The Friedel-Crafts acylation reaction is very similar to Friedel-Crafts alkylation reactions which are used to produce alkylated aromatic rings or arenes. Friedel-Crafts acylation reactions as the name suggests adds an acyl group (R-C=O) to an aromatic ring to produce aryl ketones.
The acyl group (R-C=O) is usually obtained from an acid chloride or an acid anhydride. Friedel-Crafts acylation reactions have one massive advantage over the similar Friedel-Crafts alkylation reactions in that the acyl group (R-C=O) is an electron withdrawing group and so will deactivate an aromatic ring, this solves the main polyalkylation problem which occurs with Friedel-Crafts alkylation reactions.
The mechanism of a Friedel-Crafts acylation reaction is simply what you would expect from an aromatic ring, that is electrophilic substitution. The reactive electrophile is the resonanced stabilised acylium ion (RCO+). This ion is generated by the reaction of an acyl chloride (acid chloride) or acid anhydride with a Lewis acid catalyst such as aluminium chloride (AlCl3). This is outlined in the diagram below:
The acylium ion (RCO+) is a resonance stabilised ion, this is outlined below. Note the presence of the double-headed resonance arrow in this equation:
The acylium ion is an excellent electrophile since it carries a full positive charge; this means that the delocalised electrons in an aromatic ring are able to readily attack the acylium ion and the reaction proceeds by the expected electrophilic substitution route|:
Overall we can write an equation for this the Friedel-Crafts acylation of benzene with ethanoyl chloride as shown. The product of this reaction is an aryl ketone called acetophenone or 1-phenylethanone. It's a colourless, viscous liquid with a sweet, pungent odour.
As well as using acid chlorides as the acylating agent it is also possible to use acid anhydrides. The reaction mechanism is the same and the electrophile is still the acylium ion e.g.
While discussing Friedel-Crafts alkylation reactions we looked at the preparation of ethylbenzene which was then dehydrogenated to form phenylethene or styrene; which is the monomer used to make the polymer polystyrene. This reaction can be shown as:
However due to the limitations of the Friedel-Crafts alkylation reaction, in particular the issue of polyalkylation during the synthesis of ethylbenzene this is not a particularly efficient way to produce the styrene monomer. However the problem of polyalkylation can be overcome by simply using of a Friedel-Crafts acylation reaction to produce acetophenone and then reducing this to form ethylbenzene using hydrogen and a nickel catalyst; as shown below:
