Friedel Crafts Reaction - Mechanism, Limitations, Alkylation and Acylation Reactions

We often use different pens from our childhood to write colourful texts, and for that, different colours of writing ink are used. These writing inks are made up of triarylmethane, which is used as a dye. It is also used in printing inks and stamping inks, and for dying leather, plastics, and papers.

It is used as a colouring agent in foods; it is also used in soft drinks, icings, and gelatins.

The Friedel-Crafts reaction is used to produce the important dying agent triarylmethane. So, in this article, you will get a deep knowledge of the Friedel-Crafts reaction.

TABLE OF CONTENT

• Friedel-Crafts Reaction
• Friedel-Crafts Alkylation Reaction
• Mechanism of Friedel-Crafts Alkylation Reaction
• Limitations of Friedel-Crafts Alkylation reaction
• Friedel-Crafts Acylation Reactio
• Mechanism of Friedel-Crafts Acylation Reaction
• Limitations of Friedel-Crafts Acylation Reaction
• Practice Problems
• Frequently asked questions

Friedel-Crafts Reaction

Friedel-Crafts reaction is an electrophilic aromatic substitution of an aromatic compound. This popular reaction was developed by two chemists, namely French Charles Friedel and American James Crafts. In a Friedel-crafts reaction, the aromatic compound undergoes an electrophilic substitution, In the Friedel-Crafts reaction, the aromatic molecule experiences an electrophilic substitution, and the hydrogen atom in the benzene is substituted with an electrophile in the presence of a Lewis acid such as anhydrous aluminium chloride.

The two types of Friedel-Crafts Reactions are Friedel-Crafts Alkylation and Friedel-Crafts Acylation.

Friedel-Crafts Alkylation Reaction

Friedel-Crafts Alkylation is a chemical reaction where the proton in the aromatic compound gets substituted with the alkyl group only. The reaction takes place in the presence of anhydrous aluminium chloride. Anhydrous aluminium chloride may also be replaced with any other Lewis acids such as Ferric chloride.

Mechanism of Friedel-Crafts Alkylation Reaction

The mechanism of the Friedel-Crafts Alkylation reaction may be illustrated in the steps given below.

Step 1: Formation of an electrophile

The Lewis acid is taken for the reaction i.e. either anhydrous aluminium chloride or ferric chloride reacts with the alkyl halide. Generally, an electrophilic carbocation is formed as a result.

The process's Lewis acid, either anhydrous aluminium chloride or ferric chloride, reacts with the alkyl halide. An electrophilic carbocation is consequently created.

Step 2: Formation of an intermediate 

The electrophilic carbocation formed by the reaction of Lewis acid and alkyl halide attacks the aromatic ring. Once it attacks the aromatic ring, an intermediate called cyclohexadienyl cation is formed. The aromatic ring loses its aromaticity because the carbon-carbon double bond breaks.

Step 3: Formation of product

The cyclohexadienyl cation undergoes deprotonation i.e. it loses one proton. The carbon-carbon double bond of the aromatic ring is reformed, thereby restoring the aromaticity of the ring too. The proton released by deprotonation regenerates the aluminium chloride catalyst.

The loss of one proton by the cyclohexadienyl cation is known as deprotonation. The carbon-carbon double bond of the aromatic ring is repaired, regaining the ring's aromaticity as well. The proton released during deprotonation regenerates the aluminium chloride catalyst.

Limitations of Friedel-Crafts alkylation reaction

Friedel-craft’s alkylation reaction offers the following limitations.

• The carbocations formed by vinyl and aryl halides are not stable. Hence, they cannot be used in Friedel-Crafts alkylation reaction.
• Aromatic rings containing deactivating groups may not be suitable for Friedel-Crafts alkylation because deactivating groups withdraw the electron density from the benzene ring, hence the nucleophilic attack of the benzene ring will not be feasible.
• The amine group present in aniline reacts with anhydrous aluminium chloride and forms a complex which deactivates the ring. The reaction is thus incomplete.
• When benzene reacts with an alkyl halide in the presence of anhydrous lewis acid alkylation occurs on the benzene ring but polyalkylation may occur because of the activation nature of the alkyl group. To avoid this phenomenon, the aromatic sample must be taken in large quantities.

Compounds such as mono halobenzenes do not respond or participate in Friedel-Crafts alkylation reaction because they are least reactive.

Friedel-Crafts acylation reaction

Friedel-Crafts acylation reaction is similar to the alkylation reaction. The only difference is that the Friedel-Crafts acylation reaction leads to the formation of a ketone, unlike the alkylation reaction.

The Friedel-Crafts acylation reaction is comparable to the alkylation reaction. The Friedel-Crafts acylation reaction merely differs from the alkylation reaction in that it generates a ketone instead.

Friedel-Crafts acylation reaction is suitable for various aromatic compounds. But if the reactant is alcohol or amine, the oxygen and nitrogen atoms undergo acylation respectively.

Mechanism of Friedel-Crafts Acylation reaction

The mechanism of the Friedel-Crafts Acylation reaction may be illustrated in the steps given below.

Step 1: Formation of acylium ion

The anhydrous aluminium chloride reacts with the acyl halide resulting in the formation of an acylium ion. The acylium ion formed is stabilized by resonance.

Step 2: Formation of an intermediate

The acylium ion formed by the reaction of Lewis acid and acyl halide attacks the aromatic ring. Once it attacks the aromatic ring, an intermediate is formed. The aromatic ring loses its aromaticity because the carbon-carbon double bond breaks.

The acylium ion formed by the reaction between Lewis acid and acyl halide attacks the aromatic ring. When it attacks the aromatic ring, an intermediate is produced. The aromatic ring loses its perfume as a result of the carbon-carbon double bond breaking.

Step 3: Formation of the Product

The intermediate complex undergoes deprotonation i.e. it loses one proton. The carbon-carbon double bond of the aromatic ring is reformed, thereby restoring the aromaticity of the ring too. The proton released by deprotonation regenerates the aluminium chloride catalyst.

The general reactivity order of Friedel-Crafts Acylation reaction is

Limitation of Friedel-Crafts Acylation reaction

Friedel-craft’s acylation reaction offers the following limitations.

• Friedel-Crafts acylation reaction results in the formation of ketone products only.
• Compounds such as mono halobenzenes do not respond or participate in Friedel-Crafts acylation reaction because they are least reactive.
• Similarly, aryl amines are also not suitable for Friedel-Crafts acylation reaction because they react with Lewis acid catalyst and give rise to unreactive complexes.

Practice Problems

Q 1. Write the product of the following reaction?

a. 

b.  

c.  

d. Both A and B

Answer: The given reaction is a Friedel-craft alkylation reaction which is an electrophilic substitution reaction. The groups which direct the incoming electrophile to ortho and para positions are called ortho and para directing groups. Electron density is more on ortho and para position. Therefore, substitution takes place mainly on ortho and para positions.

-OMe is ortho-para directing group.

So, the correct option is C. 

Q 2. Benzene reacts with CH3COCl in the presence of anhydrous AlCl3 to give:

a. 

b.  

c.  

d.  

Answer: Benzene reacts with CH₃COCl in the presence AlCl₃ to give C6H₅COCH₃.

So, the correct Option is D.

Q 3. Write the product of the following reaction.

Answer: When the phenyl chloride reacts with AlCl₃, it should give phenyl cation which is unstable because formed cation is not stabilized by resonance, so it will not be formed. Hence, there will be no reaction.
Hence, Option D is the correct answer.

Q 4. Which is the major product of the given reaction?

Answer: This is a Friedel-craft reaction that is used for the formation of alkylbenzenes by using alkyl halides as reactants in presence of anhydrous Aluminium chloride.

This is done through an electrophilic attack on the aromatic ring with the help of a carbocation. The secondary carbocation formed will rearrange to a tertiary carbocation.

So, the correct option is option B.

Frequently Asked Questions - FAQ

Q 1. What is the use of anhydrous aluminium chloride in the Friedel-Crafts reaction?

Answer: Anhydrous aluminium chloride is used to generate electrophile which is attacked by the electron-rich aromatic ring to form the product.

Q 2. Friedel-Crafts reaction is an example of which reaction?

Answer: Friedel-Crafts reaction is an example of an electrophilic substitution reaction.

Q 3. What happens when weak lewis acids are used in Friedel-crafts reaction? 

Answer: In the first step of the reaction, carbocation is formed which undergoes rearrangement(if possible) to form more stable carbocation, but in the presence of weak Lewis acid, rearrangement of the carbocation is not possible.

Q 4. Give some Lewis acid which can be used in the Friedel-Crafts reaction.

Answer: Strong Lewis acids such as AlCl₃ , BF₃, SbCl₅ and weak Lewis acids such as FeCl₃ , ZnCl₂ are used in Friedel-Crafts reaction.

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