Naming a cycloalkane | Organic chemistry | Khan Academy

Let's see if we can name this guy right over here. And so, like always, we always want to look for the longest carbon chain or the longest carbon cycle. I think it's pretty obvious from this picture that we have a very long carbon cycle here that we can start. We can just look at this: we have one, two, three, four, five, six, seven, eight carbons in a cycle right over here.

So we can call this backbone, which is the backbone of our molecule right over here. This is cyclooctane, cyclocyclooctane. 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 could 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 bute. And it is in a cycle, so this is a cyclobutyl group, cyclocyclobutyl butyl. And we also have another cyclobutyl right over here. So this is another cyclobutyl, cyclo. I'm spelling 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.

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 butte. But this isn't in a cycle, and it's not just in a straight chain; it has this y shape.

The common name when we end up with this y shape, when we're attached down here and then that's we're attached to one carbon here, and that's attached to kind of this carbon right over here, this then attached to these other two, this is an isobutyl group, which we've seen multiple times before. Iso butyl, 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 want to go back to our main carbon chain, which in this case is a cycle, and think about how we want to number. The convention is to start numbering at the thing that is first in alphabetical order, and when we consider 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 sec-butyl or tert-butyl, but for isobutyl, you do consider the "i." For the cycles, you consider the "c." And so when you look at it that way, all of the "c's" come before the "i's."

Amongst the "c's," these all have cyclo, so cyclobutyl comes before cyclopropyl or cyclooctane. "B" comes before "p" or "o." So the thing that is first alphabetically are both of these cyclobutyls. We can start numbering; we can start numbering at where either of them are attached to the main carbon backbone.

But we have to—those are, I guess we should say those are both options. But we want to number from the one that is going to give us the total lowest numbering. So let's think about that a little bit. 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, because remember we're trying to minimize the numbers here. We might want to 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 1, at the 2, or where do we have groups attached to the chain? 1, 2, 4, four, six, and seven.

So that's one option; we'll have groups attached one, two, four, six, and seven if we start right over there. And if we were to go clockwise, our other option is to start—and let me erase those—our other option is to start at this other cyclobutyl.

At 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 is one, and then we would also go clockwise to hit those cyclopropyl as soon as quick as 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—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 going to 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 they're two cyclobutyl groups, we would say dicyclobutyl. One for 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 two, two cyclopropyl, two cyclopropyl.

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—we can say five, seven. And since there's two of them, we'd say diisobutyl, five, seven, diisobutyl.

So di iso bu, diisobutyl. And then we just give the name of the backbone. We've tackled, we've done the butyls, the dicyclobutyl right there, the cyclopropyl, the two isobutyls, and now we just write cyclooctane, cyclocyclooctane, cyclooctane.

And we are done. We have named—successfully named the molecule, and this was kind of fun. It's like a secret code or something. You wouldn't expect naming organic molecules to be, uh, I think quite so satisfying, but it is.