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

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 makes it possible to eliminate the -SO3H group by treatment with aqueous sulfuric acid. This property is used to protect benzene positions, occupying them with the -SO3H group.