Friedel Crafts Reaction

The Friedel-Crafts reaction is an aromatic chemical reaction that undergoes an electrophilic aromatic substitution. 

Two scientists, French Charles Friedel and American James Crafts invented this well-known reaction. The aromatic compound undergoes an electrophilic substitution in the Friedel-Crafts reaction. 

In the presence of a Lewis acid, such as anhydrous aluminium chloride, the hydrogen atom in benzene is swapped with an electrophile.

The two forms of Friedel-Crafts Reactions are-

• Friedel-Crafts Alkylation

• Friedel-Crafts Acylation 

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

Steps and Limitations

• Friedel-Crafts Alkylation

Friedel-Crafts Alkylation is a chemical reaction in which an aromatic compound's proton is substituted with an alkyl group. In the presence of anhydrous aluminium chloride, this reaction takes place. Other Lewis acids, such as Ferric chloride, can be used in place of anhydrous aluminium chloride.

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

Aromatic ring + Alkyl halide  Lewis acid→  Alkyl aromatic compound 

                                                                      (Alkylbenzene)

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

Steps:

The steps below illustrate the mechanism of the Friedel-Crafts Alkylation process.

1. The formation of an electrophilic carbocation is the first step.

The alkyl halide reacts with the Lewis acid used for the process, which is either anhydrous aluminium chloride or ferric chloride. As a result, an electrophilic carbocation is produced.

2. Intermediate cation formation

The aromatic ring is attacked by the electrophilic carbocation generated by the interaction of Lewis acid and alkyl halide. When it hits the aromatic ring, it forms a cyclohexadienyl cation intermediate. Because the carbon-carbon double bond breaks, the aromatic ring loses its scent.

3. Alkyl halide formation

Deprotonation occurs when the cyclohexadienyl cation loses one proton. The aromatic ring's carbon-carbon double bond is reformed, restoring the ring's aromaticity as well. The aluminium chloride catalyst is regenerated by the proton released during deprotonation.

Limitations

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

2. Deactivating groups in aromatic rings may not be suitable for Friedel-Crafts alkylation. Because the deactivating group can create a compound with the Lewis acid, inactivating it. For example, aniline's amine group deactivates anhydrous aluminium chloride. It's a half-reaction

3. Polyalkylation occurs commonly in alkyl halide and aromatic chemical reactions. To avoid this, take the aromatic sample in huge quantities.

4. Since mono halobenzenes are least reactive, they do not respond or participate in Friedel-Crafts alkylation.

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

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

• Friedel-Crafts Acylation

The acylation reaction of Friedel-Crafts is analogous to the alkylation reaction. The only difference is that, unlike the alkylation reaction, the Friedel-Crafts acylation reaction produces a ketone.

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

Aromatic ring + RCOX  Lewis acid →  Acyl aromatic compound

Various aromatic compounds can be acylated using the Friedel-Crafts procedure. When the reactant is an alcohol or an amine, the oxygen and nitrogen atoms are acylated.

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. 

Steps:

The steps below illustrate the mechanism of the Friedel-Crafts Acylation process:

1. The formation of the acylium ion is the first step.

An acylium ion is formed when anhydrous aluminium chloride combines with an acyl halide. Resonance stabilises the acylium ion that results.

2. Intermediate cation formation

The aromatic ring is attacked by the acylium ion generated by the interaction of Lewis acid and acyl halide. An intermediate is generated when it attacks the aromatic ring. Because the carbon-carbon double bond breaks, the aromatic ring loses its scent.

3. Intermediate complex deprotonation

Deprotonation occurs in the intermediate complex, which means it loses one proton. The aromatic ring's carbon-carbon double bond is reformed, restoring the ring's aromaticity as well. The aluminium chloride catalyst is regenerated by the proton released during deprotonation.

4. The ketone molecule is released.

The carbonyl oxygen is attacked by anhydrous aluminium chloride that has been regenerated by proton addition. The ketone product is produced and released in the presence of excess water.

Limitations

1. The Friedel-Crafts process produces exclusively ketone compounds. Formyl chloride breaks into HCl and CO2 under certain conditions (CO).

2. Since mono halobenzenes are the least reactive, they do not respond or participate in Friedel-Crafts acylation reaction. 

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

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

5. Acylation reactions generally form only ketones.