Friedel-Crafts acylation

Electrophilic substitution-acylation of benzene rings

The Friedel-Crafts acylation reaction is very similar to Friedel-Crafts alkylation reactions which are used to produce alkylated aromatic rings. 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 with Friedel-Crafts alkylation reactions.

The mechanism of a Friedel-Crafts acylation reaction is simply what you would expect from an aromatic rings, that is electrophilic substitution. The reactive electrophile in these reactions is the resonanced stabilised acylium ion (RCO+). This ion is generated by the reaction of an acyl chloride or acid anhydride with a Lewis acid catalyst such as aluminium chloride (AlCl3). This is outlined in the diagram below:

mechanism for the production of an aclium ion

Reaction mechanism

The acylium ion is an excellent electrophile since it carries a full positive charge, this means that an aromatic ring is able to readily attack the acylium ion and the reaction proceeds by the expected electrophilic substitution route:

mechanism of Friedel-Crafts acylation reaction

Overall we can write an equation for the Friedel-Crafts acylation of benzene using ethanoyl chloride as:

general equation for Friedel-Crafts acylation reactions

Using acid anhydrides as acylating agents

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.

Friedel-Crafts acylation using an acid anhydride as the acylating agent

Preparation of ethylbenzene

While discussing Friedel-Crafts alkylation reaction we looked at the preparation of ethylbenzene, however due to the limitations of the Friedel-Crafts alkylation reaction, in particular the issue of polyalkylation the synthesis of ethylbenzene can be done more efficiently using a Friedel-Crafts acylation reaction. The phenylethanone produced above, either using the ethanoyl chloride or the ethanoic anhydride can be reduced using hydrogen and a nickel catalyst to produce ethylbenzene e.g.

equation for the reduction of phenylethanone to ethylbenzene using hydrogen and a nickel catalyst

Key Points

Practice questions

Check your understanding - Questions on Friedel-Crafts acylation reactions