Problem 143 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Reagent) 1\. \(\mathrm{Cl}_{2}+\mathrm{AlCl}_{3}\) 2\. \(\mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4}\) 3\. \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7}\) 4\. \(\mathrm{Br}_{2}+\mathrm{Fe}\) List II (Electrophiles) (i) \(\mathrm{NO}_{2}\) (ii) \(\mathrm{Cl}_{\ldots} . \mathrm{Cl} \ldots \mathrm{AlCl}_{3}\) or \(\mathrm{Cl}^{+}\) (iii) \(\mathrm{SO}_{3} \mathrm{H}\left(\right.\) or \(\left.\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{SO}_{3}\right)\) (iv) \(\mathrm{SO}_{3}\) (v) Br...Br...FeBr \(_{3}\) or \(\mathrm{Br}^{+}\)

Step-by-Step Solution

Answer

1-(ii), 2-(i), 3-(iv), 4-(v).

Step 1: Analyze Reagent 1

For the reagent \( \mathrm{Cl}_{2}+\mathrm{AlCl}_{3} \), the \( \mathrm{AlCl}_{3} \) acts as a Lewis acid and helps to form the electrophilic chloride ion \( \mathrm{Cl}^{+} \). Thus, the electrophile corresponding to this reagent is \( \mathrm{Cl}^{+} \). Therefore, it matches with List II (ii).

Step 2: Analyze Reagent 2

\( \mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \) is a nitrating mixture. \( \mathrm{H}_{2} \mathrm{SO}_{4} \) protonates \( \mathrm{HNO}_{3} \) to form the nitronium ion \( \mathrm{NO}_{2}^{+} \), which serves as the electrophile. Therefore, it matches with List II (i).

Step 3: Analyze Reagent 3

\( \mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7} \), also known as oleum, contains \( \mathrm{SO}_{3} \) as part of its composition. The electrophile in this case is the \( \mathrm{SO}_{3} \) group. Thus, it corresponds to List II (iv).

Step 4: Analyze Reagent 4

For \( \mathrm{Br}_{2}+\mathrm{Fe} \), the iron assists in forming the bromonium ion \( \mathrm{Br}^{+} \) or \( \mathrm{Br}\ldots \mathrm{Br}\ldots \mathrm{FeBr}_{3} \.\). As the electrophile formed is \( \mathrm{Br}^{+} \), it associates with List II (v).

Key Concepts

Reagent Analysis

When we talk about reagents in electrophilic substitution reactions, we are essentially referring to the chemicals that will interact with a complex molecule to initiate a reaction. These chemicals play pivotal roles in creating the right conditions for the formation of electrophiles. To understand the core of reagent analysis, consider the following:

• Every reagent has a unique way of interacting with molecules and can be analyzed to predict the kind of electrophile it may form.
• It's crucial to identify the purpose of each component in a mixture. Whether it provides a source of a functional group, acts as a catalyst, or serves as a catalyst activator.
• Examining the reagent allows for predicting the type and strength of the electrophile, and therefore, the nature of the expected chemical reaction.

For example, in the reagent \(\mathrm{Cl}_2 + \mathrm{AlCl}_3\), chlorine gas isn't potent enough on its own as an electrophile in aromatic substitution reactions. However, when combined with \(\mathrm{AlCl}_3\), the Lewis acid helps to enhance chlorine's ability, forming a more active electrophile.

Electrophile Formation

Electrophiles are positively charged species that seek out electrons. In chemical reactions, they play the crucial role of attacking electron-rich areas. The formation of an electrophile can often be attributed to the combination of a neutral molecule with a Lewis acid, creating a positively charged ion. Let's delve further into this process:

• An electrophile is usually formed when a more stable, neutral molecule reacts with an activating agent, such as a Lewis acid. This reaction generates a highly reactive, positively charged species.
• In the reaction of \(\mathrm{HNO}_3 + \mathrm{H}_2 \mathrm{SO}_4\), sulfuric acid protonates nitric acid. This transforms \(\mathrm{HNO}_3\) into \(\mathrm{NO}_2^+\), an active electrophile ready to participate in further chemical reactions.
• Meanwhile, \(\mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7\) or oleum contains \(\mathrm{SO}_3\) which can directly function as an electrophile when it reacts with an organic substrate.

By understanding how electrophiles are formed, students gain insights into predicting the outcomes of chemical reactions and the mechanisms behind them.

Lewis Acid

Lewis acids play a fundamental role in the formation of electrophiles, as they are the catalysts or activators in many chemical reactions. Understanding what makes a Lewis acid such an effective partner in these reactions is crucial.

• A Lewis acid is defined as a species that can accept a pair of non-bonding electrons. This acceptance creates an activated complex that can facilitate the formation of a positively charged species.
• For instance, \(\mathrm{AlCl}_3\) in \(\mathrm{Cl}_2 + \mathrm{AlCl}_3\). Here, \(\mathrm{AlCl}_3\) accepts a pair of electrons from chlorine, forming \(\mathrm{Cl}^+\), an active electrophile.
• Similarly, in the system \(\mathrm{Br}_2 + \mathrm{Fe}\), the iron acts like a Lewis acid and assists in the polarization of the bromine molecule, leading to the formation of \(\mathrm{Br}^+\).

The role played by Lewis acids in electrophilic substitution reactions provides a foundation for predicting and designing new reactions where electrophiles are necessary. They effectively enable the transformation of neutral molecules into reactive species.