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A coupling organic reaction that involves an electrophilic aromatic substitution which is utilised for the bonding of substituents to aromatic rings is called a Friedel-Crafts reaction. We can say that alkylation and acylation reactions are the two major kinds of Friedel-Crafts reactions. These reactions were first discovered by Charles Friedel and the American chemist James Crafts.
Friedel Crafts Alkylation-
When adding an alkyl group to an aromatic ring occurs it is called the Friedel Crafts alkylation. The reaction is done in the presence of aluminium chloride which is an acid catalyst. The alkyl group is added to the carbon atom which is conterminous to the ring nitrogen atom.
In other words, we can say that a type of replacing electrophilic reaction that includes the replacement of an alkyl group with another alkyl group is termed the Friedel Crafts alkylation reaction. In this kind of reaction, Capitalization occurs by an acid catalyst, generally aluminium chloride, and occurs in a two-step.
Friedel Crafts Acylation-
As we know Friedel Crafts acylation is a reaction that can be defined by adding an acyl group to an aromatic ring. In this reaction, we make use of an acyl chloride or an acyl anhydride as a reactant. The acyl group is then added to the aromatic ring, and the performing product is a ketone or an ester.
The technique used in Friedel Crafts Acylation
The above-mentioned process i.e Friedel crafts acylation is used to add an acyl group to an aromatic ring. The reaction is initiated by the addition of an acyl chloride to an aromatic ring. The acyl chloride is also replaced by a nucleophile, like alcohol, to form an acylated aromatic ring.
A three-step mechanism is involved in the Friedel-Crafts alkylation reaction:
Step 1: (AlCl3) which is also termed as 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.
Mechanism
Four steps are involved in the Friedel-Crafts acylations. The steps are-
Step 1
In this step, a complex is generated and the acyl halide faces the loss of a halide ion when a reaction takes place between the Lewis acid catalyst and the acyl halide, forming an acylium ion that is stabilized by resonance.
Step 2
Now 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 which are formed in the previous step. Therefore, via the Friedel-Crafts acylation reaction, the needed acyl benzene product is attained.
Conclusion
Negotiation of an alkyl group for a hydrogen atom on an aromatic ring can be termed Friedel Crafts alkylation. Similar to aluminium chloride, the reaction is catalyzed by a Lewis acid. The alkyl group is generally a methyl or ethyl group. whereas, a reaction used to add an acyl group to an aromatic ring is known as Friedel Crafts acylation. The reaction is named for Charles Friedel and James Crafts, who first described it. An acyl chloride or an acyl anhydride is used in this reaction as a reactant. The acyl group is added to the aromatic ring, and the performing product is a ketone or an ester.
