Ether
:
This article is about ether as a general class of chemical compounds. For other meanings, see Ether (disambiguation)Ether is the general name for a class of
chemical compounds which contain an ether
group â€" an
oxygen atom connected to two (substituted)
alkyl groups. A typical example is the
solvent and
anesthetic diethyl ether, commonly referred to simply as "ether", (ethoxyethane, CH
3-CH
2-O-CH
2-CH
3).
Ether molecules cannot form
hydrogen bonds among each other, resulting in a relatively low
boiling point comparable to that of the analogous
alkanes. Ethers are more
hydrophobic than esters or
amides of comparable structure.
Ethers can act as
Lewis bases. For instance, diethyl ether forms a complex with
boron compounds, such as
boron trifluoride diethyl etherate .F
3B:O(CH
2CH
3)
2. Ethers also coordinate to magnesium in
Grignard reagents.
In the
IUPAC nomenclature system, ethers are named using the general formula
"alkoxyalkane", for example CH
3-CH
2-O-CH
3 is methoxyethane. If the ether is part of a more complex molecule, it is described as an alkoxy substituent, so -OCH
3 would be considered a
"methoxy-" group. The nomenclature of describing the two alkyl groups and appending
"ether", e.g.
"ethyl methyl ether" in the example above, is a
trivial usage.
 |
Not all compounds of the formula R-O-R are ethers |
Ethers are not to be confused with the following classes of compounds with the same general structure R-O-R.
*
Aromatic compounds like
furan where the oxygen is part of the aromatic system.
* Compounds where one of the carbon atoms next to the oxygen is connected to oxygen,
nitrogen, or
sulfur:
**
Esters R-C(=O)-O-R
**
Acetals R-CH(-O-R)-O-R
**
Aminals R-CH(-NH-R)-O-R
**
Anhydrides R-C(=O)-O-C(=O)-R
The terms
"primary ether",
"secondary ether", and
"tertiary ether" are occasionally used and refer to the carbon atom next to the ether oxygen. In a
primary ether this carbon is connected to only one other carbon as in diethyl ether CH
3-CH
2-O-CH
2-CH
3. An example of a
secondary ether is
diisopropyl ether (CH
3)
2CH-O-CH(CH
3)
2 and that of a
tertiary ether is
di-tert-butyl ether (CH
3)
3C-O-C(CH
3)
3.
|
A primary ether (diethyl ether) |
|
A secondary ether (diisopropyl ether) |
|
A tertiary ether (di-tert-butyl ether) |
Dimethyl ether, a primary, a secondary, and a tertiary ether.Polyethers are compounds with more than one ether group. While the term generally refers to
polymers like
polyethylene glycol and
polypropylene glycol, low molecular compounds such as the
crown ethers may sometimes be included.
Synthesis
Ethers can be prepared in the laboratory in several ways.
*
Dehydration of
alcohols::
R-OH + R-OH â†' R-O-R +
H2O: This direct reaction requires drastic conditions (heat and an acid catalyst) and is usually not applicable. Such conditions can destroy the delicate structures of some
functional groups. There exist several milder methods to produce ethers.
*
Nucleophilic displacement of
alkyl halides by
alkoxides:
R-O- +
R-X â†' R-O-R +
X-: This reaction is called the
Williamson ether synthesis. It involves treatment of a parent
alcohol with a strong
base to form the alkoxide
anion followed by addition of an appropriate aliphatic compound bearing a suitable
leaving group (R-X). Suitable leaving groups (X) include
iodide,
bromide, or
sulfonates. This method does not work if R is aromatic like in
bromobenzene. Likewise, this method only gives the best yields for primary carbons, as secondary carbons will undergo E2 elimination on exposure to the basic alkoxide anion used in the reaction. Aryl ethers can be prepared in the
Ullmann condensation.
*
Electrophilic addition of alcohols to
alkenes.:
R2C=CR2 + R-OH â†' R
2CH-C(-O-R)-R
2:
Acid catalysis is required for this reaction.
Tetrahydropyranyl ethers are used as
protective groups for alcohols.
Cyclic ethers which are also known as
epoxides can be prepared:
* By the oxidation of alkenes with a
peroxyacid such as
m-CPBA.
* By the base intramolecular nuclephilic substitution of a halohydrin.
Reactions
 |
Structure of the polymeric diethyl ether peroxide |
Ethers in general are of very low chemical
reactivity. Organic reactions are:
*
Hydrolysis.: Ethers are
hydrolyzed only under drastic conditions like heating with
boron tribromide or boiling in
hydrobromic acid. Lower mineral acids containing a halogen, such as
hydrochloric acid will cleave ethers, but very slowly. Hydrobromic acid and
hydroiodic acid are the only two that do so at an appreciable rate. Certain aryl ethers can be cleaved by
aluminium chloride.
*
Nucleophilic displacement.
Epoxides, or cyclic ethers in three-membered rings, are highly susceptible to nucleophilic attack and are reactive in this fashion.
*
Peroxide formation.
Primary and secondary ethers with a CH group next to the ether oxygen easily form highly
explosive organic peroxides (e.g.
diethyl ether peroxide) in the presence of oxygen, light, and metal and
aldehyde impurities. For this reason ethers like diethyl ether and
THF are usually avoided as
solvents in industrial processes.
*
Ethylene oxide, the smallest cyclic ether:
|
Chemical structure of ethylene oxide |
*
Dimethyl ether, an
aerosol spray propellant:
|
Chemical structure of dimethyl ether |
*
Diethyl ether, a common low boiling solvent:
|
Chemical structure of diethyl ether |
*
Dimethoxyethane, a high boiling solvent:
|
Chemical structure of dimethoxyethane |
*
Dioxane, a cyclic ether and high boiling solvent:
|
Chemical structure of dioxane |
*
THF, a cyclic ether, one of the most polar simple ethers that is used as a solvent:
|
Chemical structure of THF |
*
Anisole (methoxybenzene), a major constituent of the
essential oil of
anise seed:
|
Chemical structure of anisole |
*
Crown ethers, cyclic polyethers that are used as
phase transfer catalysts:
|
Chemical structure of 18-crown-6 |
*
Polyethylene glycol, a linear polyether, e.g. used in
cosmetics:
|
Chemical structure of polyethylene glycol |
*
Functional group*
Methoxy*
Petroleum ether, not an ether but a low boiling alkane mixture.
*
Thioether, analogs of ethers with the oxygen replaced by sulfur.
*
Luminiferous ether*
ILPI page about ethers.
*
An Account of the Extraordinary Medicinal Fluid, called Aether, by M. Turner, circa 1788, from
Project Gutenberg
