Nomenclature ethers epoxides
Epoxides involve an oxygen and two carbon atoms in a three-atom ring structure, as illustrated below. Whereas ethers are relatively stable molecules, epoxides are highly reactive. Nomenclature The fundamental functional group for naming ethers is the alkoxy group, several examples of which are shown below with their corresponding names.
Note that the names closely resemble the names of the similar alkyl groups. To properly name an ether according to IUPAC rules, identify the shortest alkyl chain attached to the oxygen atom and consider that portion the alkoxy group.
Then follow the usual rules for naming molecules, where the alkoxy group is a substituent of the alkane. Another acceptable naming procedure is to write the two alkyl group names followed by the word ether similarly to how we named alcohols, such as ethyl alcohol. Cyclic ethers have non-systematic names, so we will not focus on this aspect of the nomenclature, but the IUPAC names for several cyclic ethers are given below.
Oxolane also goes by the name tetrahydrofuran. For epoxides, the functional group name is epoxy when used in the same manner as, for example, ethyl or methyl that is, following the rules of substitutive nomenclature. When used in the manner of, for instance, ethyl alcohol functional class nomenclature , the term oxide is used. In the substitutive approach, the epoxy- prefix is preceded by the numbers of the two carbons to which the oxygen is bound.
Also note that because of the structure of epoxies, these molecules can exhibit stereoisomerism. For instance, 3,4-epoxyhexane can actually be either cis-3,4-epoxyhexane or trans-3,4epoxyhexane, as shown below. In addition to cyclic structures such as epoxides , ethers can involve multiple oxygen atoms in a carbon chain.
For instance, so-called crown ethers are cyclic ethers with multiple oxygen atoms in the ring. An example is shown below, along with its simplified name we will not discuss nomenclature for crown ethers. Interested in learning more? Why not take an online Organic Chemistry course? Solution: This molecule is an ether with two four-carbon chains butyl groups attached to the central oxygen atom.
Thus, it can be legitimately called either dibutyl ether or butoxybutane. Solution: This compound is 2,3-epoxymethylheptane. Note that the main carbon chain is numbered as shown below. Synthesizing Ethers and Epoxides As mentioned previously, we have already studied the acid-catalyzed synthesis of ethers from alcohols. Another method is the Williamson ether synthesis, which involves a reaction between a metal alkoxide and an alkyl halide. For instance, consider sodium ethoxide and bromopropane.
The overall reaction is shown below. The mechanism for this reaction involves dissociation of the metal from the alkoxide and then nucleophilic attack SN2 on the alkyl halide. The alkoxide ion acts as a Lewis base with respect to the alkyl halide. Again, note that this reaction is a nucleophilic substitution involving two molecules SN2. For epoxides, one approach to synthesis essentially follows this mechanism but involves a single molecule in which a hydroxyl group and a halide attached to adjacent carbon atoms these functional groups are said to be vicinal, and this particular type of molecule is called a vicinal halohydrin.
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. 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. 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. So this is pentane. The oxygen is bonded to the 1 and the 2 carbons.
So we call this 1,2-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. 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 pentene, 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.
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And there's one on this substituent. So once again, it would be methoxy. So I'll go ahead and write "methoxy" in hear. So "methoxy propane. And that group is coming off of carbon, too. Let's do a much more complicated one that has a little bit of stereochemistry in it. So if this was the molecule that I was trying to name-- and let's go ahead and put a bromine here like that. So for this one, once again, I have to think about the larger group as my parents name.
So if I look at those two alkyl groups, the alkyl group on the left looks like the longest one to me. And I want to number to give my substituents the lowest number possible. So if I look over here on the left, I can see that there are four carbons in my larger substituent.
So four carbons is going to be my parent name here. So I'm going to call this "butane. So this would be "butane" so far. Now, when I number that butane, I want to give the lowest number as possible to my substituents. So I could start from the left, or I could start from the right. And starting from the right makes more sense because I have a substituent coming off of carbon one. I have a substituent coming off of carbon two. And then, three and four. If I'm thinking about those two substituents, let's think about how I would name them.
Over here on the right, for my alkoxy substituent, this time, there are two carbons in my alkoxy substituent right here. So two carbons would be s, so that would be "ethoxy. So I can go ahead and write that in here. So "1-ethoxy butane" is what I have so far.
And I also have a bromine coming off of carbon two. So it'd be "2-bromo. Because B comes before E. So "2-bromoethoxy butane. So carbon two is a chirality center. So we need to think about how to assign priority to those 4 groups.
So if I think about the atoms directly attached to my chirality center-- first, let's go ahead and identify my chirality center. That would be this one right here. Four different groups attached to it because there's also a hydrogen going away from me in space. And I think about atomic numbers. So I have carbon versus carbon versus bromine right here.
So bromine, of course, has the highest atomic number. It gets highest priority. Now, my hydrogen, of course, is going to get lowest priority. So that's priority number 4. And now, I have 2 groups to worry about.
I have two carbons to worry about. Let's go ahead and mark those carbons again. So which one of these carbons is going to get higher priority? Well, it's all about what they're attached to. So the carbon on the left is attached to another carbon and two hydrogens. The carbon on the right is attached to an oxygen and two hydrogens. So in terms atomic number, carbon versus oxygen, the oxygen will win. And this substituent on the right would get the highest priority.
So this would get a two over here. All the stuff on the right would get a two. And over here on the left, this would be a three. So we have one, two, three going around this way-- going around counterclockwise, which is the s absolute configuration. So this is " s bromoethoxy butane" for the final name. Let's do one more example of naming an ether here.
So let's go ahead and look at one that has a ring. And we'll put a double bonds in our ring. And then, we'll have our ether over here on the right. So if I wanted to name this molecule, I would think about my two alkyl groups and think about which one is the larger one.
And of course, all this stuff on the left is going to be my parent name. And then, this is going to be my alkoxy substituent. So on the left, I know what that molecule is. I know that's called "cyclohexene" from an earlier video. So this is "cyclohexene" as my parent name. I now need to number my ring to give my alkoxy substituent the lowest number possible. So if I wanted to number my ring to give my alkoxy substituent the lowest number possible, I should start here and make that one, two, three and four.
So we think about-- what is that alkoxy substituent? It is an ethoxy substituent because I have two carbons right there. So I have an ethoxy off of carbon four. Note that the main carbon chain is numbered as shown below. Synthesizing Ethers and Epoxides As mentioned previously, we have already studied the acid-catalyzed synthesis of ethers from alcohols. Another method is the Williamson ether synthesis, which involves a reaction between a metal alkoxide and an alkyl halide.
For instance, consider sodium ethoxide and bromopropane. The overall reaction is shown below. The mechanism for this reaction involves dissociation of the metal from the alkoxide and then nucleophilic attack SN2 on the alkyl halide. The alkoxide ion acts as a Lewis base with respect to the alkyl halide. Again, note that this reaction is a nucleophilic substitution involving two molecules SN2. For epoxides, one approach to synthesis essentially follows this mechanism but involves a single molecule in which a hydroxyl group and a halide attached to adjacent carbon atoms these functional groups are said to be vicinal, and this particular type of molecule is called a vicinal halohydrin.
In the presence of a base such as hydroxide ions , the halohydrin donates the proton an acid-base reaction bound to the oxygen atom. Consider the case below of 3-iodobutanol. The remaining molecule can then dissociate an iodide ion as follows, creating the epoxide. The product in this case is 2,3-epoxybutane.
The stereoisomer of the product depends on the configuration of the starting molecule. Another method of synthesizing an epoxy is through epoxidation of an alkene. The reaction involves a peroxy acid, a generic example of which is shown below. Consider the example of cyclohexene. In the presence of peroxyacetic acid, the following reaction yields 1,2-epoxycyclohexane and acetic acid.
Reactions of Ethers and Epoxides Let's consider a couple reactions that involve ethers and epoxides: in particular, cleavage of ethers by hydrogen halides and acid-catalyzed ring opening of epoxides two similar reactions. The first reaction, cleavage of ethers by hydrogen halides, is exemplified overall below for the case of ethoxypropane and hydrogen bromide. The reaction mechanism in this case proceeds in two steps to yield an alkyl halide and an alcohol. The next step is a cleavage that, in this non-symmetrical case, can yield two slightly different results.
Thus, in this case, the final products of the reaction are ethyl bromide, propyl bromide, and water. Epoxides can undergo ring opening in the presence of hydrogen bromide in a reaction very similar to that shown above. The mechanism is illustrated below for 1,2-epoxybutane. In the first step, the dissociated bromide ion attacks the less substituted carbon bound to the oxygen atom.
A proton originating from dissociation of the hydrogen and bromine atoms in hydrogen bromide then bonds with the oxygen to form 1-bromobutanol. Practice Problem: Describe the relationship between cleavage of ethers in hydrogen bromide and acid-catalyzed ring opening of epoxides in hydrogen bromide. Solution: In each case, an oxygen-carbon bond is broken, and a halide ion replaces this broken bond. The oxygen atom also bonds to a hydrogen in both cases.
These steps occur in a different order for epoxides compared with ethers.
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