Question

The most reactive species among the following towards sulphonation is: (a) Toluene (b) Nitrobenzene (c) 1,3 -dimethyl benzene (d) Chlorobenzene

Short answer

1,3-dimethylbenzene is the most reactive towards sulphonation.

Step-by-step solution

Understand the Context

Sulphonation is an electrophilic aromatic substitution reaction, where an electrophile (usually SO_3 or H_2SO_4 ) substitutes a hydrogen atom on the benzene ring.

Identify the Groups and Effects

Analyze each species: (a) Toluene has a methyl group, which is an electron-donating group. (b) Nitrobenzene has a nitro group, which is an electron-withdrawing group. (c) 1,3-dimethylbenzene has two methyl groups, both electron-donating. (d) Chlorobenzene contains a chlorine atom, which is an electron-withdrawing group.

Recognize Activation or Deactivation

Electron-donating groups activate the benzene ring towards electrophilic substitution, while electron-withdrawing groups deactivate the ring.

Evaluate the Species Reactivity

The presence of methyl groups in toluene and 1,3-dimethylbenzene makes these compounds more reactive compared to nitrobenzene and chlorobenzene because the methyl groups donate electrons, making the ring more nucleophilic.

Determine the Most Reactive Compound

1,3-dimethylbenzene (mesitylene) has two methyl groups donating electrons, increasing electron density more significantly than in toluene. Hence, it is the most reactive towards sulphonation.

Key concepts

Electrophilic Aromatic Substitution

In organic chemistry, electrophilic aromatic substitution is a key reaction mechanism that involves the replacement of a hydrogen atom on an aromatic ring, such as benzene, with an electrophile. This process fundamentally alters the structure of the aromatic compound. The aromatic ring, rich in electrons, serves as the nucleophilic component that attracts the electrophile. Typically, a strong electrophilic reagent is needed to initiate this reaction, due to the stability of the benzene ring. Some typical electrophiles include:

• Sulfur trioxide ( SO_3 )
• Nitronium ion ( NO_2^+ )
• Halogens (e.g., Cl^+, Br^+ )

The reaction proceeds through two main steps:

• Formation of a Carbocation Intermediate: The electrophile attacks the π-electron cloud of benzene, leading to a highly unstable carbocation intermediate.
• Restoration of Aromaticity: The aromaticity is restored via deprotonation, thereby substituting the hydrogen with the electrophile.

Understanding the nature of substituents is crucial. Compounds with electron-donating groups make the ring more susceptible to electrophilic attack, increasing the reaction rate.

Sulphonation Reaction

Sulphonation is a specialized type of electrophilic aromatic substitution where a sulfonyl group ( SO_3 H) is introduced onto the aromatic ring. This reaction is typically conducted using concentrated sulfuric acid ( H_2SO_4 ) or sulfur trioxide ( SO_3 ). The sulphonation process involves several stages:

• Generation of Electrophile: In commonly employed methods, heating sulfuric acid induces the generation of SO_3, which is the active electrophile in the process.
• Attack by Benzene: The electrophile SO_3 adds to the benzene ring, forming a sigma complex, a non-aromatic intermediate.
• Deprotonation: Finally, a proton ( H^+ ) is released to restore aromaticity.

Because the sulphonation reaction is reversible, controlling reaction conditions like temperature is vital. Sulphonation tends to favor electron-rich aromatic rings, influenced primarily by the substituents present. Compounds with electron-donating groups react more rapidly, making them highly reactive under sulphonation conditions.

Reactivity of Aromatic Compounds

The reactivity of aromatic compounds in electrophilic aromatic substitution reactions, such as sulphonation, depends largely on the nature of substituents attached to the benzene ring. These substituents can either donate or withdraw electrons. Electron-Donating Groups (EDGs):

• Groups like methyl ( CH_3 ), hydroxyl ( OH ), and amino ( NH_2 ) increase electron density on the ring, thereby enhancing its reactivity.
• These generally make the aromatic compound more nucleophilic and reactive towards electrophiles.

Electron-Withdrawing Groups (EWGs):

• Groups such as nitro ( NO_2 ), carbonyl ( COOH ), and halogens withdraw electron density, reducing ring reactivity.
• EWGs typically cause the aromatic ring to be less nucleophilic and less reactive.

Compounds with multiple substituents require careful consideration. For instance, 1,3-dimethylbenzene (mesitylene) is significantly more reactive than toluene because it bears two activating methyl groups. This enhanced reactivity is crucial when predicting the outcome of sulphonation and other electrophilic aromatic substitution reactions.