Ether

In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R′ represent the organyl groups. Ethers can again be classified into two varieties: if the organyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers. A typical example of the first group is the solvent and anaesthetic diethyl ether, commonly referred to simply as "ether" (). Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.

Structure and bonding

Ethers feature bent linkages. In dimethyl ether, the bond angle is 111° and C–O distances are 141  pm. 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 electronegative than carbon, thus the alpha hydrogens of ethers are more acidic than those of simple hydrocarbons. They are far less acidic than alpha hydrogens of carbonyl groups (such as in ketones or aldehydes), however.

Ethers can be symmetrical of the type ROR or unsymmetrical of the type ROR'. Examples of the former are dimethyl ether, diethyl ether, 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 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 simpler 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 anisole, because it was originally found in aniseed. The 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 polymers containing ether linkages in their main chain. The term polyol generally refers to polyether polyols with one or more functional end-groups such as a 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.

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: Esters (R–C(=O)–O–R′), hemiacetals (R–CH(–OH)–O–R′), carboxylic acid anhydrides (RC(=O)–O–C(=O)R′).

There are compounds which, instead of C in the linkage, contain heavier group 14 chemical elements (e.g., Si, Ge, Sn, Pb). Such compounds are considered ethers as well. Examples of such ethers are silyl enol ethers (containing the linkage), disiloxane (the other name of this compound is disilyl ether, containing the linkage) and stannoxanes (containing the linkage).

Physical properties

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

Reactions

The C-O bonds that comprise simple ethers are strong. They are unreactive toward all but the strongest bases. Although generally of low chemical reactivity, they are more reactive than alkanes.

Specialized ethers such as epoxides, ketals, and acetals are unrepresentative classes of ethers and are discussed in separate articles. Important reactions are listed below.Wilhelm Heitmann, Günther Strehlke, Dieter Mayer "Ethers, Aliphatic" in ''Ullmann's Encyclopedia of Industrial Chemistry'' Wiley-VCH, Weinheim, 2002.

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. O(H)CH₃sup>+Br⁻.

Some ethers undergo rapid cleavage with boron tribromide (even aluminium chloride is used in some cases) to give the alkyl halide. 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 explosive peroxides, such as 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

Ethers serve as Lewis bases. For instance, diethyl ether forms a complex with boron trifluoride, i.e. borane diethyl etherate (). 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:
: 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
Acid catalysis is required for this reaction. Commercially important ethers prepared in this way are derived from isobutene or 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).

Solid acid catalysts are typically used to promote this reaction.

Epoxides

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:
* By the oxidation of alkenes with a peroxyacid such as ''m''-CPBA.
* By the base intramolecular nucleophilic substitution of a halohydrin.

Many ethers, ethoxylates and crown ethers, are produced from epoxides.

Williamson and Ullmann ether syntheses

Nucleophilic displacement of alkyl halides by alkoxides
: R–ONa + R′–X → R–O–R′ + Na X
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 phenoxides. The R–X cannot be used to react with the alcohol. However phenols can be used to replace the alcohol while maintaining the alkyl halide. Since phenols are acidic, they readily react with a strong base like sodium hydroxide to form phenoxide ions. The phenoxide ion will then substitute the –X group in the alkyl halide, forming an ether with an aryl group attached to it in a reaction with an S_N2 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.

Important ethers

See also

* Ester
* Ether lipid
* Ether addiction
* History of general anesthesia
* Inhalant
* Chemical paper pulping processes: Kraft process (and Soda pulping), Organosolv pulping process and the Sulfite process

References

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