Alkyation results in the substitution of an acyl (RCO) group with an H atom.
The electrophile is an acylium ion. The acylium ion is generated by the Lewis acid/base reaction of the acidhalide (R-Cl) with AlCl3.
The mechanism proceeds as follows.
The acylation reaction does not have all the limitations as Friedel-Crafts alkylation. Since it uses AlCl3 it does have compatibility issues with aromatics with a nitro (NO2) group.
1) Carbocation rearrangement is not an issue with acylium ions.
2) Over acylation is not an issue since the acyl group deactivates the ring so further acylation does not occur. We will see soon that an acyl group is an EWG (Electron Withdrawing Group). This is easily shown by considering the resonance structures for acetophenone below. Notice how the curved arrows show the electrons being pulled from the aromatic ring, resulting in a positive charge in the ring.
Aryl ketones and aldehydes can be reduced to the corresponding alkyl groups by way of the Clemmenson reduction. Recall that propyl benzene would be difficult to synthesize with a Friedel-Crafts alkylation since rearrangement would occur and overalkylation is a problem. This acylation/reduction sequence is useful for creating difficult alkyl arenes.