Ether

Source: Wikipedia, the free encyclopedia.
For the substance whose common name is ether, see Diethyl ether. For the digital asset, see Ethereum. For other uses, see Aether (disambiguation).
aryl
).

In

anaesthetic diethyl ether, commonly referred to simply as "ether" (CH3−CH2−O−CH2−CH3). Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.[2]

Structure and bonding

Ethers feature bent C−O−C linkages. In dimethyl ether, the bond angle is 111° and C–O distances are 141 pm.[3] The barrier to rotation about the C–O bonds is low. The bonding of oxygen in ethers, alcohols, and water is similar. In the language of valence bond theory, the hybridization at oxygen is sp3.

Oxygen is more

aldehydes
), however.

Ethers can be symmetrical of the type ROR or unsymmetrical of the type ROR'. Examples of the former are

dipropyl ether etc. Illustrative unsymmetrical ethers are anisole (methoxybenzene) and dimethoxyethane
.

Vinyl- and acetylenic ethers

Vinyl- and acetylenic ethers are far less common than alkyl or aryl ethers. Vinylethers, often called enol ethers, are important intermediates in organic synthesis. Acetylenic ethers are especially rare. Di-tert-butoxyacetylene is the most common example of this rare class of compounds.

Nomenclature

In the

alkyl
radical is written in front, so CH3–O–CH2CH3 would be given as methoxy(CH3O)ethane(CH2CH3).

Trivial name

IUPAC rules are often not followed for simple ethers. The trivial names for simple ethers (i.e., those with none or few other functional groups) are a composite of the two substituents followed by "ether". For example, ethyl methyl ether (CH3OC2H5), diphenylether (C6H5OC6H5). As for other organic compounds, very common ethers acquired names before rules for nomenclature were formalized. Diethyl ether is simply called ether, but was once called sweet oil of vitriol. Methyl phenyl ether is

aromatic ethers include furans. Acetals
(α-alkoxy ethers R–CH(–OR)–O–R) are another class of ethers with characteristic properties.

Polyethers

Polyethers are generally

hydroxyl
group. The term "oxide" or other terms are used for high molar mass polymer when end-groups no longer affect polymer properties.

Crown ethers are cyclic polyethers. Some toxins produced by dinoflagellates such as brevetoxin and ciguatoxin are extremely large and are known as cyclic or ladder polyethers.

Aliphatic polyethers
Name of the polymers with low to medium molar mass Name of the polymers with high molar mass Preparation Repeating unit Examples of trade names
Paraformaldehyde
Polyoxymethylene
(POM) or polyacetal or polyformaldehyde
Step-growth polymerisation of formaldehyde
 –CH2O– Delrin from DuPont
Polyethylene glycol (PEG) Polyethylene oxide (PEO) or polyoxyethylene (POE) Ring-opening polymerization of ethylene oxide –CH2CH2O– Carbowax from Dow
Polypropylene glycol (PPG) Polypropylene oxide (PPOX) or polyoxypropylene (POP) anionic ring-opening polymerization of propylene oxide –CH2CH(CH3)O– Arcol from Covestro
Polytetramethylene glycol (PTMG) or Polytetramethylene ether glycol (PTMEG) Polytetrahydrofuran (PTHF) Acid-catalyzed ring-opening polymerization of tetrahydrofuran −CH2CH2CH2CH2O− Terathane from Invista and PolyTHF from BASF

The phenyl ether polymers are a class of aromatic polyethers containing aromatic cycles in their main chain: polyphenyl ether (PPE) and poly(p-phenylene oxide) (PPO).

Related compounds

Many classes of compounds with C–O–C linkages are not considered ethers:

carboxylic acid anhydrides
(RC(=O)–O–C(=O)R′).

There are compounds which, instead of

chemical elements (e.g., Si, Ge, Sn, Pb). Such compounds are considered ethers as well. Examples of such ethers are silyl enol ethers R3Si−O−CR=CR2 (containing the Si−O−C linkage), disiloxane H3Si−O−SiH3 (the other name of this compound is disilyl ether, containing the Si−O−Si linkage) and stannoxanes
R3Sn−O−SnR3 (containing the Sn−O−Sn linkage).

Physical properties

Ethers have boiling points similar to those of the analogous alkanes. Simple ethers are generally colorless.

Selected data about some alkyl ethers
Ether Structure m.p. (°C) b.p. (°C) Solubility in 1 liter of H2O Dipole moment (D)
Dimethyl ether CH3–O–CH3 −138.5 −23.0 70 g 1.30
Diethyl ether CH3CH2–O–CH2CH3 −116.3 34.4 69 g 1.14
Tetrahydrofuran O(CH2)4 −108.4 66.0 Miscible 1.74
Dioxane O(C2H4)2O 11.8 101.3 Miscible 0.45

Reactions

Structure of the polymeric diethyl ether peroxide

The C-O bonds that comprise simple ethers are strong. They are unreactive toward all but the strongest bases. Although generally of low chemical

alkanes
.

Specialized ethers such as

ketals, and acetals are unrepresentative classes of ethers and are discussed in separate articles. Important reactions are listed below.[4]

Cleavage

See also: Ether cleavage

Although ethers resist hydrolysis, they are cleaved by hydrobromic acid and hydroiodic acid. Hydrogen chloride cleaves ethers only slowly. Methyl ethers typically afford methyl halides:

ROCH3 + HBr → CH3Br + ROH

These reactions proceed via

onium
intermediates, i.e. [RO(H)CH3]+Br−.

Some ethers undergo rapid cleavage with boron tribromide (even aluminium chloride is used in some cases) to give the alkyl bromide.[5] Depending on the substituents, some ethers can be cleaved with a variety of reagents, e.g. strong base.

Despite these difficulties the chemical paper pulping processes are based on cleavage of ether bonds in the lignin.

Peroxide formation

When stored in the presence of air or oxygen, ethers tend to form

diethyl ether hydroperoxide. The reaction is accelerated by light, metal catalysts, and aldehydes. In addition to avoiding storage conditions likely to form peroxides, it is recommended, when an ether is used as a solvent, not to distill it to dryness, as any peroxides that may have formed, being less volatile than the original ether, will become concentrated in the last few drops of liquid. The presence of peroxide in old samples of ethers may be detected by shaking them with freshly prepared solution of a ferrous sulfate followed by addition of KSCN. Appearance of blood red color indicates presence of peroxides. The dangerous properties of ether peroxides are the reason that diethyl ether and other peroxide forming ethers like tetrahydrofuran (THF) or ethylene glycol dimethyl ether
(1,2-dimethoxyethane) are avoided in industrial processes.

Lewis bases

Structure of VCl3(thf)3.[6]
  Vanadium, V
  Chlorine, Cl
  Carbon, C
  Hydrogen, H
  Nitrogen, N

Ethers serve as

Lewis bases. For instance, diethyl ether forms a complex with boron trifluoride, i.e. borane diethyl etherate (BF3·O(CH2CH3)2). Ethers also coordinate to the Mg center in Grignard reagents. Tetrahydrofuran is more basic than acyclic ethers. It forms with many complexes
.

Alpha-halogenation

This reactivity is similar to the tendency of ethers with alpha hydrogen atoms to form peroxides. Reaction with chlorine produces alpha-chloroethers.

Synthesis

Dehydration of alcohols

The dehydration of alcohols affords ethers:[7]

2 R–OH → R–O–R + H2O at high temperature

This direct nucleophilic substitution reaction requires elevated temperatures (about 125 °C). The reaction is catalyzed by acids, usually sulfuric acid. The method is effective for generating symmetrical ethers, but not unsymmetrical ethers, since either OH can be protonated, which would give a mixture of products. Diethyl ether is produced from ethanol by this method. Cyclic ethers are readily generated by this approach. Elimination reactions compete with dehydration of the alcohol:

R–CH2–CH2(OH) → R–CH=CH2 + H2O

The dehydration route often requires conditions incompatible with delicate molecules. Several milder methods exist to produce ethers.

Electrophilic addition of alcohols to alkenes

Alcohols add to electrophilically activated alkenes. The method is atom-economical:

R2C=CR2 + R–OH → R2CH–C(–O–R)–R2

isoamylene, which protonate to give relatively stable carbocations. Using ethanol and methanol with these two alkenes, four fuel-grade ethers are produced: methyl tert-butyl ether (MTBE), methyl tert-amyl ether (TAME), ethyl tert-butyl ether (ETBE), and ethyl tert-amyl ether (TAEE).[4]

Solid acid catalysts
are typically used to promote this reaction.

Epoxides

Main article: epoxide

Epoxides are typically prepared by oxidation of alkenes. The most important epoxide in terms of industrial scale is ethylene oxide, which is produced by oxidation of ethylene with oxygen. Other epoxides are produced by one of two routes:

Many ethers,

ethoxylates and crown ethers
, are produced from epoxides.

Williamson and Ullmann ether syntheses

alkyl halides by alkoxides

R–ONa + R′–X → R–O–R′ + NaX

This reaction, the Williamson ether synthesis, involves treatment of a parent alcohol with a strong base to form the alkoxide, followed by addition of an appropriate aliphatic compound bearing a suitable leaving group (R–X). Although popular in textbooks, the method is usually impractical on scale because it cogenerates significant waste.

Suitable leaving groups (X) include iodide, bromide, or sulfonates. This method usually does not work well for aryl halides (e.g. bromobenzene, see Ullmann condensation below). Likewise, this method only gives the best yields for primary halides. Secondary and tertiary halides are prone to undergo E2 elimination on exposure to the basic alkoxide anion used in the reaction due to steric hindrance from the large alkyl groups.

In a related reaction, alkyl halides undergo nucleophilic displacement by

SN2
mechanism.

C6H5OH + OH− → C6H5–O− + H2O
C6H5–O− + R–X → C6H5OR

The Ullmann condensation is similar to the Williamson method except that the substrate is an aryl halide. Such reactions generally require a catalyst, such as copper.[8]

Important ethers

Chemical structure of ethylene oxide Ethylene oxide A cyclic ether. Also the simplest epoxide.
Chemical structure of dimethyl ether Dimethyl ether A colourless gas that is used as an
aerosol spray propellant. A potential renewable alternative fuel for diesel engines with a cetane rating
as high as 56–57.
Chemical structure of diethyl ether Diethyl ether A colourless liquid with sweet odour. A common low boiling
perfumery
.
Chemical structure of dimethoxyethane Dimethoxyethane (DME) A water miscible solvent often found in lithium batteries (b.p. 85 °C):
Chemical structure of dioxane
Dioxane
 A cyclic ether and high-boiling solvent (b.p. 101.1 °C).
Chemical structure of THF Tetrahydrofuran (THF) A cyclic ether, one of the most polar simple ethers that is used as a solvent.
Chemical structure of anisole Anisole (methoxybenzene) An aryl ether and a major constituent of the essential oil of anise seed.
Chemical structure of 18-crown-6 Crown ethers Cyclic polyethers that are used as
phase transfer catalysts
.
Chemical structure of polyethylene glycol Polyethylene glycol (PEG) A linear polyether, e.g. used in
pharmaceuticals
.
Polypropylene glycol A linear polyether, e.g. used in
polyurethanes
.
Platelet-activating factor An ether lipid, an example with an ether on sn-1, an ester on sn-2, and an inorganic ether on sn-3 of the glyceryl scaffold.

See also

References

  1. doi:10.1351/goldbook.E02221
  2. ISBN 978-0-470-77107-5
    .
  3. PMID 15303175
    .
  4. ^
    doi:10.1002/14356007.a10_023
  5. ^ J. F. W. McOmie and D. E. West (1973). "3,3′-Dihydroxylbiphenyl". Organic Syntheses; Collected Volumes, vol. 5, p. 412.
  6. doi:10.1016/S0020-1693(00)86863-2
    .
  7. ISBN 978-0-19-850346-0
    .
  8. doi:10.1055/s-2006-942440
    .
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