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Naming Alkenes-Naming Alkynes 11,487 views

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

The rules in naming alkenes and alkyles are generally similar to naming alkanes, but also include denoting multiple bonds. First, it is necessary to count the number of atoms in the longest chain. Then we number the branches, always using the lowest numbers possible. Finally, chains or branches that include multiple bonds have different suffixes.

Alright. We're going to talk about naming alkenes and alkynes. And alkenes and alkynes are types of hydrocarbons. Alkenes contain double bonds and alkynes contain triple bonds.

So let's talk about naming them. I'll start with methane. We know that meth talks about one carbon. It's a prefix for one carbon. Well, if you have one carbon it can't be double bonded or single bo-, triple bonded to another carbon because it's only one carbon, so there is no such thing as methene. Okay, so we're going to start with ethene. Ethene has, is double bonded, it's two carbons double bonded together. Propene or three carbon [IB] bonded together or what double bond within this compound. And let's go down to butene. Butene is where things get sort of confusing. The double bonds can be located in several different locations because there's four carbons. The double bonded can, the double bond can be between any of these carbons. If you get back to propene, if the double bond was here it's actually the same compound just flipped around. So the double bond in different places is okay. It's still propene. But let's look at butene a little more in depth.

So if we were to go back to our naming, our naming rules back when we talked about them in alkanes. We were taught to number our parent carbon chain. So we're going to number our carbons one, two, three, four. And we were to name this, we would say that the double bond is between the first and second carbon making it one butene. Okay. Fair enough. Let's say it was between the middle two carbons. We again numbered it one, two, three, four and we would call this two butene and that would tell us that the double bond is between the two and not the second and the third carbon. Making it two butene.

Let's go over here. Let's say it's betwen the last two carbons meaning we would name it three butene. Well, actually, this is exactly the same as this guy overe here. It just flipped, okay? So it's the same exact thing here, it's just turned around. So this is actually the same as one butene. So I'm going to put a one in front of it because if I were to rename, renumber this and make this the first carbon, it would actually, you would notice, that was actually the same thing. One, two, three, four. And in reality, actually, you're going to want the double bond between the lowest carbon. so you are not going to want the double bond between the third one, you are going to want it at the one. So you're going to have to renumber it to make it, make sure the double bond is between the lowest numbers, lowest number to carbons. Okay. so we'll get in this more detailed when we go into practice problems.

So what about cyclical alkenes, things that are in a ring such as this one. So we know this is pentene. This, inbetween each intersection which you'll see, you're actually going to see it looking without the carbons there but just to let you know, there are carbons at each intersection of this points. Notice there's a double bond here, so we are going to, and there are methyl groups on either side. so we're going to call this a pentene ring. I'm not going to get into how they name it quite yet, but this is a pentene ring. It's a five carbons and there's a double bond between them. Okay. So let's see them get branched like when we talked about these branches on the pentene, there are branches that come off, okay? So we have our parent chain and some branched substitual groups coming off of it. So what I want to do is again just like how we name alkanes, we're going to number our carbons. So our first, our double bond gets the lowest number. So we notice the double bond is over here. so we're going to start nymbering our carbons from the left hand side to the right hand side. Okay? So I'm going to say this is carbon one, two, three, four, five, six, seven. Okay. So their number, number is going to be double bonds [IB] going to be between the second and the third carbon.

Now we have some branching going on. these are methyl groups. Because they are, they're two of them, we're going to name this, it's on the sixth and the fourth carbon. So we're going to name it four six dimethyl four six, don't forget your hyphen dimethyl two, this is heptene. That's four six dimethyl two heptene. This tells me that the methyls are in the fourth and the sixth carbon and the double bond heptene tells me it's a double bonded carbon. It's in the second location. Between the second and third. So this tells me all the directions I need to know. When I thought, let's say I've more than one double bond within my carbon chain. So for example I have, and I didn't draw on the hydrogen just for a cumbersome reasons. I have this carbon chain, there are seven of them so I'm going to, it's going to be a prefix of hept beacuse that's a prefix for seven. You want to get your double bond at the lowest number as always. So let's say I number it this way, one, two, three, four, five, six, seven, going from left to right. My double bond's between the third and the fourth and then the fifth and the sixth. So I'm going to say three five heptadiene, the reason I use the prefix di, is because there are two double bonds there. If I had three double bonds within the carbon chain, it'd be triene, so and so forth. So this is three five heptadiene. Hepta means seven, di means two double bonds, ene means that [IB] that they are double bonds. Or it could look like this. This is the same exact chain except it's just flipped the other way. So this would give my carbon chain a double bond between the two and the four. So which would I like better? Three five heptadiene or two four heptadiene? This is actually the correct answer because it gives my numbers in the lowest, the lowest numbers of the double bond within the chain.

Okay. Let's actually put all these stuff into practice. Alright. Let's say we have this compound. It's a little complicated. How are we going to go about naming it? I'm actually going to pick up a different color. Let's pick up a purple and we're going to, how are we going to go about naming this? Well, first things as always we are going to figure out which is our parent carbon chain? A parent carbon chain must contain that double bond. So I'm going to have it start from down here. One, two, three, four, five, six, seven. Okay. We know it's a heptene. So because it has seven carbons. Hept is a prefix for seven ene is the suffix for double bond. So we know it's a heptene. Oops. Oh this pen is not working. Heptene. We know that double bond's between the third and fourth carbon. so we're going to say three heptene. Okay. so then we're going to talk about our branches. We have a methyl group here, we have a methyl group here and we have an ethyl group, two carbons, here. So we're going to deal for ethyl group first because our ethyl is group alphabetically it comes before methyl. E comes before m in the alphabet. So we're going to deal with, this is four. I'm actually going to do this. Four ethyl, four ethyl and this is three six, dimethyl, to show me that there are two methyl groups. And we said that was three heptene. This is the name of that compound. Notice I put the prefix di for two methyls. I had, the ethyl come before the methyl because alphabetically that make sense. There are seven hyd- there are seven carbons in there and the parent chain and it ends in [IB] telling me that there's a double bond in there.

Alright. Let's actually draw this one out. We have three four diethyl one six heptadiene. Now yne tells me there's a triple bond. Doesn't matter. Triple bond [IB] naming is exactly the same thing. It's just that it tells you where the triple bond is rather than double bond same exact thing. So heptadiene tells me that there are seven carbon three, four, five, six, seven. One, two, three, four, five, six, seven. Okay. There is two triple bonds. So between the first and second and this last one. They're triple bonds telling me one's six and the diene tells me that they're two of them and the carbon three and four, because this is one, two, three, four, five, six, seven a carbon three and four, they are ethyl groups, which is two carbons. C C and [IB] C C. Alright? This is what your new this is what your structure will look like of this with this name, okay?

So these basically are just great instructions on how to draw my carbon, my carbon compound. Now I will just then fill in the rest of them with hydrogens to make sure that it looks complete. So this, we have a hydrogen. And don't forget every carbon just wants four bonds. That's how just figure it out. Every carbon wants four bonds. Fill. And just kind of fill it all in and you should be set. that's three, four. I know it can get kind of messy if hydrogens [IB] just sometimes [IB] don't draw them. It depends on what kind of structure yo do. But anyway the naming of the alkenes and alkynes it's actually pretty straight forward and it's basically math to draw, to figure out what the actual compound is.