Sulphonation is a chemical reaction involving introducing a sulfonic acid group (SO3​H) into a molecule. When applied to benzene, sulphonation results in the formation of benzenesulfonic acid. This reaction is highly significant in synthesizing various industrial chemicals, detergents, and dyes. Here is a detailed note on the sulphonation reaction of benzene:

Sulphonation Reaction of Benzene

1. Conditions for Sulphonation:

Reagent: Concentrated sulfuric acid (H₂SO₄) serves as both the solvent and the reagent.

Temperature: The reaction is typically carried out at elevated temperatures, generally around 50-60°C.

2. Reaction Mechanism:

The sulphonation of benzene involves the following steps:

Formation of Sulfuric Acid-Arene Complex:

H2​SO4​+C6​H6​​ →H(C6​H5​)SO4⁻​+H3​O⁺

Protonation of Benzene:

H(C6​H5​)SO4⁻​+H3​O⁺​→C6​H5​HSO4​+H2​O

Loss of Water and Formation of Benzenesulfonic Acid:

C6​H5​HSO4​​→C6​H5​SO3​H+H2​O

3. Formation of Benzenesulfonic Acid:

The final product of sulphonation is benzenesulfonic acid (C6H5SO3H).

The sulfonic acid group (SO₃H) is introduced at the position originally occupied by a hydrogen atom on the benzene ring.

4. Isomer Formation:

Due to the high symmetry of benzene, all hydrogen atoms are equivalent. Therefore, sulphonation of benzene produces only one product – benzenesulfonic acid.

There are no positional isomers in the product.

5. Regioselectivity:

Benzene is regioselective in sulphonation, meaning that the substitution predominantly occurs at one specific position.

The sulfonic acid group (SO₃H) tends to attack the benzene ring at the ortho and para positions, with para being the major product.

6. Reaction Outcome:

The overall balanced chemical equation for the sulphonation of benzene is:

C6​H6​+H2​SO4​​→C6​H5​SO3​H+H2​O

Significance

– Benzenesulfonic acid is a valuable intermediate in synthesizing various organic compounds, including detergents, pharmaceuticals, and dyes.

– Sulphonation is a reversible reaction; benzenesulfonic acid can be converted back to benzene under certain conditions.

– The regioselectivity and the absence of side products make the sulphonation of benzene a useful tool in organic synthesis.

– This reaction is part of a broader class of electrophilic aromatic substitution reactions, which benzene and its derivatives frequently undergo.