Electrophilic aromatic substitution — reversible
Sulfonation of benzene
How the Sulfonation of benzene works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: fuming H₂SO₄ (oleum, SO₃ in H₂SO₄). Conditions: 25–80 °C; reverse with dilute H₃O⁺, heat. Bonds formed: C–S (Ar–SO₃H); bonds broken: Ar C–H. Stereochemistry: Not applicable. Rate and selectivity: Attack on SO₃ is rate-limiting; fully reversible.
At a glance
Step 1: Ring π attacks sulfur of SO₃
Sulfur trioxide, a strongly electrophilic sulfur center (or protonated as H₂SO₄'s more reactive form), is attacked directly by the ring's π electrons — the same EAS pattern as every other electrophile in this library, just with sulfur as the electrophilic atom instead of carbon, nitrogen, or halogen.
Step 2: Arenium (σ-complex) intermediate
This forms the familiar resonance-stabilized arenium intermediate.
Step 3: Deprotonation gives the sulfonic acid
Loss of a proton gives benzenesulfonic acid — unusually among EAS reactions, sulfonation is fully reversible: heating the product in dilute aqueous acid runs the identical mechanism backward, which is why sulfonate groups are used industrially as removable, temporary blocking groups during multi-step aromatic synthesis.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
How it compares with related reactions
| Sulfonation of benzene | Bromination of benzene (FeBr₃-catalyzed) | Duff reaction | |
|---|---|---|---|
| Reagents | fuming H₂SO₄ (oleum, SO₃ in H₂SO₄) | Br₂ with FeBr₃ (Lewis-acid catalyst) | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ |
| Conditions | 25–80 °C; reverse with dilute H₃O⁺, heat | Anhydrous, 25 °C, dark | Heat, 100–150 °C; aqueous hydrolysis |
| Stereochemistry | Not applicable | None (planar, achiral) | Ortho-selective (OH directs; H-bonded iminium) |
| Rate / selectivity | Attack on SO₃ is rate-limiting; fully reversible | Slow step = arenium formation; deactivated rings react poorly | Moderate; electron-rich phenols only |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Sulfonation of benzene?
fuming H₂SO₄ (oleum, SO₃ in H₂SO₄). Conditions: 25–80 °C; reverse with dilute H₃O⁺, heat.
Which bonds are formed and which are broken?
Formed: C–S (Ar–SO₃H). Broken: Ar C–H.
What is the stereochemical outcome?
Not applicable
What controls the rate or the selectivity?
Attack on SO₃ is rate-limiting; fully reversible
Study guides that help with this reaction
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam