Reactions of Friedel-crafts were invented in 1877 by French scientists Charles Friedel &  James Crafts. A Friedel-Crafts reaction is an organic coupling process that attaches substituents to aromatic rings using an electrophilic aromatic substitution. The two most common Friedel-Crafts reactions are alkylation and acylation.

Mechanism of Friedel Crafts Alkylation

Mechanism of Friedel-Crafts Alkylation is the process of replacing a type of aromatic proton with a group of alkyl. Then, an aromatic ring is taken through an electrophilic attack via a carbocation. Alkyl halides are used as reactants in the Friedel-Crafts alkylation reaction to create alkylbenzenes.

A catalyst of Lewis acid, like FeCl3 or AlCl3, is utilized in the reaction to help eliminate the halide and create a carbocation. The output carbocation goes through a reformation before proceeding to the alkylation phase.

Mechanism

The reaction mechanism of Friedel-Crafts alkylation is governed by a three-stage mechanism.

• An electrophilic carbocation is formed when the alkyl halide combines with Lewis acid catalyst (AlCl3)
• The carbocation generates a cyclohexadienyl cation intermediate after attacking the aromatic ring
• The arene’s aromaticity is lost for a short time due to breaking the C-C double bond
• The C-C double bond regenerates after the intermediate is deprotonated, restoring the aromaticity of the molecule
• When this proton interacts with hydrochloric acid to create hydrochloric acid, the AlCl3 catalyst is regenerated

What is the mechanism of  Friedel-Crafts Alkylation Reaction’s Limitations?

The following are some of the significant drawbacks of the mechanism of Friedel-Crafts alkylation:

• This reaction cannot utilize the carbocations produced by aryl & vinyl halides, as they are exceedingly unstable
• Aromatic rings having a deactivating group can cause the catalyst to deactivate due to complex formation
• More of the aromatic components must be utilized in these processes to prevent polyalkylation
• The Friedel-Crafts alkylation reaction does not include aromatic compounds less reactive than mono-halobenzenes
• It’s worth noting that this particular reaction, like any other reaction involving carbocations, is susceptible to carbocation readjustments

Mechanism of Friedel Crafts Acylation

The Friedel-Crafts acylation uses a strong Lewis acid catalyst to react an arene using acyl chlorides or anhydrides. This process produces monoacetylated compounds through electrophilic aromatic substitution.

The electrophilic aromatic substitution is the basis for the Acylation reaction’s mechanism. As previously stated, acylation is the process or mechanism of inserting an acyl group (RCO-) into a chemical in Organic Chemistry. A nucleophile replaces the electrophilic carbonyl group of a carboxylic acid derivative in this reaction. An addition-elimination reaction is frequently used to carry out the replacement.

While there are a variety of acylating reagents, acid halides and anhydrides are the most commonly used. Alcohols or phenols are the nucleophiles in the acylation reaction. Both of them produce Esters, ammonia, and amines. Most of the time, acyl halides are used as acylating agents because they create potent electrophiles when combined with metal catalysts.

Mechanism

A 4-stage process governs this response: 

• The acyl halide reacts with the catalyst of Lewis acid, resulting in a complex and the loss of halide iron by the acyl halide
• Due to resonance, an acylium ion is produced, which becomes stable
• The acylium ion attacks the ring electrolytically
• As a result, during the development of a complex, the ring’s aromatic quality is lost
• At this point, the intermediate complex undergoes deprotonation, and the ring regains its aromaticity
• The proton is released and reacts with a chloride ion to create HCl and the catalyst AlCl3
• The retrieved catalyst tries to attack the oxygen of the carbonyl group
• As a result of water addition to Step 3, a ketone product is produced

There is no rearrangement because the acylium ion is stabilized by resonance. Furthermore, because the product has been deactivated, it is no longer vulnerable to electrophilic assault and undergoes no further reaction. As a result, the Friedel-Crafts acylation procedure produces the acyl benzene needed.

Friedel-Crafts Acylation Reaction Limitations

The Friedel-Crafts acylation reaction shows some of these drawbacks: 

• The only results of the acylation process are ketones

• Formyl chloride (H(C=O)Cl) breaks down into CO & HCl when put under these conditions

• This reaction cannot take place if the aromatic compound is not much reactive in comparison to a mono-halobenzene

• Aryl amines can’t be employed in this method, as they form non-reactive complexes with the catalyst of Lewis acid

• Acylations on the oxygen or nitrogen atoms can occur when amines or alcohols are utilized

Friedel Crafts Acylation of Benzene Mechanism

An aromatic electrophilic substitution process is the Friedel crafts acylation reaction. Benzene is easily subjected to an electrophilic substitution reaction because benzene’s aromaticity, which is related to its stability, remains intact due to its participation in aromatic electrophilic substitution reactions. As a result, benzene undergoes Friedel’s acylation process.

Mechanism

• The production of the acylium ion, which would later react with benzene, is the initial step
• In the second step, the acylium ion attacks benzene as a new electrophile, forming one complex
• In the third stage, the proton must leave for aromaticity to revert to benzene
• In the third stage, AlCl4 returns to extract a proton from the benzene ring, enabling it to revert to aromaticity
• The initial AlCl3 and HCl are both recreated due to this process
• Most significantly, we have the first portion of the reaction’s eventual outcome, a ketone
• The final step is to add water to liberate the acyl benzene as the final product

There is no rearrangement because the acylium ion is stabilized by resonance. Also, because the product has been deactivated, it is no longer susceptible to electrophilic attack. However, the Friedel-Crafts Acylation of benzene is not observed when the strong deactivating group is present in the ring. 

Conclusion

We learned that Friedel-Crafts alkylation and acylation reactions are essential electrophilic aromatic substitution processes because they form carbon-carbon bonds. Also, note that they complete the six key electrophilic aromatic substitution processes, including bromination, chlorination, nitration, and sulfonylation.