Ethers and Epoxides

Ethers are organic molecules consisting of an ${\rm {R{-}O{-}R}}$. In common nomenclature, simple ethers are named by naming the alkyl groups on either side of the ether. In IUPAC nomenclature, ethers are named as a prefix with a number indicating the position of the shorter carbon chain and given the name alkyloxy.
An ether is an organic molecule containing an oxygen atom bound by two alkyl or aryl groups through ${\rm {C{-}O}}$ bonds. If the two alkyl or aryl groups are the same, it is considered symmetrical. If they are not equal, it is considered unsymmetrical. Ethers can form cyclic compounds called cyclic ethers or, in some cases, epoxides. An epoxide is a compound that contains a cyclic ether made of two carbons and an oxygen. An oxirane is a cyclic three-membered ether ring, C2H4O. The bond angles of oxygen are similar to carbon atoms, and the cyclic rings have a conformation similar to cycloalkane rings.
In common nomenclature, simple ethers are named by naming the alkyl groups on either side of the ether, such as diethyl ether and dimethyl ether. In IUPAC nomenclature, ethers are named as a prefix with a number indicating the position of the shorter carbon chain and given the name alkyloxy, such as methoxy and ethoxy. Examples include 1-ethoxyethane, which goes by the common name diethyl ether, and 2-methoxy-2-methylpropane, which goes by the common name tert-butyl methyl ether (MTBE). The smaller alkyl group is the alkoxy substituent, and the larger alkyl group side is the alkane base name. Each alkyl group is numbered separately, while the carbon closest to the oxygen has the highest numbering priority, assuming there is not a higher-priority functional group in the molecule, such as an alcohol, alkene, or alkyne. In 1-ethoxyethane (diethyl ether), for example, the numbering starts at C1 closest to the oxygen. In 2-ethoxyethanol, however, C2 is closest to the oxygen of the ethoxy because the ethanol has higher priority. The smaller alkoxy side has an -oxy ending with its matching alkyl group. The rule also applies when there is cis or trans stereochemistry. In common nomenclature, simple ethers are named by listing the alkyl groups on either side of the ether in alphabetical order. For example, tert-butyl methyl ether is correct, but methyl tert-butyl ether would be technically incorrect. However, methyl tert-butyl ether is commonly used for this compound because of the abbreviation, MTBE.
American organic chemist Charles Pedersen (1904–89) first discovered crown ethers, which have six oxygens in the ring. A crown ether is a cyclic compound consisting of repeating ${-}{\rm {CH_2}{CH_2}{O}{-}}$ units. Crown ethers are named so that the first number is the total number of carbon and oxygen atoms in the ring and the second number is the number of oxygens in the ring. Today, crown ethers have a variety of sizes and uses. Crown ethers are used as phase-transfer catalysts and for binding metal cations. The negative oxygens inside the ring make it very selective for various ions. The high degree of selectivity enables the crown ether to "identify" the guest ion in a solution and capture it. Crown ethers are used to remove certain radioactive elements, such as strontium, and regulate sodium in the blood. Crown ethers are useful in certain organic reactions to remove spectator cations from an organic reaction.
Intermolecular forces are the forces that attract or repel neighboring particles. Ethers have a permanent dipole-dipole force, giving them stronger intermolecular forces than hydrocarbons, which do not have a permanent dipole because of a lack of electronegative atoms such as oxygen. However, since ethers do not contain a hydrogen bonded to an oxygen, ethers do not form hydrogen bonds, and thus they have weaker intermolecular forces than alcohols, which have a hydrogen bonded to an oxygen. Their boiling points are similar to alkanes with similar molecular weights but lower than those of alcohols. Alcohols bond more strongly because of their hydrogen-bonding properties. Adding carbon atoms to the alkyl chain of an ether decreases the water solubility of ethers because of the increased hydrophobic nature of the carbons.

Ethers are widely found in nature and in medicine. Diethyl ether was originally used as an anesthetic in the 1840s. However, ether is very flammable and has since been replaced by less-flammable anesthetics. Ethers are used as spray propellants, refrigerants, and plasticizers and as solvents in waxes, perfumes, and dyes. Some ethers are used as insecticides and fumigants in soils. Codeine is a methyl ether of morphine used to alleviate mild to moderate pain and is classified as an opiate. Ether is used as a starting fluid in diesel and gasoline engines.

Products containing epoxides, such as resins, adhesives, and polymers, are very durable and water repellant. Epoxides are often cross-linked with themselves to create durable polyepoxides, resins, and polymers. They are used to make fumigants and refrigerants, such as ethylene glycol. One very common use for epoxides is for strong adhesives, including marine adhesives. Epoxides are used in metal coatings, LED components, and electrical insulators. They are used to create thermosetting polymers.