Download a printable version of this document here

A Friedel-Craft acylation is a reaction to add an acyl group to an aromatic ring such as benzene. This requires the presence of AlCl₃ to allow the electrophilic substitution to occur.

An example of this reaction along with its mechanism can be seen below. 

The reaction requires the presence of AlCl_3, acting as a Lewis acid, for the reaction to proceed. This is due to it being crucial in the formation of the highly reactive acyl cation intermediate. This acyl cation intermediate is key to allowing the not so reactive aromatic ring to attack the cation. 

Because the AlCl₃ is required for the reaction to proceed, it is often described as a catalyst. Also, because the HAlCl₄ could be broken down into HCl and AlCl₃, the AlCl₃ is often described as “not being used up” or acting “catalytically”.  Practically however the salt HAlCl₄ is often  the main bi-product/waste product of this reaction and does not reform AlCl₃. The HAlCl₄ salt usually requires removal from the reaction in purification steps. 

AlCl₃ is also not used in catalytic amounts i.e. approximately < 1 mol%. It is often actually used in slight excess, approximately 1.05 mol compared to the aromatic reagent. This coupled with the fact that AlCl₃ is not actually reformed practically in the reaction is why AlCl₃ is not actually a catalyst, it technically should be described as a reagent in the reaction instead. 

Learn more about designing sustainable synthesis here.

This work is licensed under a Creative Commons Attribution 4.0 International License.