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The Friedel-Crafts Acylation Reaction

Core Concepts

In this tutorial on the Friedel-Crafts acylation reaction, you will learn about the acylation of an aromatic ring. Furthermore, you will learn about the way resonance stabilization affects the reaction mechanism. We will also discuss the similarities and differences between Friedel-Crafts acylation and Friedel-Crafts alkylation.

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Introduction to the Friedel-Crafts Reactions

In 1877, Charles Friedel and James M. Crafts published “Sur une Nouvelle Méthode Générale de Synthèse d’Hydrocarbures, d’Acétones, etc” or “On a New Method to Synthesis Hydrocarbons, Acetones, etc.,” which included the two Friedel-Crafts reactions – the alkylation and acylation reactions.

Friedel-Crafts Acylation

The purpose of the Friedel-Crafts acylation reaction is to synthesize a monoacylated aromatic ring. This reaction occurs when a carboxylic acid chloride (R-COCl) reacts with an aromatic ring in the presence of aluminum chloride (AlCl3). This reaction can only occur when the aromatic ring is not substituted with electron-withdrawing groups (also called meta-directing groups).

To better understand the mechanism of the Friedel-Crafts acylation reaction, let’s use benzene and acetyl chloride as example substrates. In the presence of an aluminum chloride catalyst, these compounds react to form acetophenone.

Friedel-Crafts Acylation Reaction

The mechanism for the reaction consists of two steps: the formation of the acyl cation, followed by acylation. 

In the first step, chlorine forms a Lewis acid-base complex with aluminum chloride, which then acts as a leaving group. This forms a resonance-stabilized acyl cation.

During acylation, the acyl cation reacts with benzene. A benzene π-bond acts as a nucleophile, forming a new C-C bond with the electrophilic acyl carbon. To re-form the aromatic double bond, AlCl4– abstracts the hydrogen (a beta-hydride elimination). This regenerates the aluminum chloride catalyst and generates HCl. 

Friedel-Crafts Acylation Reaction Mechanism

Comparison of Friedel-Crafts Acylation and Alkylation Reactions

Both types of Friedel-Crafts reaction fail when a strong electron-withdrawing group is present. This is because the aromatic π-bonds require a certain degree of electron density to engage in a nucleophilic attack on the acyl/alkyl cation, which they do not have in the presence of a withdrawing group. Therefore, a ring can only be acylated once, as acyl groups are electron-withdrawing.

This differs from the alkylation reaction, which often polyalkyates the aromatic ring. Because alkyl groups are electron-donating, products of Friedel-Crafts alkylation reactions are generally better substrates than the starting material.

The acylation reaction does not result in a carbocation rearrangement because the acyl cation in the Friedel-Crafts acylation reaction is resonance stabilized. Conversely, the alkyl cation in Friedel-Crafts alkylations is not resonance-stabilized and will readily rearrange.

Examples of Friedel-Crafts Acylation Reactions

Friedel-Crafts acylation reactions
Additional Examples of the Friedel-Crafts Acylation Reaction