Friedel–Crafts Reaction was discovered by French chemist Charles Friedel and the American chemist James Crafts in the late 19th century. There are two main types of Friedel-Crafts reactions, namely, Friedel-Crafts Alkylation and Friedel-Crafts Acylation.

In this article, we will read in detail about the Friedel Craft Reaction, its types, mechanisms, and examples.

What is Friedel Craft’s Reaction?

When substituents are attached to aromatic rings, an organic coupling process called a Friedel-Crafts reaction takes place. This reaction involves an electrophilic aromatic substitution. An alkyl or acyl group can be chemically attached to an aromatic ring via the Friedel-Crafts reaction. To accomplish electrophilic aromatic substitution, a Lewis acid catalyst—typically aluminium chloride—is used.

There are two types of Friedel-Crafts reactions:

• Alkylation reaction
• Acylation reaction

What is Friedel Craft’s Alkylation Reaction?

Friedel-Crafts Alkylation is a chemical reaction in which the proton of an aromatic compound is substituted with an alkyl group (for example: CH₃Cl). In the presence of anhydrous aluminium chloride (AlCl₃), this reaction takes place. We can also use other Lewis acids, such as ferric chloride, instead of anhydrous aluminium chloride.

Mechanism of Friedel-Crafts Alkylation Reaction of a Benzene

The mechanism of the Friedel-Crafts Alkylation Reaction of benzene is discussed in the steps mentioned below:

Step 1: The formation of an electrophilic carbocation is the first step.

In this step, alkyl halide reacts with the Lewis acid such as anhydrous aluminium chloride or ferric chloride which results in the generation of electrophilic carbocation.

Step 2: Intermediate cation formation

The electrophilic carbocation produced by the reaction of Lewis acid and alkyl halide attacks the aromatic ring. It creates a cation intermediate when it comes into contact with the aromatic ring. The aromatic ring loses its perfume because the carbon-carbon double bond splits.

Step3: Alkyl halide formation

Deprotonation also known as loose of an electron from the chemical reaction, raises into the formation of alkyl halide.

Use of catalyst (AlCl3) in the reaction: It acts as a Lewis acid and coordinates with the halogens, generating an electrophile in the process. It helps to speed up the process of reaction. Also, it brings back the stability of the benzene ring and stabilizes the reaction.

Limitations of Friedel-Crafts Alkylation

The limitations of Friedel-Crafts Alkylation are mentioned below:

• Since their carbocations are extremely unstable and reactive, aryl and vinyl halides cannot be employed in this procedure.
• For Friedel-Crafts alkylation, deactivating groups in aromatic rings might not be appropriate. because the Lewis acid can form a combination with the deactivating group, rendering it inactive. Anhydrous aluminium chloride, for instance, is deactivated by the amine group of aniline. A half-reaction.
• In alkyl halide and aromatic chemical reactions, polyalkylation frequently takes place. Large amounts of the aromatic sample are taken to prevent this.
• Mono halobenzenes do not react or take part in Friedel-Crafts alkylation since they are the least reactive.
• If the substituent group in benzene is more deactivating than halogens, no reaction occurs.
• Polyalkylation occurs because alkyl benzene is more reactive than benzene.

Application of Friedel-Crafts Alkylation

The applications of Friedel-Craft Alkylation are mentioned below:

• It is also used in Haworth reactions.
• It is used to synthesis organic compounds like ketone, aldehydes, etc.

What is Friedel-Craft’s Acylation Reaction?

The only difference is Between Acylation and alkylation is, unlike the alkylation reaction, the Friedel-Crafts acylation reaction produces a ketone. Friedel-Craft’s Acylation is used in acylation reaction of various aromatic compounds.

Mechanism of Friedel-crafts acylation reaction of a benzene

On treating benzene with an acyl halide, in the presence of Lewis acid, it forms acyl benzene. This is known as Friedelcraft’s acylation reaction of Benzene. The mechanism of the same is discussed below:

Step 1: The formation of the acylium ion is the first step.

In this step, anhydrous aluminium chloride reacts with an acyl halide which results in the formation of acylium ion. This acylinium ion so formed is stabilized by resonance.

Step 2: Intermediate cation formation

The acylium ion produced by the reaction of acyl halide and Lewis acid attacks the aromatic ring. Attacking the aromatic ring results in the generation of an intermediate. The aromatic ring loses its perfume because the carbon-carbon double bond splits. As it creates a partial bond to carbon=carbon link with benzene, the oxygen becomes partially connected to the carbon.

Use of catalyst (AlCl₃) here: It acts as a Lewis acid and helps to regenerate the ion formation and stabilize the reaction, also speeding up the process.

Step 3: Deprotonation and the ketone molecule is released.

In this step, carbonyl oxygen is attacked by anhydrous aluminium chloride which result in the release of proton and formation of ketone product.

Limitations of Friedel-Crafts Acylation Reaction

The limitations of the Friedel-Crafts Acylation Reaction are discussed below:

• The halide must be an alkyl halide. Vinyl or aryl halides do not undergo Friedel Craft Acylation.
• Alkylation reactions are prone to carbocation rearrangements.

Application of Friedel-Crafts Acylation Reaction

The applications of Friedel-Crafts Acylation are mentioned below:

• It is used in making many different kinds of aromatic compounds with different substituents.
• It is used in the synthesis of dyes called triarylmethane, and testing of aromatic compounds.
• This reaction is related to several name reactions like Clemmensen Reduction.

Uses of Friedel-Crafts Reactions

The uses of Friedel-Craft Reactions are mentioned below:

• One of the most crucial processes in organic chemistry for C-H activation and C-C bond formation is the Friedel-Crafts reaction.

• Friedel-Crafts type alkylations, which include adding an alkyl group to an arene molecule, serve as the building block for the synthesis of a wide range of industrial goods. Common chemical feedstocks like benzene are transformed by the chemistry into a diverse range of intermediate and end products.

• The production of ethylbenzene by the acid-catalyzed reaction of benzene and ethylene is one of the traditional industrial Friedel-Crafts alkylations. Millions of tons of ethylbenzene are produced each year, indicating a massive industrial scale. It is the styrene precursor molecule, which is utilized to create polystyrene polymers.

• Friedel-Craft-type reactions can yield cumene, xylene, and toluene.

Points to Remember about Friedel-Crafts Reactions

The following points need to be remembered about Friedel-Craft Reactions:

1. Lewis Acid Power: Friedel-Crafts reactions need a Lewis acid catalyst (like AlCl3) to activate the reaction and make it work.
2. Alkyl vs. Acyl: Two main types: Alkylation adds a short carbon chain, while acylation adds an acyl group (often forming a ketone). Acylation generally gives more product.
3. Yield vs. Regioselectivity: Acylation often has a better yield (product amount) than alkylation (prone to overalkylation). Both can be tricky to control where the new group attaches to the aromatic ring.
4. Limitations: Not ideal for very deactivated aromatic rings, and Lewis acid catalysts are corrosive and require caution.
5. Alternatives and Importance: Though not perfect, Friedel-Crafts reactions are still useful for making various chemicals. Greener catalysts and alternative methods like Suzuki-Miyaura coupling are being developed.

Also, Check

• Reactions of Haloarenes
• Benzene Reactions
• Chemical reactions of Haloalkanes and Haloarenes

Samples Questions on Friedel-Crafts Reactions

Example 1. How are ethers formed by Friedel-crafts reaction?

Solution:

Formation of Ether by Friedel-crafts reaction can be formed through acylation or alkylation of anisole. The mechanism of alkylation and acylation of anisole by Friedel-crafts reaction are as follows:

Friedel-crafts Alkylation of Anisole

The Friedel Craft alkylation of anisole is given as:

Friedel-crafts acylation of Anisole

The Friedel craft acylation of anisole is given as follows:

Example 2: Show the Friedel-crafts acylation and alkylation reaction of Chloro-benzene.

Solution:

The Friedel-crafts alkylation of chloro-benzene is as follows:

Here, CH₃Cl is a halogen and catalyst(AlCl3) speeds up the process of reaction. It is reacted with chlorobenzene to give two products.
As, the halogen can be attacked by both para- and ortho-position, two products are formed.

The Friedel-crafts acylation reaction of chlorobenzene is:

Similarly, here, chlorobenzene is reacted with acetyl chloride, which undergoes a reaction with a catalyst (Anhydrous AlCl3) and gives two products. One with acetyl attached on ortho-position, and the other with para-position.

FAQs on Friedel–Crafts Reaction

What is the Friedel-Crafts reaction?

It’s a reaction in organic chemistry that adds a functional group (alkyl or acyl) to an aromatic ring using a Lewis acid catalyst.

What are the two main types of Friedel-Crafts Reaction?

The two main types are:

• Friedel-Crafts Alkylation: Attaches a short carbon chain (alkyl group) to the aromatic ring.
• Friedel-Crafts Acylation: Attaches a carbonyl group (acyl group), often forming a ketone.

What is the role of the Lewis acid catalyst?

The Lewis acid (like aluminium chloride) activates the reactant molecule, making it more electrophilic (positively charged) and ready to react with the aromatic ring.

Can the Friedel-Crafts reaction be reversed?

No, it’s an irreversible substitution reaction.

What are the advantages of Friedel-Crafts acylation over alkylation?

Acylation often gives a higher yield (amount of product formed) and is less prone to polyalkylation (adding multiple alkyl groups).

What are some alternative methods to Friedel-Crafts reactions?

Friedel-Crafts reactions using greener catalysts are being developed. Other methods for aromatic substitution are such as Suzuki-Miyaura coupling or Negishi coupling.

What are some applications of Friedel-Crafts reactions?

These reactions are used in the synthesis of many important chemicals, including Pharmaceuticals, Dyes, Polymers and Fine chemicals

Are there any safety considerations when performing a Friedel-Crafts reaction?

Yes, Lewis acids can be corrosive and toxic. It’s crucial to follow proper laboratory procedures and safety protocols when working with these reactions.