Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

Amongst the following, the compound that can most readily get sulphonated is (a) benzene (b) toluene (c) nitrobenzene (d) chlorobenzene

Step-by-Step Solution

Answer

Toluene (b) can most readily get sulphonated.

Step 1: Identify the Reaction Conditions

Sulphonation is an electrophilic substitution reaction where a sulphonyl group (SO₃H) is introduced to an aromatic ring using sulphuric acid or oleum as the electrophile. The presence of electron-donating groups or electron-withdrawing groups on the aromatic ring affects the ease of sulphonation.

Step 2: Evaluate Electronic Effects

Benzene is the standard reference for aromatic compounds. Toluene has a methyl group which is an electron-donating group, increasing the electron density on the ring and making it more reactive towards electrophilic substitution compared to benzene. Chlorobenzene and nitrobenzene have electron-withdrawing groups (chlorine and nitro), which reduce the electron density on the ring, making them less reactive.

Step 3: Compare Reactivities

Considering the electronic effects, toluene, with an electron-donating group, will encourage the formation of the cationic intermediate required for electrophilic substitution. On the other hand, nitrobenzene and chlorobenzene will slow down this process due to their electron-withdrawing effects.

Step 4: Conclusion and Final Choice

Toluene will be the compound that can most readily undergo sulphonation due to the activating effect of the methyl group, which makes it more reactive towards electrophilic substitution than benzene, nitrobenzene, or chlorobenzene.

Key Concepts

Understanding Sulphonation

Sulphonation is a fascinating reaction that involves the introduction of a sulphonyl group \( \text{SO}_3 \text{H} \) into an aromatic ring. This transformation is a type of electrophilic substitution reaction. What happens here is that the aromatic ring reacts with an electrophile, which in this case is sulfur trioxide (\( \text{SO}_3 \)) or oleum. Oleum is essentially sulfuric acid containing added \( \text{SO}_3 \). Once the sulphonyl group attaches, it replaces a hydrogen atom on the aromatic ring. This is a reversible process and the conditions can significantly influence the reaction outcome.

• Electrophiles: In sulphonation, \( \text{SO}_3 \) or oleum serves as the electrophile.
• Substitution Process: The attached sulphonyl group leads to the release of a hydrogen atom.
• Reversibility: Under different conditions, the reaction can proceed backward.

Aromatic Compounds Explained

Aromatic compounds are a class of compounds known for their distinct cyclic structure and stability, partially due to conjugated double bonds. Benzene is the prototype of such aromatic compounds, characterized by a six-carbon ring with alternating single and double bonds, often represented with a circle to indicate resonance. This structure allows the electrons to delocalize, giving aromatic compounds their unique properties such as enhanced stability and distinctive reactivity patterns.

• Resonance Stability: Electrons are shared across the ring, leading to lower energy and increased stability.
• Benzene: Acts as the basic template and reference for studying other aromatic derivatives.
• Reactivity: Aromatic compounds typically undergo electrophilic substitution rather than addition reactions, maintaining the aromaticity.

Role of Electron-Donating Groups

Electron-donating groups (EDGs) are crucial players in determining the reactivity of aromatic compounds in electrophilic substitution reactions. EDGs increase electron density on the aromatic ring, making it more susceptible to attack by electrophiles. These groups, such as the methyl group in toluene, donate electrons to the ring through inductive or resonance effects.

• Inductive Effect: EDGs push electron density through sigma bonds.
• Resonance Effect: Certain groups can stabilize carbocations via delocalization.
• Reactivity Enhancement: EDGs accelerate reactions by stabilizing transition states.

Impact of Electron-Withdrawing Groups

Contrastingly, electron-withdrawing groups (EWGs) decrease the electron density of the aromatic ring, making it less reactive towards electrophilic substitution. These groups, such as the nitro group in nitrobenzene or the chlorine in chlorobenzene, pull electrons away from the ring either by inductive effects or resonance. This reduction in electron density renders the ring less attractive to electrophiles.

• Inductive Effect: EWGs withdraw electron density through sigma bonds, reducing reactivity.
• Resonance Effect: Some EWGs can exert an electron-withdrawing strength through resonance, often stabilizing negative charges outside the ring.
• Reactivity Decrease: EWGs destabilize the transition state in electrophilic substitution, slowing the reaction.