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

What is Friedel crafts alkylation reaction ?

When hydrogen atom of benzene ring be substituted by the alkyl group in presence of anhydrous aluminium chloride ( AlCl₃ ) catalyst to produced alkyl benzene, then the reaction is is Friedel crafts alkylation reaction. 

It is an example of aromatic electrophilic substitution reaction.

Friedel crafts alkylation reaction is a very important reaction for the preparation of alkyl benzene or many other aromatic compound with alkyl group.

Catalysts used in Friedel crafts alkylation reaction :

Generally, the best catalyst used in Friedel crafts alkylation reaction is anhydrous Lewis acid aluminium chloride ( AlCl₃ ) .

Besides, another Lewis acid like boron trifluoride ( BF₃ ) , anhydrous ferric chloride ( FeCl₃ ) , anhydrous zinc chloride ( ZnCl₂ ) is also used in Friedel crafts alkylation reaction . 

Few proton acid , like HF , H₂SO₄ H₃PO₄ etc are also used as a catalyst in Friedel crafts alkylation reaction .

Mechanism of Friedel crafts alkylation reaction: 

In Friedel crafts alkylation reaction , the effective electrophile is R⁺ . Secondary or tertiary alkyl halide can form carbocation with  anhydrous aluminium chloride ( AlCl₃ ) ,but R⁺ carbocation does not form by the reaction of 1ᵒ or methyl halide with AlCl₃.

Because, 1ᵒ or primary carbo cation is very much less stable . In that case the primary complex obtained by the reaction of RX and  AlCl₃ is used as electrophile .

In Friedel crafts alkylation reaction , besides alkyl halides , alcohol, alkene  are used as alkylating reagent.  

In that case , anhydrous AlCl3 and also BF₃  , HF or H₂SO₄ are used as catalys.

Friedel crafts alkylation reaction of benzene

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

Friedel crafts alkylation reaction of benzene is an example of electrophilic substitution reaction. That is the hydrogen atom of benzene be substituted by electrophile . 

Here alkyl group act as an electrophile. The reaction mechanism of Friedel crafts alkylation reaction of benzene is shown above . 

But few example of Friedel crafts alkylation reaction of benzene are given below.

Friedel crafts alkylation reaction of phenol

When phenol is react with alkyl halide in presence of anhydrous AlCl₃ , The alkyl substituted phenol is obtained .  

The –OH  group of phenol is ortho and para directing .Because –OH  group of phenol  increase the electron  density on ortho and para carbon due its +R effect . 

So the electrophile attact in ortho and para position and hence the products obtained are  ortho cresol and p-cresol . 

But para isomer is the main product . Because, in ortho cresol , a steric hindrance is operated between  – OH  and – CH₃ group .

Friedel crafts alkylation reaction of anisole 

When anisole is react with alkyl halide in presence of anhydrous AlCl₃ , The alkyl substituted anisole is obtained . Due to +R effect of –OCH₃  group ,the density of electron on ortho and para carbon increases. 

Hence , –OCH₃  group of anisole  is ortho and para directing , so 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 alkylation reaction of chloro benzene 

When chloro benzene is react with alkyl halide in presence of anhydrous AlCl₃ , The alkyl substituted chloro benzene is obtained .Chlorine atom have +R effect and also -I effect .

 But +R effect is more effective than - I effect . So due to +R effect, the density of electron on ortho and para carbon atom increases.  

Hence , –Cl  group of chloro benzene  is ortho and para directing . So the electrophile attact on ortho and para carbon  to give product ortho chloro toluene  and p-chloro toluene . 

But para isomer is the main product . Because, in o-chloro toluene , a steric hindrance is operated between  – Cl  and – CH₃ group.

Limitation of Friedel crafts alkylation reaction 

(I)Friedel crafts alkylation reaction is not eligible for the preparation of           alkyl benzene . Because , the alkyl group is a activating group . 

They can increase the density of electron of benzene ring  due to their +I effect .

As a result , benzene ring becomes more reactive and further under goes Friedel crafts alkylation reaction . Hence in that case, poly alkylation reaction takes place . 

(II) Friedel crafts alkylation reaction is not suitable for the preparation of long chain alkyl benzene .

For example, if we use the 1ᵒ alkyl halide with three or more carbon atom , then the expected carbocation is obtained , under goes rearrangement reaction and it change into more stable 2ᵒ or 3ᵒ carbocation .

As a result , the main alkyl benzene obtained  must be contain 2ᵒ or 3ᵒ alkyl group.

( III) Friedel crafts alkylation reaction is an electrophilic substitution reaction . 

Therefore , the benzene ring with strong electron withdrawing group that is with deactivating group , does not under go Friedel crafts alkylation reaction . 

Because, due to lack of electron density ,the benzene ring have no affinity toward weak electrophile ( R⁺) .

For the above reason , C₆H₅NO₂ , C₆H₅ COOH , C₆H₅ COR , C₆H₅N⁺( CH₃ )₃ compounds does not exhibit Friedel crafts alkylation reaction.

(iv) Vinyl halide or halo benzene [ vinyl chloride , chloro benzene ] are not used as alkylating reagent in Friedel crafts alkylation reaction .



Summary:

What is Friedel-crafts alkylation reaction?

Friedel crafts alkylation of benzene

Friedel crafts alkylation of phenol

Friedel crafts alkylation of anisole

Friedel crafts alkylation of chloro benzene

Limitation of Friedel crafts alkylation.

Mechanism for Friedel crafts alkylation.

Why picric acid is more acidic than phenol ? click here.