Description: Treatment of an aromatic molecule such as benzene with sulfur trioxide and a strong acid leads to formation of sulfonic acids.
Notes: Although H2SO4 is shown here, essentially any strong acid will do.
Mechanism: This is an electrophilic aromatic substitution reaction and proceeds in two phases, the “activation phase” and the “substitution phase”.
Treatment of sulfur trioxide (SO3) with strong acid (H2SO4) leads to protonation of the oxygen (Step 1, arrows A and B) making the sulfur a better electrophile than it was initially. The sulfur of the “activated” HSO3(+) is then attacked by a PI bond from the aromatic, leading to breaking of aromaticity and formation of a carbocation (Step 2, arrows C and D). Aromaticity is regenerated by removal of a proton, which leads to formation of the C1-C2 PI bond (Step 3, arrows E and F).
Notes: Note that strong acid (H2SO4) is regenerated in this reaction. Note also that there are other reasonable ways of showing the deprotonation reaction besides using HSO4(–) as base.