Friedel Craft Alkylation of Aromatic Rings
This reaction is discovered by the two Scientists ” Charles Friedel & James Craft ” in 1877.
This Reaction is used to formation of the Alkyl benzenes from the alkyl halides with the help of Metal Chlorides. Friedel Craft Alkylation is a most difficult aromatic Electrophillic Substitution reaction to carried out on Laboratory.
Before seeing Reaction Mechanism, you must be known with some other phenomena.
Lewis Acid :-
Lewis acid is substance which is ready to accept an pair of electron i.e Aluminium Chloride ( AlCl₃)
Resonance Structure :-
• Usually all aromatic compounds has resonance (Delocalization of pi electrons).
In this reaction we see with Benzene, So Benzene creates carbocation in their Ortho and Para position only. So Alkylation takes place only on Ortho and Para Position.
• We can alkylate on Meta Position also but extra supply of energy is required.
General Reaction :
C₆H₆ + RX + AlCl₃ → C₆H₅CH₃ + AlCl₃ + HCl
Reaction Mechanism :-
Friedel Craft Alkylation :-
Formation of Electrophile :-
This the initial step in the mechanism. In this the Alkyl halide is treated with the Lewis acid (Aluminium Chloride) AlCl₃
By the end it will forms the Electrophile
Reaction Between Aromatic Ring and Electrophile :-
In this process, formed Electrophile is treated with the Aromatic compound like Benzene.
It will form the resonance structure due to the delocalizing of pi electrons present in Aromatic ring
Deprotonation :- ( Final Product )
In this process, the proton present nearby to the substituted Alkyl group is leaving.
It will end in reformation of Catalyst (Lewis acid) and Acid.
Limitations of Friedel Craft Alkylation :-
• Carbocation Rearrangement :
Whenever we try to add Carbon chain containing more than 2 carbon atoms, it will rearrange themselves Iso – structures
• Ring with Deactivating Species :
Friedel Craft Alkylation fails when used with compounds like nitrobenzene and other any strong deactivating species.
Because nitrobenzene has an electron withdrawing nature that deactivates the ring towards another Electrophile Substitution.
• Polyalkylation :
Friedel Craft Alkylation can undergo Polyalkylation until the Reacting Alkyl group finish their quantity.
The Alkylation is possible in Ortho and Para Position of Benzene, Because of the resonance structure of benzene.
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