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: \(\quad[\mathbf{2 0 0 7}]\) (a) benzyl chloride (b) 0 - and p-chlorotoluene (c) m-chlorotoluene (d) benzoyl chloride

Step-by-Step Solution

Answer

The reaction produces predominantly ortho- and para-chlorotoluene.

Step 1: Understanding the Reaction

The reaction given involves toluene, which is an aromatic compound, interacting with \( \mathrm{Cl}_2 \) in the presence of a catalyst \( \mathrm{FeCl}_3 \). This setup suggests an electrophilic aromatic substitution, common in aromatic compounds with catalysts that facilitate the substitution of hydrogen atoms by other substituents.

Step 2: Analyzing the Catalyst Function

In electrophilic aromatic substitutions involving aromatic hydrocarbons like toluene, the presence of \( \mathrm{FeCl}_3 \) helps to generate the electrophile \( \mathrm{Cl}^+ \). This electrophile will attack the aromatic ring of toluene.

Step 3: Identifying Directive Effects

Toluene has a methyl group which is an electron-donating group through hyperconjugation and an inductive effect. This causes the methyl group to be an ortho-para director in electrophilic aromatic substitution reactions, meaning that the chlorine will most likely attach to the ortho or para positions relative to the methyl group on the benzene ring.

Step 4: Predicting the Major Products

Due to the directing influence of the methyl group, the major products will be ortho-chlorotoluene and para-chlorotoluene. Meta substitution is typically less favored in this scenario.

Step 5: Concluding the Reaction Outcome

Considering the conditions and the directive effects, the correct answer to what toluene predominantly forms under these specific conditions is ortho- and para-chlorotoluene.

Key Concepts

Toluene

Toluene is a simple aromatic hydrocarbon, much like benzene, but with an added twist—it carries a methyl group. This small modification impacts its chemical behavior significantly.
Unlike benzene, the presence of the methyl group in toluene makes it more reactive towards electrophilic aromatic substitution reactions. This is because the methyl group is an electron-donating group. Through hyperconjugation and an inductive effect, it increases the electron density on the aromatic ring. This makes the ring more attractive to electrophiles, the reactive species looking to capture electrons from other atoms.

• The methyl group activates the benzene ring, providing extra electrons.
• Toluene is used extensively in industries as a precursor to other chemicals.

This unique property of toluene accounts for its reactivity and utility in various chemical processes, particularly in environments where electrophilic aromatic substitution is involved.

FeCl3 catalyst

The iron(III) chloride, or FeCl₃, acts as a crucial catalyst in the process of electrophilic aromatic substitution. Its role is pivotal in facilitating the generation of a strong electrophile.
When chlorine gas (\( \text{Cl}_2 \)) is present, FeCl₃ encourages the formation of the chlorine cation (\( \text{Cl}^+ \)) from molecular chlorine. This is because FeCl₃ assists in breaking the Cl-Cl bond by forming a complex.

• FeCl₃ catalyzes reactions by helping to polarize bonds.
• It's often used when chlorinating aromatic compounds like toluene.

Once the electrophile is generated, it is free to attack the electron-rich areas of the toluene ring, setting the stage for further transformation and substitution. This makes FeCl₃ indispensable when looking to add chlorine atoms to an aromatic compound efficiently.

Ortho-para directive effects

One of toluene's intriguing features is how it's affected by ortho-para directive effects. This influence comes from the methyl group attached to the aromatic ring.
Methyl groups donate electron density through mechanisms like hyperconjugation, enhancing the nucleophilicity of the aromatic ring at specific positions.
Thus, when an electrophile such as chlorine cation approaches, the reaction tends to occur at positions that are ortho or para relative to the methyl group on the aromatic ring.

• Ortho and para positions are more electron-dense due to the methyl group.
• The probability of substitution at these positions is higher compared to the meta position.

These directive effects ensure that the chlorine is predominantly added at these positions, resulting in ortho-chlorotoluene and para-chlorotoluene as the major products of the reaction. These effects simplify the prediction of products in electrophilic aromatic substitution reactions.