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Alkane and cycloalkane nomenclature I

Using IUPAC rules to name linear, branched, and cyclic hydrocarbons. Created by Jay.

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Video transcript

Let's start with an introduction to alkanes. Alkanes all have the general molecular formula of CnH2n plus 2, where n is the number of carbons that are in your molecule. For example, if you have one carbon in your molecule you plug one into your formula there, and you would get C1, and you would get 1 times 2 plus 2, which is of course 4, so CH4, which is methane. And we've seen the dot structure for methane many times, right. So carbon with four hydrogens around it like that. So methane is the simplest alkane. What about two carbons? Well, plug it into your formula. That would be C2. And that'd be 2 times 2, which is 4, plus 2, which is 6. So C2H6 is the molecular formula for a two carbon alkane which we call a ethane. So this would be the dot structure for ethane, carbon bonded to another carbon with six hydrogens around it like that. So there are millions upon millions of organic compounds, and you have to have some sort of logical way to name them. So one carbon is methane, two carbons is ethane, and these names were determined by what's called the IUPAC nomenclature system. IUPAC stands for International Union of Pure and Applied Chemistry, which was a bunch of chemists getting together and saying we're going to name all these molecules in a very systematic way. So for the rest of this course we're going to focus on IUPAC nomenclature for naming organic molecules. And I've kind of summarized IUPAC nomenclature up in this little table here. If you have one carbon your parent name is meth. And if you're working with an alkane your ending is an ane. So therefore a one carbon alkane is called methane as we have already seen. A two carbon alkane, the root is eth, and so that would be ethane. And so, of course, your ending would change depending on what functional group you're working with, but here we're just working with alkanes. Three carbons, the root is prop. So if I were to draw a three carbon alkane like that, that would be the dot structure for propane. Four carbons is but. So if I were to draw a four carbon alkane like that, that would be butane. And of course five carbons would be pentane. So I could go ahead and draw pentane. And we could keep going here, but I think you get the idea. These are all straight chain alkanes, meaning it's just one line of carbons, one carbon right after the other. Six carbons would be hex or hexane, seven carbons hept or heptane, eight octane, nine nonane, ten is decane, and we can see the rest of them here. Undec for 11 carbons, dodec, tridec, tetradec, pentadec, and eicos for 20. So this holds true not only for alkanes, but for other functional groups. So it's important to memorize all of these parent names here. So, so far we've talked about straight chain alkanes. What about if you get a branched chain alkane like this? So you can see it's no longer just one carbon one after another in a straight chain. You can see there's something coming off of that carbon chain. So first let's find the longest carbon chain. So this is a skill you have to develop when you're doing IUPAC nomenclature. So if I start over here, I find the longest carbon chain, and if I go like this you can see the carbon chain that I'm going for like that. How many carbons are there in that chain? Well, there was one right here, and then two, and then three, and then four, and then five. So there are five carbons in this chain. And, when we go back up here to our IUPAC nomenclature table, we see that five carbons will have pent, and it will be pentane. So the parent name will be pentane for this example. So for this molecule you're going to name this pentane. And what is coming off of my straight chain alkane? Right, I have something coming off of it right here. I have a one carbon CH3 group branching off of my pentane molecule. This is called a substituent. So a substituent is something coming off of your parent chain. Right? It's a group or groups that is connected to your parent chain. So how are we going to name this substituent? Well, it's one carbon, and this is what's called an alkyl group. And notice it's not an alkane, it's an alkyl. So the ending for an alkyl group will be Y-L. But we're still going to use our parent name to name alkyl groups. So if there's one carbon, I go back up here to my IUPAC table here and I say, well one carbon in organic chemistry the parent name of meth, and this is an alkyl group which has a Y-L ending, so I have meth plus Y-L, so this is called a methyl group, which we've said several times already in these videos. So we have a methyl group attached to my straight chain, to my parents chain here, which is called pentane. So there's a methyl group coming off of pentane in the second position. We will get into details about IUPAC nomenclature in the next video. So for right now let's just be able to identify substituents coming off of our parent chain. And the first step when you're doing IUPAC nomenclature is always to find the longest carbon chain possible. So for this molecule it is a five carbon chain. Let's look at a more challenging example. Here are three dot structures for the exact same molecule, and let's see if we can find some carbon chains for this molecule here. So let's find the longest carbon chain we possibly can. So I'm going to start with the top left one, and I'm going to look at this portion of the molecule here, and I'm going to try to find my longest carbon chain. So that might be my first guess here. So how many carbons in that chain? Well this would be one, two, three, four, five, six, and seven. So if you were to name this as a parent chain, this would be heptane since I have seven carbons on it like that. What about these substituents coming off of heptane? Well I have a one carbon alkyl group coming off of the second position. So one carbon we've seen, that would be called a methyl group. I have a two carbon substituent coming off of the third carbon. So let's go back up here and refresh our memory. What would be the parent name for two carbons for organic chemistry? So two carbons would be eth, and since this is an alkyl group it would be called a ethyl group. So we have an ethyl group coming off of carbon three. So this is a methyl group right here, and then this is an ethyl group. So we have a methyl group coming off of carbon two, and we have an ethyl group coming off of carbon three, and coming off of carbon four is yet another ethyl group like that. Let's say we chose a different way to find our longest carbon chain. Let's say we started down here. So let's say we said, oh that looks like that might be the longest carbon chain to me at first glance. So let's see what we have. Let's see how many carbons we have if we said this is our longest carbon chain. So let's number them. Let's call this carbon one. Let's call this carbon two, three, four, five, six, and seven. So once again this would be called heptane. What sort of substituents do we have coming off this molecule? We have a methyl group coming off of carbon two. We have an ethyl group coming off of carbon three. And we have another ethyl group coming off of carbon four. So that's the exact same situation we had for the first example here. So these are the same thing. So it doesn't really matter which one of those you chose, you'd be naming it the exact same name. Let's compare those two to the molecule to the example down here. Again it's the same molecule, but let's say you chose a different path. Let's say you chose down here. So you said, oh, this looks like it's the longest carbon chain to me. So you go like this, and you say, all right, that's my longest carbon chain. How many carbons are in that? Well, this would be one, two, three, four, five, six, and seven. So what sort of substituents do we have in this molecule? Well coming off of carbon four we can see there is an ethyl group. Coming off of carbon three we can see this looks kind of complicated. It's not really a straight chain. This is much more complex substituent, which we'll get to naming in a future video. So for this molecule we have a total of two substituents. For the top molecule we have an example of three substituents. So the question is which one of these will be the correct way to name my molecule according to IUPAC nomenclature? So I have two chains of equal length. Both of these chains are seven carbons. So how do I break that tie? IUPAC rules state you choose the parent chain with the greatest number of subtituents. So the top one has three subtituents, the bottom example has two substituents. So if you were to name this molecule using IUPAC packed nomenclature you would choose the top way of naming it, which again we will get to in more detail in the next few videos here. So let's look at cyclo alkanes now. So we've just done straight chain alkanes. We looked at branched chain alkanes. Let's look at cyclo alkanes. So this is a pretty funny dot structure here. Let's see how many carbons are in this triangle. Well, of course, there's one, two, and three carbons. So if I were to draw what this molecule looks like, if I were to draw all the atoms involved there'd be three carbons like that. And to complete the octet around carbon there'd have to be two hydrogens on each carbon like that. So that's a cyclo alkane. So it's carbon forming rings now. So the molecular formula for this molecule there'd be three carbons and a total of six hydrogens, and we can see that the pattern for a cyclo alkane would therefore have to be, if you have n carbons you must have 2n hydrogens for a cyclo alkane. This cyclo alkane has three carbons so we go back up here to our IUPAC nomenclature table and we say that three carbons should be prop, right? And since it's an alkane it'd be propane, but it's a cyclo alkane, so we would actually call this cyclopropane. So this molecule is called cyclopropane, like that. The next molecule looks like a square. How many carbons are in it? There are four, so that would be butane. But since this is a cyclic molecule it would be called cyclobutane. And let's do two more examples. So the next one's a pentagon. So that's five carbons. So it would be pentane. So it'd be cyclopentane. And probably the most important cyclo alkane would of course be six carbons, and six carbons would be cyclohexane. So this guy down here would be named as cyclohexane. So that's an introduction to alkanes and cyclo alkanes. The next few videos will get into much more detail about nomenclature.