Ethers by intermolecular dehydration

Complications of Intermolecular Dehydration The method of synthesizing ethers by intermolecular dehydration has some important limitations. 

Attempts to synthesize ethers by intermolecular dehydration of secondary alcohols are usually unsuccessful because alkenes form too easily. 

Attempts to synthesize ethers with 2° alkyl groups by intermolecular dehydration... 

Diethyl ether is prepared commercially by intermolecular dehydration of ethanol with sulfuric acid. The Williamson ether synthesis, another route to ethers, involves preparation of an alkoxide from an alcohol and a reactive metal, followed by an SN2 displacement between the alkoxide and an alkyl halide. 

ETHERS, ROR, would result from replacing both H s of H2O by an R group. They may be formed by intermolecular dehydration of alcohols. Diethyl ether, called ether, an anesthetic, is made this way from ethanol ... 

Dehydration to olefins, which sometimes accompanies the reaction of alcohols with DAST [95, 108], is seldom as extensive as with a-fluoroamines (FAR and 1,1,2,3,3,3 hexafluoropropyldiethylamine) but occurs in a few cases to the exclusion of fluonnation, thus, 9a-fluoro-11-hydroxysteroids give 9a fluoro-A -steroids [127, 128] Dehydration accompanied by Wagner-Meerwein rearrangement occurs during the fluonnation of testosterone [129] Intermolecular dehydration to form ethers in addition to fluorides is observed in the reaction of benzhydryl alcohols [104] (Table 6)... 

Dehydration of alcohols over solid catalysts can yield alkenes by intramolecular dehydration, whereas ethers are the product of an intermolecular process. The catalysts used can be acidic or basic solids or bifunctional acid-base materials. Although selective synthesis of any desired product is possible, complications can arise as a result of side-reactions-dehydrogenation and decomposition of the starting alcohol, decomposition and consecutive transformations of intermediates and products (j9-cleavage of carbocations, oligomerization of alkenes). 

Intramolecular dehydration (alkene formation) and intermolecular dehydration are competitive processes but selective ether synthesis is possible by applying appropriate catalysts under suitable reaction conditions. 

Simple ethers can be prepared by acid-catalyzed intermolecular dehydration of alcohols. 

This reaction competes with the formation of alkenes by acid-catalyzed alcohol dehydration (Sections 7.7 and 7.8). Intermolecular dehydration of alcohols usually takes place at lower temperature than dehydration to an alkene, and dehydration to the ether can be aided by distilling the ether as it is formed. For example, diethyl ether is made commercially by dehydration of ethanol. Diethyl ether is the predominant product at 140 °C ethene is the predominant product at 180 °C. 

Intermolecular Dehydration of Alcohols to Form an Ether 517 The Williamson Ether Synthesis 518 Ether Cleavage by Strong Acids 522 Aikene Epoxidation 524... 

Diethyl ether and several other commercially available ethers are synthesized on an industrial scale by the acid-catalyzed dehydration of primary alcohols. Intermolecular dehydration of ethanol, for example, gives diethyl ether. 

Yields of ethers from the acid-catalyzed intermolecular dehydration of alcohols are highest for symmetrical ethers formed from unbranched primary alcohols. Examples of symmetrical ethers formed in good yield by this method are dimethyl ether, diethyl ether, and dibutyl ether. From secondary alcohols, yields of ether are lower because of competition from acid-catalyzed dehydration (Section 10.6). In the case of tertiary alcohols, dehydration to an alkene is the only reaction. 

Methanol conversion is a special case, because it caimot form unsaturated species by dehydration. Therefore, the primary product of methanol conversion is dimethyl ether, which is produced by an intermolecular dehydration reaction [174]. For this reaction a contribution of the lattice oxygen atoms (basic sites) is also proposed [175], which woiUd rule out the application of methanol conversion as a probe reaction for acid sites. 

Alcohols can undergo dehydration to produce ethers in an intermolecular reaction (Scheme 4) [1,3]. Characteristically only symmetrical ethers can be prepared in this way, usually via a bimolecular reaction (8 2 mechanism). When a mixture of two alcohols is reacted a mixture of three ethers is produced. Mixed ethers, however may be synthesized by reacting a tertiary alcohol with a primary or secondary alcohol. In this reaction an S l mechanism is operative, with involvement of the tertiary carbocation formed from the tertiary alcohol. 

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