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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 AlCl3, 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.
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