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Friedel Crafts reactions are a type of reactions which is meant for the attachment of substituents on the aromatic ring. Earlier in 1887 Charles Friedel and James Mason Crafts was the first to obtain amylbenzene from a reaction of amyl chloride with AlCl 3 in presence of benzene. There are basically two varieties of friedel crafts reaction alkylation and acylation. Both of these reactions come under electrophilic aromatic substitution. In case of alkylation, there is alkylation of an aromatic ring with an alkyl halide and the process is catalysed via strong Lewis acids like- Aluminium chloride, ferric chloride. Both friedel crafts alkylation and acylation mainly involve the replacement of a hydrogen atom with an electrophile. The main catalyst which is mostly used in friedel crafts reactions is Aluminium trichloride(AlCl3) as it helps in an easy generation of an electrophile during the process.
Friedel Crafts Alkylation
This type of reaction involves the substitution of an aromatic proton with an alkyl group. This step is made possible via an electrophilic attack on the aromatic ring along with the formation of a carbocation. By using alkyl halides as a reactant we can generate alkylbenzene via friedel crafts alkylation reaction.
In order to obtain a carbocation, a Lewis acid like FeCl3 or AlCl3 is used for the removal of the halide ion. Carbocation rearrangement precedes alkylation reaction. It is a form of coupling reaction. In presence of protons many different types of alkylating agents can be used in place of alkyl halides like enones or epoxide. The major disadvantage of this reaction is that the product which is formed in this reaction is more nucleophilic than the reactant. This is possible because of the alkyl group which acts as an activator of friedel crafts alkylation reaction. Steric hindrance helps in limiting the number of alkylations as observed in t-butylation of 1,4 dimethoxybutane. These types of reactions are suitable mainly for primary alkyl halides during the formation of a 5-6 membered ring. In primary alkyl halides the carbocation complex like (R(+)—X—Al(-)Cl3)undergoes carbocation rearrangement reactions in order to give a completely organised product which is derived from secondary or tertiary carbocation.
Mechanism of Alkylation
Step 1:-
During this step a carbocation is formed which acts as an electrophile in the reaction. This carbocation is formed by the secondary and tertiary halides. This step helps in the activation of haloalkanes.
Step 2:-
In this step an electron pair of the aromatic ring attacks the carbocation thereby producing a new C-C bond. The arenium ion intermediate is the result of stabilization from multiple resonance forms. The loss of a proton leads to the formation of a neutral alkylated substitution product.
Step 3:-
Reactivity of haloalkanes increases while moving up a periodic table. Thus, RF refers to a most reactive haloalkane followed by RCl, RBr and RI.
Limitations of Alkylations
The halide ions can only be alkyl halides. Vinyl or Aryl halides do not react with the carbocation.
Alkylation reactions always include carbocation rearrangement.
The deactivated benzenes cannot be reactivated via Friedel Crafts reactions, the benzene should be always more reactive than a mono -halobenzene.
AlCl3 helps in making aryl amines more reactive.
Friedel Crafts Acylation
This reaction includes addition of an acyl group to an aromatic ring. Acylation is accompanied by an acid chloride (R-(C=O)-Cl) and catalyst AlCl3 . These reactions help in the transformation of the aromatic ring to a ketone group. Alternatively acid anhydrides can also be used in friedel crafts acylation reaction. When we use an alcohol or an amine then oxygen and nitrogen atoms are acylated. Main difference between acylation and alkylation reactions is the formation of a ketone group.
Mechanism of Acylation
Step 1:-
During this step an acylium ion is formed via combination of anhydrous aluminium chloride with acyl halide. This acylium ion is a resonance stabilised structure.
Step 2:-
The interaction of a Lewis acid and acyl halides helps the acylium ions to attack the aromatic ring. This leads to the breakdown of carbon-carbon double bonds and formation of an intermediate cation.
Step 3:-
There is a loss of one proton from the intermediate complex, this is also known as deprotonation. The aromaticity of the ring is restored, after the reformation of carbon-carbon double bonds present on the aromatic ring. Proton again releases the catalyst aluminium chloride during deprotonation.
Step 4:-
The carbonyl group was attacked via AlCl3 which was released upon proton addition. Ketone product was released during excess water.
Limitations of Acylation
These types of reactions mainly form ketones.
In this reaction, aromatic compounds that are less reactive than mono halobenzene, are not used.
Alongwith Lewis acid catalyst aryl amines can only form unreactive complexes which is also not used in the reaction.
Conclusion
The friedel crafts alkylation helps in the synthesis of polyalkylated products. These acylated products can be converted into alkanes via Clemmensen Reduction or Wolf Kishner reaction. Friedel Crafts Alkylation of Biphenyl via electrophilic substitution of a Lewis base in presence of Ferric chloride as a catalyst leads to the formation of 4,4 di-tert-butylbiphenyl. We hope that through this article you were able to get a clear concept of Friedel Crafts reactions, Alkylation and Acylation reactions and many more things.
