The electrophile generated in sulphonation of benzene from fuming sulp

The electrophile generated in sulphonation of benzene from fuming sulphuric acid is

SO₃⁺

SO₃H

SO₃

SO₂H

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UPSC Geoscientist – 2023

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Sulphonation of benzene is an electrophilic aromatic substitution reaction. When fuming sulfuric acid (H₂SO₄ containing dissolved SO₃) is used, the active electrophile is sulfur trioxide (SO₃). SO₃ is a highly electron-deficient species due to the electron-withdrawing nature of the oxygen atoms and the formal positive charge on sulfur (though it can be considered neutral overall with resonance structures). It is sufficiently electrophilic to attack the pi electron system of the benzene ring.

The electrophile in the sulphonation of benzene by fuming sulfuric acid is SO₃.

In concentrated sulfuric acid without fuming (less SO₃), the electrophile is still primarily SO₃, generated from the equilibrium 2H₂SO₄ ⇌ H₃O⁺ + HSO₄⁻ + SO₃. However, fuming sulfuric acid has a much higher concentration of SO₃, making the reaction faster. SO₃H and SO₂H are not the active electrophilic species attacking the benzene ring.

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