Question

Amongst the following, the compound that can most readily get sulphonated is: (a) Benzene (b) Toluene (c) Nitrobenzene (d) Chlorobenzene

Short answer

Toluene is most readily sulphonated.

Step-by-step solution

Understand Sulphonation

Sulphonation is a type of electrophilic aromatic substitution (EAS) reaction where a sulfonyl group (\(-SO_3H\)) is introduced into an aromatic ring. The ease of this reaction depends on the electron density of the aromatic ring.

Compare Electron Density

Analyze the electron-donating or withdrawing nature of each substituent: - **Benzene** has no substituents affecting its electron density. - **Toluene** has a methyl group, which is electron-donating, increasing the electron density on the aromatic ring. - **Nitrobenzene** has a nitro group, which is strongly electron-withdrawing, decreasing the electron density. - **Chlorobenzene** has a chlorine atom, which is an electron-withdrawing group, although it can donate electrons via resonance.

Determine Reactivity Order

Reactivity in sulphonation typically increases with electron density. Therefore, toluene, which has the electron-donating methyl group, is more prone to sulphonation compared to benzene, nitrobenzene, and chlorobenzene.

Identify the Most Reactive Compound

Based on the electron densities analyzed: - Toluene has the highest electron density due to the methyl group. - Therefore, toluene is the most reactive towards sulphonation.

Key concepts

Sulphonation reaction

Sulphonation is a fascinating reaction type that falls under electrophilic aromatic substitution (EAS). In chemistry, EAS describes a family of organic reactions where an electrophile replaces a hydrogen atom on an aromatic ring. During sulphonation, a sulfonyl group \(-SO_3H\) is introduced into the aromatic system. This is usually accomplished using fuming sulfuric acid or sulfur trioxide, which serves as the sulfonating agent.
In the grand scheme of EAS reactions, sulphonation stands out because it is quite reversible. That means the sulfonic acid group can also be removed under the appropriate conditions, restoring the aromatic compound. Sulphonation's reaction rate is highly dependent on the electron density of the aromatic ring. Understanding this concept is crucial for predicting which aromatic compound will sulphonate most rapidly.
This concept may sound complicated, but at its core, it's all about how quickly and effectively a sulfonyl group can join up with an aromatic ring. It's an essential reaction in industrial chemistry for producing sulfonic acids, which have various applications.

Electron density

The term "electron density" simply refers to the concentration of electrons in a particular area of a molecule. For aromatic compounds, electron density plays a key role in determining how susceptible they are to chemical reactions.
In electrophilic aromatic substitution, such as sulphonation, the process is greatly influenced by how much electron density surrounds the aromatic ring. Aromatic rings, like benzene, with higher electron density will attract electrophiles more eagerly.
Several factors can affect the electron density in a molecule, including:

• The presence of substituents: Substituents like methyl groups donate electrons, increasing electron density.
• Resonance and inductive effects: Groups such as nitro and chlorine may pull electrons from the ring, decreasing its electron density.

Increasing the electron density usually means increasing the reactivity of the compound towards electrophilic attack.

Reactivity of aromatic compounds

Reactivity in aromatic compounds, particularly for electrophilic aromatic substitution, hinges heavily on the electron density present in the ring. More electron density means a compound is generally more reactive.
This is because electrophiles seek out areas of high electron density to initiate the reaction process. In simpler terms, an aromatic compound with more available electrons in its ring will typically react faster and more readily with an electrophile.
To illustrate this, comparing different compounds can provide insight:

• Toluene, with its electron-donating methyl group, exhibits increased reactivity compared to benzene.
• Benzene, lacking substituents, sits in the middle, with moderate reactivity.
• Compounds like nitrobenzene, with electron-withdrawing groups, are less reactive.

Identifying the reactivity order of compounds towards sulphonation helps in predicting outcomes in chemical processes and reactions.

Substituent effects on aromatic rings

Substituents attached to aromatic rings can have a tremendous impact on the physical and chemical properties of the molecule. Their influence is often understood through either electron-donating or electron-withdrawing effects.
Essentially, these substituents can increase or decrease the electron density of the aromatic ring, thereby affecting the compound's reactivity. Here's how it works:

• Electron-donating groups (EDGs), such as methyl, increase electron density, making the aromatic ring more reactive to electrophiles. This is often due to the inductive effect or resonance where electrons are pushed into the ring.
• Electron-withdrawing groups (EWGs), on the other hand, like nitro or chlorine, pull electrons away due to resonance or inductive effects, leading to a decrease in reactivity.

Understanding these effects is crucial when predicting how a particular aromatic compound will react, especially in reactions like sulphonation, where electron density is a key component.