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 is the compound that most readily gets sulphonated.

Step 1: Understand Electrophilic Aromatic Substitution (EAS)

Sulphonation is a type of Electrophilic Aromatic Substitution (EAS) reaction, where a hydrogen atom in the aromatic ring is replaced with a sulfonyl group (-SO3H). This reaction requires an electrophile which is typically generated using sulfur trioxide in the presence of sulfuric acid. The rate of sulphonation depends on the reactivity of different substituents on the aromatic ring towards EAS.

Step 2: Analyze the Effect of Substituents

Different substituents on a benzene ring can either activate or deactivate the ring towards EAS reactions. Activating groups, such as alkyl groups, increase the electron density on the ring, making it more reactive, while deactivating groups, such as nitro or chloro groups, decrease the electron density, making the ring less reactive.

Step 3: Compare the Compounds

Let's compare the compounds: - **Benzene** has no substituents and is the baseline for reactivity. - **Toluene** has a methyl group, which is an activating group. - **Nitrobenzene** has a nitro group, which is a strong deactivating group. - **Chlorobenzene** has a chloro group, which is a moderate deactivating group. Since toluene contains an activating methyl group, it will make the benzene ring more reactive compared to the other options.

Step 4: Determine the Most Reactive Compound

Considering the activating and deactivating effects, toluene with its electron-donating methyl group activates the benzene ring the most, making it more susceptible to the sulphonation reaction compared to benzene, nitrobenzene, and chlorobenzene.

Key Concepts

Sulphonation

Sulphonation is one of the classic examples of Electrophilic Aromatic Substitution (EAS) reactions, where an aromatic compound undergoes substitution to introduce a sulfonyl group (-SO₃H) into the ring. It involves using a strong electrophile, often sulfur trioxide ( SO₃ ) in conjunction with sulfuric acid, to facilitate the reaction.
In this reaction, the aromatic ring acts as a nucleophile and attacks SO₃ , forming a sigma complex. The stability of this intermediate is vital for the reaction's progression. The final product is an aromatic sulfonic acid.
Key steps in sulphonation:

• Generation of the electrophile using SO₃ and sulfuric acid.
• Formation of the sigma complex as the aromatic compound interacts with the electrophile.
• Reformation of the aromatic system with the release of a proton.

Sulphonation is reversible; by changing the conditions, such as using dilute acid, the reaction can be driven backward to regenerate the original aromatic compound.

Activating and Deactivating Groups

Substituents attached to aromatic rings significantly influence their reactivity in electrophilic aromatic substitution reactions. These groups can either enhance (activate) or reduce (deactivate) the rate of these reactions by altering the electron density of the ring.
Activating groups, like alkyl groups, increase electron density via inductive effects or by resonance, thereby making the ring more reactive to electrophiles. For example, a methyl group ( -CH₃ ) donates electron density, stabilizing the positively charged intermediates.
Conversely, deactivating groups decrease the ring's reactivity by either withdrawing electron density or by destabilizing the intermediate. Nitro groups ( -NO₂ ) are classic deactivators due to their strong electron-withdrawing effects, significantly reducing the effectiveness of the ring in such reactions.

• Activating Groups: Increase reactivity, usually directing new substituents to ortho/para positions.
• Deactivating Groups: Decrease reactivity and often direct substitutions to the meta position.

Aromatic Compounds

Aromatic compounds, exemplified by benzene, are fundamental in chemistry due to their stable ring structures characterized by delocalized π -electrons. This unique electron configuration gives rise to exceptional stability known as "aromatic stability" or "aromaticity."
These cyclic compounds adhere to Huckel's rule, which states that they must contain (4n + 2) π -electrons, making them less likely to participate in reactions that would disrupt their aromatic system.
Despite their stable nature, aromatic compounds can undergo reactions like electrophilic aromatic substitution, where they maintain their aromatic core. The stability is preserved by substituting a hydrogen atom instead of adding to the ring or altering it significantly.
Examples of aromatic compounds include not just benzene, but also toluene and anisole, each modified by different substituents that influence their chemical behavior during reactions, such as sulphonation.