Courses
Courses for Kids
Free study material
Offline Centres
More
Store Icon
Store

Friedel Crafts Reaction

ffImage
Last updated date: 29th Mar 2024
Total views: 443.4k
Views today: 6.43k
hightlight icon
highlight icon
highlight icon
share icon
copy icon

Friedel Crafts Reaction and its Types

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.

  1. 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.

  1. 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.

  1. 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.

  1. 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.

  1. 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. 

FAQs on Friedel Crafts Reaction

1. Is it possible to reverse the Friedel-Crafts alkylation of benzene?

Friedel-Crafts alkylation of benzene is a reversible reaction that can occur. A benzene molecule is alkylated using the Friedel Crafts method, which involves electrophilic aromatic substitution to attach the alkyl group to the benzene molecule. These reactions can be reversible if an excessive amount of Halo acid is produced. 

2. Friedel Crafts Acylation Is Preferable to Friedel Crafts Alkylation for what reason?

Friedel and crafts observed that in the presence of an aluminium halide, a haloalkane interacts with benzene in 1877.


Friedel Crafts acylation is favoured over Friedel Crafts alkylation because there is oxygen with a double bond along with R in Friedel Crafts acylation, resulting in a drop in electron density. In Fridel craft alkylation, however, there is a +I effect that causes the electron density on the benzene to grow.

3. Choose the substance that does not produce the Friedel Craft reaction from the list below:

Toluene

Xylene

Cumene

Nitrobenzene

Nitrobenzene is the right answer.


It's hard for Nitrobenzene to go through the Friedel Craft reaction because of the presence of the nitro group, which is a strong electron-puller. 


As the electrons from the nitrogen are taken away by the oxygens, the benzene ring is no longer benzene. For the Friedel Craft reaction to happen, the electrophile that is coming in must be attracted to it.

4. What is the role of a Lewis acid in Friedel Crafts acylation?

The Friedel-Crafts acylation reaction is one of the most widely used reactions in aromatic chemistry, and it is also utilised in the synthesis of aryl ketones. 


The use of stoichiometric amounts of Lewis acid leads to the development of a complex between the aryl ketone produced and the Lewis acid at the end of the reaction after the reaction is completed.

5. What is the difference between an acyl group and an alkyl group?

When comparing alkyl and acyl groups, the most significant difference is that the acyl group has an oxygen atom linked to the carbon atom through a double bond, whereas the alkyl group does not have an oxygen atom coupled to the carbon atoms at all. 


When it comes to organic chemistry, the acyl group (IUPAC name: alkanoyl) is most typically produced from a carboxylic acid.