In Friedel-Crafts acylation, why is water needed? Why does the entire step 4 mechanism need to occur? Also in the step 4 mechanism, what are the exact arrow-pushing mechanisms for the addition of water? MECHANISM 17-6 Friedel-Crafts AcylationFriedel-Crafts acylation is an electrophilic aromatic substitution with an acylium ion acting as the electrophile.Step 1: Formation of an acylium ion.acylbenzeneîR-C -AICI,R C 1: + AlCl3 →acyl chloridecomplexacylium ionSteps 2 and 3: Electrophilic attack forms a sigma complex, and loss of a proton regenerates the aromatic system.(+)CofkoelenoRHCl AICI₂Hsigma complexR+ AlCl3AICI4+ [R[R-C=O(+)acylbenzeneStep 4: Complexation of the product. The product complex must be hydrolyzed (by water) to release the free acylbenzene.ĀIC13DodRproduct complexR + AlCl3+HC1excess H₂OR-C=0:]Rfree acylbenzene+ aluminum salts

We have to explain the role of water in Friedel-Crafts acylation product. Also, we have to give the mechanism of step 4.We know that, AlCl3 is a Lewis acid, as it has vacant orbitals. It is used as a catalyst in the Friedel-Crafts acylation reaction. Now, the final product of the Friedel-Crafts acylation reaction has a carbonyl group (C=O). The carbonyl oxygen of the final product having two lone pairs. So, it acts as a Lewis base. As a result of it, the final product of the Friedel-Crafts acylation reaction and the catalyst forms a bond with the acylated product (acid-base reaction). To get the final acylated product from this complex product, we need water for the Lewis acid catalysed Friedel-Crafts acylation reaction. Hence, the role of water is to break the bond between the acylated product and the Lewis acid, to produce the desired acylated product.The required mechanism is shown below