Electrophilic aromatic substitution
Friedel-Crafts acylation (contrast with alkylation)
How the Friedel-Crafts acylation (contrast with alkylation) 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: RCOCl, AlCl₃ (≥1 equiv). Conditions: CH₂Cl₂ or CS₂, 0–25 °C; aqueous work-up. Bonds formed: Ar–C(=O)R (C–C σ); bonds broken: C–Cl; Ar–H. Stereochemistry: No rearrangement; ketone deactivates ring (no poly-acylation). Rate and selectivity: Acylium formation fast; arenium formation is rate-determining.
At a glance
Step 1: AlCl₃ ionises the acid chloride
AlCl₃ helps an acid chloride ionize, forming a resonance-stabilized acylium ion — unlike a simple alkyl carbocation, positive charge here is shared between carbon and oxygen, so it never rearranges.
Step 2: Ring π attacks the acylium carbon
The ring's π electrons attack the acylium carbon, forming the same kind of resonance-stabilized arenium ion seen in every EAS reaction.
Step 3: Deprotonation restores aromaticity
Loss of a proton restores aromaticity, giving an aryl ketone. Because the acylium ion can't rearrange (unlike alkyl cations in Friedel-Crafts alkylation), this reaction is far more reliable for installing unbranched chains.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- AlCl3
- Aluminium chloride. Strong Lewis acid for Friedel–Crafts reactions.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
- Wheland
- Wheland intermediate (arenium ion). The cyclohexadienyl cation formed when an electrophile adds to a benzene ring.
How it compares with related reactions
| Friedel-Crafts acylation (contrast with alkylation) | Bromination of benzene (FeBr₃-catalyzed) | Duff reaction | |
|---|---|---|---|
| Reagents | RCOCl, AlCl₃ (≥1 equiv) | Br₂ with FeBr₃ (Lewis-acid catalyst) | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ |
| Conditions | CH₂Cl₂ or CS₂, 0–25 °C; aqueous work-up | Anhydrous, 25 °C, dark | Heat, 100–150 °C; aqueous hydrolysis |
| Stereochemistry | No rearrangement; ketone deactivates ring (no poly-acylation) | None (planar, achiral) | Ortho-selective (OH directs; H-bonded iminium) |
| Rate / selectivity | Acylium formation fast; arenium formation is rate-determining | 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 acylation (contrast with alkylation)?
RCOCl, AlCl₃ (≥1 equiv). Conditions: CH₂Cl₂ or CS₂, 0–25 °C; aqueous work-up.
Which bonds are formed and which are broken?
Formed: Ar–C(=O)R (C–C σ). Broken: C–Cl; Ar–H.
What is the stereochemical outcome?
No rearrangement; ketone deactivates ring (no poly-acylation)
What controls the rate or the selectivity?
Acylium formation fast; arenium formation is rate-determining
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam