Sulphonation reaction Arenes

Sulphonation reaction Arenes:

in this Sulphonation reaction Arenes, a sulphonic acid group is introduced on the benzene or arene ring by using the concentrated sulphuric acid or oleum. The reagents can be represented as

1. Concentrated H₂SO₄ 
2. Concentrated H₂SO₄ + SO₃
3. Oleum

There is equilibrium between concentrated sulphuric acid and Sulphur trioxide.

the mechanism of the reaction starts with the formation of Sulphur trioxide gas. Protonation of Sulphur trioxide occurs by forming sulphonium ion, this is due to the fact that there is two dπ-dπ bonds in Sulphur trioxide. These bonds are weak bonds and readily react with a proton to form a sulphonium ion (an electrophile). 

the sulphonium ion formed reacts with an aromatic ring to produce resonating forms of the arene. This positive charge on the aromatic ring a stabilized by resonating forms.
In resonance hybrid structure the positive charges are distributed over five carbon atoms so relative positive charge density on each carbon atom is very less and thus it is stabilized.

The structure three formed in the resonating forms further stabilizes to give mono substituted Sulpho derivative.

The H+ ion release in this reaction joins with the conjugate base to form a byproduct here it is termed as spent acid.

H⁺  + HSO₄ ^-1 → H₂SO₄
The monosubstituted derivative of arenes uses a Fuming sulphuric acid always uses a higher concentration of SO₃ as it is a hygroscopic material and it absorbs water formed in the first step as a result of which more forward reaction takes place and concentration often Sulphonium ion increases the reaction mixture thus the rate of reaction increases and formation of product is higher yield.

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