Ethers symmetrical, preparation

Diethyl ether and other simple symmetrical ethers are prepared industrially by the sulfuric acid-catalyzed dehydration of alcohols. The reaction occurs by SN2 displacement of water from a protonated ethanol molecule by the oxygen atom of a second ethanol. Unfortunately, the method is limited to use with primary alcohols because secondary and tertiary alcohols dehydrate by an El mechanism to yield alkenes (Section 17.6). 

Problem 18.2 Why do you suppose only symmetrical ethers are prepared by the sulfuric add-catalyzed dehydration procedure What product(s) would you expect if ethanol and 1-propanol were allowed to react together In what ratio would the products be formed if the two alcohols were of equal reactivity ... 

Ethers are prepared from alkyl halides by the treatment of metal alkoxide. This is known as Williamson ether synthesis (see Sections 4.3.6 and 5.5.2). Williamson ether synthesis is an important laboratory method for the preparation of both symmetrical and unsymmetrical ethers. Symmetrical ethers are prepared by dehydration of two molecules of primary alcohols and H2SO4 (see Sections 4.3.7 and 5.5.3). Ethers are also obtained from alkenes either by acid-catalysed addition of alcohols or alkoxymercuration-reduction (see Section 5.3.1). 

Williamson ether synthesis preparation of ether The sodium or potassium alkoxides are strong bases and nucleophiles. Alkoxides (RO ) can react with primary alkyl halides to produce symmetrical or unsymmetrical ethers. This is known as Williamson ether synthesis. The reaction is limited to primary alkyl halides. Higher alkyl halides tend to react via elimination. For example, sodium ethoxide reacts with ethyl iodide to produce diethyl... 

Diethyl ether and other simple symmetrical ethers are prepared industrially by the sulfuric acid-catalyzed dehydration of alcohols ... 

Preparation and Uses of Ethers Symmetrical ethers, such as diethyl ether, can be prepared by the elimination of a water molecule from between two alcohol molecules with a strong dehydrating agent, such as concentrated H2SO4. 

The symmetrical dibromo- and diodo-methyl ethers were prepared by Tischenko by reacting polyoxymethylene with hydrogen bromide and iodide respectively. Symmetrical dibromomethyl ether, (CHaBr)20, is reported to boil at 148.5-151.5°C the dnodoether, (CH3l)20, at 218-219°C.. -Vccoiding to Henryk, the dibromoether solidifies at and boils at... 

Formation of, -dinitrobenzoates. Aliphatic ethers are broken up by heating with ZnClg, and a 3,5--dinitrobenzoate of the residue can then be prepared. This is suitable only for symmetrical ethers. 

A), -Dinitrohenzoates. Only suitable for symmetrical aliphatic ethers. Preparation, see above. The yields are usually very low. 

In general this method is limited to the preparation of symmetrical ethers m which both alkyl groups are primary Isopropyl alcohol however is readily available at low cost and gives high enough yields of diisopropyl ether to justify making (CH3)2CHOCH(CH3)2 by this method on an industrial scale... 

Alcohols can be dehydrated with strong acid catalysts and high reaction temperatures to produce ethers. This method is particularly useful for the preparation of symmetrical lower alkyl ethers, such as ethyl ether. 

Symmetrical labile ethers such as cycloalkenyl ethers (15) or mixed acetals (16) can also be prepared from the 3-hydroxyl group by acid-catalyzed exchange etherification or by acid-catalyzed addition to cyclohexanone methyl enol ether. 

The following section describes a versatile method for preparing either symmetrical or unsymmetrical ethers that is based on the principles of bimoleculai nucleophilic substitution. 

This reaction, which is named after W. Williamson, is the most important method for the synthesis of unsymmetrical ethers 3. For this purpose an alkoxide or phenoxide 1 is reacted with an alkyl halide 2 (with R = alkyl, allyl or benzyl). Symmetrical ethers can of course also be prepared by this route, but are accessible by other routes as well. 

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

Simple 1,2,4-triazole derivatives played a key role in both the synthesis of functionalized triazoles and in asymmetric synthesis. l-(a-Aminomethyl)-1,2,4-triazoles 4 could be converted into 5 by treatment with enol ethers <96SC357>. The novel C2-symmetric triazole-containing chiral auxiliary (S,S)-4-amino-3,5-bis(l-hydroxyethyl)-l,2,4-triazole, SAT, (6) was prepared firmn (S)-lactic acid and hydrazine hydrate <96TA1621>. This chiral auxiliary was employed to mediate the diastereoselective 1,2-addition of Grignard reagents to the C=N bond of hydrazones. The diastereoselective-alkylation of enolates derived from ethyl ester 7 was mediated by a related auxiliary <96TA1631>. 

In the preparation of iodides, but not bromides, PMHS may be substituted for the TMDO. Chlorides can be obtained if thionyl chloride and zinc iodide are added to suppress the formation of symmetrical ethers.314 An example of this type of reductive chlorination is shown by the TMDO-mediated conversion of p-tolualdehyde into p-methylbenzyl chloride (Eq. 201).313 To obtain chlorides from aldehydes having electron-withdrawing groups such as nitro or carbomethoxy, the initial reaction is first carried out at —70° and the mixture is then heated to reflux in order to reduce the formation of symmetrical ether by-products. Zinc chloride is substituted for zinc iodide for the synthesis of chlorides of substrates with electron-donating groups such as methoxy and hydroxy.314... 

The main method for the preparation of different types of organostannanes is the ancient Wilrtz reaction between triorganotin halides or diorganotindihalides. Reactions proceed with or without solvent. Usual solvents are benzene, toluene, xylene, diethyl ether and ethanol. Occasionally hquid ammonia is also used. The preferred metals for this reaction are sodium and lithium. In the case of triorganotin halides, the reaction leads to symmetric hexaorganoditin compounds72 ... 

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