Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.
The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.
On treatment with an acid chloride (Acyl chloride) in the presence of anyhydrous aluminium chloride, acylation occurs.
Benzene + Acetyl chloride --> Acetophenone + HCl
The attacking electrophile an acyl carbocation RCO is supplied by acid chloride RCOCl in the presence of anhydrous AlCl3