Problem 163 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct statements. (a) \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{O}-\mathrm{CH}_{3}\) on cleavage with \(\mathrm{PCl}_{5}\) gives equimolar mixture of ethyl chloride and methyl chloride (b) \(\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\) is more reactive than \(\mathrm{Br} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) to prepare ether using Williamson's synthesis (c) Anisole can be prepared by the reaction of \(\mathrm{CH}_{3} \mathrm{ONa}\) on chlorobenzene (d) Sodium metal can be used to dry diethyl ether, benzene and ethyl alcohol.

Step-by-Step Solution

Answer

Statements (a) and (b) are correct.

Step 1: Analyzing Statement (a)

The compound \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{O}-\mathrm{CH}_{3} \) is ethyl methyl ether. Upon reaction with \( \mathrm{PCl}_{5} \), it cleaves to form different products. The cleavage results in the formation of ethyl chloride (\( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{Cl} \)) and methyl chloride (\( \mathrm{CH}_{3}\mathrm{Cl} \)). Therefore, statement (a) is correct as both chlorides are formed in equimolar amounts.

Step 2: Analyzing Statement (b)

To determine the reactivity for ether preparation, in Williamson's synthesis, the order of reactivity is typically allyl halides > benzyl halides > primary alkyl halides. Here \((\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2})\), an allylic halide, is more reactive than \(\mathrm{Br} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\), a primary alkyl halide. Therefore, statement (b) is correct.

Step 3: Analyzing Statement (c)

Anisole (methoxybenzene) cannot be prepared by the reaction of \( \mathrm{CH}_{3} \mathrm{ONa} \) on chlorobenzene due to the lack of reactivity of chlorobenzene with nucleophiles. Chlorobenzene's ring is stable and resistant to nucleophilic substitution, so statement (c) is incorrect.

Step 4: Analyzing Statement (d)

Sodium metal can indeed be used to dry diethyl ether and benzene due to their lack of reactivity with sodium. However, sodium cannot be used to dry ethyl alcohol because it reacts vigorously with alcohols, forming hydrogen gas and alkoxides. Therefore, statement (d) is incorrect.

Key Concepts

Ether Cleavage

Ether cleavage is a core concept in organic chemistry, focusing on the breaking of an ether bond to form smaller molecules. In the case of ethyl methyl ether, the reaction with phosphorus pentachloride (\( \mathrm{PCl}_{5} \)) leads to cleavage.

• Cleavage occurs between the oxygen and one of the carbon atoms, resulting in different alkyl halides depending on the ether involved.
• For ethyl methyl ether (\( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{O}-\mathrm{CH}_{3} \)), cleavage results in equimolar amounts of ethyl chloride (\( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{Cl} \)) and methyl chloride (\( \mathrm{CH}_{3}\mathrm{Cl} \)).

This type of reaction is useful for the synthesis of a variety of compounds in organic synthesis. Understanding ether cleavage is essential for predicting the outcome of reactions involving ethers and choosing the appropriate conditions and reagents for specific transformations.

Reactivity in Williamson's Synthesis

Williamson's synthesis is a method used to prepare ethers from alcohols and alkyl halides. The reactivity of different halides in this synthesis varies significantly.

• Allylic halides, like \( \mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2} \), are more reactive and lead to successful ether formation.
• In contrast, primary alkyl halides like \( \mathrm{Br} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \) tend to be less reactive and might not be suitable for the synthesis.
• The success of Williamson's synthesis relies on the nucleophilic substitution reaction, where a nucleophile replaces a leaving group in an alkyl halide.

Understanding the reactivity of different halides is critical for selecting the right materials to achieve efficient ether synthesis.

Nucleophilic Substitution Resistance

Nucleophilic substitution is an essential reaction in organic chemistry. However, not all compounds readily undergo this type of reaction.

• Ring systems like chlorobenzene display resistance towards nucleophilic substitution due to resonance stability and the strengthening of the carbon-halide bond.
• This stability makes chlorobenzene a poor candidate for reactions requiring nucleophilic attack, such as the attempted preparation of anisole using \( \mathrm{CH}_{3} \mathrm{ONa} \).

The inability of chlorobenzene to participate in nucleophilic substitution reactions demonstrates the importance of predicting compound reactivity based on its structure and the surrounding electronic environment.

Drying Solvents with Sodium Metal

Sodium metal is commonly used to dry certain solvents by removing traces of water. However, the suitability of sodium varies among different solvents.

• Non-reactive solvents like diethyl ether and benzene can safely be dried with sodium as they do not react significantly with sodium metal.
• In contrast, alcohols like ethyl alcohol react vigorously with sodium, producing hydrogen gas, making sodium unsuitable for drying alcohols.

For safe and effective drying, it's crucial to understand the reactivity of the solvent in question. Sodium's inability to be used with reactive solvents such as alcohols highlights the need to choose compatible drying agents for specific chemical environments.