Concentrated sulfuric acid does not sulfonate benzene at room temperature. However a more reactive form called fuming sulfuric acid, permits electropilic attack by SO₃. Fuming sulfuric acid is made by adding about 8% of sulfur trioxide, SO₃, to the concentrated acid. Because of the strong electron withdrawing effect of the three oxygens, the sulfur in SO₃ is electrophilic enough to attack benzene directly. Proton transfer results in the sulfonated product, benzenesulfonic acid.

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