Understanding Friedel Crafts Halogenation

Friedel crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. These usually are of two types, namely Friedel crafts alkylation and acylation, where alkyl and an acyl group, respectively, are added to benzene rings. These reactions usually proceed by electrophilic aromatic substitution mechanisms and occur at negligible/no rate without the presence of a Lewis acid.  

What Is Friedel Crafts Reaction

 The Friedel crafts reactions are essentially used to refer to a set of reactions developed by the two chemists, Charles Friedel and James Crafts, in 1877. This is a reaction well studied in organic chemistry in order to substitute/add certain functional groups in the molecule. These reactions proceed by electrophilic aromatic substitution mechanism.  

Friedel Crafts Acylation And Alkylation

Friedel crafts reactions are of two types, alkylation and acylation, based on the functional group that is to be added. 

In the Friedel crafts alkylation reaction, an alkyl group (R-X) is added to the aromatic ring, whereas in the Friedel crafts acylation, an acyl group (R-CO-X) is added to the aromatic ring. 

In the Friedel crafts reactions, a catalyst is required to be added for the reaction to happen at a considerable rate.

For Friedel crafts alkylation, the alkylating agents usually are alkyl halides, as shown in the reaction above. However, there are many other alkylating agents which can be used, like enones and epoxides, in the presence of protons. Nevertheless, the important point to remember about Friedel crafts alkylation reaction is that there occurs an additional reaction at the benzene ring, with the help of R-X and a Lewis acid like aluminium chloride. R-X is an alkyl halide like alkyl chloride, bromide or iodides. The Lewis acid can be aluminium chloride, AlCl3, or some unconventional ones like FeCl3 and ZnCl2. In the Friedel crafts alkylation reactions, carbocation rearrangements can occur. 

Friedel crafts acylation involves the addition of an acyl group at the benzene ring. Common acylating agents include acyl chlorides, although acid anhydrides and carboxylic acids can also be used. The Lewis acid catalyst usually provided is aluminium trichloride, and because the ketone product forms a stable complex with such lewis acids, a significant stoichiometric amount of the lewis acid must be employed, unlike the Friedel crafts alkylation reaction where the catalyst is constantly regenerated. 

Friedel crafts acylation has many advantages over the alkylation reaction. 

1. Multiple acylations do not occur because the ketone group is often less reactive than the original molecule due to the electron-withdrawing effect of the carbonyl group. 

2. There are no carbocation rearrangements because the acylium ion R-C≡O(+) is stabilised by resonance.  

Role of Lewis Acid:

Lewis acid basically works to activate the electrophile by coordinating with the leaving group, making the leaving group a better base and a better leaving group. In the end, the result is that the coordination of the Lewis acid to the electrophile makes the species a better electrophile.

For example, if the alkyl halide is isopropyl chloride and a lewis acid-like AlCl3 is used, the lewis acid will form a coordination complex with chloride, weakening the C-Cl bond and chlorine departs. We thus get a secondary carbocation, a better electrophile than isopropyl chloride itself. It has been seen that no reaction occurs in the absence of a lewis acid.  

Conclusion 

In conclusion, it can be said that the basic reactions in organic chemistry are very important since they drive the synthetic downstream processes of various other compounds. Friedel crafts reaction is one such basic reaction wherein alkyl halides and acyl halides are added to aromatic rings in the presence of Lewis acids to give acyl halides. These reactions follow the electrophilic substitution mechanism.