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Naming a cycloalkane

Naming conventions for cycloalkanes. Created by Sal Khan.

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  • blobby green style avatar for user ashleylau8
    could you also name 4,5-diisobutyl as 5,7-bis(2-methylpropyl) ? so the full name is 1,4-dicyclobutyl-2-cyclopropyl-5,7-bis(2-methylpropyl)cyclooctane?
    (48 votes)
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  • leaf blue style avatar for user Chris Habgood
    What is the reason for the second numbering option exactly?
    (7 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      Names like isopropyl, isobutyl, sec-butyl, n-butyl, tert-butyl, etc., originated in the early days of organic chemistry. They named straight chain “normal”, as in n-propyl. Its isomer became “iso”, as in isopropyl. When they got to butyl, they had to invent two new prefixes, “sec” and “tert”. By the time they reached pentyl, it became obvious that they couldn’t keep on inventing new prefixes. They weren’t even using the old prefixes consistently to mean the same thing. They had to come up with a new system on which everyone could agree and which would automatically apply to new groups. This evolved into the IUPAC system of nomenclature.
      (41 votes)
  • leafers ultimate style avatar for user Justin Callahan
    Would the systematic naming of the molecule in the video be 1,4-dicyclobutyl-2-cyclopropyl-5,7-bis(2-methylpropyl)-cyclooctane?
    (10 votes)
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  • leaf grey style avatar for user Adam Baker
    At the beginning of the video, he says that the "backbone" of the group, or the longest chain of carbons, is the cyclooctane. This isn't true though, because if you start on the rightmost isobutyl group, you can go all the way around the cyclooctane to the rightmost cyclobutyl, making a 15-length chain. Is this correct?
    (6 votes)
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    • leaf green style avatar for user hasan.ali2014
      If I understood the previous video (Organic chemistry naming examples 3) correctly, when deciding the "backbone", you must decide between the cycloalkane or the linear hydrocarbon, you can't 'combine' them so to speak to get the longest chain. Also, ignoring the isobutyl(s), that's also wrong because you wouldn't combine two cycloalkanes for deciding the longest chain. Hope that helps!
      (9 votes)
  • orange juice squid orange style avatar for user Nazish
    Is it possible one could label the carbons anticlockwise instead? Or can you only label them clockwise from the first one in alphabetical order?
    (3 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      You set each carbon in turn as Number 1 and number around the ring in both directions.
      From all ten possibilities (in this example), you choose the one that gives the lowest numbers at the first point of difference.
      Then you name the substituents in alphabetical order.
      If there is a tie in the numbering choices, you choose the one with the substituent that comes first alphabetically.
      (6 votes)
  • blobby green style avatar for user KironTJames
    Sal says the reason for numbering the carbons the way he selected to was to keep the numbers lowest. What if you were working with a ring that had 1, 2, 5 and 1, 3, 4? Which would you select? I assume this shape itself can not exist, but I just wanted to illustrate my question with an example.
    (3 votes)
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  • male robot donald style avatar for user Suraj P
    What is "iso" preffix?Could you explain please?
    (1 vote)
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  • old spice man green style avatar for user Aaron White
    also, how about calling the isobutyl, diethyl? or is something more right about isobutyl? if so, what?
    (2 votes)
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  • purple pi purple style avatar for user Taruna Ramnath
    Wouldn't the name be 1,4 dicyclobutly-5,7 diisobutyl-2cyclopropyl cyclooctane? I know sec and tert aren't considered when naming alphabetically, however, I believe "iso" is included.
    Please verify, Thank you! :)
    (2 votes)
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  • blobby green style avatar for user Jakub Niemirski
    What about "1,7-di-Isobutyl- 2,5 dicyclobutyl - 4 propyl-cyclooctane? It also have 1,2,4,5,7?
    (2 votes)
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    • leaf red style avatar for user Richard
      In the event that two numbering systems yield the smallest sum of numbers for the substituents, the tie breaker is given the system where the first substituent in alphabetical order has the smaller number. So the correct system is one where cyclobutyl group is in the #1 position because it comes before cyclopropyl and isobutyl. So Sal's name in the video would technically be the sole correct answer.

      Hope that helps.
      (2 votes)

Video transcript

- [Instructor] Let's see if we can name this guy right over here. And so like always, we always wanna look for the longest carbon chain or the longest carbon cycle, and I think it's pretty obvious from this picture that we have a very long carbon cycle here that we can start, or we can just look at this. We have one, two, three, four, five, six, seven, eight carbons in a cycle right over here, and so we can call this backbone, which is the backbone of our molecule right over here, this is cyclooctane. Cyclooctane. It's an octane in a cycle. It's an alkane because it's all single bonds right over here, so that is cyclooctane. Now let's think about the different groups that are attached to this backbone right over here. So first we can look at this one right over here, and we can ask ourselves, well, how many carbons are there? How many carbons? Well, there's one, two, three, four carbons that are attached to the backbone. So our prefix for when we're dealing with four carbons is but-, and it is in a cycle, so this is a cyclobutyl group. Cyclobutyl. And we also have another cyclobutyl right over here. So this is another cyclobutyl. Cyclobutyl. Bu-, I'm misspelling it, cyclobutyl group. Now let's think about this one right over here. How many carbons are there? We have one, two, three carbons. So our prefix when we're dealing with three carbons is prop-, and they're in a cycle, as well, so this is a cyclopropyl group. Cyclopropyl. This is a cyclopropyl group. And now we can jump over to these two groups right over here. We see that these are clearly the same groups. Let's count the number of carbons they have. We have one, two, three, four carbons. So if we're dealing with four carbons, we would think about the prefix but-, but this isn't in a cycle and it's not in just a straight chain. It has this Y shape. And so the common name when we end up with this Y shape, when we're attached down here and then that, we're attached to one carbon here and that's attached to kind of this carbon right over here that's then attached to these other two, this is a isobutyl group, which we've seen multiple times before. Isobutyl group. And of course this one is an isobutyl group, as well. So these are both isobutyl groups. Now the next thing we need to think about is how we can number it. We wanna go back to our main carbon chain, which in this case is a cycle, and think about how we wanna number. And the convention is to start numbering at the thing that is first in alphabetical order. And when we're considering alphabetical order, we think even the C in cyclo and we think of the I in isobutyl. If this was tert- or sec-, you wouldn't think about the T or the S in secbutyl or tertbutyl, but for isobutyl, you do consider the I. For the cyclos, you consider the C. And so when you look at it that way, all of the Cs come before the Is, and then amongst the Cs, these all have cyclos, but the cyclobutyl comes before cyclopropyl or cyclooctane. B comes before P or O. So the thing that is first alphabetically are both of these, both of these cyclobutyls. So we can start numbering at where either of them attach to the main carbon backbone. But we have to, or those are, I guess we should say those are both options, but we wanna number from the one that is gonna give us the total lowest numbering. So let's think about that a little bit. So we could say, let's number from here. Let's make this the one carbon. And so that we hit something else as soon as possible, 'cause remember we're trying to minimize the numbers here, we might wanna go clockwise, so we hit a two right over here, three, four, five, six, seven. So in this situation, where do we hit interesting things? We hit interesting things at the one, at the two, or where do we have groups attached to the chain? One, two, four, six and seven. So that's one option. We'll have things at groups attached at one, two, four, six, and seven if we start right over there and if we were to go clockwise. Our other option, one, two, four, six, seven. Our other option is to start, and let me erase those. Our other option is to start at this other cyclobutyl, this other cyclobutyl group here on the left. Let me erase these so I don't mess up the diagram too much. The other option is to start here, make this the one, number this as one, and then we would also go clockwise to hit the cyclopropyl as soon as quick, to hit something else as soon as possible. So two, three, four, five, six, seven. So now where are we hitting interesting things? We have groups at the one carbon, the two carbon, the four carbon, the five carbon, and the seven carbon. So actually, this second numbering is preferable. Both of them have something at the one, the two, the four, and the seven, but the second one has something at a five while the first one had something at a six. So we would actually want to do this numbering right over here, and this is the numbering that we now have on our, that we have now listed right over here. Let's now write what the name of this molecule actually is. So once again, we start first in alphabetical order. So we're gonna start with the cyclobutyl groups, and we have two of them, one at the one carbon and one at the four carbon. So we could say one comma four, and since there are two cyclobutyl groups, we would say dicyclobutyl. 1,4-Dicyclobutyl. And then what comes next in alphabetical order is the cyclopropyl. That comes before isobutyl. C comes before I. That's at the two carbon. So then we can say 2-cyclopropyl. And now we can get to the two isobutyls, so there's an isobutyl at the five carbon and at the seven carbon. So we can say five, seven, and since there's two of them, we'd say diisobutyl. 5,7-diisobutyl. So diisobutyl. And then we just give the name of the backbone. We've tackled, we've done the butyls, the dicyclobutyls right there, the cyclopropyl, the two isobutyls, and now we just write cyclooctane. Cyclooctane. And we are done. We have successfully named the molecule. And this was kind of fun. It's like a secret code or something. Now you wouldn't expect naming organic molecules to be, I think, quite so satisfying, but it is.