Friedel-Crafts acylation of aromatic groups to give ketones
Description: In the presence of a catalyst such as FeCl3 or AlCl3 , acyl chlorides will add to aromatic groups to form aromatic ketones. This is called the Friedel-Crafts acylation reaction.
Notes: Acyl chloride is the same as “acid chloride”, a name you might be more familiar with.
For our purposes, FeCl3 and AlCl3 are equally effective for this reaction.
Notes: In all cases the byproduct is HCl and FeCl3 is regenerated.
Note that when a nitro group is present, the aromatic is generally too deactivated for a Friedel-Crafts to occur.
Mechanism: The reaction occurs in two phases, the “activation phase” and the “substitution phase”. In the activation phase, chlorine is removed from the acyl chloride with the Lewis acid. Chloride attacks FeCl3 (Step 1, arrow A) followed by cleavage of the C–Cl bond to give an acyl carbocation (“acylium ion”) (Step 2, arrow B). This is an excellent electrophile (much better than the acyl chloride itself).
In the substitution phase, a PI bond from the aromatic attacks the acyl carbocation (Step 3, arrow C) leading to formation of a carbocation. Aromaticity is regained through deprotonation of the carbon (Step 4, arrows D and E), re-forming the PI bond.
Notes: Note that FeCl3 is regenerated at the end (it is a catalyst). Note that there’s other (equivalent) ways of depicting the deprotonation in the final step, but they all have the same effect.