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Studying for a test? Prepare with these 5 lessons on Alkanes, cycloalkanes, and functional groups.
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So here are the rules for naming alkanes and cycloalkanes. You first find the longest carbon chain and you name it. So let's go ahead and do that. So we can see that the longest carbon chain for this example has five carbons, which we know is pentane. The second step is to number the chain to give the substituents the lowest number possible. So I could number this chain from the left or I could number it from the right. If I number it from the left, I give that substituent a number of two. If I numbered it from the right that substituent would have a number of four, so numbering it from the left, for this example, is the correct way to approach it. Identify and name your substituents is step three. Well, I know that this is a methyl group and that methyl group is found on carbon two. So I would write 2-methylpentane, and that would be the correct IUPAC name for this dot structure. I don't have to worry about step four because I have only one substituent. So when we see multiple substituents we'll have to think about using the alphabet rule. So 2-methylpentane is the correct IUPAC name for this molecule, and from that name you should be able to draw the dot structure. Let's do another one here. So let's follow our steps. Find the longest carbon chain and name it. So I have one, two, three, four, five, six. So this would be hexane for my longest carbon chain. Number to give your substituents the lowest number possible. So once again I have a choice of numbering from the left or from the right. In this case numbering from the left would give my substituents the lowest number possible. What are my substituents? Well, I have two methyl groups this time, so I have to use a prefix. I have two methyl groups so I'm going to call this dimethyl. So dimethylhexane. And those methyl groups are coming off of carbon two, so I have to write 2, 2-dimethylhexane. So when you have prefixes you would use di for two, you would use tri for three, you would use tetra for four, you would use penta for five, and hexa for six. So let's do another nomenclature example. Let's look at this molecule. Find the longest carbon chain. So let's see, that would be one, two, three, four, five, six, seven. And name it. So seven would be heptane. So I have heptane here. Number to give your substituents the lowest number possible. My options are to start from the left or to start from the right. This time it makes more sense to start from the right because that gives my first substituent a number of two, which would be lower than if I started from the left. So one, two, three, four, five, six, and seven. So that is heptane. Identify your substituents and name them is the third step. So that's a methyl group coming off of carbon two, and that is an ethyl group coming off of carbon four. Which one comes first? The ethyl group or the methyl group? Step four says it's the alphabet. You arrange them alphabetically. So e comes before m, so you're going to put ethyl before methyl. So you have an ethyl group coming off of carbon four, so 4 ethyl. And a methyl group coming off of carbon two, so 2 methyl if I can squeeze it in here. So I have 4-ethyl-2-methylheptane is the official IUPAC name for this molecule. What about something like this? Right, so this is a little bit more complicated. Let's look at my longest carbon chain and how many carbons are in my longest carbon chain. Right? That would be eight. This is the exact same dot structure, so both of these are going to be octane. Number your carbon chain to give the substituents the lowest number possible. Well, if I number from the left I would have my substituent with a number of two. If I number from the right I would have a substituent with a number of two. So it's not immediately obvious to me which would be the correct way to start from. So let me go ahead and just number all of them. One, two, three, four, five, six, seven, eight. And if I go this way one, two, three, four, five, six, seven, eight. So let's go ahead and name both of these and let's see what we get. Well what kind of substituents do I have on the left? I have, let's see, how many methyl groups? A total of five methyl groups. So it'd be pentamethyl, so pentamethyl-octane. And obviously they both would be pentamethyl-octanes. It's the same dot structure here, so pentamethyl-octane. Where are those methyl groups? Well on the left they are on two, three, three, seven, seven, so 2, 3, 3, 7, 7-pentamethyl-octane. But when I'm on the right my methyl groups are on two, two, six, six, seven. So let me go ahead and put that down. 2, 2, 6, 6, 7-pentamethyl-octane. So I want to give my substituents the lowest number possible, but I've already seen that that first number, right, two versus two, is a tie. So that doesn't quite work. What I need to do is go to the next number. So I have a three over here and I have two over here. And you always want to do the lowest number possible. So since two is a lower number than three this is actually the correct IUPAC name for this molecule, 2, 2, 6, 6, 7-pentamethyl-octane. This is called the first point of difference rule. You want to give your substituents the lowest number possible. So if there's a tie with the first number you go to the second number and compare those numbers. And if there's a tie with the second number you go on to the third number, and so on. So, in this case, the first point of difference came with the second number. Let's look at cycloalkanes. So here I have a cycloalkane, and you name it the same way you would name a straight chain alkane. You first find the longest number of carbons. In this case it's in a ring. So we have a total of six carbons in a ring, which we know is cyclohexane. So that's my parent name. So cyclohexane is my parent name. Number the ring to give the substituents the lowest number possible. Well here's my substituent, and I know that's an ethyl group. I want to number my ring to give that substituent the lowest number possible, so obviously that must be number one. So which way do I go around my ring? Well it doesn't matter for this example because it'd be the exact same since I have only one substituent. So six carbons all the way around with an ethyl group coming off of carbon one. So I could call this 1-ethylcyclohexane. Or you could even leave off the one and just say ethylcyclohexane, and it's implied that the ethyl group is coming off of carbon one here. So that's how to name a cycloalkane. Let's look at a more complicated cycloalkane here. So now again I can see that it's cyclohexane. That's my base name here. So this is going to be cyclohexane. So I can go ahead and put that down. Next I have to number to give my substituents the lowest number possible. So which one of these two carbons is going to be number one? I could make this carbon number one, or I could make this carbon number one. Well first point of difference rule, if I make the top carbon number one, that's going to give me two number ones, versus if I make this carbon down here number one I have only one substituent coming off of carbon one. So the first point of difference rule says this could not be carbon number one. So carbon number one has to be this top carbon up here like that. So how do I number my ring? Well I could go this way. I could go one, two, three, four, five, six, or I could go the other way. I could say that's number one, two, three, four, five, six. Which way is correct? Well if I think about my substituents, this would be an ethyl group coming off of carbon three whereas this would be an ethyl group coming off of carbon five. So the one on the left is the correct way to number your ring because it gives your substituents the lowest number possible. So now we're ready to go ahead and name it. Let's look at all my substituents on this first example here. I have two methyl groups coming off of carbon one, so that'd be dimethyl. I have an ethyl group coming off of carbon three, so when I think about the alphabet rule, I'm going to put the ethyl group first. So I have 3-ethyl-1, 1-dimethylcyclohexane as the official name. Now sometimes students get confused with the alphabet rule. Think about the parent name. Think about m versus e. E comes before m in the alphabet. Prefixes don't matter. So don't think about alphabetizing something with the D for di. Think about ethyl versus methyl, and that will give you 3-ethyl-1, 1-dimethylcyclohexane as the correct IUPAC name for this molecule. Let's look at another example here. All right, so what do I do? What do I do for this one? Well, first thing, identify the longest carbon chain, this case what kind of cycloalkane is it? Well five carbons, so it's going to be cyclopentane. So they're both going to be cyclopentane here since it's the same molecule. Well which one gets a number one? I could make this a number one and this a number two, or I could make this a number one and this a number two. So let's just go ahead and name them both, and then let's see which one is the correct name. So on the left I have 1-ethyl-2-methylcyclopentane. And on the right I would have 2-ethyl-1-methylcyclopentane. And the question is which is the correct IUPAC name? First point of difference rule doesn't really work because I have one that's one then I have one that's two, so there is no first point of difference for this. So if the first point of difference rules fails, then you go to the alphabet rule, and you say to yourself, all right, so which one is going to get a number one? Is it the ethyl that's going to get a number one, or is it the methyl that's going to get a number one? If first point of difference rule fails, you go to the alphabet rule. So e comes before m, so the ethyl group gets priority. So ethyl is number one, and therefore this is the correct IUPAC name for the molecule like that. So let's look at one more here. So how do I name this guy? So it's kind of funny looking. Well let's see how many carbons are in my longest carbon chain. So there's one, two, three, four, five, six, and seven. So there are seven carbons in that chain. How many carbons are there in the cyclo portion? There's one, two, and three. So I have more carbons in my straight chain alkane than I do in my cycloalkane. And, therefore, I'm going to name this as an alkane. So there are seven carbons, so it's going to be heptane. And I have a cycloalkane as an alkyl group now. So if this wasn't an alkyl group I'd call it cyclopropane. I drop the ane ending because it's now an alkyl group, so it becomes cyclopropyl. So I have a cyclopropyl group coming off of my heptane. Let's see which carbon is that? Let's go ahead and number it. One, two, three, four, five, six, seven. So I have a cyclopropyl group coming off of carbon four. So it's 4-cyclopropyl-heptane. So when you have more carbons in your chain than you do in your cycloalkane you name it as a cycloalkyl group, and then your alkane. Let's remind ourselves an example up here where we had six carbons in our cycloalkane , only two carbons in our alkyl. In that case you name it as an alkyl cyclohexane. And if there's a tie, let's say if there's a tie, the tie goes to the cycloalkane. So If you had six carbons in both you would name it as an alkyl cyclohexane molecule.