EAS-Sulfonation

Sulfonation of Benzene: A Step-by-Step Mechanism

1. Generation of the Electrophile: The electrophile in a sulfonation reaction is sulfur trioxide (SO3H+​), which can be generated directly from fuming sulfuric acid or from concentrated sulfuric acid under certain conditions. SO3 or SO3H+​ is the actual sulfonating agent, acting as a strong electrophile.

   

2. Electrophilic Attack: The sulfur trioxide (SO3H+​) reacts with the aromatic ring (e.g., benzene) to form a high-energy carbocation intermediate, also known as the sigma complex or arenium ion. This intermediate is stabilized by resonance, similar to other EAS mechanisms, allowing the positive charge to be delocalized across the ring.
   

3. Formation of Sulfonic Acid Group: The sigma complex then undergoes deprotonation, which restores the aromaticity of the ring and results in the attachment of the sulfonic acid group ($S{O}^{3}H$) to the aromatic ring.

4. Equilibrium and Reversibility: Unlike most other EAS reactions, sulfonation is reversible. By heating the sulfonated aromatic compound with dilute sulfuric acid, the sulfonic acid group can be removed, reverting the compound back to the original aromatic hydrocarbon. This reversibility is useful for protecting groups or temporary modification of aromatic compounds.

Sulfonation provides a method for introducing sulfonic acid groups into aromatic rings, enhancing the solubility of aromatic compounds in water and enabling further functionalization. This reaction is widely used in the synthesis of dyes, detergents, and pharmaceuticals, where the $S{O}^{3}H$ group plays a crucial role in the chemical properties and activity of the final compounds.