Epoxide Nomenclature

There are 3 distinct ways to name cyclic ethers. But before we get into them, let’s first specifically define what an epoxide is.  

Concept: Defining what an epoxide (oxirane) is.

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Video Transcript

All right guys, so now let's talk about naming epoxides. It turns out that epoxides are just cyclic ethers. That's basically the definition. Some types of cyclic ethers, remember that an ether is ROR, are going to be named as their own functional group due to increased reactivity.
The specific ones that we usually name as their own functional group are three-membered ethers 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. What that means is that they're very reactive and it's very easy to open them up.
What we call these three-membered cyclic ethers is – there's actually two common names for them. We call them epoxides, so go ahead and write that down. They're also called, in some textbooks, some professors like to use the word oxirane. These are really synonyms for each other. An epoxide and an oxirane are the same exact thing. It's just a three-membered cyclic ether. 

Cycloalkane Convention

Name the ring as a cycloalkane, adding the prefix oxa- and location if necessary.

Concept: How to name cyclic ethers using the cycloalkane convention.  

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Video Transcript

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's a lot of substituents. So it turns out that there's three 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. As you can see here I have a six-membered ring, but how many of those atoms are actually carbons? Only five 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 – maybe I did, but I'm just using that as an example right now just to count carbons.
I have five carbons, but what I'm telling you is that we should actually name it as a cycloalkane, not by the number of carbons. What that means is that usually when we're naming an alkane we would say there's five 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. Then we're going to add the prefix oxa-. What oxa- is going to tell us is there's one member of this ring that is an oxygen. So if I call it oxacyclohexane, what I'm saying is that I have a six-membered ring where one of the atoms is an oxygen, not a carbon. Then, obviously, location if necessary.
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 location. How do we know where to put those guys? Well, it turns out that the oxygen is always going to get your one spot. 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. Then, obviously, I should number to give the lowest overall number or to go to the next highest priority, etcetera. This would be 3-methyl-1-oxacyclohexane. Cool so far?
Just so you guys know, this also applies to rings that have more than one oxygen. If I had two oxygens, that would be what was called a dioxide. Just putting that out there. You could use prefixes as well.
Now let's go ahead and talk about another naming system, by the way, one word really quick. This is going to be commonly used for non-three-membered rings. If not three-membered, as you can see I was dealing with a six-membered one here, this is usually the one we use. If it's four-membered, five-membered, six-membered, etcetera, you would use this naming system. Now if it is a three-membered ring, we could still use it, but this is not going to be the most common way to name it. 

Epoxy Convention

Name as a typical alkane, and then include epoxy as a di-located substituent. 

Concept: How to name epoxides using the epoxy convention.

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Video Transcript

If we are dealing with a three-membered ring, there's much more common ways. One is the epoxy convention. What the epoxy convention basically says is this: we have a substituent named an epoxy group. We're just going to name our longest carbon chain as normal and then label the three-membered ring as just a 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. 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 is – 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 to 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? That's just something to consider that you could also use the epoxy convention. It's perfectly legit. 

Oxide Convention

Name as an imaginary alkene, then follow with the word oxide

Concept: How to name epoxides using the oxide convention.

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Note: I forgot to address in the videos that both of these would be trans due to the alkyl groups facing opposite sides of the ring.

Problem: Which of the following is not the correct name of the following compound:

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Problem: Which of the following is not the correct name of the following compound:

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