Friedel Crafts Alkylation Reaction

Charles Freidel and James Crafts invented the Friedel–Crafts reactions in 1877 to attach substituents to aromatic rings. The alkylation and acylation responses are the two most common Friedel–Crafts responses. Friedel–Crafts Alkylation refers to the substitution of an alkyl group for an aromatic proton. With the assistance of a carbocation, an electrophilic attack on the aromatic ring is completed. The Friedel Crafts alkylation reaction is a response that involves alkyl halides as reactants to create alkylbenzenes.

Friedel–Crafts Alkylation

Adding an alkyl group to an aromatic ring is called the Friedel Crafts alkylation. The reaction is carried out in the presence of aluminum chloride, which is an acid catalyst. The alkyl group is added to the carbon atom, adjacent to the ring nitrogen atom. 

In other words, we can say that a type of electrophilic replacement reaction that includes the replacement of an alkyl group with another alkyl group can be termed the Friedel Crafts alkylation reaction. 

In this type of reaction, catalyst used is chloride of aluminum

Mechanism of Friedel–Crafts Alkylation

A three-step process is involved in the Friedel-Crafts alkylation reaction- 

Step 1: (AlCl₃), also termed the Lewis acid catalyst along with alkyl halide, undergoes a reaction process. The result of which is the confirmation of an electrophilic carbocation. 

Step 2: In this step, the aromatic ring is attacked by carbocation to form an intermediate, i.e. cyclohexadienyl cation. It results in a temporary loss of aromaticity of the arena because of the breakage of the carbon-carbon double bond present.  

 Step 3: In this step, due to the deprotonation of the intermediate, reformation of the carbon-carbon double bond occurs, leading to the restoration of aromaticity to the emulsion. This proton goes on to form hydrochloric acid, regenerating the AlCl₃ catalyst. 

Friedel Crafts Acylation 

Friedel Crafts acylation is a reaction that can be stated as adding an acyl group to an aromatic ring. In this reaction, the use of an acyl chloride or an acyl anhydride as a reactant is made. The acyl group is added to the aromatic ring, and the product which is performing is a ketone or an ester. 

The technique of Friedel Crafts Acylation

The process mentioned above adds a group of acyl to an aromatic ring. The reaction is initiated by adding an acyl chloride to an aromatic ring. The acyl chloride is also replaced by a nucleophile, like alcohol, forming an acylated aromatic ring.

Mechanism

Four steps involved in the Friedel-Crafts acylations process are-  

Step 1 

In this step, a complex is generated. The acyl halide faces a loss of a halide ion when a reaction occurs between the Lewis acid catalyst and the acyl halide, forming an acylium ion stabilized by resonance. 

Step 2 

The ring’s aromaticity is temporarily lost because of the formation of a complex due to an electrophilic attack done by the acylium ion on the aromatic ring.

Step 3 

In this step now, the AlCl₃ catalyst is reformed. In this step, deprotonation of the intermediate complex takes place, restoring the aromaticity to the ring. HCI is formed as this proton attaches itself to a chloride ion (from the complexed Lewis acid).

Step 4 

In this last step, Carbonyl oxygen can be attacked by the regenerated catalyst. Therefore, the ketone product should be liberated by adding water to the products formed in the previous step. Thus, via the Friedel-Crafts acylation reaction, the needed acyl benzene product is attained.

Limitation of Friedel Craft Reaction

1. In Friedel craft Alkylation of Alkyl Halide, Alkene or alcohol, Carbocation will form and it may rearrange to attain stability product will form with respect to stable carbocation.

2. Friedel-Crafts alkylation and acylation reactions usually give poor yields when powerful electron-withdrawing groups are present on the aromatic ring.

3. Aniline or benzene rings containing –NR₂, –NHR group do not give Friedel-craft reaction because they form complexes with Lewis Acid. 

4. Aryl Halide and vinylic halide do not give Friedel craft reaction because they form carbocation on sp2 carbon which is not stable.

5. In case of polyalkylation, Alkyl group increases inductive effect so that as first alkylation completed on the benzene ring it became easier for further substitution.

Conclusion

Friedel Crafts acylation is a reaction used to add an acyl group to an aromatic ring. The reaction is named after Charles Friedel and James Crafts, who first described and discovered it. In The reaction, the use of an acyl chloride or an acyl anhydride is made as a reactant. The acyl group is added to the aromatic ring, and the product is a ketone or an ester. Friedel Crafts alkylation is the substitution of an alkyl group for a hydrogen atom on an aromatic ring. The reaction is catalyzed by a Lewis acid, similar to aluminum chloride. The alkyl group is generally a methyl or ethyl group.