2.6: Ethers, Epoxides and Sulfides

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Ethers

Ethers are compounds having two alkyl or aryl groups bonded to an oxygen atom, as in the formula R¹–O–R². The ether functional group does not have a characteristic IUPAC nomenclature suffix, so it is necessary to designate it as a substituent. To do so the common alkoxy substituents are given names derived from their alkyl component (below):

Alkyl Group	Name	Alkoxy Group	Name
CH₃–	Methyl	CH₃O–	Methoxy
CH₃CH₂–	Ethyl	CH₃CH₂O–	Ethoxy
(CH₃)₂CH–	Isopropyl	(CH₃)₂CHO–	Isopropoxy
(CH₃)₃C–	tert-Butyl	(CH₃)₃CO–	tert-Butoxy
C₆H₅–	Phenyl	C₆H₅O–	Phenoxy

The smaller, shorter alkyl group becomes the alkoxy substituent. The larger, longer alkyl group side becomes the alkane base name. Each alkyl group on each side of the oxygen is numbered separately. The numbering priority is given to the carbon closest to the oxgen. The alkoxy side (shorter side) has an "-oxy" ending with its corresponding alkyl group. For example, CH₃CH₂CH₂CH₂CH₂-O-CH₂CH₂CH₃ is 1-propoxypentane. If there is cis or trans stereochemistry, the same rule still applies.

Example
Examples are: CH₃CH₂OCH₂CH₃, diethyl ether (sometimes referred to as ether), and CH₃OCH₂CH₂OCH₃, ethylene glycol dimethyl ether (glyme).

Example

Examples are: CH₃CH₂OCH₂CH₃, diethyl ether (sometimes referred to as ether), and CH₃OCH₂CH₂OCH₃, ethylene glycol dimethyl ether (glyme).

Common names

Simple ethers are given common names in which the alkyl groups bonded to the oxygen are named in alphabetical order followed by the word "ether". The top left example shows the common name in blue under the IUPAC name. Many simple ethers are symmetrical, in that the two alkyl substituents are the same. These are named as "dialkyl ethers".

Epoxides

An epoxide is a cyclic ether with three ring atoms. These rings approximately define an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in chloromethyloxirane. As a functional group, epoxides feature the epoxy prefix, such as in the compound 1,2-epoxycycloheptane, which can also be called cycloheptene epoxide, or simply cycloheptene oxide.

A generic epoxide.

The chemical structure of the epoxide glycidol, a common chemical intermediate

A polymer formed by reacting epoxide units is called a polyepoxide or an epoxy. Epoxy resins are used as adhesives and structural materials. Polymerization of an epoxide gives a polyether, for example ethylene oxide polymerizes to give polyethylene glycol, also known as polyethylene oxide.

Sulfides (Thioethers)

A thioether is a functional group in organosulfur chemistry with the connectivity C-S-C as shown below. Like many other sulfur-containing compounds, volatile thioethers have foul odors.[1] A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen. The grouping of oxygen and sulfur in the periodic table suggests that the chemical properties of ethers and thioethers are somewhat similar.

General structure of a thioether with the blue marked functional group.

Nomenclature

Thioethers are sometimes called sulfides, especially in the older literature and this term remains in use for the names of specific thioethers. The two organic substituents are indicated by the prefixes. (CH₃)₂S is called dimethylsulfide. Some thioethers are named by modifying the common name for the corresponding ether. For example, C₆H₅SCH₃ is methyl phenyl sulfide, but is more commonly called thioanisole, since its structure is related to that for anisole, C₆H₅OCH₃.

Structure and properties

Thioether is an angular functional group, the C-S-C angle approaching 90°. The C-S bonds are about 180 pm.

Thioethers are characterized by their strong odors, which are similar to thiol odor. This odor limits the applications of volatile thioethers. In terms of their physical properties they resemble ethers but are less volatile, higher melting, and less hydrophilic. These properties follow from the polarizability of the divalent sulfur center, which is greater than that for oxygen in ethers.

Thiophenes

Thiophenes are a special class of thioether-containing heterocyclic compounds. Because of their aromatic character, they are non-nucleophilic. The nonbonding electrons on sulfur are delocalized into the π-system. As a consequence, thiophene exhibits few properties expected for a thioether - thiophene is non-nucleophilic at sulfur and, in fact, is sweet-smelling. Upon hydrogenation, thiophene gives tetrahydrothiophene, C₄H₈S, which indeed does behave as a typical thioether.

Contributors

Wikipedia (used with permission)