Name: Chemical Reactions of Ethers: Nucleophilic Substitution
Uploaded: 2019-07-19
Duration: 4 min 9 s
Description: This video illustrates the chemical reactions of ethers. In the presence of strong acids, ethers undergo nucleophilic substitution reactions to give alkyl ha...

Question

Describe Williamson's synthesis of ethers. What are its limitations? Discuss the mechanism.

Accepted Answer

Williamson's synthesis of ether: In this method, an ether is obtained by heating an alkyl halide with sodium alkoxide.

Example: $\underset{Sodium ethoxide}{C_{2} H_{5} ONa} + \underset{Ethyl iodide}{C_{2} H_{5} I \overset{}{\to}} \underset{diethyl ether}{C_{2} H_{5} {OC}_{2} H_{5}} + NaI$

Limitations of Williamson's synthesis of ethers:

(i) In Williamson synthesis for the preparation of ethers, alkoxide ion acts as a nucleophile and attacks a primary alkyl halide. However, when secondary or tertiary alkyl halide is used instead of primary alkyl halide, along with substitution, elimination is possible. Elimination will give alkenes as alkoxide ion can also act as a strong base.

(ii) Sodium alkoxide and aryl halides cannot be used for the preparation of phenolic ethers because, in aryl halides, the $C - X$ bond acquires partial double bond character due to resonance. Hence, phenolic ethers are prepared by treating sodium phenoxide with alkyl halides.

Mechanism of Williamson's synthesis of ethers:

This reaction involves substitution nucleophilic bimolecular ( $S_{N} 2$ ) attack of an alkoxide ion on primary alkyl halide as shown below:

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