Rules for naming alkynes (molecules with a carbon-carbon triple bond). Created by Jay.
Before we get into the nomenclature of alkynes, let's review a few of the things we learned in the video on sp hybridization. So we learned that when a triple bond is present in a molecule, these carbon atoms right here are sp hybridized. And we talked about how the geometry around our sp hybridized atoms must be linear. So this bond angle right here is 180 degrees, which makes this portion of the molecule with the triple bond in it linear, like that. And in that triple bond, we know that one of those bonds is a sigma bond, and two of those bonds are pi bonds, OK? So if you don't remember that stuff, you need to go back and watch the video on sp hybridization. So this particular alkyne right here has alkyl groups on either side of my triple bond. So it's a disubstituted alkyne, and the more common term you'll see for that would be an internal alkyne. So let's go ahead and write that. So it's an internal alkyne. Which just means that the triple bond is found in the interior of the molecule in the interior of the chain. The triple bond is not found on the end. So that's an internal alkyne, which is different from a terminal alkyne. So let's go ahead and draw a terminal alkyne. And this is just where your triple bond is at the end of your carbon chain. So if I put a hydrogen on one side, now it's only monosubstituted, and the triple bond is on the end, or the terminal, of my alkyne. So this is a terminal alkyne, like this. So it's important to distinguish between these, because in future videos we'll see how things like terminal alkynes have special properties. So let's talk about the nomenclature of alkynes now. And let's start with the simplest alkyne, a two carbon alkyne-- so a triple bond between two carbons, like that. And let's just clear up that triple bond there a little bit. So this is our triple bond between our two carbons, like that, and I have hydrogens on either side. So when we talked about two carbons for nomenclature, our root was eth-, and since we're dealing with alkynes, our ending is going to be -yne here. So I stick the root together with the ending, so I take eth- and I add the -yne to it, so it's called ethyne. So ethyne would be the IUPAC name for this molecule. That's probably not the name that's used most frequently. This is the most famous alkyne, also called acetylene. So you've heard of acetylene torches before, and you can do a very cool demonstration called underwater fireworks where you use acetylene in there. So this is acetylene, which is the common name or ethyne, which is more the IUPAC name, and this is the simplest alkyne. So let's name another one here. Let's get an actual chain, a carbon chain in here, and let's see how to name this one like that. Well, the first thing to notice is the straight line that surrounds our triple bond there. And remember, that's because that portion of the molecule is linear, because those carbons are sp hybridized. So when you're drawing Lewis Dot Structures, you'll notice that almost everyone will draw that portion of the molecule straight to better reflect the actual geometry of the molecule. All right, so I need to figure out how many carbons are in this alkyne. So I find my longest carbon chain including my triple bond. So let's see, here is one carbon, two carbons, three carbons, four, and five. So a five carbon alkyne. So remember your root for five carbons would be pent-. So it would be pentane if it was an alkane, but since it's an alkyne, it would be pentyne. So this is pentyne. So I have that much so far. Next, I need to figure out how to number my chain. So I could number it from the left, or I could number it from the right. My goal is to give my triple bond the lowest number possible, so it's a lot like double bonds here. So I want to give my triple bond the lowest number possible, and that means, of course, I need to start from the right here. So if I start from the right, and I say this is carbon one, then my triple bond starts at carbon two. And then this would be carbon three, carbon four, and carbon five, like that. So the triple bond starts at carbon two, so all I have to do is put a two in here and say 2-pentyne. So in that respect, it's just like naming alkenes here. So that's 2-pentyne. Let's do another one, one that's a little bit more complicated than that. So let's look at this molecule here. So there's my triple bond, and then I have-- let's see, I'll put in a chain and some methyl groups there-- and so this is my molecule. All right, so once again find the longest carbon chain that includes your triple bond. So that would be one, two, three, four, five, six, and seven. So seven carbons would be hept-, so it's heptyne. So let's go ahead and write heptyne in here. So this molecule would be heptyne, like that. And let's go ahead and number it, right? I want to number to give my triple bond the highest priority here. So in this case, if I start from the left or start from the right, there'd be a two for either that methyl group or for the start of my triple bond. The triple bond is going to win. You want to give the triple bond the highest priority, and that's the reason you're going to start numbering from the left. OK, so you're going to go one, two, three, four, five, six, and seven, like that. And so now we can see that our triple bond starts at carbon two. So I'm going to write 2-heptyne here, like that. And what else do I have? Well at carbon five and at carbon six, I have methyl groups. So it'd be 5, 6-dimethyl, so go ahead and put that in here, 5, 6-dimethyl-2-heptyne, like that. And this is kind of the old school way of naming it. So old IUPAC recommendations would say to name it this way, and maybe most organic chemistry professors will still name this molecule this way. But there's a newer way that IUPAC recommends that you name this molecule. So it would still be 5, 6-dimethyl, except now you would go ahead and write hept, and then you would put the 2 in between the ending. So it would be hept-2-yne, like that. So the first way is more old school, the second way is more the new way of doing it. Either way is perfectly acceptable, just make sure to do the one that your professor prefers.