Sulfonation Sulfur trioxide,SO3,in fuming sulfuric acid is the electrophile. benzenesulfonic acid Chapter 17 9Chapter 17 9 Sulfonation Sulfur trioxide, SO3 , in fuming sulfuric acid is the electrophile. S O O O S O O O S O O O S O O O + + + _ _ _ S O O O H S O O O H + _ S HO O O benzenesulfonic acid =>