Fused and Bridged Bicyclic Nomenclature

Bicyclic rings possess multiple ringed systems within the same molecule

Types of Bicyclic Molecules

Concept: The two types of bicyclic molecules 

2m
Video Transcript

As we talked about before, bicyclics basically come in two different categories. There's what I call the normal bicyclics and what that would be is just one structure with two rings that are attached by one bond. I could say this is ring number one, this is ring number two. This would be a normal bicyclic.
Then we have the other type of bicyclic called bridged bicyclic and these actually have three rings total. I know it can be difficult to visualize, but basically imagine that you have one ring down here that is basically the bottom, the base, and then you'd have a second ring formed on this side. Then you'd have your last ring, your third ring, formed on that side.
What that means is that you have three rings that are all on this compound structure and they're all connected by what this is called the bridgehead atoms. Remember that the bridgehead atoms are simply these right here. We call those bridgehead.
Now it can be difficult to visualize this, but it turns out that both of these structures, both the 3-D one that I drew here and this one over here are both the same thing. They're just two different ways to represent the same molecule. Basically, the bottom part down here, that's actually cyclohexane that I have drawn right here. So that would be that blue part right there. And then the bridge, which is the part that comes over the top that I'm drawing in red, would be right here. So this is basically the 2D way to represent it or planar and this would be the 3-D way to represent it. But in either case, they're both the same thing.
Now let's go on and figure out how to name these guys. It turns out that bicyclics because of their added complexity of multiple rings, are going to be named completely differently as monocycloalkanes. 

Basically there two general categories of bicyclics:

Bridged Bicyclic Nomenclature (Three Rings)

Concept: How to name a bridged bicyclic 

5m
Video Transcript

So we're going to have to kind of figure out what this nomenclature is. The first step is we have to figure out which atom gets the one position. It turns out that the bridgehead atom must always be in the 1 position. Now there are two different bridgehead atoms in all of these molecules, so the one you would pick would depend on maybe how close it is to a substituent, but, regardless, you're always going to give your 1 position to one of the bridgeheads.
Let's go ahead and look at this compound. The bridgehead would be either this one or this one. In this case, there are no substituents, so it doesn't matter which one I pick, but one of those would have to get the 1 position. So let's just give the back one the 1 position.
Then the next thing we do is we look at the actual format of the name and what you'll notice is that a few things have changed. First of all, the prefix used to be cyclo- if it was one ring, but now we're going to exchange that for bicyclo-. Bicyclo- just stands for the fact that it's a bicyclic compound, so any time you have bridgehead atoms, any time you have two rings or more co-joined together, that's going to be bicyclo-.
Then notice that the n or the functional group would just be an alkane. And what's interesting about this is that we're going to sum all of the carbons together to get the name of this alkane, meaning that I'm going to count up how many carbons are in this entire structure and that's going to be the name of my alkane.
If we wanted to, we could go ahead and just use these first few steps on this compound. We could say well, this is a bicyclo- compound because of the fact that I have bridgehead atoms. It also is going to be an octane because I have 1, 2, 3, 4, 5, 6, 7, 8 carbons. Now is that the correct numbering? Absolutely not. Don't take that as me saying that's the way to number the alkane. I'm just saying that there are eight total. I was just counting. That means this is definitely going to be a bicyclo octane.
But now let's talk about these funky numbers in the middle because I know you're already thinking, how do I do that. Basically what we do is we figure out how many carbons are inside of these rings that are not bridgehead atoms.
What I would do is I would say in this case there's three rings. Remember I taught you guys that in a bridged compound there's three rings. So you would say, “Okay, how many non-bridgehead atoms are in ring number one, ring number two, and ring number three.” Then you put them together using periods and we list them from largest to smallest. So I would start off with my largest ring here end off with my smallest ring size there and you might guess that the middle one would be in the middle at ring two.
Let's go ahead and look at this structure now. What I see is that I have my bridgeheads that I'll label right here, bridgeheads, and then what I've got is a few different rings. I've got the bigger ring, which is right here, so I'll call that one number 1. And what we'll notice is that I have 1, 2, 3, carbons that are not bridgehead atoms on that ring. That means the ring size of this would be equal to three.
Then let's go to the next one. The next one would be over on this side. On this side, I would have one, two atoms that are not bridgehead atoms on that side. That means the size of this ring would be two.
Then finally, at the top, notice that I only have one carbon in between my bridgehead atoms, so all I'm looking for is how many different carbons are in between these bridgehead atoms. In this case, I only have one so that would have a ring size of one.
Now, what I do is I go ahead and I put them together in order of largest to smallest and that indicates that, all right, we've got an eight-membered structure. Two carbons are bridgehead atoms because we always have two bridgehead atoms and then three carbons are in one ring, two carbons are in another ring and one carbon is in the last ring. That's what this color coding has to to with, so 3 would be here, 2 would be here and then lastly, 1 would be up here.
Now what I've done is I've just taught you guys how to basically name an unsubstituted bicyclic, but what if we actually need to number it in terms of substituents? How about if I have a methyl group coming off of this location right here? What location does that methyl get? Is it a 1, is it a 2, is it an 8?

  • The bridgehead atom (one of the circled ones) must always be in the “1” position
  • The name is formatted as bicyclo[ringsize1.ringsize2.ringsize3]alkane

Fused Bicyclic Nomenclature (Two Rings)

*Error: The name of this molecule was misstated in the video. Please refer to the above name (pentane)!

Awesome job! These are some of the hardest molecules to name, so if you got them, you’re ahead of the curve.