Electrophilic aromatic substitution
Friedel-Crafts alkylation of benzene
How the Friedel-Crafts alkylation 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: tert-butyl chloride, AlCl₃ (Lewis acid). Conditions: Anhydrous, 0–25 °C, excess benzene. Bonds formed: C–C σ (aryl–alkyl); H–Cl; bonds broken: C–Cl; aromatic C–H. Stereochemistry: None here; 1° cations rearrange (hydride/methyl shift). Rate and selectivity: Rate-limiting: attack of arene on R⁺; product more reactive → polyalkylation.
At a glance
Step 1: AlCl₃ ionises the alkyl chloride
AlCl₃, a strong Lewis acid, helps ionize the alkyl chloride, generating a carbocation electrophile plus AlCl₄⁻.
Step 2: Ring π attacks the cation
The ring's π electrons attack the carbocation. This breaks aromaticity briefly, forming a resonance-stabilized arenium ion.
Step 3: Deprotonation restores the ring
A base removes the proton from the substituted carbon; those electrons flow back into the ring, restoring aromaticity and giving the alkylbenzene.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- AlCl3
- Aluminium chloride. Strong Lewis acid for Friedel–Crafts reactions.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
How it compares with related reactions
| Friedel-Crafts alkylation of benzene | Bromination of benzene (FeBr₃-catalyzed) | Duff reaction | |
|---|---|---|---|
| Reagents | tert-butyl chloride, AlCl₃ (Lewis acid) | Br₂ with FeBr₃ (Lewis-acid catalyst) | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ |
| Conditions | Anhydrous, 0–25 °C, excess benzene | Anhydrous, 25 °C, dark | Heat, 100–150 °C; aqueous hydrolysis |
| Stereochemistry | None here; 1° cations rearrange (hydride/methyl shift) | None (planar, achiral) | Ortho-selective (OH directs; H-bonded iminium) |
| Rate / selectivity | Rate-limiting: attack of arene on R⁺; product more reactive → polyalkylation | 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 Friedel-Crafts alkylation of benzene?
tert-butyl chloride, AlCl₃ (Lewis acid). Conditions: Anhydrous, 0–25 °C, excess benzene.
Which bonds are formed and which are broken?
Formed: C–C σ (aryl–alkyl); H–Cl. Broken: C–Cl; aromatic C–H.
What is the stereochemical outcome?
None here; 1° cations rearrange (hydride/methyl shift)
What controls the rate or the selectivity?
Rate-limiting: attack of arene on R⁺; product more reactive → polyalkylation
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