Naming Bicyclic Compounds - Online Tutor, Practice Problems & Exam Prep

Bicyclic compounds are categorized into normal bicyclics, which consist of two rings connected by one bond, and bridged bicyclics, which contain three rings connected by bridgehead atoms. Naming these compounds involves identifying the bridgehead atom as position one and using the prefix "bicyclo." The structure is described by the number of non-bridgehead carbons in each ring, listed from largest to smallest. For normal bicyclics without a bridge, the third ring is denoted as zero, indicating the absence of a bridge structure.

The two types of bicyclic molecules

As we discussed earlier, bicyclics essentially fall into two different categories. There are what I refer to as the normal bicyclics, which is simply a structure consisting of two rings attached by a single bond. Imagine this as ring number 1 and this as ring number 2; this configuration represents a normal bicyclic.

Then, we have another type of bicyclic called the bridged bicyclics. These structures actually comprise three total rings. It may be challenging to visualize, but imagine having one ring at the base. A second ring is formed on one side, and then the third ring is formed on the opposite side. In this setup, you have three rings that are all part of this compound structure, interconnected by what are known as bridgehead atoms. Note that these are the atoms right here, referred to as bridgehead atoms.

Visualizing this might be difficult, but it turns out that both of these structures, the three-dimensional one drawn here and the other one over here, represent the same molecule; they are merely two different ways to depict the same molecular structure. Specifically, the lower part here represents a cyclohexane, shown in blue, and the bridge, which crosses over the top and is depicted in red, is right here. These drawings exemplify the two-dimensional or planar representation versus the three-dimensional representation. However, regardless of the depiction method, they are the same molecular entity.

Let's proceed to examine how to name these compounds. Bicyclics, with their added complexity due to multiple rings, follow a distinct nomenclature system compared to monocycloalkanes.

How to name a bridged bicyclic

So we're going to have to figure out what this nomenclature is. The first step is to determine which atom gets the one position. It turns out that the bridgehead atom must always be in the one 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 assign the one position to one of the bridgeheads.

Let's proceed and look at this compound. The bridgehead could be either this one or that 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 one position. Let's just give the back one the one position. Then the next thing we do is look at the actual format of the name and you'll notice a few changes. Initially, the prefix used to be 'cyclo' if it was one ring, but now we exchange that for 'bicyclo'. Bicyclo simply indicates that it's a bicyclic compound. Anytime you have bridgehead atoms, anytime you have two rings or more conjoined together, it's considered bicyclic. Notice the carbon structure throughout; that's going to define the name of my alkane. If we wanted, we could just use these first few steps on this compound and say, well, this is a bicyclo compound because of the bridgehead atoms. It is also going to be an octane because it has eight carbons. Now that is not the correct numbering; I'm just counting to illustrate. So that means this would definitely be a bicyclooctane.

But now let's discuss these specific numbers in the middle, because I know you're wondering how to determine that. Essentially, we figure out how many carbons are inside these rings that are not bridgehead atoms. In this case, there are three rings. Remember, in a bridge compound there are three rings. So you would count the non-bridgehead atoms in ring number one, ring number two, and ring number three. Then we catalog them using periods and list them from largest to smallest. My largest ring here ends off with my smallest ring size there, and the middle one would be ring two.

Looking at this structure now, I have my bridgeheads labeled here. And then I see a few different rings. I'll call the bigger ring here number 1. Notice that I have three carbons that are not bridgehead atoms on that ring. This implies the ring size is three. Then the next ring over on this side would include two atoms that are not bridgehead atoms. Thus, the size of this ring is two. Finally, at the top, notice I only have one carbon between my bridgehead atoms. So, the ring size here is one. Now, I put them together in order of largest to smallest, showing that we've got an eight-membered structure with three carbons in one ring, two carbons in another ring, and one carbon in the last ring. That's what this color coding indicates.

Now about naming an unsubstituted, bicyclic structure. But what if we need to number it in terms of substituents? Suppose I have a methyl group coming off this location right here. What position does that methyl group occupy? Is it at 1, 2, or 8?

How to name a fused bicyclic

Well, the way we figure that out is by using the next rule. So let's scroll down a little bit. The next rule states that we're going to number when it comes to substituents. If we have to number the locations of substituents, we're going to number from the largest ring to the smallest ring. Okay? And remember that your bridgehead atom always gets the one position. So what that means is that if we were to go ahead and number these carbons up here, okay, let's go ahead and number all 8. The first one, the one that would get the number one position would be one of these bridgehead atoms. Okay? I'm just going to put it on the back, but if you put it on the front that would be equally acceptable. Then which atom should get my 2? Should it be the other bridgehead? Actually, no. I should go around the largest ring first. So that means it's going to be 2, 3, 4, 5. Okay? So those are the numbers so far. Now what I need to do is I need to go around my 2nd largest ring. So my 2nd largest ring would be the blue one. So then this would be 6 and 7. And then finally, I would go to my least, my smallest ring and that would be 8 up here. So it's a little bit unusual, but it makes sense if you just think we're going from the largest to the smallest. Okay? So in this case, if we were to start numbering this carbon, the very first one would be here. Okay. And then as I've drawn in this diagram, we would go to the largest ring, which is this 2 carbon one, and then to the smaller ring, so it would be 2, 3, 4, and then to the smaller substituent in the 5 position, but it's okay. That wouldn't get the 2 in the 5 position, but it's okay. That wouldn't get the 2 position. Why? Because I have to be consistent. I always have to start off from the biggest ring and work to the smallest one. Now a question you guys might be wondering is okay, now I know how to do this for a bridge compound, but what about a normal bicyclic like this one? Notice that this one doesn't have 3 rings. It only has 2. It only has this 2 carbon ring on one side. And then if you go around, these are your bridge heads. Right? And if you go around, it only has one carbon on the other. Okay? That's actually part of the ring. The methyl group is a substituent. It doesn't count towards the ring size. So then what you're thinking is what happened to the 3rd ring? Remember I always have to have 3 rings? Well, it turns out that that only happens when you have a bridge. If you have a bridge, then you're going to have 3 rings. But what if you don't have a bridge? Well, if you don't have the bridge, then the 3rd ring is just going to count as 0. What you're basically telling the reader there is if you put a 0 in the 3rd position, that means this is not a bridge structure. This is a normal bicyclic where I only have 2 rings. Okay? So as you can see in this case, the way we numbered this was that we had 5 methyl because it was a methyl in the 5 position, then bicycl because I have 2 rings conjoined by 1 bond and then 210, where what that means is that I have one ring that has 2 carbons that are not bridgehead, another ring that has one carbon that is not bridgehead and then I'm missing the 3rd ring. Why am I missing the 3rd ring? Because there's no bridge at all. Okay. So I'm just going to write here once again that 0 means no bridge. Okay? And then finally, we call this a hexane because of the fact that there's 6 carbons total in the structure.

Here’s what students ask on this topic:

What are the two main categories of bicyclic compounds?

Bicyclic compounds are categorized into two main types: normal bicyclics and bridged bicyclics. Normal bicyclics consist of two rings connected by a single bond. In contrast, bridged bicyclics contain three rings connected by bridgehead atoms. The bridgehead atoms are the points where the rings are joined, forming a more complex structure. Understanding these categories is crucial for correctly naming and identifying bicyclic compounds in organic chemistry.

How do you determine the numbering of atoms in bicyclic compounds?

In bicyclic compounds, the numbering starts with one of the bridgehead atoms, which is always assigned the number one position. The numbering then proceeds through the largest ring first, followed by the second largest, and finally the smallest ring. This method ensures a consistent and systematic approach to naming, especially when dealing with substituents on the bicyclic structure.

What is the significance of the prefix 'bicyclo' in naming bicyclic compounds?

The prefix 'bicyclo' is used in the nomenclature of bicyclic compounds to indicate that the structure contains two or more rings conjoined together. This prefix replaces 'cyclo,' which is used for monocyclic compounds. The 'bicyclo' prefix helps differentiate between single-ring and multi-ring structures, providing clarity in the compound's name.

How do you name a bridged bicyclic compound?

To name a bridged bicyclic compound, start by identifying the bridgehead atoms and assigning one of them the number one position. Use the prefix 'bicyclo' followed by brackets containing the number of non-bridgehead carbons in each ring, listed from largest to smallest, separated by periods. Finally, add the name of the parent alkane based on the total number of carbons in the structure. For example, a compound with eight carbons and rings containing 3, 2, and 1 non-bridgehead carbons would be named bicyclo[3.2.1]octane.

What does a zero in the third position of a bicyclic compound's name indicate?

A zero in the third position of a bicyclic compound's name indicates the absence of a bridge structure. This notation is used for normal bicyclic compounds that do not have a third ring. For example, in a compound named bicyclo[2.1.0]hexane, the zero signifies that there is no bridge, and the structure consists of only two rings.