Here’s a explanation of the Friedel-Crafts acylation reaction:
What It Is
- Friedel-Crafts acylation is a reaction in which an acyl group (–COR) is introduced into a benzene ring.
- Like other reactions of benzene, it is a type of electrophilic aromatic substitution, so a hydrogen atom on the ring is replaced.
Step 1: Formation of the Electrophile
- The acyl group is usually provided by an acid chloride (like R–COCl) or acid anhydride.
- A Lewis acid catalyst (like AlCl₃) is used to generate a positively charged acylium ion (R–C⁺=O), which acts as the electrophile.
Step 2: Attack on the Benzene Ring
- The electron-rich benzene ring attacks the acylium ion.
- This forms a temporary sigma complex (arenium ion) where the ring loses aromaticity temporarily.
Step 3: Restoration of Aromaticity
- A hydrogen atom is removed from the carbon that bonded to the acyl group.
- This restores the aromatic character of the benzene ring.
- The final product is an aromatic ketone (benzene with a carbonyl group attached).
Key Points
- Friedel-Crafts acylation does not usually cause rearrangements, unlike alkylation.
- The reaction requires a Lewis acid catalyst to generate the reactive acylium ion.
- Acyl groups are deactivating, so the substituted ring is less reactive for further substitutions.