Friedel-Crafts Reaction: Mechanism, Limitation and Sample Questions

The Friedel-Crafts Reaction refers to the alkylation of an aromatic ring with the use of the alkyl halide in the presence of a strong Lewis acid which forms the catalyst. French chemist Charles Friedel and the American chemist James Crafts developed these reactions in the year 1877.

Key Terms: Friedel-Crafts Reaction, alkylation, acylation, Lewis acid, Lewis acid catalyst, alkyl halide, electrophilic substitution reaction

---

Friedel-Crafts Reaction

[Click Here for Sample Questions]

Friedel-Crafts Reaction is an electrophilic substitution reaction that is used to attach substituents to aromatic rings. Alkylation and Acylation are the two primary types of Friedel-Crafts reactions.

The picture above shows the Friedel-Crafts Reaction, both alkylation and Acylation undergone by Benzene. You can notice that in both these reactions (alkylation and acylation), the hydrogen atom which was at first attached to the aromatic reaction was replaced by an electrophile Aluminium trichloride (AlCl₃). In Friedel-Crafts reactions Aluminium trichloride is used as a catalyst most of the time as it synchronizes well with the halogens by acting as a Lewis Acid. It then ends up generating an electrophile in the process. 

Also Read:

---

Friedel-Crafts Alkylation

[Click Here for Previous Year Questions]

Friedel-Crafts Alkylation like mentioned above is a basic type of Friedel-Crafts Alkylation. It refers to the attachment of an alkyl group by replacing an aromatic proton which is done through an electrophilic attack on the aromatic ring with the help of carbocation. In Friedel-Crafts alkylation reaction, alkyl halides are used as reactants to generate alkylbenzenes. The picture below illustrates Friedel-Crafts alkylation reaction of Benzene. 

In the above reaction, Aluminium trichloride (AlCl₃) is used as a catalyst which is a Lewis acid catalyst so as to remove the halide and form a carbocation. The carbocation that is formed as a result undergoes a rearrangement reaction and then proceeds with the alkylation reaction. Ferric Chloride (FeCl₃) is also a Lewis acid and so can also be used as a catalyst here.

Friedel-Crafts Alkylation Mechanism

The mechanism of Friedel-Crafts Alkylation follows three steps.

Step 1: An electrophilic carbocation is formed as a result of the reaction between a Lewis acid catalyst with an alkyl halide. 

Step 2: The carbocation starts attacking the aromatic ring which results in the formation of a cyclohexadienyl cation as an intermediate. The aromaticity of the arene is temporarily lost as a result of the breakage of the double bond of carbon-carbon. 

Step 3: The intermediate deprotonates causing the reformation of the carbon-carbon double bond. This leads to the restoration of aromaticity to the compound. The proton then proceeds to form Hydrochloric acid and thus regenerates the Lewis acid catalyst.

The above image represents the mechanism of Friedel-Crafts Alkylation. 

Friedel-Crafts Alkylation Limitations

Here are some of the limitations of Friedel-Crafts Alkylation.

• The carbocations that are formed by aryl and vinyl halides are extremely unstable and hence cannot be used in the reaction. 
• If there is a deactivating group on the aromatic ring, such as NH₂, it can deactivate the catalyst by forming complexes. 
• In Friedel-Crafts alkylation reaction, excess alcohol must be used to avoid polyalkylation. 
• Aromatic compounds are found to be less reactive than mono-halobenzenes. 

One must also note that the Friedel-Crafts alkylation is prone to carbocation rearrangements.

---

Friedel-Crafts Acylation

[Click Here for Sample Questions]

Friedel-Crafts Acylation is another basic type of Friedel-Crafts reaction in which an acyl group is added to the aromatic ring. The process is done by using an acid chloride (R-(C=O)-Cl) and a Lewis acid Catalyst. The aromatic ring gets transformed to ketone in the Friedel-Crafts Acylation reaction. The diagram below represents the reaction between an acyl chloride and benzene.

Friedel-Crafts Acylation

An acid anhydride can also be used as an alternative to acyl halide in Friedel-Crafts acylations. A highly electrophilic acylium ion which has the general formula, RCO⁺ is formed as a result of a complex formed by the halogen belonging to the acyl halide with the Lewis Acid Catalyst and it is stabilised by resonance. 

Friedel-Crafts Acylation Mechanism

The mechanism of Friedel-Crafts Acylation follows four steps.

Step 1: The acyl halide and Lewis acid catalyst (here AlCl₃) react together to form a complex. The acyl halide thus loses a halide ion to form an acylium ion which is then stabilised by resonance. 

Step 2: The acylium ion that is formed (RCO⁺) does an electrophilic attack on the aromatic ring. A complex is formed and the aromaticity of the compound is lost temporarily. 

Step 3:The intermediate complex that is formed gets now deprotonated which restores the aromaticity to the ring. The proton then attaches itself to a chloride ion from the formed Lcomplexed Lewis acid and forms HCl. Thus, the regeneration of the AlCl₃ catalyst happens. 

Step 4: Now, the regenerated catalyst proceeds to attack the carbonyl oxygen. So, the ketone product must be released by adding water to the products that were formed in the previous step.

Thus using Friedel-Crafts Acylation reaction, acyl-benzene can be formed. 

Friedel-Crafts Alkylation Limitations

Friedel-Crafts acylation has some limitations of its own even though it resolved the shortcoming of the related alkylation reaction. 

• Only ketones are yielded from Friedel-Crafts acylation reaction. This is a result of the decomposition of formyl chloride (H(C=O)Cl) into CO and HCl when exposed to these conditions. 
• If the aromatic compound is less reactive than a mono-halobenzene, then it won’t be able to participate in the reaction. 
• Aryl amines cannot be used in Friedel-Crafts acylation reactions as they can only form complexes that are highly unreactive with the Lewis acid catalyst.

---

Things to Remember

• Friedel Crafts Reaction is a part of CBSE class 12 chemistry syllabus 2022.
• It is a part of Unit 11 Organic Chemistry Chapter, Alcohols, Phenols and Ethers and carries a total of 9 periods and 4 to 6 marks. 
• Friedel Crafts Reaction is one of the Electrophilic Substitution reactions.
• This organic coupling reaction involves an electrophilic substitution reaction which is used to attach substituents to aromatic rings. 

Also Read: