11.3:
Ethers from Alcohols: Alcohol Dehydration and Williamson Ether Synthesis
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 SN2 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 SN2 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 SN2 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 SN2 reactivity.
In contrast, secondary or tertiary alkyl halides are less preferred, as they are more hindered and undergo elimination instead.
11.3:
Ethers from Alcohols: Alcohol Dehydration and Williamson Ether Synthesis
Overview
Ethers can be prepared from organic compounds by various methods. Some of them are discussed below,
Preparation of Ethers by Alcohol Dehydration
In this method, in the presence of protic acids, alcohol dehydrates to produce alkenes and ethers under different conditions. For example, in the presence of sulphuric acid, dehydration of ethanol at 413 K yields ethoxyethane, whereas it yields ethene at 443 K.
This method is a nucleophilic substitution reaction. The two alcohol molecules involved in the reaction play two roles—one alcohol molecule acts as a substrate while the other acts as a nucleophile. The reaction follows an SN2 mechanism. The dehydration of secondary and tertiary alcohols to get corresponding ethers is unsuccessful as alkenes are formed easily in these reactions.
Preparation of Ethers by Williamson Ether Synthesis
It is the most versatile method for the preparation of asymmetrical ethers in laboratories. In this method, initially, the alcohol is deprotonated to form an alkoxide ion. Further, the alkoxide ion functions as a nucleophile and attacks an alkyl halide, leading to the formation of ether. The reaction generally follows the SN2 mechanism for primary alcohol.
Williamson synthesis exhibits higher productivity when the halide to be displaced is on a methyl or a primary carbon. In the case of secondary alkyl halides, elimination competes with substitution, whereas the formation of elimination products is the only case in tertiary alkyl halides.