Friedel Crafts Acylation Reaction: Definition, Examples, Benzene, Mechanism, Applications, and Limitations

Friedel Crafts Acylation Reaction Overview

A Friedel-Crafts reaction is an organic coupling reaction that attaches different substituents to aromatic rings by using an electrophilic aromatic substitution. Electrophilic aromatic substitution occurs in each of these processes.

What is Friedel Crafts Reaction?

A Friedel-Crafts reaction is a type of organic coupling reaction that uses an electrophilic aromatic substitution to attach substituents to aromatic rings. The French scientist Charles Friedel and the American chemist James Crafts invented these reactions in 1877. The electrophilic aromatic substitution of an aromatic substance is the Friedel-Crafts reaction. In the Friedel-Crafts reaction, the aromatic molecule undergoes an electrophilic substitution. The hydrogen atom in the benzene is substituted with an electrophile in the presence of a Lewis acid such as anhydrous aluminum chloride. Friedel-Crafts reactions are classified into two types- 

1. Friedel-Crafts Alkylation
2. Friedel-Crafts Acylation

Friedel Crafts Acylation 

As with the F.C. alkylation, the specific Lewis acid in the Friedel-Crafts acylation can vary. Aluminum chloride (AlCl₃)is often used, butFeCl₃and other Lewis acids will also do the job.

Example of Friedel Crafts Acylation

Friedel and Crafts discovered Friedel-Crafts acylation, a technique related to Friedel-Crafts alkylation, at the same time (1877). When a Lewis acid is introduced to an acyl halide in the presence of an aromatic ring, an electrophilic aromatic substitution reaction occurs in which the acyl group attaches to the aromatic ring (with HX loss). 

Who invented Friedel Crafts Acylation Reaction?

The Friedel-Crafts reactions were invented in 1877 by Charles Friedel and James Crafts to attach substituents to aromatic rings. There are two types of Friedel-Crafts reactions: alkylation reactions and acylation processes. Both are accomplished by electrophilic aromatic substitution.

Read more about Gabriel's Phthalimide Reaction.

Friedel Crafts Acylation Reaction

The acylation reaction of Friedel-Crafts is analogous to the alkylation reaction. The sole difference is that, unlike the alkylation process, the Friedel-Crafts acylation reaction results in the creation of a ketone. The Friedel-Crafts acylation procedure is applicable to a wide range of aromatic chemicals. However, if the reactant is alcohol or amine, the oxygen and nitrogen atoms are acylated. The Friedel acylation reaction can be summarized as follows-

Acyl Aromatic Compound + Aromatic Ring + RCOX Lewis Acid

The Friedel-Crafts process may be used to acylate a variety of aromatic substances. When an alcohol or an amine is used as the reactant, the oxygen and nitrogen atoms are acylated.

Friedel's Acylation of Benzene

When benzene is treated with an acyl halide in the presence of Lewis acid, it creates acyl benzene. Friedel Craft's acylation reaction is what this is. 

Friedel Crafts Acylation Reaction Highlights

The following table gives details about the Friedel Crafts Acylation reaction-

Friedel Crafts Acylation Reaction Mechanism

The Friedel-Crafts Acylation reaction mechanism is depicted in the stages below-

Step 1: Acylium Ion Formation

The anhydrous aluminum chloride combines with the acyl halide to create the acylium ion. The generated acylium ion is stabilized via resonance.

Step 2: Create an Intermediate

The aromatic ring is attacked by the acylium ion generated by the interaction of Lewis acid and an acyl halide. An intermediate is generated when it attacks the aromatic ring. Because the carbon-carbon double bond breaks, the aromatic ring loses its aromaticity.

Step 3: Product Development

The intermediate complex is deprotonated, which means it loses one proton. The aromatic ring's carbon-carbon double bond is repaired, restoring the ring's aromaticity. The proton produced by deprotonation is used to renew the aluminum chloride catalyst. The Friedel-Crafts Acylation process has the following typical reactivity order:

Applications of Friedel Craft's Acylation Reaction

The following are some of the applications of Friedel Craft's acylation reaction-

1. It is also employed in the Haworth reactions.
2. It is employed in the synthesis of aromatic compounds with functional groups such as aldehydes and ketones.
3. One of the most prominent applications of this reaction is in the manufacturing of dyes, such as xanthene dyes.

Limitations of Friedel Craft's Acylation Reaction

The following are some of the limitations of Friedel Craft's acylation reaction-

1. Due to their low reactivity, mono halobenzene does not respond to or participate in the Friedel-Crafts acylation process.
2. In most cases, acylation processes produce solely ketones. 
3. The Friedel-Crafts technique only yields ketone molecules. Under certain conditions (CO), formyl chloride degrades into HCl and CO₂.
4. This synthesis cannot utilize the aromatic compound, which is less reactive than mono halobenzene.
5. We can't employ aryl amines because they create very unreactive compounds with Lewis acid catalysts. 

What makes Friedel Craft Acylation superior to Alkylation?

In the Friedel craft reaction, acylation is favored over alkylation. Because the acyl group is an electron-withdrawing group in acylation, it reduces the electron density, so further electrophilic substitution is prohibited. Because the carbon chain of acyl halide does not rearrange, isomeric ketone compounds cannot be formed. In contrast, in alkylation, the methyl group acts as an electron donor, increasing the electron density and electrophilic reactivity of the substrate. As a result, poly alkylation occurs rather than mono alkylation.

Points to remember

1. Friedel and Crafts discovered Friedel-Crafts acylation, which is similar to Friedel-Crafts alkylation in that an electrophilic aromatic substitution reaction can occur when a Lewis acid is added to an acyl halide in the presence of an aromatic ring, with the acyl group being added to the aromatic ring (with -HX loss).
2. The acylation process converts the aromatic ring to a ketone.
3. The Lewis acid in the Friedel-Crafts acylation, like the Lewis acid in the Friedel-Crafts alkylation, can vary. While aluminum chloride (AlCl₃) is the most commonly used, FeCl₃and other Lewis acids can also be used.
4. Friedel-Crafts acylation is demonstrated via the reaction of acetyl chloride and benzene catalyzed by aluminum chloride.