Problem 197 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of toluene with \(\mathrm{Cl}_{2}\) in presence of \(\mathrm{FeCl}_{3}\) gives predominantly: (a) benzyl chloride (b) 0 - and p-chlorotoluene (c) m-chlorotoluene (d) benzoyl chloride

Step-by-Step Solution

Answer

The correct answer is (b) o- and p-chlorotoluene.

Step 1: Understanding the Reaction Process

The reaction involves toluene and chlorine in the presence of a catalyst, ferric chloride (\(\mathrm{FeCl}_3\)). This setup is typical of an electrophilic aromatic substitution reaction, which introduces a chlorine atom into the aromatic ring of toluene.

Step 2: Identifying the Electrophilic Aromatic Substitution Mechanism

Electrophilic aromatic substitution in the presence of \(\mathrm{FeCl}_3\) leads to the formation of a chloronium ion, which attacks the aromatic ring. Toluene, being an aromatic compound, will have its methyl group acting as an electron-donating group (EDG) and directing the incoming chlorine atom towards the ortho and para positions. This is due to the methyl group increasing electron density at these positions relative to the meta position.

Step 3: Recognizing Substitution Products

For aromatic substitution reactions in toluene, the major products are ortho- and para-chlorotoluene. This occurs because the methyl group on toluene directs electrophilic attack to the ortho and para positions, favoring these products.

Step 4: Selecting the Correct Option

Upon understanding that the reaction predominantly yields ortho- and para-chlorotoluene, we can see that the correct answer choice must be (b) o- and p-chlorotoluene.

Key Concepts

Toluene chlorination

When toluene undergoes chlorination, the chemical reaction takes place in the presence of chlorine gas ( \(\mathrm{Cl}_2 \)) and a catalyst like ferric chloride ( \(\mathrm{FeCl}_3 \)). The chlorination of toluene is an example of an electrophilic aromatic substitution reaction. In this process, chlorine atoms are introduced into the aromatic ring of toluene, forming chlorotoluene isomers.

• The catalyst, \(\mathrm{FeCl}_3 \), plays a crucial role by generating the electrophile, which is the chloronium ion in this context, making the aromatic ring more receptive to substitution.
• Toluene has a methyl group attached to the benzene ring, enhancing the electron cloud and making it more reactive under these conditions.

Overall, the presence of a catalyst and the nature of the toluene structure lead to specific placement of chlorine atoms during the chlorination process, emphasizing the need for understanding reaction mechanisms and resulting products.

Ortho-para directors

In electrophilic aromatic substitution reactions, the presence of substituents affects where new atoms are added to the aromatic ring. Substituents can be categorized as ortho-para directors or meta directors based on how they influence incoming groups.

• Ortho-Para Directors: These are typically electron-donating groups (EDGs), such as the methyl group on toluene, which direct incoming electrophiles to the ortho (adjacent) and para (opposite) positions on the ring relative to themselves.
• The increased electron density at the ortho and para positions makes them more attractive to electrophiles, leading to faster and more favorable reactions at these sites.

The methyl group in toluene is a classic example of an ortho-para director, which explains why chlorination results in ortho- and para-chlorotoluene as the major products.

Reaction mechanisms

Understanding reaction mechanisms is crucial to predicting the outcomes of chemical reactions. In electrophilic aromatic substitution, the reaction mechanism involves several key steps that transform reactants into products.

• Step 1: Formation of Electrophile - The chlorine molecule ( \(\mathrm{Cl}_2 \)) interacts with the catalyst ( \(\mathrm{FeCl}_3 \)) to form a chloronium ion, which is the active electrophile.
• Step 2: Electrophile Attack - The chloronium ion attacks the aromatic ring of toluene. The electron-donating nature of the methyl group directs the attack to the ortho and para positions.
• Step 3: Deprotonation - To restore aromaticity, a proton is removed from the intermediate cationic complex, finalizing the substitution process and yielding chlorotoluene.

This sequential process highlights why certain products are favored and explains the predominance of ortho- and para-chlorotoluene, offering a clear insight into the molecular dance occurring during chlorination of toluene.