Ether

Encyclopedia : E : ET : ETH : Ether

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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 — 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, CH₃-CH₂-O-CH₂-CH₃).

Physical properties

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 .F₃B:O(CH₂CH₃)₂. Ethers also coordinate to magnesium in Grignard reagents.

Nomenclature

In the IUPAC nomenclature system, ethers are named using the general formula "alkoxyalkane", for example CH₃-CH₂-O-CH₃ is methoxyethane. If the ether is part of a more complex molecule, it is described as an alkoxy substituent, so -OCH₃ 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.

Similar structures

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

Primary, secondary, and tertiary ethers

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 CH₃-CH₂-O-CH₂-CH₃. An example of a secondary ether is diisopropyl ether (CH₃)₂CH-O-CH(CH₃)₂ and that of a tertiary ether is di-tert-butyl ether (CH₃)₃C-O-C(CH₃)₃.

Dimethyl ether, a primary, a secondary, and a tertiary ether.

Polyethers

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.

Organic reactions

Synthesis

Ethers can be prepared in the laboratory in several ways.

• Dehydration of alcohols:

R-OH + R-OH → 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 → 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 → R₂CH-C(-O-R)-R₂

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

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.

Important ethers

• Ethylene oxide, the smallest cyclic ether:

• Dimethyl ether, an aerosol spray propellant:

• Diethyl ether, a common low boiling solvent:

• Dimethoxyethane, a high boiling solvent:

• Dioxane, a cyclic ether and high boiling solvent:

• THF, a cyclic ether, one of the most polar simple ethers that is used as a solvent:

• Anisole (methoxybenzene), a major constituent of the essential oil of anise seed:

• Crown ethers, cyclic polyethers that are used as phase transfer catalysts:

• Polyethylene glycol, a linear polyether, e.g. used in cosmetics:

See also

• 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

External links

• [ILPI] page about ethers.

• For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].

 

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