Question

Amongst the following, the compound that can most readily get sulphonated is (a) benzene (b) toluene (c) nitrobenzene (d) chlorobenzene

Short answer

Toluene is the compound that can most readily get sulphonated.

Step-by-step solution

Understand the Concept of Sulphonation

Sulphonation is a chemical process where a sulfonyl group (SO₃H) is introduced into an organic compound. In aromatic compounds, the process typically involves an electrophilic substitution, where one hydrogen atom is replaced by the sulfonyl group. The rate of this reaction depends on the substituents on the aromatic ring.

Evaluate Electron Donating and Withdrawing Groups

Substituents on the benzene ring can either donate or withdraw electrons. Electron-donating groups, like methyl groups in toluene, activate the ring towards electrophilic substitution by increasing electron density. Electron-withdrawing groups, like nitro groups in nitrobenzene or chlorine in chlorobenzene, deactivate the ring by reducing electron density.

Identify the Effect of Each Substituent

- Benzene has no substituents, thus it is considered a standard reference for substitution reactions. - Toluene has a methyl group that donates electrons, thus activating the ring. - Nitrobenzene contains a nitro group, which withdraws electrons and strongly deactivates the benzene ring. - Chlorobenzene has a chlorine atom that withdraws electrons due to its inductive effect, making the ring less reactive.

Conclusion Based on Reactivity

Toluene, with its electron-donating methyl group, increases the electron density of the benzene ring, making it more reactive towards sulphonation. As such, toluene can be sulphonated more readily than benzene, nitrobenzene, or chlorobenzene.

Key concepts

Electrophilic Substitution

Electrophilic substitution is a fundamental process in the chemistry of aromatic compounds. It involves the replacement of a hydrogen atom on an aromatic ring with an electrophile. This type of reaction is crucial for introducing various functional groups into aromatic systems.
The process begins when the aromatic compound, acting as a nucleophile, attacks an electrophile. The aromaticity of the compound contributes significantly to its reactivity. Compared to alkanes and alkenes, aromatic compounds have a delocalized ring of electrons, which stabilizes the compound and makes it more reactive towards electrophiles.
An important feature of electrophilic substitution reactions is their ability to increase or decrease the compound's reactivity based on the nature of the substituents on the aromatic ring. Therefore, understanding the electronic nature of these substituents can help predict the rate and position of the substitution.

• During sulphonation, the electrophile is typically sulphur trioxide ( SO₃ ) or its electrophilic equivalent.
• The sulphonyl group ( SO₃H ) is introduced to the aromatic ring in place of a hydrogen atom, creating sulfonated aromatic compounds.
• This substitution retains the aromatic character of the compound, allowing it to engage in further reactions.

Electron Donating and Withdrawing Groups

Substituents on an aromatic ring can significantly influence its reactivity. They can be classified as either electron-donating or electron-withdrawing, depending on how they interact with the electronic cloud of the benzene ring.
Electron-donating groups (EDGs) increase the electron density of the ring, which enhances its reactivity towards electrophiles. An example is the methyl group in toluene. This group pushes electrons into the ring, making it more attractive to electrophiles and thus more reactive in processes like sulphonation.

• They generally position new substituents in ortho and para positions due to the stability provided by electron donation to these sites.

Conversely, electron-withdrawing groups (EWGs) reduce the electron density, making the aromatic ring less attractive to electrophiles. Such is the case with the nitro group in nitrobenzene and the chlorine atom in chlorobenzene. These groups pull electrons away from the ring, thereby deactivating it toward further reactions.

• EWGs tend to guide substituents to meta positions, where the inductive and resonance effects are mitigated.

Understanding the electronic effects of these substituents is crucial for predicting the outcomes of electrophilic substitution reactions.

Aromatic Compounds

Aromatic compounds are a unique class of molecules characterized by their stable ring structures and distinctive chemical properties. The prototypical example of an aromatic compound is benzene, a six-carbon ring with alternating double bonds distributed over the ring.
The stability and reactivity of aromatic compounds arise from their delocalized pi-electron cloud. This electron conjugation across the ring grants the molecule both increased stability and a unique set of reactivity characteristics compared to non-aromatic compounds.
Aromaticity is a core concept in chemistry, explaining the behavior of many less common aromatic structures like toluene, nitrobenzene, and chlorobenzene. In the context of sulphonation, these monomers exhibit varied reactivities based on their structural compositions.

• Toluene, with a methyl group, is highly reactive towards electrophilic substitution due to increased electron density.
• Nitrobenzene, with an electron-withdrawing nitro group, resists such substitutions.
• Chlorobenzene, though deactivated by chlorine, still allows for reactions depending on conditions.

The inherent nature of aromatic compounds makes them a central topic of study in organic chemistry, applicable in contexts extending from industrial processes to biological systems.