Conversion of Alcohols to Ethers

Primary alcohols are converted to ethers on heating in the presence of an acid catalyst, usually sulfuric acid. 

This kind of reaction is called a condensation. A condensation is a reaction in which two molecules combine to form a larger one while liberating a small molecule. In this case two alcohol molecules combine to give an ether and water. 

When applied to the synthesis of ethers, the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols. 

Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C. At higher temperatures elimination predominates, and ethylene becomes the major product. The formation of diethyl ether is outlined in Mechanism 15.2. The individual steps of this mechanism are analogous to those seen earlier. Nucleophilic attack on a protonated alcohol was encountered in the reaction of primary alcohols with hydrogen halides (Section 4.12), and the nucleophilic properties of alcohols were discussed in the context of solvolysis reactions (Section 8.5). Both the first and the last steps are proton-transfer reactions between oxygens. 

Diols react intramolecularly to form cyclic ethers when a five-membered or six-membered ring can result. 

Summary of Reactions of Alcohols Discussed in Earlier Chapters 

Reaction with hydrogen haiides (Section 4.8) The order of alcohol reactivity parallels the order of carbocation stability R3C+ R2CH+ RCH2+ CHb. Benzylic alcohols react readily. 

Reaction with thionyl chloride (Section 4.14) Thionyl chloride converts alcohols to alkyl chlorides. 

Reaction with phosphorus trihaiides (Section 4.14) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides. 

The individual steps in the formation of diethyl ether are outlined in Mechanism 15.1 and each is analogous to steps seen in earlier mechanisms. Both the first and the last steps 

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Alcohols represent a very versatile class of compounds for organic synthesis, so a variety of synthetic pathways utilize alcohols. Some examples appear in Figure 3-16. The conversion of alcohols to ethers appears later in this chapter. 

Scheme 8.3 Conversion of alcohols to ethers—the Williamson ether synthesis.

Upon further examination of the functional group transformations summarized in Figure 8.1, there are a number of additional conversions applicable to the product functional groups. Among these are the conversions of alcohols to ethers illustrated in Scheme 8.3. 

Dehydration of organic molecules can occur on surface acids, for example, the conversion of alcohols to ethers and ketones. A good example is the reaction of ethanol on modified AI2O3 catalysts of various acidities (Table 5-28). 

Nucleophilic substitution at aliphatic carbon is a reaction that plays an important role in organic synthesis. The use of the reaction in construction of carbon-carbon bonds will be discussed in Part B, but the role of the reaction in interconverting certain functional groups has such a basic place in organic chemistry that it is appropriate to illustrate some examples of the synthetic utility of nucleophilic substitution. The conversion of alcohols to alkyl halides and the conversion of alcohols to ethers are among the most fundamental organic transformations. 

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