Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Heteroatomic Functional Groups and Organic Nomenclature
3.15 Structure and Properties of Ethers

Ethers have the functional group R─O─R, where R represents alkyl saturated groups or unsaturated groups, including the phenyl group. These alkyl groups may be the same or different. Ethers have very strong C─O─C bonds, and as a result, their toxicity effects are low, but they can depress the central nervous system and have been used as anesthetic for surgery. Diethyl ether was first used as an anesthetic in 1846. It is a powerful anesthetic and not very toxic to patients, but some patients had symptoms of nausea and vomiting after surgery. Another ether that was widely used for patients as well as in veterinary medicine is isoflurane. Structures of common ethers are shown below.

Image

Ethers are relatively unreactive compounds and as a result are used as common organic solvents in the lab. Owing to the low boiling points of ethers that have low molecular weights, they are typically flammable and explosive liquids. Diethylether, commonly referred to as ether, is used as a starter fluid for diesel and gasoline engines. Methyl tert-butyl ether (MTBE) is a common organic solvent, which is also used as an octane-boosting additive for gasoline engines.

Image

Cyclic ethers are also possible. The smallest cyclic ether, also called an epoxide or oxirane, is one that contains an oxygen atom and two carbons to form a three member-ring molecule. These molecules are very reactive and are used frequently as reagents of organic synthesis. One of the reasons for the reactivity of these molecules is due to the release of the ring strain found in these molecules. Carbons, which are sp3 hybridized, would prefer to have bond angles of approximately 109.5°, but the bond angles in oxiranes are approximately 60°.

Image

Larger cyclic ethers include tetrahydrofuran (THF) and 1,4-dioxane, which are used as common solvents in industry and the organic research labs. Many sugar molecules have the pyran structure and are usually referred to as a pyranose ring, as we will see later when we study monosaccharides and carbohydrates in Chapter 20. Shown below are examples of common cyclic ethers.

Image

Crown ethers are large polycyclic ethers, owing to their size, they are often referred to as macromolecules.

Image

Depending on the number of oxygen atoms, the size of the cavity in the middle of crown ethers can be different. Crown ethers form stable complexes with different metal ions, such as Na+, K+, or Hg2+. Owing to different sizes of the cavity in the middle of these compounds, they are often used to extract specific metal ions that can fit in the cavity.

Image

12-Crown-4 is shown below, and the size of the cavity is just the right for the lithium cation, as shown below. It is labeled as 12-crown-4 since there are 12 atoms in the complete structure and 4 oxygen atoms that are involved in the interaction with the lithium cation.

Image

For larger cations, such as sodium and potassium, larger crown ethers would be needed. For the sodium cation, 15-crown-5 is used, and for potassium, which is even larger, would require 18-crown-6.

Problem 3.26

Give the structures of the crown ether that would be necessary for sodium cation ion (15-crown-5).

Epoxides are also important intermediates in biosynthesis of compounds, such as steroid hormones, which is illustrated below.

Image

trans-4,5-Epoxy-(E)-2-decenal is the compound in human blood that gives it its characteristic metallic odor. The metallic odor of trans-4,5-epoxy-(E)-2-decenal is used by predators to locate blood or prey.

Image

3.15.1 Nomenclature of Ethers

The common method of naming ethers is to name the two groups on either side of the ether functionality followed by the word ether. So CH3-O-CH3 is dimethyl ether and CH3OCH2CH3 is ethyl methyl ether. Note that since the letter “e” comes before the letter “m” in the alphabet, ethyl comes before methyl in the name of this compound. However, for the IUPAC system, the longest chain that does not include the RO is named as an alkane and the other portion is named as an alkoxy substituent. Thus, CH3OCH2CH3 is methoxyethane. Other examples are shown below.

Image

3.15.2 Nomenclature of Oxiranes

In naming epoxides, or three member-ring ethers, a numbering system must be established and the numbering starts at the oxygen and proceeds in the direction that contains a group, or the group that has the highest priority. Shown below is the numbering of a substituted epoxide.

Image

In naming epoxides, identify the group on the carbon and name it as a substituent of oxirane. If there are two groups on the same carbon or on different carbons, numbers are used to indicate the locations of these groups. That is, if there are two groups on carbons 2 and 3 of an oxirane, it is named as a 2,3-disubstituted oxirane. If two groups are on the same carbon of an oxirane, it is named as 2,2-disubstituted oxirane. Examples are shown below.

Image

Since the three-member oxirane ring is a rigid system, groups can be bonded to either side of the of epoxide ring, resulting in different isomers. The cis and trans notations are used to name such isomers.

Problem 3.27

Give the structures of the following molecules.

a) 2-Methoxybutanetrans

b) Butyl ethyl ether

c) -2-Ethyl-3-methyloxirane