Question

Amongst the following compounds which can be most casily sulphonatcd? (1) Benzene (2) Nitrobcnzene (3) Toluene (4) Chlorobenzene

Short answer

Toluene

Step-by-step solution

Understand Sulphonation

Sulphonation is a chemical reaction where a sulfonic acid group \(\ce{SO3H}\) is introduced into an organic compound. This reaction requires the compound to be reactive towards the electrophilic reagent \(\ce{SO3}\).

Identify Activating and Deactivating Groups

In electrophilic aromatic substitution reactions like sulphonation, substituents on the benzene ring influence the reactivity. Activating groups (like alkyl groups) increase reactivity, while deactivating groups (like nitro or chloro groups) decrease reactivity.

Analyze the Compounds

Consider the compounds: Benzene (neutral), Nitrobcnzene (strongly deactivating due to \(\ce{NO2}\)), Toluene (activating due to \(\ce{CH3}\)), Chlorobenzene (moderately deactivating due to \(\ce{Cl}\)).

Determine the Most Reactive Compound

The presence of the \(\ce{CH3}\) group in Toluene makes it more reactive towards sulphonation compared to Benzene, Nitrobenzene, and Chlorobenzene because \(\ce{CH3}\) is an activating group.

Conclusion

Given the comparison, Toluene is the compound that can be most easily sulphonated due to the electron-donating nature of the \(\ce{CH3}\) group.

Key concepts

electrophilic aromatic substitution

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry. It allows substituents to be introduced onto an aromatic ring, like benzene, through the action of an electrophile. The sulfonation reaction specifically introduces a sulfonic acid group \(\text{SO}_3\text{H}\) onto an aromatic compound. This reaction involves placing the electrophile, sulfur trioxide \(\text{SO}_3\), with the aromatic ring. The benzene ring, with its delocalized electrons, acts as a nucleophile and attacks the electrophile. This results in the aromatic ring temporarily losing its aromaticity, forming a sigma complex. A subsequent loss of a proton then restores the aromatic system, completing the substitution.

activating and deactivating groups

In electrophilic aromatic substitution reactions, different substituents on the benzene ring can influence its reactivity. Activating groups increase the ring's reactivity toward electrophiles. They generally donate electrons through resonance or induction. For example, the methyl group \(\text{CH}_3\) in toluene donates electron density to the ring, making it more reactive. On the other hand, deactivating groups withdraw electron density, making the ring less reactive. Examples include the nitro group \(\text{NO}_2\) in nitrobenzene and the chloro group \(\text{Cl}\) in chlorobenzene. These groups decrease the electron density on the benzene ring, hindering its ability to react with electrophiles like \(\text{SO}_3\).

organic chemistry reactions

Organic chemistry reactions encompass a broad range of transformations involving organic molecules. Electrophilic aromatic substitution (EAS) is one such reaction, crucial for introducing various substituents into aromatic systems. Sulfonation is a specific type of EAS. Other types include nitration, halogenation, and Friedel-Crafts alkylation and acylation. Understanding these reactions involves knowing how different groups on an aromatic ring affect the reactivity and position of incoming substituents. For instance, activators direct new substituents to the ortho and para positions, while deactivators tend to direct them to the meta position. Knowing how to predict the outcomes of these reactions is vital for synthesizing complex organic compounds and designing pathways for chemical synthesis.

sulfonic acid group introduction

The introduction of a sulfonic acid group \(\text{SO}_3\text{H}\) into an aromatic ring via sulfonation is essential in making compounds more water-soluble or to introduce a functional group for further reactions. The sulfonation reaction typically uses sulfur trioxide \(\text{SO}_3\) or concentrated sulfuric acid as the sulfonating agent. This reaction is more feasible on aromatic rings with activating groups such as \(\text{CH}_3\) in toluene. The presence of the electron-donating \(\text{CH}_3\) group facilitates the attack on the \(\text{SO}_3\) molecule, making the reaction faster and more efficient compared to benzene or compounds with deactivating groups like nitrobenzene or chlorobenzene. Hence, toluene can be sulfonated more easily than benzene, nitrobenzene, or chlorobenzene.