Sulfonation of Benzene SO3H +S03 H2S04 Sulfur trioxide (SO3)is the electrophile in the reaction. A 7%mixture of SO3 and H2SO4 is commonly referred to as "fuming sulfuric acid". The-SO3H groups is called a sulfonic acid. Chapter 17 13Chapter 17 13 Sulfonation of Benzene ▪ Sulfur trioxide (SO3) is the electrophile in the reaction. ▪ A 7% mixture of SO3 and H2SO4 is commonly referred to as “fuming sulfuric acid”. ▪ The —SO3H groups is called a sulfonic acid. SO3 H + SO3 H2 SO4