11.3 : Éthers d'alcools : déshydratation de l'alcool et synthèse de l'éther de Williamson

The two most common methods for preparing ethers from alcohols are alcohol dehydration and Williamson ether synthesis.

Alcohol dehydration is used for the industrial preparation of ethers. The method involves acid-catalyzed dehydration of alcohols via the S_N2 mechanism.

Consider the dehydration of ethanol in the presence of sulfuric acid to give diethyl ether.

The reaction begins with a proton transfer from the acid catalyst to the hydroxyl group of ethanol, forming an oxonium ion. The proton transfer converts hydroxyl, a poor leaving group, into an oxonium ion, a better leaving group.

Subsequently, a second ethanol molecule, acting as a nucleophile, attacks the oxonium ion in an S_N2 reaction and displaces a water molecule, forming a new oxonium ion.

Finally, proton transfer from the new oxonium ion to water completes the reaction, giving ethoxyethane as the final product.

The alcohol dehydration method is limited to the preparation of symmetrical ethers derived from primary alcohols. Secondary and tertiary alcohols cannot be used because they dehydrate to alkenes.

Additionally, the method is unfit for the preparation of asymmetrical ethers because it yields a mixture of ethers.

A more general method for preparing ethers is the Williamson ether synthesis. It is a two-step process and provides an important route for preparing asymmetrical ethers.

A specific example is the synthesis of ethoxypropane from propanol.

The reaction begins with alcohol deprotonation, where propanol reacts with sodium hydride, a strong base, to form an alkoxide ion.

The second step is an S_N2 reaction wherein the alkoxide ion acts as a nucleophile and displaces the iodide ion, forming the ether.

The Williamson ether synthesis prefers methyl or primary alkyl halides as substrates because they are less hindered and have high S_N2 reactivity.

In contrast, secondary or tertiary alkyl halides are less preferred, as they are more hindered and undergo elimination instead.