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Naming simple linear and cyclic alkanes. Created by Sal Khan.
Video transcript
So when we name organic molecules, the core of what we think about is just how many carbons form the chain or form the ring that we're looking at? So let's say I have something like this. And hopefully, you're reasonably familiar with what this represents. And I'll review it a little bit. So let's say I have something like this. Let's say that that is the molecular structure right there. And so the first question you should be asking is how many carbons are there? And some of you might say, wait, how is that even a molecule? And just as a review, the end point of every line represents a carbon. So that's a carbon, that's a carbon, that's a carbon, that's a carbon, that's a carbon, that's a carbon. And we have one, two, three, four, five, six carbons and we have no double bonds. So if you have all of that information you're ready to name this molecule. And before I actually name it, let me just kind of give you all of the different prefixes. So if you have one carbon, the prefix is meth-. If you have two carbons, the prefix is eth-. And it's good to memorize at least up to about 10. And actually it kind of repeats after that. If you have three carbons, the prefix is prop-. Prop- like propane, and you've heard of ethane and methane. So you'll see all of that soon. Four, you're talking about but-. Five, and after five it kind of becomes the traditional prefixes that we associate with a lot of these numbers. So at five, it's pent-, like pentagon. Six, it's hex- like hexagon. Seven is hept-. Eight is oct-, like octagon. Nine is non-. Ten is dec-. And then after that it kind of starts to have a pattern here, and you're not going to really deal with things much beyond the teens, but I'll just write them down here just out of interest. Eleven is undec-. And for those of you who know French, I'm not one of them, but I know that one in French is une, or in Spanish, uno. So it's 1 and 10, 11. Twelve is dodec-. Do or dos, if you're speaking Spanish, for 2. Dec for 10, 2 and 10, that's 12. Thirteen, you could imagine what it's going to be. It is tridec-. Fourteen is tetradec-. A tetrapod is something with four legs. And after that it, becomes very systematic. At fifteen is pentadec-. Notice pent-, 5 and 10. Sixteen is hexadec-. Seventeen is heptadec-. So it just goes on and on and on. I don't think I've got to go-- anyway, it's hexadec-, heptadec-, octodec- is eighteen, nineteen is nonadec-, and then twenty is actually iso-. But we won't even go into that. This'll probably serve our purposes. I mean, I could go up to sixteen is hexadec-. So this is just how many carbons are in our longest chain. What I drew here is just one chain. So we could immediately-- let's try to name it. Well, how many carbons do we have here? Well, we have one, two, three, four, five, six carbons. So we'll be dealing with hex- as a prefix. And then to get, I guess, the post-fix on this prefix, or the kind of the root, you look to see if there are any double bonds here. And there are no double bonds here. And if we have no double bonds in this carbon chain, we're dealing with an alkane. This is called an alkane, which is a general term for all of the chains of carbons that have no double bonds on them, or no triple bonds, all single bonds. So in this situation, you take hex- for six, so this is hex-. And then, because it's an alkane, it gets the -ane from alkane. So this is hex-, this is hexane. Let's do another one. Let's say I have this thing right here. I'll draw-- let's make it even longer. So let's say I have that thing right there. So how many carbons do we have? We have one, two, three, four, five, six, seven carbons. They're all single bonds, so it's an alkane. So this will be seven carbons. It is heptane because we have all single bonds. Now, if things form a chain, or if things form a ring, I should say, we put the prefix cyclo- in front of it. So if I have-- let me show you what I'm talking about. So if I just have five carbons, one, two, three, four, so one, two, three, four, five. OK, so that's five right there. I have one, two, three, four, five carbons in a chain. If I just have five carbons in a chain like this, this would be pentane. But if I have five carbons and they form a ring, so let me draw it. So it's one, two, three, four, five carbons and it forms a ring. Let me make the drawing a little bit better. So it's really, I'm just drawing a pentagon. But notice, this has five carbons on it. I can draw the carbons here. Carbon, carbon, carbon, carbon, carbon. And just as a review, what you don't see is the hydrogens they're bonded to. Each of these guys have two bonds, so they must have two bonds with something else and those are going to be with hydrogen, And I'lll draw it here just as a bit of a review, but you notice very quickly, the drawing gets extremely messy when you draw the two hydrogens on each of these carbons. So it's a little bit over-- maybe I shouldn't be doing that. But there you go. So it becomes very messy when you draw the hydrogens, so it's better to just assume that they're there. If we don't draw all four bonds of the carbon, the other two bonds are going to be with hydrogen. So here, you might say, OK, this is an alkane, because I don't have any double bonds here. All of these are single bonds with the carbon. I have five carbons, so you might say this is pentane, but you have to think about one more thing. It's in a ring, so we add the prefix cyclo- to it. So this is, because it's a ring, we write cyclopentane. So let me just break that apart. This tells us that we're dealing with a ring. You see that this is a ring right there. This tells us that we're dealing with five carbons, and then this tells us right here, the -ane part, that tells us that they are all single bonds. All carbon-carbon single bonds. No double or triple bonds. All single bonds. So let's go the other way. Let's start with the word and let's see if we can figure out what the actual structure would look like. Let's say I have cyclononane. So what is this telling me? This tells me I'm dealing with a ring. That is a ring. It's going to have a ring structure. It's going to have nine carbons, nine C's, and then it's an alkane, so they're all going to be single bonds. So if I want to draw it, I want to draw nine carbons in a ring, it's not a trivial thing to draw. I'll try my best, so let's see, that's one, two, three, four, five, six, seven, eight. Let's see, let me draw it. I'll try a little a better shot at it. So, let's see, you have one, two, three, four, five, six, seven, eight, and then nine. And then you can connect the last. So let me make sure that this is-- and obviously, I could have drawn it better than that, but hopefully, you can see all of the edges here. So I have one, two, three, four, five, six, seven, eight, nine carbons. It's in a cycle. It's in a ring. I have nine carbons. They're all single bonded, so this is cyclononane, although there's probably better ways to draw that ring right there. So if someone were to tell you, octane, and that word might feel familiar to you from the gas station. They are literally talking about the molecule octane. And now you know, or at least you have a sense of what they're talking about. The oct- tells you that you have eight carbons. There's no cyclo- in front of it, so it's not a cyclo. It's just going to be a chain. And then the -ane part tells you that they're all single bonds, so it's just going to be eight carbons in a chain. One, two, three, four, five, six, seven, eight. It's just going to look like that. One, two, three, four, five, six, seven, eight. In the next few videos, we'll add more to these molecules and make the names even more complex.