylethyl if you do systematic naming. We have two of these 1-methylethyl groups, just like we had two isopropyl groups. If you're using common naming, you can say diisopropyl to say you have two of these groups. When you're using systematic naming, you don't say di-1-methylethyl, although that would probably would get the point across. You use bis-. Since we have two of them, instead of writing di-, you write bis-. You write bis-(1-methylethyl) That means you have two of these things right there, and it's still in the four and the five position. If you look at it in alphabetical order now, methylethyl comes after methyl, right? So the order will now change. So now if you want to write it with systematic naming, it would be written as 2,2-dimethyl. That's these two guys. 2,2-dimethylethyl, then you would write this guy, so the order changed for the two groups just based on how they're named. Bis-, and then over here you have two 1-methylethyl groups. I know it's confusing, but when you just break it down, it actually makes a reasonable amount of sense. You have two of these methylethyl groups. Oh, sorry, I forgot where they're located. So we have them at the four and five position, so in the four and five position, we have two, so bis-(1methylethyl) groups. I know it's a little daunting now, but it all makes sense when you break it down. And then we can just add the octane at the end. Let me scroll over to the right a little bit. Octane, and we're done.. Now this might seem more confusing, but when you break it down, it makes sense. We have octane as the backbone. We have two methyls. They're both sitting on the two position. So you have two methyls sitting on the two position, and then you have two 1-methylethyl groups sitting at the four and five positions. So in the four and five position, 1-methylethyl. You have an ethyl, and in the one position you have a methyl, so that's all it's saying. Or another way to think about it, just in case this doesn't confuse you enough, you could call it that, or you could say 4,5-diisopropyl. these two things are the same thing: common naming, systematic. Hopefully, you found that useful.