Friedel-crafts alkylation reaction-examples 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 .
