Friedel-Crafts acylation has a few advantages over Friedel-Crafts alkylation and uses a Lewis acid catalyst and an acyl chloride to add an acyl group to benzene. The ketones produced can be reduced to alkyl groups using Clemmensen reduction.
Say you want to get to n-propylbenzene. How would you do it? Could you use Friedel-Crafts alkylation? Nope! The carbocation that would result would rearrange from a primary carbocation to a secondary like in the mechanism below:
Notice that the substituent added is actually an isopropyl instead of a straight-chain propyl group because of the hydride shift. Additionally, the added alkyl group would actually activate the benzene to react further and add more isopropyl groups. Let’s actually look at this chart to see the relative disadvantages of Friedel-Crafts alkylation:
No reaction with deactivated benzene
*Another limitation is that acylation doesn't work with vinyl or aryl halides.
Okay, so we know that our direct and obvious limitation in this case is the carbocation rearrangement. Friedel-Crafts acylation is not subject to carbocation rearrangement, so let’s try that instead:
Let’s break this down step by step. The chloride first dissociates to form a bond with the AlCl3 to form the acylium ion (the carbocation on the carbonyl carbon) which does not rearrange. The benzene then attacks the cation, and the chlorine removes a hydrogen to restore aromaticity.
That ketone makes that substituent an electron-withdrawing group, making the benzene deactivated, so the benzene will only be monoacylated. From there, all that’s left is to remove that carbonyl by using Clemmensen reduction (zinc amalgam in HCl) or Wolff-Kishner reduction.