Mechanisms of benzene sulfonation

Answer 1

Benzene sulfonation is a chemical reaction that involves the substitution of a hydrogen atom on a benzene ring with a sulfonic acid group (-SO3H). This reaction is typically carried out in the presence of a strong acid catalyst, such as concentrated sulfuric acid (H2SO4).

There are two main mechanisms by which benzene can be sulfonated: the electrophilic aromatic substitution mechanism and the free radical mechanism.

1. Electrophilic Aromatic Substitution (EAS) Mechanism:
In this mechanism, the strong acid catalyst, such as concentrated sulfuric acid, protonates the sulfonic acid group to form a more reactive electrophile, known as the sulfonium ion (RSO3H+). The sulfonium ion then reacts with benzene via electrophilic aromatic substitution, where the electrophile attacks the electron-rich benzene ring to form a sigma complex. This intermediate is then deprotonated by the strong acid catalyst, which regenerates the sulfonium ion and releases a proton. Finally, a base is added to neutralize the excess acid and remove the sulfonic acid group, resulting in the product, benzenesulfonic acid.

2. Free Radical Mechanism:
In this mechanism, the concentrated sulfuric acid acts as both an acid catalyst and a source of sulfate radicals (SO4·^-). The sulfate radical is generated via the decomposition of sulfuric acid under high temperature conditions. Once formed, the sulfate radical reacts with benzene to form a benzene radical. This benzene radical then reacts with another sulfate radical to form a sigma complex intermediate. Upon protonation by the strong acid catalyst, the sulfonic acid group is added to the benzene ring, and the product, benzenesulfonic acid, is formed. Finally, a base is added to neutralize the excess acid and remove the sulfonic acid group.

Overall, both mechanisms involve the initial generation of an electrophile or a radical species, which then reacts with benzene to form an intermediate complex. This complex is then protonated and deprotonated to transfer the sulfonic acid group onto the benzene ring, resulting in the sulfonation reaction. The choice of mechanism depends on the reaction conditions and the presence of suitable reactants.