The general structure of an ether. R and R' represent mostorganylsubstituents.
Inorganic chemistry,ethers are a class ofcompounds that contain an ethergroup—a singleoxygen atom bonded to two separate carbon atoms, each part of anorganyl group (e.g.,alkyl oraryl). They have the general formulaR−O−R′, 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.[1] A typical example of the first group is thesolvent andanaestheticdiethyl ether, commonly referred to simply as "ether" (CH3−CH2−O−CH2−CH3). Ethers are common in organic chemistry and even more prevalent inbiochemistry, as they are common linkages incarbohydrates andlignin.[2]
Ethers feature bentC−O−C linkages. Indimethyl ether, thebond 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 ofvalence bond theory, the hybridization at oxygen is sp3.
Oxygen is moreelectronegative 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 inketones oraldehydes), however.
Vinyl- and acetylenic ethers are far less common than alkyl or aryl ethers. Vinylethers, often calledenol ethers, are important intermediates inorganic synthesis. Acetylenic ethers are especially rare.Di-tert-butoxyacetylene is the most common example of this rare class of compounds.
In theIUPAC Nomenclature system, ethers are named using the general formula"alkoxyalkane", for example CH3–CH2–O–CH3 ismethoxyethane. 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 simpleralkyl radical is written in front, so CH3–O–CH2CH3 would be given asmethoxy(CH3O)ethane(CH2CH3).
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 calledsweet oil of vitriol. Methyl phenyl ether isanisole, because it was originally found inaniseed. Thearomatic ethers includefurans.Acetals (α-alkoxy ethers R–CH(–OR)–O–R) are another class of ethers with characteristic properties.
Polyethers are generallypolymers containing ether linkages in their main chain. The termpolyol generally refers to polyether polyols with one or more functionalend-groups such as ahydroxyl group. The term "oxide" or other terms are used for high molar mass polymer when end-groups no longer affect polymer properties.
There are compounds which, instead ofC in theC−O−C linkage, contain heaviergroup 14chemical elements (e.g.,Si,Ge,Sn,Pb). Such compounds are considered ethers as well. Examples of such ethers aresilyl enol ethersR3Si−O−CR=CR2 (containing theSi−O−C linkage),disiloxaneH3Si−O−SiH3 (the other name of this compound is disilyl ether, containing theSi−O−Si linkage) andstannoxanesR3Sn−O−SnR3 (containing theSn−O−Sn linkage).
The C-O bonds that comprise simple ethers are strong. They are unreactive toward all but the strongest bases. Although generally of low chemicalreactivity, they are more reactive thanalkanes.
Specialized ethers such asepoxides,ketals, andacetals are unrepresentative classes of ethers and are discussed in separate articles. Important reactions are listed below.[4]
These reactions proceed viaonium intermediates, i.e. [RO(H)CH3]+Br−.
Some ethers undergo rapid cleavage withboron tribromide (evenaluminium 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 chemicalpaper pulping processes are based on cleavage of ether bonds in thelignin.
When stored in the presence of air or oxygen, ethers tend to formexplosiveperoxides, such asdiethyl ether hydroperoxide. The reaction is accelerated by light, metal catalysts, andaldehydes. 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 liketetrahydrofuran (THF) orethylene glycol dimethyl ether (1,2-dimethoxyethane) are avoided in industrial processes.
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.
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:
This reaction, theWilliamson ether synthesis, involves treatment of a parentalcohol with a strongbase to form the alkoxide, followed by addition of an appropriatealiphatic compound bearing a suitableleaving group (R–X). Although popular in textbooks, the method is usually impractical on scale because it cogenerates significant waste.
Suitable leaving groups (X) includeiodide,bromide, orsulfonates. 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 byphenoxides. The R–X cannot be used to react with the alcohol. Howeverphenols can be used to replace the alcohol while maintaining the alkyl halide. Since phenols are acidic, they readily react with a strongbase likesodium 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 anSN2 mechanism.
C6H5OH + OH− → C6H5–O− + H2O
C6H5–O− + R–X → C6H5OR
TheUllmann 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]
A colourless liquid with sweet odour. A common low boilingsolvent (b.p. 34.6 °C) and an earlyanaesthetic. Used as starting fluid for diesel engines. Also used as arefrigerant and in the manufacture ofsmokeless gunpowder, along with use inperfumery.
^Vojinović, Krunoslav; Losehand, Udo; Mitzel, Norbert W. (2004). "Dichlorosilane–Dimethyl Ether Aggregation: A New Motif in Halosilane Adduct Formation".Dalton Trans. (16):2578–2581.doi:10.1039/b405684a.PMID15303175.
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