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 \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) to prepare ether using Williamson's synthesis (c) Anisole can be prepared by the reaction of 1 \(\mathrm{CH}_{3} \mathrm{ONa}\) on chlorobenzene (d) Sodium metal can be used to dry diethyl ether, benzene and ethyl alcohol.

Step-by-Step Solution

Answer

Correct statements: (a) and (b).

Step 1: Analyze Statement (a)

Statement (a) involves the reaction of ethyl methyl ether, or ethoxyethane \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{O}-\mathrm{CH}_{3} \), with phosphorous pentachloride \( \mathrm{PCl}_{5} \). Phosphorous pentachloride cleaves ethers to give alkyl chlorides. On cleavage, it should yield two products: methyl chloride \( \mathrm{CH}_{3}\mathrm{Cl} \) and ethyl chloride \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{Cl} \). Thus, statement (a) is correct.

Step 2: Examine Statement (b)

To evaluate statement (b), we need to compare the reactivities of halides for the Williamson's synthesis, a method to synthesize ethers by reaction of haloalkanes with alkoxides. Chlorinated allylic compounds like \( \mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2} \) are generally more reactive towards nucleophilic substitution reactions than saturated brominated compounds like \( \mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \), due to the conjugation in allylic systems. Thus, statement (b) is correct.

Step 3: Evaluate Statement (c)

Anisole cannot be prepared from 1-methoxy sodium (\( \mathrm{CH}_{3} \mathrm{ONa} \)) and chlorobenzene because chlorobenzene is extremely unreactive towards nucleophilic substitution due to the electron-withdrawing nature of the chlorine, as well as the aromaticity of benzene. Therefore, statement (c) is incorrect.

Step 4: Assess Statement (d)

Statement (d) concerns the use of sodium metal to dry various organic solvents. Sodium metal reacts violently with alcohols, such as ethyl alcohol, and does not react with benzene, but it is often used to dry diethyl ether. Therefore, sodium cannot be used to dry ethyl alcohol safely, making statement (d) incorrect.

Key Concepts

Ether Cleavage with PCl5

Phosphorous pentachloride, often abbreviated as PCl5, is a powerful reagent used for cleaving ethers. When ethers like ethyl methyl ether ( C_2H_5-O-CH_3 ) react with PCl5, they are broken down into alkyl chlorides. This process is known as ether cleavage. For ethyl methyl ether, the reaction with PCl5 results in the formation of ethyl chloride ( C_2H_5Cl ) and methyl chloride ( CH_3Cl ).
This reaction illustrates the general tendency of PCl5 to convert alkoxy (O-R) bonds in ethers to alkyl chloride ( R-Cl ) and alcohol chloride ( R'-Cl ), making it a valuable tool in organic synthesis and transformations.
Ether cleavage is an important reaction in organic chemistry because it allows chemists to break down ethers into their component parts, providing simpler molecules that can be used in further chemical reactions.

Reactivity of Halides

The reactivity of halides varies significantly depending on various factors like their structure and the nature of the carbon-halogen bond. In the case of Williamson's synthesis, a reaction widely used for preparing ethers, the choice of halide is crucial. Aliphatic halides are typically good at undergoing nucleophilic substitution, a key step in this synthesis.
However, the structure greatly influences their reactivity. For instance, chlorinated allylic compounds such as ClCH_2CH=CH_2 are more reactive compared to saturated brominated compounds like BrCH_2CH_2CH_3 . This increased reactivity is because allylic halides are stabilized by resonance, allowing for easier substitution by nucleophiles, such as alkoxides.
This principle of reactivity is essential for predicting the outcomes of reactions and for selecting the best reagents in synthetic routes.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental concept in organic chemistry, characterized by the replacement of a leaving group by a nucleophile. In Williamson's synthesis, which is an example of such a reaction, an alkoxide ion (the nucleophile) attacks a haloalkane to form an ether.
This reaction proceeds via two main pathways: S_N1 and S_N2 , the choice of mechanism largely depending on the structure of the haloalkane and the conditions. S_N2 involves a direct displacement of the leaving group, making it faster for primary substrates.
Understanding this substitution process is vital because it forms the backbone of many synthetic strategies, enabling the construction of complex organic molecules and contributing to fields such as pharmaceuticals and materials science.

Drying Organic Solvents with Sodium

Drying organic solvents is a regular task in organic synthesis, and choosing the right drying agent is important. Sodium metal is often used to dry ethers such as diethyl ether, but not all solvents can be safely dried with sodium.
Sodium reacts violently with water and alcohols, such as ethyl alcohol, because it forms hydrogen gas, which is highly flammable. However, for ethers, sodium reacts with minimal violence, efficiently removing water and other impurities by forming insoluble compounds.
This drying process is crucial for ensuring the purity of solvents, which impacts the success and reliability of chemical reactions.