Acylium cations electrophilic aromatic substitution

Aromatic rings are usually not good enough nucleophiles to react with this sink. If a Brpnsted acid or a Lewis acid is added to improve the electron sink, then electrophilic aromatic substitution occurs, usually via the acylium cation as an electrophile. 

A Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction that attaches a carbon-carbon bond to the ring.The electrophile is R, which will add to the aromatic ring to produce a cyclohexadienyl cation. Aromaticity is regained when that intermediate cyclohexadienyl cation is deprotonated. That s all there is to it—all the rest is details. Remember to watch out for rearrangements, because this Friedel-Crafts alkylation is especially prone to them. In the Friedel-Crafts acylation, an acid chloride is used to generate the acylium ion which is the reactive electrophile. No rearrangements are observed in Friedel-Crafts acylation. 

In Summary The problems of Friedel-Crafts alkylation (multiple substitution and carboca-tion rearrangements) are avoided in Friedel-Crafts acylations, in which an acyl halide or carboxylic acid anhydride is the reaction partner, in the presence of a Lewis acid. The intermediate acylium cations undergo electrophilic aromatic substitution to yield the corresponding aromatic ketones. 

Now the electrophilic attack of the aromatic system onto the acylium cation. The product is para substituted, so go for that site of attack. We then need removal of a proton, and chloride is the obvious base, but since this is complexed as AlC, we need the latter to decompose to chloride and AICI3. 

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