Problem 5 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

In case of chlorination of benzene in the presence of \(\mathrm{FeCl}_{3}\) the electrophile is: 1CHSE 1996] (a) \(\mathrm{Cl}^{+}\) (b) \(\mathrm{Cl}^{-}\) (c) \(\mathrm{Cl}\) (d) \(\mathrm{FeCl}_{3}\)

Step-by-Step Solution

Answer

The electrophile is \(\mathrm{Cl}^{+}\), so the answer is (a) \(\mathrm{Cl}^{+}\).

Step 1: Understand the Reaction Context

When benzene undergoes chlorination in the presence of a catalyst like \(\mathrm{FeCl}_3\), the aim is to substitute one hydrogen atom in benzene with a chlorine atom. This requires the generation of an electrophile that can attack the electron-rich benzene ring.

Step 2: Identify the Catalyst's Role

\(\mathrm{FeCl}_{3}\) is a Lewis acid and acts as a catalyst in the reaction. It helps in the formation of the electrophile by accepting a lone pair of electrons from the chlorine molecule \(\mathrm{Cl}_2\).

Step 3: Formation of the Electrophile

In the presence of \(\mathrm{FeCl}_3\), \(\mathrm{Cl}_2\) forms \(\mathrm{Cl}^{+}\) and \(\mathrm{FeCl}_{4}^{-}\). Here, \(\mathrm{Cl}^{+}\) is the electrophile that is formed which is capable of attacking the benzene ring.

Step 4: Select the Correct Option

From options (a) \(\mathrm{Cl}^+\), (b) \(\mathrm{Cl}^-\), (c) \(\mathrm{Cl}\), and (d) \(\mathrm{FeCl}_3\), know that \(\mathrm{Cl}^+\) is the electrophile that attacks the benzene ring, resulting in the chlorination reaction.

Key Concepts

Chlorination of Benzene

Chlorination of benzene is a specific type of chemical reaction known as electrophilic aromatic substitution. This process involves replacing one hydrogen atom in a benzene ring with a chlorine atom. Benzene, being an aromatic compound, possesses a stable ring of electrons, which makes direct substitution reactions challenging unless a powerful electrophile is involved. In the chlorination reaction, this is achieved by generating a highly reactive chlorine species that can effectively substitute a hydrogen atom from the benzene ring.

In practical terms, the chlorination of benzene requires the presence of a catalyst and a chlorine source like \( \mathrm{Cl}_2 \). The benzene ring's inherent stability necessitates this mechanism—ensuring that the chlorine atom is introduced into the ring effectively and efficiently.

Lewis Acid Catalysis

Lewis acids play a crucial role in catalytic processes like chlorination by accepting electrons and facilitating the formation of an electrophile. When chlorinating benzene, \( \mathrm{FeCl}_3 \) acts as a Lewis acid, which means it has vacant orbitals ready to accept electron pairs. This ability is vital as it helps to transform the chlorine molecule into a more reactive form.

During the reaction, \( \mathrm{FeCl}_3 \) interacts with \( \mathrm{Cl}_2 \). By accepting electron pairs from \( \mathrm{Cl}_2 \), \( \mathrm{FeCl}_3 \) effectively facilitates the breakdown of the molecule, contributing to the production of the active electrophile, chlorine cation \( \mathrm{Cl}^+ \). This transformation would be difficult without the involvement of a Lewis acid catalysis provided by \( \mathrm{FeCl}_3 \).

Thus, the role of Lewis acids is crucial for manipulating the electronic environment to promote the progress of the reaction.

Electrophile Formation

The creation of an electrophile is crucial in the electrophilic aromatic substitution used in benzene's chlorination. An electrophile is a chemical species that seeks out electrons due to its electron-poor nature, allowing it to react with the electron-rich benzene ring.

The electrophile's formation begins with \( \mathrm{Cl}_2 \) interacting with \( \mathrm{FeCl}_3 \). In this interaction, \( \mathrm{FeCl}_3 \) facilitates the reversible separation of \( \mathrm{Cl}_2 \) into a positively charged chlorine ion \( \mathrm{Cl}^+ \) and a chloride ion \( \mathrm{FeCl}_4^- \). Here, \( \mathrm{Cl}^+ \) serves as the electrophile, primed to attack the benzene ring.

Without such a specific electrophile, the benzene's stable structure would resist alteration. Therefore, the precise formation of \( \mathrm{Cl}^+ \) is indispensable in ensuring the chlorination process proceeds efficiently.