Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Allyl phenyl ether can be prepared by heating: [Main Online April 9, 2014] (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}+\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{ONa}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{Br}+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{ONa}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{Br}+\mathrm{CH}_{3}-\mathrm{ONa}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{Br}+\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{ONa}\)

Step-by-Step Solution

Answer

Option (b) is correct.

Step 1: Identify the desired reaction product

The problem requires us to prepare allyl phenyl ether, which has the structure \( \mathrm{C}_6\mathrm{H}_5\mathrm{O}\{\mathrm{CH}_2\mathrm{CH}=\mathrm{CH}_2} \). We need to recognize this structure to identify the best reaction option for its production.

Step 2: Analyze reactants for suitable ether preparation

For preparing allyl phenyl ether, we need an allyl group \( \mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_2 \) and a phenoxide group \( \mathrm{C}_6\mathrm{H}_5\mathrm{O}^- \). We are looking to pair these appropriately: a good nucleophile (like phenoxide) and a good leaving group (like bromide) on the allyl compound.

Step 3: Evaluate option (a)

Option (a) uses \( \mathrm{C}_{6}\mathrm{H}_5\mathrm{Br} \) and \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{ONa} \). Here, bromine can be replaced by alkoxy, forming a phenyl ether. However, it's phenyl bromide, not allyl bromide. So this would not form allyl phenyl ether.

Step 4: Evaluate option (b)

Option (b) involves \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{Br} \) and \( \mathrm{C}_{6}\mathrm{H}_5\mathrm{ONa} \). The allyl bromide can react with sodium phenoxide to swap the bromide for the phenoxide, forming allyl phenyl ether.

Step 5: Check remaining options

Option (c) uses alkyl-bromo and methoxy sodium, resulting in a different product. Option (d) involves \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{Br} \) and \( \mathrm{C}_{6}\mathrm{H}_5-\mathrm{CH}_2-\mathrm{ONa} \), which would not give the ether of interest because of incorrect pairing. Thus these options aren't suitable.

Step 6: Conclusion

Based on the analysis, option (b) is the only choice where the substituents are correctly aligned to form allyl phenyl ether through a nucleophilic substitution reaction.

Key Concepts

Nucleophilic Substitution Reaction

Nucleophilic substitution reactions are a fundamental concept in organic chemistry that are crucial in many synthesis pathways, including the formation of ethers. In these reactions, a nucleophile, which is a molecule or ion with a pair of electrons to donate, attacks a positively charged or partially positive region of another molecule. This interaction results in the replacement of a leaving group, usually a halide or another good leaving group, with the nucleophile.

The general process involves steps such as nucleophilic attack, formation of an intermediate, and the departure of the leaving group. In the context of our exercise, allyl bromide serves as the substrate with the bromide acting as a good leaving group.

• Key Players: Nucleophiles (e.g., phenoxide ions) and leaving groups (e.g., bromides).
• Outcome: Replacement of the leaving group by the nucleophile to form a new molecule.

Understanding the roles of different molecules and ions in these reactions helps predict the products and understand reaction mechanisms.

Ether Synthesis

Synthesizing ethers involves the combination of an alkyl or aryl group with an alcohol group. The Williamson ether synthesis is a popular method for creating ethers and involves nucleophilic substitution reactions. In this technique, an alkoxide ion (a deprotonated form of an alcohol) reacts with a primary alkyl halide or tosylate.

In the case study of allyl phenyl ether synthesis, we observe the allyl bromide reacting with sodium phenoxide. This reaction uses the nucleophilic nature of phenoxide to replace the bromide group on the allyl compound, thus forming an ether linkage.

• Steps: Formation of alkoxide ion, nucleophilic attack on alkyl halide, ether formation.
• Requirements: A good nucleophile (like an alkoxide) and a good leaving group (often a halide).

Correct pairing of reactants is critical; the nucleophile must have the strength to displace the leaving group, ensuring successful ether formation.

Organic Chemistry Reactions

Understanding organic chemistry reactions requires knowledge of various reaction mechanisms and the types of reactants needed. These reactions are governed by the chemical properties of the reactants, such as nucleophilicity, electrophilicity, and the stability of leaving groups.

Organic reactions often involve electron-rich species (nucleophiles) interacting with electron-deficient species (electrophiles). The nature of these substances dictates the outcome of reactions such as nucleophilic substitutions and the synthesis of various organic compounds. Steps often include initiation by a catalyst or reactant, transformation over intermediate stages, and final product formation.

• Key Concepts: Electron flow, reaction intermediates, and product stability.
• Examples: Synthesis of esters, amides, and ethers through nucleophilic substitution.

By dissecting these reactions into underlying steps and interactions, one gains a clearer understanding, enabling better prediction and control over synthetic outcomes.