Ringed structures are easy to name, you just need to use a new prefix (aka –cyclo)!
Hint: Benzene and a cyclohexane are NOT the same thing. Remember, benzene has double bonds in it!
Concept: How to find the root name for cycloalkanes2m
So now we're just going to start layering stuff onto these alkanes and making the names more complex. Let's talk about what happens when you have a ring structure.
Cycloalkanes are the name given to any time that you have a ring inside of your alkane. We're going to start off with the easy ones, which is just monocyclic compounds. Monocyclic just means one ring. These are easy. All we're going to do is we're just going to attach cyclo- to the beginning of the root chain. All of the sudden hexane becomes cyclohexane if it's a ring.
The root is assigned to the portion of the alkane with the greater number of carbons. Now where this comes into play is that usually, it's really obvious which one is bigger or which one is going to get the root name, but sometimes it's not as obvious, meaning that sometimes you have both a long chain and a ring on the same structure. Most of the time it's just going to be either a chain or it's going to be a ring, but sometimes some structures combine both.
What do we do if we combine both? Then what we want to do is we want to give the part with the greater number of chains the root.
In general, we assign the root name to the portion of the alkane that has the greater number of carbons.
Example: Determine the root carbon name for the following structure1m
Example: Determine the root carbon name for the following structure1m
If you only have one substituent on your ring, the numerical location is unnecessary!
Concept: Why it is okay to omit a single location for monocyclics2m
Then lastly, if there's only one substituent on your ring. Let's say you have a ring and you have one thing coming off of it. The location of that thing can be omitted.
How does that make sense? Well, because if you have a chain – let me give you an example chain. Obviously, this is like the ugliest chain ever. I didn't even do the zigzags. But if you have a chain and you add one thing to it, that thing could be in a lot of different places. It could be there or I could erase it and I could put it there or I could erase and I could put it right at the end. Those are all different possibilities of where that stick could be. Do you just see how I'm saying that the location is going to matter? That is a different location than that.
But if I have a ring and I put it here, that's the same thing as if I put it here and that's the same thing as if I put it here. All of them are the same because the ring I can rotate as much as I want, whereas the chain, if I put it in the middle, it's stuck in the middle. It's never going to go to the end. Does that kind of make sense? For a chain, you always have to say the location. Always note location. But for a ring, location can be omitted. Does that make sense?
Now, this is only true if I have one group. If I have more than one branch, let's say I have two branches, now you need to say what the locations are. Why? Because that is going to be a very much different structure than that. And that's going to be a different structure than that. So then once I have two things, it breaks that rule. I'm just trying to say if you only have one thing coming off of your ring, many times that location will be omitted. Does that make sense? Cool.
Example: Name the following alkane2m
Example: Name the following alkane1m
Great job! Did you remember to include the location for the first example? Remember, that location is not optional!
Bicyclics are also forms of cycloalkanes, but since they are not monocycles, they have completely different rules for naming! (See next topic)
Concept: What is a bicyclic molecule?3m
Now I just want to introduce bicyclic. I'm not going to rigorously teach you how to name them here. In fact, I'm not going to teach you how to name them unless your professor specifically asks because bicyclics are kind of iffy. Some professors want you to know them, some professors don't. But I'm just going to teach you – no matter what you should know the basics of what a bicyclic is.
Bicyclics are composed of two distinct rings attached along one bond. This would be an example of a bicyclic and it's made out of two cyclohexanes. The actual name for a bicyclic of two cyclohexanes is called a declin. Declin just equals cyclohexane bicyclic.
Some professors also take a special interest in declins and I will also be monitoring your class to see if I have to teach a separate section on declins as well. Some professors don't really care.
What's important is I just want you to know that a bicyclic, by the way, this dotted bond here is the same thing as a regular bond I'm just pointing out that this is the bond that's shared. That would be a bicyclic molecule.
Now a bridged compound is a type of bicyclic and it's actually composed of three compound rings attached by what we call two bridgehead – I know this is getting a little weird – bridgehead atoms.
Here's an example. This one's called norbornane. It's a very common – this is actually one of the most common bridge structures. And you're asking me, “Johnny, where are the three rings? I do not see three rings.” Well, there actually are. There's this main thing down here. That's actually just a weird way to draw cyclohexane. That's just a six-membered ring. Then I've got a five-membered ring if I go along one side and then up like that. That's one five-membered ring. Then it turns out that I have another five-membered ring if I go up the other side and up that thing.
This thing in the middle that I keep pointing to is called the bridge. It's like, I don't know, think about that you're walking over a bridge and you're going from one side of the molecule to the other, the atoms that attach all of those are called the bridgehead atoms. That's what I meant by two bridgeheads, so this is called a bridged compound.
These are going to get – we have different ways of naming bicyclics. These have a certain way of naming, but like I said, that's going to be a separate section that I teach you only if your professor requires that you know that. I just want you to be familiar with what a bridge is.
Awesome guys. So with that said let's go ahead and move on to the next topic.
For each of the following IUPAC names draw the skeletal structure in the space provided below.
For the two questions below, an incorrect name has been given for an alkane. To answer these questions, you will need to draw the structure described by the incorrect name. Once you have drawn the structure, use the nomenclature rules to assign the correct name to the alkane.
The incorrect name is: 2-methyl-3-ethylcyclohex-3-ene. What is the correct name of this alkane?
E. None of the above
Please answer the following questions for the compound given below.
a) Provide a name
b) How many methine groups are there?
c) Give the molecular formula for this compound
d) For the number of Cs that you gave in the moleular formula for this problem, what is the maximum possible number of Hs?
e) How many secondary Hydrogens are there?
f) How many quaternary Cs are there?
Write the IUPAC name of the following line structure. Ignore R and S while naming.
Which one is the molecular formula of a cyclohexyl group?
For the compound given below, provide a name.