Friedel-Crafts acylation header image

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 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) to be added to the aromatic molecule is usually obtained from an acid chloride or an acid anhydride. Friedel-Crafts acylation reactions have one massive advantage over Friedel-Crafts alkylation reactions in that the acyl group (R-C=O) is an electron withdrawing group and so will deactivate an aromatic ring and so reduce the likelihood of any further substitution onto the aromatic ring, this solves the main polyalkylation problem which occurs with Friedel-Crafts alkylation reactions.


The mechanism of Friedel-Crafts acylation 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 resonance 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 using the acid chloride ethanoyl chloride as an example:

mechanism and eqaution for the production of an aclium ion from the reaction of the Lewis acid aluminium chloride and ethanoyl chloride

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:

Image shows the mechanism to form an acylium ion and also the fact that it is a resonance stabilised ion

Reaction mechanism

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:

mechanism of Friedel-Crafts acylation reaction

Overall we can write an equation for this the Friedel-Crafts acylation of benzene using the acid chloride ethanoyl chloride; this is outlined below. The product of this reaction is an aryl ketone acetophenone or 1-phenylethanone. It's a colourless, viscous liquid with a sweet, pungent odour.

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. Acid anhydrides are very common acylating agents in Friedel-Crafts acylation reactions, especially in industrial settings because they are generally less corrosive and easier to handle than acyl chlorides (acid chlorides), though they are generally less reactive. The reaction mechanism is the same and the electrophile is still the acylium ion, for example the aryl ketone acetophenone can also be prepared by reacting benzene using the acid anhydride ethanoic anhydride, this is outlined below.

Eqiation to Friedel-Crafts acylation using an acid anhydride as the acylating agent, reaction produces acetophenone from benzene and ethanoic anhydride

Acid anhydride or acid chloride as acylating agents?

The use of acid anhydrides or acid chlorides as acylating agents in an industrial setting will often come down to a balance between a number of factors which include:


Preparation of ethylbenzene

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:

Image shows the dehydrogenation of ethylbenzene to form styrene or phenylethene

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:

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

Limitations of Friedel-Craft acylation reactions

Like Friedel-Crafts alkylation reactions Friedel-Crafts acylation reactions also have their limitations; these include:


Self-check- summary

Use the flashcards below to test your understanding of some of the main points on Friedel-Crafts acylation covered above:

What functional group is added to the aromatic ring in an acylation reaction? A comic style image of benzene.
An acyl group (R-C=O).
What type of reaction mechanism is Friedel-Crafts acylation? Student doing a test, she is stuck on the questions.
Electrophilic substutition. Student doing a test, she is stuck on the questions.
What is a Lewis acid? Chemistry teacher asking students questions
An electron pair acceptor. Chemistry teacher asking students questions
How is electrophile generated. Students working in the chemistry lab.
By the reaction of an acid chloride/anhydride with a Lewis acid catalyst. Students working in the chemistry lab.
Why are acid anhydrides often used instead of acid chlorides in acylation reactions? Chemical reaction in a flask.
Acid anhydrides are generally cheaper, safer to handle and less corrosive and produce less corrosive by-products.
Draw the structure of the acylium ion Student working on problems in the chemistry lab
The acylium ion has the formula R–C≡O⁺, where R is an alkyl group.
Why does polyacylation not occur in Friedel-Crafts acylation? A happy molecule
The acyl group deactivates the ring and prevents further reactions. A happy molecule
The acylium ion is described as resonance stabilised. What does this mean? Molecule with delocalised electrons
Resonance stabilisation means the positive charge on the acyium ion is delocalised across more than one structure.
Explain why aromatic rings with a nitro group (-NO₂) do not undergo Friedel-Crafts acylation reactions.
The nitro group is strongly electron withdrawing, reducing the electron density of the aromatic ring, so it is less able to act as a nucleophile.
Suggest why acid chlorides must be handled with great care. comic style graph.
They react violently with water, releasing corrosive HCl gas .

Key Points


Practice questions and self-check quiz

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