Electrophilic aromatic substitution
Bromination of benzene (FeBr₃-catalyzed)
How the Bromination of benzene (FeBr₃-catalyzed) 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: Br₂ with FeBr₃ (Lewis-acid catalyst). Conditions: Anhydrous, 25 °C, dark. Bonds formed: C–Br σ; H–Br; bonds broken: C–H (aromatic); Br–Br. Stereochemistry: None (planar, achiral). Rate and selectivity: Slow step = arenium formation; deactivated rings react poorly.
At a glance
Step 1: FeBr₃ activates Br₂; ring attacks
FeBr₃ is a Lewis acid — it polarizes Br–Br, making one bromine sharply electrophilic. The ring's six delocalized π electrons act as the nucleophile and attack it.
Step 2: Arenium ion (σ-complex)
Attack breaks the ring's aromaticity, forming a resonance-stabilized arenium ion (the positive charge is spread over three carbons) and releasing FeBr₄⁻.
Step 3: Deprotonation restores aromaticity
A base pulls off the proton from the carbon that was attacked. Those electrons flow back into the ring, restoring full aromatic delocalization and giving bromobenzene.
Abbreviations and terms
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- FeBr3
- Iron(III) bromide. Lewis acid catalyst for aromatic bromination.
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
How it compares with related reactions
| Bromination of benzene (FeBr₃-catalyzed) | Duff reaction | Friedel-Crafts acylation (contrast with alkylation) | |
|---|---|---|---|
| Reagents | Br₂ with FeBr₃ (Lewis-acid catalyst) | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ | RCOCl, AlCl₃ (≥1 equiv) |
| Conditions | Anhydrous, 25 °C, dark | Heat, 100–150 °C; aqueous hydrolysis | CH₂Cl₂ or CS₂, 0–25 °C; aqueous work-up |
| Stereochemistry | None (planar, achiral) | Ortho-selective (OH directs; H-bonded iminium) | No rearrangement; ketone deactivates ring (no poly-acylation) |
| Rate / selectivity | Slow step = arenium formation; deactivated rings react poorly | Moderate; electron-rich phenols only | Acylium formation fast; arenium formation is rate-determining |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Bromination of benzene (FeBr₃-catalyzed)?
Br₂ with FeBr₃ (Lewis-acid catalyst). Conditions: Anhydrous, 25 °C, dark.
Which bonds are formed and which are broken?
Formed: C–Br σ; H–Br. Broken: C–H (aromatic); Br–Br.
What is the stereochemical outcome?
None (planar, achiral)
What controls the rate or the selectivity?
Slow step = arenium formation; deactivated rings react poorly
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