Cyclic ethers and epoxide naming. Created by Sal Khan.
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- 5:22why is it "ene? cant you say 1-pentaneoxide and still get the point across? same thing with6:40cant it be called cyclohexaneoxide?(3 votes)
- It turns out that there are two popular chemical reactions that form Epoxides (which in turn, leads to two different naming conventions that are convenient in each scenario).
One method is to think of the epoxide as a substituent (attached group) of the molecule: This method corresponds to using "epoxy" and listing the linkage to the main carbon backbone (an alkAne in this case).
The second of them, which is a little more subtle, involves the formation of the Epoxide from the corresponding alkene (backbone with a double bond) by subjecting it to some reaction mechanism. In this scenario, it seems logical to name the molecule as an Alkene Oxide, namely an alkene whose “double bond” is actually an epoxy linkage via an oxygen molecule.
To give an example, around6:00Sal shows us epoxy cyclohexane (a cyclohexane structure with a single epoxy linkage). If we think of this molecule a little differently, namely as cyclohexene whose double bond is somehow substituted with an oxygen, then we are thinking of this compound as an Alkene Oxide, and so we would name the molecule “cycloalkene oxide”
But at the end of the day, it’s important to know that whether we think of such a compound as an epoxy cycloalkane or as a cycloalkene oxide, they are really describing the same molecule.
Now, referring to the original question, this is why we use "ene" and not "ane" (because alkanes have no double bond to think of as being an epoxy linkage). In conventional nomenclature, we describe alkene structures by way of where their double bonds are located (ie. 2-butene). When we add the suffix Oxide, (2-butene oxide) this tells us "I have a double bond at the second position...no wait, it's not actually a double bond, it's an epoxy linkage."(11 votes)
- 2:05Can you call it oxalane?(4 votes)
- Why is my professor using an entirely different set of rules to name these? Example, my book and professor call the first molecule (1, 4 epoxybutane) oxalane. Isn't one of the main purpose of using IUPAC rules to make naming cohesive? Why then are there two completely conflicting sources of rules for naming epoxides?(3 votes)
- The different naming rules developed over many years. Finally, IUPAC tried to develop a single coherent system. Some of the historical naming systems include:
This compound is almost always called by its common name, tetrahydrofuran (THF).
Technically, these are restricted to 3-membered rings. IUPAC allows the term epoxy in other cases only when the O atom forms a bridge between atoms in a ring that is already present, for example, an O atom joining atoms 1 and 4 of a cyclohexane ring.
Calling THF 1,4-epoxybutane is an old system that is no longer used. If you Google "epoxybutane", you will just get hits for 1,2- and 2,3-epoxy butane.
The O atom is named as an "oxa" substituent replacing a C atom. Thus, THF could be called oxacyclopentane.
IUPAC names (Hantzsch-Widman system):
This uses the name "oxa" to refer to an oxygen atom and the ending "-olane" to refer to a five-membered saturated ring that does not contain N. The final "a" of oxa is dropped (elided) before an ending that starts with "o". So the official name of THF is oxolane (not oxalane).
It is hard to get people to change old habits.(3 votes)
- Wait didn't we just learn how to name cyclic ethers (crown ethers)? I am just a little confused about how to name what now?
- Epoxides and crown ethers are two different things. Crown ethers are a circular structure of just ethers, whereas epoxides have what is essentially an ether bound to two carbons of a longer carbon chain.(2 votes)
- Is it necessary that the carbon-oxygen bonds should be on adjacent carbon atoms ?(2 votes)
- 5:28Which one does the IUPAC reccomend(1 vote)
- IUPAC allows two systems:
1. Epoxy system: e.g., 1,4-epoxybutane.
2. Hantzsch-Widman System: e.g. oxolane (tetrahydrofuran).
oxo = oxygen; ol = 5-membered ring; ane = saturated.(3 votes)
- At5:00, Can 1-Pentene exist, As I have not come across the same in any of the textbooks.(2 votes)
- Pent-1-ene exists.
It is a colourless liquid with the odour of gasoline and a boiling point of 30 °C.(1 vote)
- Can you explain how cyclic ethers with alcohols as substituents and that are alkenes will be named? I am trying to find this everywhere. I've got a 6-membered ring with the Oxygen in the middle, and directly across is an -OH group. In between the 2 is a double bond.(2 votes)
- I believe that the convention is to act like the double bond is two bonds to the same type of molecule. For example, you have O=O. You would act like it is O-O-O. Hope that helps!(1 vote)
- How would you name the first cyclic ether as a common name?(1 vote)
- Tetrahydrofuran. It's furan which has alternating dbl bonds, without the double bonds. It takes 4 hydrogens to make it not be furan....it becomes tetrahydrofuran.
The simplest cyclic ether is ethylene oxide or ethyloxirane (epoxides are also known as oxiranes).(2 votes)
- isnt it 1,4-epoxycyclobutane for the 1 st exmaple ??(1 vote)
- No, cyclobutane would require the 4 carbon chain to be connected in a ring by itself.(1 vote)
In the last video, we named some fairly simple ethers. In this video, we're going to think about slightly more complicated ones. In particular, what happens if, in the process of having an ether, we actually have a ring as opposed to just a long chain? So you can imagine a molecule that looks something like this. You have your oxygen. On this side of the oxygen, you have this carbon chain right here. You have a carbon chain like this. But then that chain bonds back to the oxygen. So we have a ring here. It's not obvious how to name this. You can't just look it this side and call it methyl. And then that side, and call it a methyl as well. It's the same side. It connects back to itself. How do you name this type of ether? What you do is, you just number it. You number the longest carbon chain, like we've always done in the case of an alkane. We can start numbering here. 1, 2, 3, 4. If we just think about the carbon chain by itself. We know if it's one carbon, the prefix is meth-. Two, it's eth-. Three, it's prop-. Four, it's but-. So if this was just a carbon chain, we would call this butane. If we only looked at this carbon chain right here, you would call this butane. But obviously this isn't butane. We have this oxygen that's bonding to the 1 and 4 carbons of the butane. To make that clear, we call this-- Let me color code this part right here, this oxygen right there. It's bonded to the 1 and the 4 carbon. So we call this 1 comma 4. And this is our new word that we're going to learn in this video. 1,4-epoxybutane. And it doesn't just apply when the ether forms a large ring. It can actually form a little subset ring on a regular chain. So you could imagine something like this. Let me draw a chain of carbons. Let's say we have five carbons. 1, 2, 3, 4, 5, just like that. Let's say that between this carbon and this carbon, instead of having a double bond, this carbon actually bonds to an oxygen, which then bonds to this carbon over here. Obviously, every carbon has four bonds, the ones that we're not drawing, those are hydrogens. How do we name this? Well, same exact process. We actually start numbering the chain closer to where the oxygen is bonded. So we start numbering at this end over here. 1, 2, 3, 4, 5. So this is pentane. The oxygen is bonded to the 1 and the 2 carbons. So we call this 1,2-epoxypentane. 1 comma 2-epoxypentane. Now, in the last video, I told you that, in general, ethers are fairly nonreactive. They actually make for good solvents. But, what I've just drawn here is a special case of ethers called epoxides. When you just have this three atom chain right here, where it's two carbons and an oxygen. This is a special case of an ether called an epoxide. This is called an epoxide. And this, unlike most ethers, is very reactive. Another way you could think about it, it's very unstable. This is very reactive. Sometimes people consider these separate from ethers. The reason why they're very reactive, is this three member ring right here. There's a lot of strain on these bonds. These electrons, these bonds don't like to be that close to each other. If you actually tried to make it with an actual model set with molecules, you would have trouble making it bend enough to actually make this bond. So this is highly, highly, highly unstable. There's actually an alternate way to name epoxides. The alternate way, so this is a completely legitimate way. You could name it just like an ether with a ring. This is 1,2-epoxypentane. But the alternate way is to pretend like you had a double bond here. That instead of this oxygen here, you had a double bond. If you had a double bond here, this thing would be called, depending how you want to name it, it could be called 1-pentene. That's if there was not this oxygen here, but if there was a double bond here. 1-pentene would look like this. 1, 2, 3, 4, 5. This is the 1 carbon. So, 1, 2, 3, 4, 5. This is what 1-petene looks like. We've learned that many, many, many videos ago. Sometimes it's called pent-1-ene, depending on which convention. This is the more common one. We have this oxygen here, instead of this double bond. Instead of calling it just 1-pentene, we call it 1-pentene oxide. Just like that. So both of these are the names for the same exact molecule. This makes it clear that it's an epoxide. That's kind of the special ether that is more reactive. This is just the general way that we name any type of cyclic ether. So let's just do one more just to make the point clear. Let's have a cycle branching off of a cycle. Let's have an epoxide off of another ring. Just to make the point clear. These aren't too hard to name. But the first time you seen them, a little daunting. Let's say we have a cyclohexane ring right here. So this is cyclohexane. But let's say we have a little epoxy branching off of it, just like this. We have that going on. If we wanted to make it clear that this is an epoxide, we would essentially pretend. First pretend that this is just a double bond. If this was just a double bond, this would be cyclohexene. If this oxygen wasn't there, and instead we just had a double bond here. You actually don't have to specify the number when you only have one double bonded cyclohexene. Because it could have been anywhere, and it would have essentially been the same molecule. But since we have this oxygen here, instead of a double bond that's bonding to both of these carbons, we call this cyclohexene oxide. This part, right here, makes us name this cyclohexene oxide. Or if we wanted to just name this as a traditional ether, we would just name this cyclohexane and put the epoxy in front of it. Either of these are valid. Once again, you don't have to number it. Because you could call it, 1,2-epoxycyclohexane, if you made this the 1 or the 2 carbon. But you know it's going to be on adjacent carbons. And it could have really been on any of these two. It could have been on the 3 and the 4, and it would have essentially been the same molecule. So this actually makes it clear exactly what the molecular structure of the molecule is. So anyway, I thought you would enjoy that. And in the next video, I told you that epoxides are reactive. So I'll actually show you a reaction dealing with epoxides.