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Video transcript
So here's a picture of the cubane molecule, which is an alkane that's shaped like a cube. And for that reason, it's one of my favorite molecules. I just think it's really cool looking. And you can see that at each of the corners of the cube, there's a carbon. So there are eight carbons total. And then there's also a hydrogen coming off of each of those carbons for a molecular formula of C8H8. At first, a lot of chemists didn't think this molecule could be made because of the high amount of angle strain that's present in this molecule. But it was made starting in the 1960s. And it's being looked at for a lot of potential uses in medicine and explosives these days, and because you can nitrate it and make things like octanitrocubane and heptanitrocubane, which are potential explosives for the future. And we're going to look and see if we can name cubane using IUPAC nomenclature in this video. So let's first think about the rules we learned in the video on bicyclic nomenclature. And if you're trying to figure out how many rings are in the system, you have to make cuts and figure out how many cuts does it take to get to an open chain alkane. So if we start with this yellow version of cubane over here on the left, I'm going to start cutting bonds. And let's see how many cuts it takes to get to an open chain alkane. For example, I could start by cutting right here. So we'll say that's our first cut. And then our second cut, we could make a cut right back here like that. So we could make that my second cut here on my cubane. And then for my third cut, I'm going to go for this one right up here. So we'll take care of that one. So that's three cuts so far. And then, if I just go ahead and take out of this one, this bond right here, and then this bond right here, so that's cuts four and five. I now get an open chain alkane. So it took five cuts for us to do that. So there are five rings in cubane, so it's pentacyclo. So that's just not immediately obvious, to me anyway, as to why there are five rings in cubane. So let's go ahead and write pentacyclo to start the IUPAC name here, so pentacyclo, meaning five rings. And then we start our brackets, just like we did in the video on bicyclic nomenclature. And to finish naming cubane, we're going to pretend like it is a bicyclic compound. And the first thing we do is identify our bridgehead carbons. So the carbons that are common to both of the two rings here. So hopefully it's obvious those are two rings and those are the bridgehead carbons that connect those two rings. When you number a bicyclic compound, you start at one of the bridgehead carbons and then you go the longest path first. So I'm going to start at this carbon, and I'm going to go the longest path, which would be up here. So this would be number 2, this would be number 3, carbon number 4, carbon number 5, carbon number 6, which takes me to the other bridgehead carbon. And then you name your next longest path. So I'm just going to continue around and make this carbon 7, and then make this one back here carbon 8. So those are my eight carbons of cubane. And so, once again, I can continue to pretend like it's a bicyclic molecule. And the next thing I would do is I would name the number of carbons in my longest path. So the number of carbons in my longest path would be this one, so there'd be 1, 2, 3, 4 carbons. Remember, you exclude the bridgehead carbons when you're doing this, so we're going to start with a 4 right here, like that. Next, you do the number of carbons in your second longest path. So we can see my second longest path would be this one right here. And there are two carbons in my second longest past. So I go ahead and put a 2 over here like that. And then finally, it's the number of carbons between the bridgehead carbons, which in this example, of course, there are no carbons between my two bridgehead carbons. So I would put a 0 here like that. But of course, cubane is not a bicyclic compound, so we have to keep going. We have to figure out how I can continue naming this molecule. And the way to do it is to next think about how many carbons are there between carbons 2 and 5. So if I draw a dashed line in here, so I can pretend like I'm connecting those two right there. And of course, there are no carbons between 2 and 5. So I can keep going. I could make this is 0, and I could put a 2 comma 5, saying there are no carbons between carbons 2 and 5. And I can continue on. I can do that between the 3 and 8. So if I were to connect the 3 and the 8 back here like that, there are no carbons between 3 and 8, so I can write 0, and then 3 comma 8. And, of course, I can do the same thing over here on the right. So between 4 and 7 there are no carbons. So I can write 0, 4, and 7, like that. So let me just clear up that 7 there. And we're done with our brackets. So the last thing you need to do when you're naming a polycyclic alkane like this is to figure out how many total carbons are in the molecule. Well, of course, there are 8 So this is octane. So I can go ahead and write octane down here, like that. And I have my IUPAC name for cubane. It is pentacyclo[,5).0(3,8)0(4,7)]octane. And there are several other molecules that are similar to cubane, which are also very interesting. So molecules like dodecahedrane would be one to check out as well if you like the structure of cubane.