Williamson Ether Synthesis: Mechanism, Uses and Limitations

Williamson's Ether Synthesis

Williamson's Ether Synthesis is an old method in the realm of organic chemistry for the synthesis of ethers. The reaction involves the nucleophilic substitution by an alkoxide ion, RO-, by a primary alkyl halide $\mathrm{R}^{\prime}-\mathrm{X}$, to form an ether, R-O-R.

In this reaction, the alkoxide ion acts as a nucleophile and attacks the electrophilic carbon of the alkyl halide, displacing the halide ion. Under these circumstances, the choice of reagents and reaction conditions becomes of prime importance for the success of the synthesis. Usually, alkoxides are generated in situ by reacting alcohols with strong bases such as sodium or potassium hydride.

It is the best method to prepare all types of ethers, that is, simple, mixed, or aromatic ethers. Here, alkyl halides are treated with sodium alkoxide in the presence of magnesium to give ethers. It involves S_N2 mechanism during the attack of $\mathrm{R}^{-} \mathrm{O}^{-}$on $\mathrm{R}-\mathrm{X}$ that is, backside attack occurs here. The reaction occurs as follows:

Some examples include:

Various Aspects of Williamson's Ether Synthesis

1. Types of Williamson Ether Synthesis

Symmetrical Ethers: The alkyl groups at either side of the oxygen atom are the same. They are usually prepared from one type of alcohol. For example, diethyl ether $\left(\mathrm{CH}_3 \mathrm{CH}_2-\mathrm{O}-\mathrm{CH}_2 \mathrm{CH}_3\right)$ is prepared using ethanol.
Unsymmetrical Ethers: These are those containing different alkyl groups, such as R-O-R'. Their synthesis needs a proper selection of alkyl halides and alkoxides to avoid side reactions and ensure the creation of the product of interest. For instance, ethyl methyl ether, $\mathrm{CH}_3-\mathrm{OCH}_2 \mathrm{CH}_3$ is formed from methanol and ethyl bromide.

2. Mechanism and Reaction Conditions

This reaction follows the bimolecular nucleophilic substitution mechanism due to a single, concerted step in which the nucleophile attacks the electrophilic carbon of the alkyl halide. The factors that affect this reaction are:

1. Choice of Alkyl Halide: Primary alkyl halides are preferred because they are more reactive with lower steric hindrance, which facilitates the nucleophilic attack.
2. Reaction Medium: Polar aprotic solvents like DMSO or acetonitrile favor alkoxide ion nucleophilicity, and hence, the rate of the reaction is enhanced.

Relevance and Applications

There is gross importance of Williamson's Ether Synthesis both in academics and in industry.

1. Academic Importance

This is a regular undergraduate laboratory test in organic chemistry classes in academic institutions, used to study the basic concepts of nucleophilic substitution and reaction mechanisms. Its simplicity and versatility make it an excellent pedagogical tool for demonstrating the principles of organic synthesis.

2. Industrial Applications

1. Pharmaceuticals: Ethers prepared by Williamson's method play a very important role in the manufacture of various pharmaceuticals. For example, anesthetic diethyl ether is prepared by this reaction.
2. Perfumery and Flavoring Agents: It is in perfumery and flavorings that others play an important role. This ability to synthesize symmetrical or unsymmetrical ethers offers very great diversity among odorant compounds.
3. Solvents: Ethers, such as diethyl ether and tetrahydrofuran, serve as common solvents both for chemical reactions and extractions. This is because physically, they have good properties, particularly low boiling points and the ability to dissolve nearly everything.

3. Complex Molecule Synthesis

Beyond simple ethers, Williamson's Ether Synthesis finds applications in synthesizing more complex molecules, including polymers and crown ethers. Among these is the crown ether, which is a type of cyclic compound that encapsulates metal ions, and hence has many chemical and biochemical applications.

Recommended topic video on(Williamson's Ether Synthesis)

Some Solved Examples

Example 1 Allyl phenyl ether can be prepared by heating which of the following combinations?

1. $( C_6H_5Br + CH_2=CH-CH_2-ONa )$
2. $\left(\mathrm{CH} \_2=\mathrm{CH}-\mathrm{CH} \_2-\mathrm{Br}+\mathrm{C} \_6 \mathrm{H} \_5 \mathrm{ONa}\right)$
3. ( $\mathrm{C} \_6 \mathrm{H} \_5-\mathrm{CH}=\mathrm{CH}-\mathrm{Br}+\mathrm{CH} \_3-\mathrm{ONa}$ )
4. ( $\left.\mathrm{CH} \_2=\mathrm{CH}-\mathrm{Br}+\mathrm{C} 66 \mathrm{H} \_5-\mathrm{CH} \_2-\mathrm{ONa}\right)$

Solution:
The correct answer is option (2)$\left(\mathrm{CH} 2=\mathrm{CH}-\mathrm{CH} \_2-\mathrm{Br}+\mathrm{C} \_6 \mathrm{H} \_5 \mathrm{ONa}\right)$.This combination undergoes a nucleophilic substitution reaction known as Williamson's Synthesis, forming allyl phenyl ether.

Example 2 Given below are two statements: one is labeled as Assertion (A) and the other is labeled as Reason (R)

Assertion (A): Synthesis of ethyl phenyl ether may be achieved by Williamson synthesis.

Reason (R): Reaction of bromobenzene with sodium ethoxide yields ethyl phenyl ether.

In the light of the above statements, choose the most appropriate answer from the options given below :

1)Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)

2) (A) is correct but (R) is not correct

3)(A) is not correct but (R) is correct

4)Both (A) and (R) are correct and (R) is the correct explanation of (A)

Solution:

Ethyl Phenyl ether can be prepared using Williamson's synthesis as

$\therefore$ Assertion (A) is correct.

Now, the reaction of bromobenzene and sodium ethoxide does not take place as there is a partial double bond character between the Benzene ring and the halogen and as a result, it is difficult for the substitution reaction to take place.

$\therefore$ Reason (R) is not correct.

Hence, the correct answer is option (2)

Example 3 Williamson's synthesis of ether is an example of:
1. Nucleophilic addition
2. Electrophilic addition
3. Electrophilic substitution
4. Nucleophilic substitution

Solution:
The correct answer is option (4) Nucleophilic substitution. Williamson's Ether Synthesis involves the treatment of alkyl halides with sodium alkoxide, proceeding via an SN2 mechanism, making it a nucleophilic substitution reaction.

Summary

One of the cardinal points of organic chemistry, Williamson's Ether Synthesis enables easy and ready production of ethers. A reaction that is key to teaching nucleophilic substitution, it finds wider applications in pharmaceuticals, fragrances, solvents, and synthetic schemes directed toward the synthesis of complex molecules. Taking an overview of its mechanism, variations, and practical application views gives a wider background from the field of organic synthesis, pointing out its remarkable relevance for both academic and industrial circles.