Problem 119 - A Complete Resource Book in Chemistry for JEE Main

Question

The most reactive species among the following towards sulphonation is (a) toluene (b) nitrobenzene (c) 1,3 -dimethyl benzene (d) chlorobenzene

Step-by-Step Solution

Answer

1,3-dimethylbenzene is the most reactive towards sulphonation due to its strong electron-donating groups.

Step 1: Understanding Sulphonation

Sulphonation is an electrophilic aromatic substitution reaction where sulfur trioxide (SO₃) adds to the aromatic ring. This reaction depends on the ability of the aromatic compound to donate electrons; therefore, electron-donating groups enhance reactivity towards sulphonation.

Step 2: Assessing Electron-Donating and Withdrawing Groups

Identify the nature of substituents on each benzene derivative: - Toluene (methyl group) is electron-donating due to hyperconjugation. - Nitrobenzene (nitro group) is electron-withdrawing due to resonance and induction. - 1,3-dimethylbenzene (xylene) has two electron-donating methyl groups, more donating than toluene. - Chlorobenzene (chloro group) is weakly withdrawing due to resonance but slightly donating due to its lone pair.

Step 3: Comparing Reactivity

The more electron-donating groups present, the more reactive the compound is towards sulphonation. Among the four compounds, 1,3-dimethylbenzene has the strongest electron-donating effect because it has two methyl groups, making it the most reactive.

Key Concepts

Sulphonation

Sulphonation is a chemical process that involves the introduction of a sulfonic acid group into an aromatic compound. This reaction is a type of electrophilic aromatic substitution, meaning it occurs when an aromatic ring is attacked by an electrophile—in this case, sulfur trioxide (SO₃). The sulfonic acid group is introduced as SO₃ is generally reactive and seeks electrons.

• This reaction typically requires the presence of sulfuric acid to generate the sulfonating agent.
• The aromatic compound, such as benzene, will undergo this substitution, replacing one of its hydrogen atoms.

The effectiveness of sulphonation strongly depends on the compound's ability to donate electrons, which increases the reactivity towards the SO₃ electrophile. Hence, compounds with electron-donating groups are more prone to undergo sulphonation.

Electron-Donating Groups

Electron-donating groups (EDGs) play a crucial role in determining the reactivity of aromatic compounds in sulphonation reactions. These are atoms or groups of atoms that can increase electron density in a benzene ring, usually through resonance or inductive effects.

• Examples include alkyl groups like methyl, which donate electrons through hyperconjugation.
• Hyperconjugation involves the overlapping of orbitals, allowing electron donation to the aromatic ring.

The presence of EDGs makes the aromatic ring more reactive towards electrophiles. This increased reactivity occurs because the additional electron density allows the aromatic compound to more easily participate in electrophilic substitution reactions, such as sulphonation.
In the context of the original exercise, both toluene and 1,3-dimethylbenzene contain electron-donating methyl groups, making them more reactive compared to other options like nitrobenzene.

Reactivity of Aromatic Compounds

The reactivity of aromatic compounds in sulphonation and other substitution reactions is heavily influenced by the groups attached to the benzene ring. In essence, the nature of these substituents dictates whether the compound becomes more or less reactive.

• Electron-donating groups (EDGs) increase reactivity by making the ring richer in electrons.
• Conversely, electron-withdrawing groups (EWGs) decrease reactivity by pulling electron density away, making the ring less attractive to electrophiles.

For example, nitrobenzene has a nitro group, which is strongly electron-withdrawing due to its oxygen atoms' high electronegativity. This makes nitrobenzene less reactive. In contrast, 1,3-dimethylbenzene, with its two electron-donating methyl groups, becomes highly reactive towards sulphonation.

Electrophilic Substitution Mechanism

Understanding the mechanism of electrophilic substitution is essential in comprehending reactions like sulphonation. This mechanism involves a few key steps:

• First, the aromatic ring encounters an electrophile, such as SO₃ in sulphonation.
• A temporary carbocation, known as a sigma complex, is formed as the ring donates electrons to the electrophile.
• Finally, the aromaticity is restored by the loss of a proton, completing the substitution.

Aromaticity refers to the stability of the benzene ring due to its unique electron configuration; hence, the mechanism seeks to preserve this stability. The presence of electron-donating groups facilitates these reactions by increasing the electron density in the ring, making it easier to form the sigma complex. This process underscores why 1,3-dimethylbenzene, with its two methyl groups, is particularly reactive in sulphonation. The sigma complex formation and reformation of aromaticity are crucial factors in determining the compound's reactivity.