Naming Epoxides: Study with Video Lessons, Practice Problems & Examples

Epoxides, also known as oxeranes, are three-membered cyclic ethers characterized by significant ring strain, making them highly reactive. Naming conventions include the cycloalkane method, where the ring is named based on its shape, and the epoxy convention, which treats the epoxide as a substituent on the longest carbon chain. Additionally, one can name them as alkenes followed by "oxide," reflecting their formation from double bonds. Understanding these naming systems is crucial for identifying and working with epoxides in organic chemistry.

Defining what an epoxide (oxirane) is.

Alright, guys. So now let's talk about naming epoxides. It turns out that epoxides are just cyclic ethers. That's basically the definition. And some types of cyclic ethers, remember that an ether is R-O-R, are going to be named as their own functional group due to increased reactivity. Okay? And the specific ones that we usually name as their own functional group are 3-membered ethers. Okay? Because there happens to be a lot of strain in those rings. They're out of their normal bonding preferences or their normal angle preferences. So what that means is that they're very reactive and it's very easy to open them up. And what we call these 3-membered cyclic ethers is there are actually 2 common names for them. We call them epoxides. So go ahead and write that down. Okay? They're also called, in some textbooks, some professors like to use the word oxirane. Okay? Oxirane. These are really synonyms for each other. Okay? An epoxide and an oxirane are the same exact thing. It's just a 3-membered cyclic ether.

How to name cyclic ethers using the cycloalkane convention.

Cool so far, right? Now, the challenge becomes how do we name these guys because sometimes, first of all, they're not always three-membered rings. And second of all, there are a lot of substituents. So it turns out that there are 3 different common ways to name epoxides and I'm going to go over all of them right now. Let's start off with what we call the cycloalkane convention. In this type of naming system, what we do is we name the entire ring as if it was an alkane first. Okay? So as you can see here, I have a 6-membered ring. But how many of those atoms are actually carbons? Only 5 of them are. I've got 1, 2, 3, 4, 5. Now you might be wondering why I started the one there. I didn't need to. I'm just using that. I mean maybe I did, but I'm just using that as an example right now just to count carbons. Okay? So I have 5 carbons, but what I'm telling you is that we should actually name it as a cycloalkane, not by the number of carbons. So what that means is that usually when we're naming an alkane, we would say there are 5 carbons, so this would be cyclopentane. But it's not. We're going to call this actually cyclohexane because we go by the shape. What we're worried about here is the shape of the molecule, not how many carbons it has in it. So this would be a cyclohexane first of all as our root. Now the difference is if we have oxygens inside of a ring, which is by definition a cyclic ether, right? Then we're going to add the prefix oxa. And what oxa is going to tell us is that there is one member of this ring that is an oxygen. Okay? So if I call it oxacyclohexane, what I'm saying is that I have a 6-membered ring where one of the atoms is an oxygen, not a carbon. Okay? And then obviously location if necessary. So let's go ahead and just talk about this for a second. The root is going to be the oxacyclohexane. I have that written here. Now we just have to talk about locations. How do we know where to put those guys? Well, it turns out that the oxygen is always going to get your one spot. So when I put the one here, that didn't really count. That wasn't true numbering. The way that I should really number it is starting from the oxygen because that's the highest priority atom inside the ring. And then obviously, I should number to give the lowest overall number or to go to the next highest priority, Okay? So this would be 3-methyl-one oxacyclohexane. Cool so far? Just so you guys know, this also applies to rings that have more than 1 oxygen. If I had 2 oxygens, that would be what was called a dioxa. Okay? Just putting that out there, you could use prefixes as well. Okay? So now let's go ahead and talk about another naming system. This is going to be commonly used for non-three-membered rings. So if not three-membered as you can see, I was dealing with a 6-membered one here. This is usually the one we use. Okay? So if it's 4-membered, 5-membered, 6-membered, etcetera, you would use this naming system. Now, if it is a 3-membered ring, we could still use it but this is not going to be the most common way to name it. Okay?

How to name epoxides using the epoxy convention.

If we are dealing with a 3-membered ring, there are much more common ways. One is the epoxy convention. So what the epoxy convention basically says is this: We have a substituent named an epoxy group. Okay? And we're just going to name our longest carbon chain as normal and then label the 3-membered ring as just the substituent coming off of that chain. And obviously, give it the lowest number. One other thing about this that's interesting is that you actually have to name the locations of both of the atoms that the three-membered ring is attached to. So as you can see here, my epoxide is going to get priority over the methyl, so I would choose this to be my first carbon over here. That means that my epoxy group, or my epoxy substituent, is across the 2 and the 3. Therefore, I'm actually going to call this a 2,3-epoxy substituent because I'm basically saying that I have a bond O across those two carbons. Then the rest of it, we're just going to name like always. So this would be 2,3-epoxy-5-methylhexane. Not so bad, right? Okay. So it's just something to consider that you could also use the epoxy convention. It's perfectly legit.

How to name epoxides using the oxide convention.

Now there's on top of that, there's even one more way to name epoxides and that this one actually comes from even further back in the history. This one is actually like a reaction. Okay? What they're basically saying is name it as an alkene. So pretend that the epoxy wasn't even there, replace it with an alkene. Okay? Name it as the alkene. Entire name. And then at the end, just add the word oxide. Okay? Now how does that make sense? The reason that makes sense is because what we're saying is that we're basically assuming that we start off with a double bond and then we did an epoxidation to put an epoxide group on that double bond. Now, you might not know how to do that yet and that's fine. We're going to actually learn that pretty soon. But I'm just saying that this is almost coming from the reactivity side of things saying, well, I could start from a double bond and if I do an epoxidation, I could get an epoxide, so then it would call it an oxide of that double bond. So in this case, I would call this this would be hexene because I've got a 6 membered chain. Notice that my double bond would be across the 2 and the 3, but the way that I name double bonds is different from the way that I name epoxides. I actually don't say that this is a 2-3 alkene. I would just start where at the lowest number. So in this case, this is actually going to be what we call a 2-hexene. So don't get them confused. This would never be called a 2-3 hexene. You only do that for the epoxy substituent. So we know you have a 2-hexene. Now we need a substituent, the 5-methyl. Right? On the 5, so this would be 5-methyl-2-hexene oxide. Now just so you know, if your professor requires stereochemistry, if your professor is asking about stereochemistry, remember that's just like cis and trans and stuff, then you would have to provide it here. Okay?

Which of the following is the correct name of the following compound:

• A. 1,1-dimethyl-1,3-dioxacyclopentane
• B. 4,4-dimethyl-1,3-dioxacyclopentane
• C. 4,4-dimethyl-dimethoxycycloethyleneoxide
• D. 1,3-dimethyl-dimethoxycycloethyleneoxide

Which of the following is the correct name of the following compound:

• A. 2,3-epoxy-3-propylhexane
• B. 3-propyl-2-hexene oxide
• C. 1-methyl-2-dipropyl epoxide
• D. A and B
• E. All are correct

Here’s what students ask on this topic:

What are epoxides and why are they highly reactive?

Epoxides, also known as oxeranes, are three-membered cyclic ethers characterized by significant ring strain. This strain arises because the bond angles in the three-membered ring are forced to be approximately 60°, which is much smaller than the ideal tetrahedral angle of 109.5°. This deviation from the preferred bond angles makes the ring highly reactive and prone to opening. The high reactivity of epoxides is utilized in various chemical reactions, making them important intermediates in organic synthesis.

How do you name epoxides using the cycloalkane convention?

In the cycloalkane convention, the entire ring is named as if it were an alkane, based on its shape rather than the number of carbons. For example, a six-membered ring with one oxygen is named oxacyclohexane. The oxygen is given the highest priority and is assigned the number 1 position. Substituents are then numbered to give the lowest possible numbers. If there are multiple oxygens, prefixes like 'dioxa' are used. For instance, a six-membered ring with two oxygens would be named dioxacyclohexane.

What is the epoxy convention for naming epoxides?

The epoxy convention treats the epoxide as a substituent on the longest carbon chain. The three-membered ring is named as an epoxy group, and its position is indicated by the numbers of the two carbon atoms it is attached to. For example, if the epoxide is attached to carbons 2 and 3 of a hexane chain, it is named 2,3-epoxyhexane. The rest of the chain is named as usual, with other substituents given their appropriate positions and names.

How do you name epoxides using the alkene oxide method?

In the alkene oxide method, the epoxide is named as if it were derived from an alkene. First, name the compound as an alkene, ignoring the epoxide. Then, add the word 'oxide' at the end. For example, if the epoxide is derived from 2-hexene, it is named 2-hexene oxide. If there are other substituents, they are named and numbered as usual. For instance, 5-methyl-2-hexene oxide indicates a methyl group on carbon 5 and an epoxide derived from 2-hexene.

What are the common names for three-membered cyclic ethers?

Three-membered cyclic ethers are commonly known as epoxides or oxeranes. These terms are synonyms and refer to the same structure, which is a three-membered ring containing one oxygen atom. The significant ring strain in these small rings makes them highly reactive, which is why they are often named as their own functional group in organic chemistry.