Problem 25 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Arrange the following in : benzene, toluene, methoxybenzene, chlorobenzene in increasing order of reactivity towards sulphonation with fuming sulphuric acid.

Step-by-Step Solution

Answer

Chlorobenzene < Benzene < Toluene < Methoxybenzene.

Step 1: Understand Sulphonation Reactivity

Sulphonation is an electrophilic aromatic substitution reaction that introduces a sulfonyl group into the aromatic ring. The reactivity towards sulphonation depends on the electron density in the benzene ring. Electron-donating groups (EDGs) increase reactivity by increasing electron density, while electron-withdrawing groups (EWGs) decrease reactivity by decreasing electron density.

Step 2: Analyze the Substituents' Effects

Identify the substituent groups attached to benzene: \(\text{toluene has a } CH_3 \) group, \(\text{methoxybenzene has an } OCH_3 \) group, and \(\text{chlorobenzene has a } Cl \) group. \(\text{CH}_3\) and \(\text{OCH}_3\) are EDGs, with \(\text{OCH}_3\) being stronger, while \(\text{Cl}\) is an EWG due to its electronegativity.

Step 3: Rank the Compounds by Reactivity

Based on the effects of the substituents, methoxybenzene, with the strongest EDG (\(OCH_3\)), will have the highest reactivity. Toluene, with a \(CH_3\) group, follows. Benzene, with no substituents, has moderate reactivity, and chlorobenzene, with an EWG (\(Cl\)), will have the lowest reactivity. Thus, the increasing order is: chlorobenzene < benzene < toluene < methoxybenzene.

Key Concepts

Sulphonation

Sulphonation is a type of electrophilic aromatic substitution (EAS) reaction where a sulfonyl group (SO₃H) is introduced into the aromatic ring of a compound like benzene. This process typically involves the use of fuming sulfuric acid or oleum as the sulfonating agent. The reaction's mechanism starts when the electrophile, SO₃, attacks the electron-rich aromatic ring, temporarily disrupting its aromaticity by forming a sigma complex. This intermediate then rapidly reverts back to a stable aromatic system by losing a proton. The overall outcome is the substitution of one hydrogen atom from the benzene ring with a sulfonic acid group.

Reactivity Order

When considering reactivity towards sulphonation, the electron density of the aromatic ring plays a crucial role. The more electron-rich the benzene ring is, the more reactive it will be toward electrophiles like the sulfonyl group. This means that the presence of substituents on the benzene ring significantly influences its reactivity. Compounds with electron-donating groups (EDGs) generally increase the reactivity of the ring. In contrast, those with electron-withdrawing groups (EWGs) reduce its reactivity. Thus, in the listed compounds: chlorobenzene, benzene, toluene, and methoxybenzene, we observe an increasing order of reactivity, whereby methoxybenzene, due to its strong EDG, ranks highest in terms of reactivity.

Electron-Donating Groups

Electron-donating groups (EDGs) significantly increase the electron density of the benzene ring, making it more attractive to electrophiles. Such groups fortify the ring's ability to attract the sulfonyl group during sulphonation. Common EDGs include alkyl groups like methyl (\(CH_3\)), and alkoxy groups like methoxy (\(OCH_3\)). The methoxy group particularly exemplifies a strong EDG since it not only provides electrons through the methoxy oxygen (by resonance) but also enhances the reaction rate through its +R (resonance) effect. As a result, methoxybenzene reacts more readily in sulphonation compared to benzene alone.

Electron-Withdrawing Groups

On the flip side, electron-withdrawing groups (EWGs) withdraw electron density from the benzene ring, decreasing its reactivity towards electrophiles. This effect makes compounds less favorable for undergoing electrophilic aromatic substitution reactions like sulphonation. A prominent EWG example is chlorine (\(Cl\)), which, despite its ability to donate electrons by resonance, substantially withdraws electrons due to its high electronegativity, exerting a strong -I (inductive) effect. This characteristic places chlorobenzene at the lower end of the reactivity scale toward sulphonation compared to the other listed compounds with electron-donating characteristics.