Friedel Crafts Reaction

In 1877, Charles Friedel and James Craft developed coupling reactions to attach substituent groups to aromatic rings. These reactions are known as Friedel craft reactions. 

Charles Friedel

James Craft 

Friedel craft reaction can be divided into following two types- 

1. Friedel craft alkylation reaction 

2. Friedel craft acylation reaction 

 Both types of reacAcetyl Chloride   Benzene tions, Friedel craft alkylation and Friedel craft acylation involve electrophilic aromatic substitution. 

Friedel crafts alkylation reaction – The reaction involves Alkylation of aromatic ring by alkyl halide in presence of lewis acid is called Friedel craft alkylation reaction. In these reactions lewis acids such as aluminum chloride and iron chloride are used. 

Friedel craft alkylation reaction can be represented in short form as follows- 

Aromatic ring + Alkyl halide Lewis acid→ Alkyl aromatic compound 

                                                                      (Alkyl benzene)


Friedel crafts alkylation of Benzene - On treating benzene with alkyl halide, in presence of lewis acid such as anhydrous aluminum chloride, it forms alkyl benzene. This reaction is known as Friedel craft alkylation reaction.

Alkylation of Benzene Reaction – 

Friedel crafts alkylation reaction mechanism – 

It takes place in 3 steps – 

Step 1. 

Formation of an electrophile (methyl carbonium) – 

Step 2. 

Electrophile (methyl carbonium) attacks on benzene (aromatic compound) and forms a cationic complex as an intermediate which is stabilized by resonance. 

Step 3. 

Removal of H+ ion or proton takes place from the complex formed above in step 2 and formation of alkyl aromatic compound (toluene) takes place through electrophilic substitution. 


Limitations of Friedel crafts alkylation reaction – 

1. Reaction doesn’t take place if benzene has a substituent group that is more deactivating than halogens. 

2. Aryl and vinyl halides can’t be used in this reaction as their carbocations are very reactive and highly unstable. 

3. As alkyl benzene is more reactive than benzene, so polyalkylation takes place. 

Friedel Crafts acylation reaction – The reaction in which addition of acyl group takes place in aromatic compound in presence of lewis acid, is called Friedel crafts acylation reaction. 

Friedel craft acylation reaction can be represented in short form as follows-

Aromatic ring + RCOX Lewis acid → Acyl aromatic compound


Friedel crafts acylation of Benzene - On treating benzene with an acyl halide, in presence of lewis acid, it forms acyl benzene. This is known as Friedel craft’s acylation reaction. 

Acylation of Benzene Reaction – 

              Acetylphenol        Hydrochloric acid


Friedel crafts acylation reaction mechanism – 

Step 1. In this step formation of electrophile takes place. 

Step 2. 

Electrophile attacks on benzene ring (aromatic compounds) and forms a cationic complex as an intermediate which is stabilized by resonance. 

Step 3. 

Positively charged complex formed in step 2 reacts with AlCl4- and forms an acyl aromatic compound. 

     


Limitations of Friedel crafts acylation reactions – 

1. Aryl amines form highly unreactive complexes with lewis acid catalysts so we can’t use them in this reaction. 

2. The aromatic compound, which is less reactive than mono halobenzene, cannot be used in this reaction.

3. Acylation reactions generally form only ketones.