Sulfonation (SEAr): practice and mechanism
SO₃ in fuming sulfuric acid is the electrophile and a sulfonic acid (–SO₃H) forms. The existing substituent decides the position as in any SEAr. The reaction is reversible.
Example
SO₃ (or HSO₃⁺) is the electrophile. It attacks the aromatic ring and after rearomatisation the sulfonic acid forms. Sulfonation is reversible: dilute acid and heat remove the SO₃H group again (protecting group).
Mechanism step by step
- 1
Forming the electrophile: Fuming sulfuric acid contains SO₃ (or protonated HSO₃⁺), a strong electrophile at sulfur.
- 2
The ring π electrons attack the electrophile. A carbocation forms (σ-complex, arenium ion) with its charge spread over three carbons; aromaticity is lost for a moment.
- 3
A base removes the H from the attacked carbon. The ring becomes aromatic again and the electrophile has replaced an H.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- An ortho/para-directing group (alkyl, OH, OR, halogen, NHCOR) sends the electrophile to ortho or para, not meta. With the bulky SO₃H, para dominates.
- Electron-withdrawing groups (NO₂, C=O, CN, SO₃H) direct to meta. The para position is particularly electron-poor here.
Often confused with: Nitration (SEAr)
Frequently asked questions
Where does the new substituent go?
Electron-donating groups (alkyl, OH, OR, NH₂, NHCOR) and halogens direct ortho/para; electron-withdrawing groups (NO₂, C=O, CN, SO₃H) direct meta. The reason is which σ-complexes are best stabilised.
What does "reversible" mean here?
With dilute acid and heat the SO₃H group comes off again. It is therefore used as a blocking group to occupy a position temporarily.