Electrophilic Aromatic Substitution - Sulfonation
Sulfur trioxide, the electrophile in sulfonation of benzene, is present in small amounts in normal sulfuric acid and sulfonation of benzene will occur with sulfuric acid. Frequently, though, a solution of sulfur trioxide and sulfuric acid is used (called oleum or fuming sulfuric acid). Electrophilic aromatic substitution begins with the addition of the electrophile into the aromatic pi system of the ring. A conjugated, carbocation intermediate is formed, a resonance combination of three forms, concentrating positive charge at three locations, the two ortho positions and one para. After this addition, a proton departs, completing the overall substitution with aromaticity restored.
Consider the case where electrophilic aromatic substitution is attempted on an aromatic ring that already contains an original sulfonate substituent. If electrophilic aromatic substitution is attempted upon rings that already contain substituents, the location of the next substitution depends on the characteristics of the original substituent. Whether the new substitution occurs ortho, para or meta to the original substituent depends on whether the original substituent either stabilizes or destabilizes a concentration of positive charge upon its carbon at the carbocation intermediate stage. If the substituent already present is electron donating, it will stabilize the carbocation by donating negative charge. New substitutions will occur ortho or para to such electron donating substituents already present on the ring. Electron withdrawing substituents destabilize a carbocation, so the new substitution will most likely be meta. Sulfonate is such an electron withdrawing substituent. Containing only electronegative atoms, sulfonate is electron-withdrawing by induction. Furthermore, the sulfur atom has no lone electron pairs to donate into the ring in resonance. A sulfonate group, already present on the ring, is a ring deactivating, meta director for further electrophilic aromatic substitution.