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Video transcript
So far we've dealt with simple chains and rings of carbons. But let's think about what happens when things get a little bit more complex. Let me just draw a molecule here and we can think about how we'll name it. So let me draw it like that. So here we don't have a simple chain. It branches off at some point. So what we do in this situation is we find the longest chain in this molecule, and let's think about what the longest chain is. If we start here we get one, two, three, four, five, six, seven, eight carbons. If we start here we get one, two, three, four, five, six, seven, eight, nine carbons. So what we do is we look at the longest chain. The longest chain is this one right here. Let me do this in a different color. So this is one, two, three, one, two, three, four, five, six, seven, eight, nine carbons. And so this will be-- that will be kind of the core of our naming. So it's nine carbons. So that right here, longest chain, has nine carbons. You can kind of view this is the backbone of our molecule and also the backbone of our naming. Nine carbons, so we're dealing with a nonane. Remember, nine, non- for nine, and then -ane, because we're dealing with an alkane. We have all single bonds over here. Now, what do we do with this thing over here? Well, what's kind of sticking off of this chain? Well, we essentially have one carbon here that's attached to the chain, right? This carbon is bonded to a carbon on the chain. If we wanted to draw the entire molecule, we could draw carbon here and it would it be bonded to three hydrogens, but that is all implicit. But what is this thing right here? Well, it's one carbon. You might be tempted to say it's methane, but it's not methane because it's attached to other things. But it would be right to use the prefix meth-. So this right here, you would use the prefix meth-, so this is one carbon, so you would want to use the prefix meth-, but because it isn't the main chain, it is added to something else, we don't write methane, we write methyl. So let me write this right here: methyl. So this means it branches off longer chain. So this molecule right here would be called-- and actually, there's one more thing that we have to think about. This thing could have been-- we could just call it methylnonane. Let me just write that down right now. So we could just call this right from the get-go, we could call it methylnonane. But there's one problem with just calling it methylnonane. This is methylnonane, but so what's-- let me draw it. So the thing I just drew here, this is, one, two, three, four, one, two, three, four, five, six, seven, eight, nine. So that's the nonane. And the one I drew here's one, two, three, the methyl is right over here. That's what I just drew. That is-- let me do it in the same color. So this is what I just drew. But maybe, what if it was something like this? What if it was one, two, three, four, five, six, seven, eight, nine? And let's say that the methyl group was, instead of being right here, let's say the methyl group was right here. So how would you name these two things differently? This name right now does not differentiate between the methyl group being on this carbon versus the methyl group being on this carbon over here or this carbon over there. And what you do in this case is you number the carbons on the longest chain, on the main backbone, and you number them so that the methyl group is attached to the lowest possible number. So there's two possible ways to number this chain. You could start here as one, two, three, four, five, six, seven. Then you would say that this would be that the methyl group is attached to the seven carbon. Or you could start numbering from this end of the chain. It would be one, two, three. Or so it could also be attached to the third. Three, four, five, six, seven, eight, nine. So you want to number it so you're closest to the methyl group. So you want to start here. One, two, three, four, five, six, seven, eight, nine. So in this case, this would not just be methylnonane. This thing right here would be three. Let me make this very clear. It would be 3-methylnonane. Because the methyl group, that one carbon, that one CH3, is attached to the third carbon on our nonane backbone, so that right there is 3-methylnonane. Since I drew this out, what is this right here? Well, once again, we have this nonane backbone, one, two, three, four, five, six, seven, eight, nine, and you want to number it so that this methyl group is at the lowest number carbon as possible. So it's closer to this end than that end. So it's one, two, three, four, I'll do that in magenta, and you can keep numbering, five, six, seven, eight, nine. So this one is going to be four, and then you have your methyl group. It's just one carbon sitting right there, so 4-methylnonane. Now, let's go the other way around. Let's say we start with the name and we want to figure out its formula. And hopefully, this gives you a good understanding how things will get more complex. And over the next few videos, they'll get more and more complex, but you'll see it's all at least reasonably logical. So let's say I were to give you let me think of one. Let's say 2-propylheptane. So how do we take this apart? So the first thing you see, it is an alkane. It's all going to be single bonds. Heptane, this is kind of the core. So the longest chain here, hept- is the prefix for seven. So this tells us-- let me make this clear. This tells us that we're dealing with all single bonds. That's what the -ane tells us. If we had double bonds, it would be -ene, triple bonds, -ine. We're going to see that in the future, but let's stay simple right now. The hept- tells us we're dealing with seven carbons. And then what does this 2-propyl mean? Well, propyl tells us-- what was the prop- prefix? If methyl is one, ethyl is two, propyl is three, butyl is four, so this is a three-carbon group, and it's going to be attached to the second carbon on the heptanes. Let me draw the heptane chain. So we have seven carbons, so one, two, three, four, five, six, seven. And then on the second carbon-- just let me pick a good color here. On the second carbon, so if we just number it one, two, three, four, five, six, seven, on the second carbon right there, we have a propyl group. We have a propyl group, but it is a three-carbon group. So let me draw the propyl group. So we have a one, two, three, three-carbon group, and it's attached at the second carbon right there. So this molecule, 2-propylheptane looks just like this. Let's do another one, maybe one that seems a little bit more difficult. Let's say that we have 6-butyltetradecane. And all of these might seem a little daunting when you see it at first, but if you really just break it up, it actually is pretty logical. So once again, we have the -ane there, so it's all going to be single bonds. What is tetradec-? What is that prefix? Well, dec- is 10 and then we have tetra-, that's 4 plus 10, that's 14. Tetradec- is 14. So this is telling us that we're dealing with a 14-carbon chain. Let's draw that. So one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen carbon chain. And if we number them, one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, this tells us that the sixth carbon will be butyl, will be a butyl group. So let's see, the sixth carbon is right here. And butyl, and you should memorize this, methyl is one, ethyl is two, propyl is three, butyl is four, so we have a four-carbon group. So let me write this in a pink maybe. So this is a four-carbon group. When you call it an alkyl group, it's attached to something else. So on the sixth carbon, we have a four-carbon group, so one-- let me draw it this way. No, this is fine. Let's say we have one, two, three, four, so that is our butyl right there, and it's attached to the sixth carbon. It's attached to the sixth carbon right there. So this is what our molecule would look like. Now, let me ask you a question. Would you ever see a 9-butyltetradecane? Would you ever see a 9-butyltetradecane? So something where-- so let me redraw it like this. So a 9-butyltetradecane, so it would have butyl over here. One, two, three, four, one, two, three, four. Would you ever see a 9-butyltetradecane? Will you ever see that written? You might say, oh, yeah, Sal, you just drew it. And the reason why you won't ever see that written is because there's a better way to number it if it's like this. Instead of starting over here at one at this end, you'd want to start one that end. So instead of saying 9-butyltetradecane, you should number it the other way. You should say this is one, two, three, four, five, six. So this is actually also 6-butyltetradecane. Let me be sure I got that. One, two, three, four, five, six. So instead of starting from the left and making this the ninth carbon, you always want to start numbering from the direction that has the lowest number for the first group.