Friedel-Crafts Acylation with Practice Problems

In the previous post, we learned about the Friedel–Crafts alkylation and discussed its mechanism and limitations. In Friedel–Crafts acylation, aromatic compounds are reacted with an acyl halide to prepare an aryl ketone:

The electrophile in this reaction is the resonance-stabilized acylium ion, which is formed when the Lewis acid AlCl₃ ionizes the C-Cl bond of the acyl chloride:

The acylium ion is a good electrophile and can be attacked by the aromatic ring according to the general mechanism of the electrophilic aromatic substitution reactions:

Carboxylic acid anhydrides can also be used for Friedel–Crafts acylations since the leaving group here is the acetate ion, which is a resonance-stabilized, good leaving group.

One important difference and advantage of the Friedel–Crafts acylation reactions is that they do not undergo rearrangements like Friedel–Crafts alkylations do:

This allows for the preparation of primary alkyl-substituted aromatic compounds since the Wolff can reduce the aryl ketones–Kishner or Clemenson reduction:

Intramolecular Friedel–Crafts Acylation is possible for aromatic compounds containing the acyl chloride functional group:

The limitation of the Friedel-Crafts alkylation not working with deactivated aromatic rings is still applied to the Friedel-Crafts Acylation as well:

Because of this, polyacylation cannot be achieved since the carbonyl added in the first acylation deactivates the ring.

We will go over this in more detail in the upcoming posts.

Notice that we have not mentioned a Friedel-Crafts reaction for introducing an aldehyde group to the aromatic ring. The reason for this is that formyl chloride (H-COCl) and formyl anhydride are not stable reagents, and therefore, alternative methods such as the Vilsmeier-Haack formylation and Gatterman-Koch reactions are used.