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Give reaction of acylation of benzene and its mechanism. |
Answer» Solution :Addition reaction of -COR (Acyl group) in benzene ring with -H is known as acylation of benzene. E.g. the reaction of formation of acetophenone from the benzene is GIVEN as follow. These reaction are knoen as Fridal Craft acylation.  This reaction is explained in THREE steps : (i) Reaction to form electrophilic `CH_(3)overset(+)(C)O`.  Such `CH_(3)overset(+)(C)O` is strong electrophile in nature. Mechanism of reaction : The above reaction COMPLETED in two steps by aromatic electrophilic substitution reaction. Slow step -I : Due to strong acylinium ion `(CH_(3)overset(+)(C)O)` it breaks `pi`- ELECTRONS clouds from stable benzene and form CARBOCATION (`sigma`-complex) having one `sp^(3)` hybridized carbon.  This `sigma`- complex is not aromatic but due to stable resonance it exist as unstable intermediate. The resonance of `sigma`-complex are given as follows :  Fast step -II : `AlCl_(4)^(-)` immediately accept the `H^(+)` from the `sigma`-complex (Arenium ion) and form acyl benzene product. 
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