The reaction of benzene [1] with a solution of sulfur trioxide in sulfuric acid produces benzenesulfonic acids [2].

The mechanism of sulfonation takes place with the following stages:

Stage 1. Attack of benzene on sulfur trioxide
Stage 2. Recovery of aromaticity by loss of a proton.

The sulfonation mechanism is reversible, which allows removal of the -SO group.3H by treatment with aqueous sulfuric. This property is used to protect benzene positions, occupying them with the -SO group. 3h.