Sulfonation of  Benzene

Reaction type: Electrophilic Aromatic Substitution

Summary.

• Overall transformation :  Ar-H to Ar-SO₃H, a sulfonic acid.
• Reagent : for benzene, H₂SO₄ / heat or SO₃ / H₂SO₄ / heat (=  fuming sulfuric acid)
• Electrophilic species :  SO₃ which can be formed by the loss of water from the sulfuric acid
• Unlike the other electrophilic aromatic substitution reactions, sulfonation is reversible.
• Removal of water from the system favours the formation of the sulfonation product.
• Heating a sulfonic acid with aqueous sulfuric acid can result be the reverse reaction, desulfonation.
• Sulfonation with fuming sulfuric acid strongly favours formation of the product the sulfonic acid.

MECHANISM FOR SULFONATION OF BENZENE
Step 1: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic S, pushing charge out onto an electronegative O atom. This destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 2: Loss of the proton from the sp3 C bearing the sulfonyl- group reforms the C=C and the aromatic system.
Step 3: Protonation of the conjugate base of the sulfonic acid by sulfuric acid produces the sulfonic acid.