Question

Which of the following method is/are not suitable to prepare ether? (A)\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH} \frac{\text { Conc }, \mathrm{H}_{2} \mathrm{SO}_{4}}{413 \mathrm{~K}}{\longrightarrow}\) (B)\(\mathrm{CH}_{3}-\mathrm{OH} \frac{\text { Conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{140^{\circ} \mathrm{C}}\) (C) (D)

Short answer

None of the given methods (A) and (B) are unsuitable for the preparation of ethers.

Step-by-step solution

1. Recap of Ether Preparation Methods

To identify if the reaction conditions provided in each method are suitable for ether formation, we need to have an idea about the common ether preparation methods. One of the most common methods for preparing ethers is the "Williamson Ether Synthesis" which includes the reaction between an alkoxide ion (RO-) and an alkyl halide (R-X). Another method is the "Dehydration of Alcohols" where an alcohol reacts with concentrated sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)) or phosphoric acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)) at a specific temperature, generally around 140°C. This method favours the formation of ethers, but at very high temperatures, it could lead to the formation of alkenes as the major product instead. Now, let's analyze each method provided in the question.

2. Analyzing Method A

The given method is: $\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH} \frac{\text { Conc }, \mathrm{H}_{2} \mathrm{SO}_{4}}{413 \mathrm{~K}}{\longrightarrow}$ This method uses Ethanol (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\)) and concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) at 413 K (140°C). These conditions are suitable for the Dehydration of Alcohols method which can form ethers at such temperatures.

3. Analyzing Method B

The given method is: $\mathrm{CH}_{3}-\mathrm{OH} \frac{\text { Conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{140^{\circ} \mathrm{C}}$ This method uses Methanol (\(\mathrm{CH}_{3}-\mathrm{OH}\)) and concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) at 140°C. These conditions are also suitable for the Dehydration of Alcohols method which can form ethers at such temperatures.

4. Conclusion

From the analysis of the given methods (A) and (B), we can conclude that both methods are suitable for the preparation of ethers. Methods (C) and (D) are not provided, therefore we cannot assess their suitability for ether preparation. So, the correct answer is: None of the given methods (A) and (B) are unsuitable for the preparation of ethers.

Key concepts

Williamson Ether Synthesis

The Williamson Ether Synthesis is a powerful and versatile method for preparing ethers. This process involves the reaction between an alkoxide ion (\(RO^-\)) and an alkyl halide (\(R-X\)). The reaction mechanism starts with the formation of the alkoxide ion through the deprotonation of an alcohol.
The alkoxide then acts as a nucleophile, attacking the electrophilic carbon in the alkyl halide. This nucleophilic substitution reaction results in the formation of the ether.
An important aspect of this synthesis is that it works best with primary alkyl halides because they undergo \(\text{S}_N2\) reactions more readily, reducing the chances of competing elimination reactions.
For example, if you take sodium ethoxide (a typical alkoxide) and react it with a primary alkyl halide such as ethyl bromide, you will obtain diethyl ether.

• This method is favored for its ability to allow the synthesis of symmetric or asymmetric ethers depending on the choice of reagents.
• It is especially popular because it can be applied under mild reaction conditions and usually gives high yields.

Dehydration of Alcohols

Dehydration of alcohols is another common route to prepare ethers, particularly in industrial chemistry. This method involves the use of concentrated acids like sulfuric acid (\(\text{H}_2\text{SO}_4\)) as a catalyst.
The process typically requires heating the alcohol with the concentrated acid to promote the intermolecular combination of alcohol molecules, effectively removing a water molecule to form the ether.

• The reaction is usually carried out at a temperature of about 140°C, which is optimal for promoting ether formation over alkene formation.
• At higher temperatures, alcohol dehydration can shift to form alkenes instead of ethers due to different mechanistic pathways.

To illustrate, when ethanol is heated with concentrated \(\text{H}_2\text{SO}_4\), the dehydration typically leads to the formation of diethyl ether. This reaction highlights the balance between temperature and reaction conditions necessary to shift the reaction pathway towards ether formation.

reaction conditions for ether formation

Reaction conditions play a pivotal role in determining the successful formation of ethers. These conditions vary depending on the method chosen.

• The Williamson Ether Synthesis requires the presence of a strong alkoxide base and a reactive primary alkyl halide. A relatively non-polar solvent, such as acetone, can help to promote the nucleophilic substitution reaction.
• The Dehydration of Alcohols involves concentrated acids and elevated temperatures around 140°C to encourage ether over alkene formation.
• For ether formation, it's crucial to control the reaction environment: in Williamson Ether Synthesis, controlling steric hindrance, and in alcohol dehydration, managing the temperature and catalyst concentration.

By closely monitoring these conditions, chemists can efficiently direct the outcomes of their reactions to obtain ether as the major product.