Epoxides

EPOXIDES

Epoxides are known as oxiranes, and are three-membered ring structures in which there are two carbon atoms and an oxygen in the middle. There can be side chains on any of the carbon molecules but, of course, there are no side chains on the oxygen molecule. The simplest and most common oxirane is ethylene oxide, which is made by the catalytic oxidation of ethylene with oxygen in air. Further chemical treatment of this molecule will make ethylene glycol, used as antifreeze and used to make polyester and plastic bottles. Figure 68 shows ethylene oxide synthesis:

Figure 68.

More complex epoxides can be made by adding an OH group and a Chlorine atom to an alkene and then adding a strong base. This takes the hydrogen and chloride off of the molecule to create an oxirane. This is shown in figure 69:

Figure 69.

There are other cyclic ethers that have common names that you need to memorize. One of these is furan, which is a five-membered ring that has an oxygen as the fifth member and two double bonds. In common usage, chemicals like 2,5-dimethyltetrahydrofuran, which is made from furan. In determining this molecule, you have to know that this can

be a cis or trans molecule, depending on the position of the methyl groups. Another is pyran, which is a five-carbon, six-membered ring that has an oxygen as the sixth member and two double bonds. Some of these molecules are shown in figure 70:

Figure 70.

In the nomenclature of epoxides, the terms oxirane, furan, dioxane, and pyran can be used to describe the more complex molecules that can be made from these basic molecules. These tend to be made by the oxidation of alkenes, making the common term “alkene oxide” used to describe these molecules. The epoxy functional group is the two carbon atoms with the oxygen in the middle of the molecule. Sometimes the nomenclature includes the term “epoxy” to describe the functional group, as in the molecule trans-4,5 epoxydecane, which is a decane molecule that has the fourth and fifth carbon atoms made into a cyclic compound with oxygen in a trans-fashion.

A crown ether is a unique ether that consists of a minimum of four oxygen atoms. All crown ethers have a central space that can take on a metal ion effectively protected by the ring of carbon atoms. They are named using both the total number of atoms in the ring plus the total number of rings. As you will see in figure 71, the middle of the crown ether is electronegative, having electron pairs that will have affinity to accommodate a large metal cation, such as potassium: