friedel-crafts acylation reaction-friedel-crafts acylation of benzene-phenol-anisole and chloro benzene

What is Friedel crafts acylation?

There are few chemical reactions, where benzene or benzenoid compounds react with acyl chloride (RCOCl) in presence of anhydrous aluminium chloride ( AlCl3 ) catalyst, through which the hydrogen atom of benzene  ring be substituted  

by acyl group (RCO–) to form acyl benzene or benzenoid compounds that is aromatic ketone. This reaction is known as Friedel crafts acylation reaction.

Mechanism for Friedel crafts acylation reaction.

This is an example of very important aromatic electrophilic substitution reaction.

In this reaction , acyl chloride ( RCOCl ) is used as acylating agents . 

Besides acyl chloride , acetic anhydride is also used as acylating agent. The mechanism of Friedel crafts acylation reaction of benzene is shown below.

Application of Friedel crafts acylation

( I ) Although poly alkylation of benzene takes place easily yet poly acylation of benzene does not . 

Because acyl group being electron withdrawing group , can attract electron towards itself . As a result , the density of electron of benzene nucleus decreases. 

Since ,acylation of aromatic compound is an example of electrophilic substitution reaction , so further acylation of acyl benzene does not occur.

That is why poly acylation of benzene does not happen .There fore , in Friedel crafts acylation reaction pure aromatic ketone is prepared .

( II) In Friedel crafts acylation reaction, rearrangement reaction of carbon chain of acyl halide takes place.  

Consequently, in that case  no any other isomers ofexpected aromatic ketone is obtained . That is in this case we get the targeted product.

For example, acylation of benzene by propanoyl chloride gives only one product, ethyl phenyl ketone, which on clemension reduction produced  the expected product, propyl benzene .

What is intra molecular Friedel crafts acylation reaction ?

There are very few example of Friedel crafts acylation reaction , which occur in the absence of acylating agents. 

In that case, rearrangement of different part of a specific molecule takes place and give the expected product. 

This type of reaction is called Friedel crafts acylation reaction.

For example, Friedel crafts acylation of 4-phenyl butanoyl chloride in presence of  aluminium chloride ( AlCl3 ) produced  α-tetralone.

Friedel crafts acylation reaction of benzene 

Friedel crafts acylation reaction is an example of aromatic electrophilic substitution reaction.Benzene under goes electrophilic substitution reaction easily . 

Because due to participate in aromatic electrophilic substitution reaction , aromaticity of benzene thatis stability of benzene remain unchanged.

Consequently, benzene  exhibits Friedel crafts acylation reaction.

Few example of Friedel crafts acylation reaction of benzene molecule are shown above.

Friedel crafts acylation reaction of phenol 

Friedel crafts acylation of phenol is an example of aromatic electrophilic substitution reaction. Here electrophilile is acyl group ( RCO– ). Now phenol is an anbidant  nucleophile. 

 The lone pair electron on oxygen atom of phenol can attact the acyl electrophile . 

So, there are two type of Friedel crafts acylation reaction of phenol may be occur ,namely ‘O’-acylation and ‘C’- acylation. 

Therefore ,when phenol react with actyl chloride or acetic anhydride, two different type of reaction is happened.  

When O-acylation takes place, the product is phenyl acetate. Although phenyl acetate under goes fries rearrangement and ultimately convert into o-hydroxy aceto phenone and p-hydroxy aceto phenone.

But when C-acylation is occurred , the products are o-hydroxy aceto phenone and p-hydroxy aceto phenone.

Friedel crafts acylation reaction of anisole 

When anisole is react with acyl halide in presence of anhydrous AlCl₃, The acyl substituted anisole is obtained .  

Due to the + R effect of –OCH₃  group ofanisole the electron density in ortho and para position of benzene ring increases.

Hence , –OCH₃  group of anisole is ortho and para directing. That is electrophile attact on ortho and para carbon atom. 

Consequentlt in this reaction ortho methoxy toluene  and p-methoxy toluene  are obtained .

But para isomer is the main product. Because, in o-methoxy anisole, a steric hindrance is operated between – OCH₃  and – CH₃ group.

Friedel crafts acylation reaction of chloro benzene 

When chloro benzene is react with acyl halide in presence of anhydrous AlCl₃ , The acyl substituted chloro benzene is obtained.

Now,chlorine atom in chloro benzene have + R effect as well  – I effect . But +R effect is greater than – I effect .

 Due to +R effect of –Cl  group of chloro benzene, the electron density in ortho and para position of benzene ring increases . 

Hence , – Cl  group of chloro benzene is ortho and para directing. That is, electrophile attact on ortho and para carbon atom. 

Consequently in this reaction o- chloro alkyl phenyl ketone and p-chloro alkyl phenyl ketone are obtained.

But para isomer is the main product . Because, in o-chloro alkyl phenyl ketone, a steric hindrance is operated between – COCH₃  and – Cl group.



Summary

• What is  friedel-crafts acylation ?
• mechanism for friedel crafts acylation.
• friedel crafts acylation of benzene.
• friedel crafts acylation of anisole.
• friedel crafts acylation of phenol.
• friedel crafts acylation of chloro benzene.
• Application of friedel crafts acylation reaction .
• Intra-molecular friedel crafts acylation reaction.

Friedel-crafts alkylation reaction. Click here.