SolutionsSulfonation

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1. Desulfonation

Propose a mechanism for the desulfonation of benzenesulfonic acid.

Desulfonation mechanism: the ring is protonated at the carbon bearing the SO₃H, HSO₄⁻ removes the proton of the SO₃H and the intermediate loses SO₃ to give benzene

It is sulfonation in reverse: H⁺ is the electrophile. It attacks the carbon bearing the SO₃H group, a benzenonium ion forms and it loses SO₃ (after the sulfonic group is deprotonated).

Desulfonation is favored by dilute sulfuric acid at high temperature, because the excess water consumes the SO₃ and shifts the equilibrium.

2. A blocking group

How would you prepare o-bromophenol from phenol taking advantage of the reversibility of sulfonation?

1) Concentrated H₂SO₄: SO₃H enters the para position (or both ortho and para if an excess is used). 2) Br₂: bromination can only take place at a free ortho position. 3) Dilute H₂SO₄ and heat: the sulfonic group is removed.

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