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French chemist, Charles Friedel, and his American collaborator James M. Crafts, discovered new methods for the preparation of alkylbenzenes (ArR) and acylbenzenes (ArCOR). These reactions are now called the Friedel - Crafts alkylation and acylation reactions.
A general equation for a Friedel–Crafts alkylation reaction is the following:
The mechanism for the reaction (shown in following steps with isopropyl chloride as R–X) starts with the formation of a carbocation (step 1).
The carbocation then acts as an electrophile (step 2) and attacks the benzene ring to form an arenium ion. The arenium ion (step 3) then loses a proton to generate isopropyl benzene.
Reaction Mechanism of Friedel-Crafts alkylation involve formation of carbocation by the help of lewis acid. The following reaction mechanims of Friedel-Crafts alkylation explain for isopropyl chloride.
When R–X is a primary halide, a simple carbocation probably does not form. Rather, the aluminum chloride forms a complex with the alkyl halide and this complex acts as the electrophile. The complex is one in which the carbon-halogen bond in nearly broken–and one in which the carbon atom has a considerable positive charge.
RCH2 ---- Cl : AlCl3
Even though this complex is not a simple carbocation, it acts as an electrophilic it transfers a positive alkyl groups to the aromatic ring. These complexes are so carbocation like that they also undergo typical carbocation rearrangements.
The order of reactivity of some catalysts is AlBr3 > AlCl3 > FeCl3 > SbCl5 > SnCl4 > BCl3 > BF3 etc. The alkylating agents may be alkyl halides, alcohols or alkenes. For more reactive halides, a less active catalyst (e.g. ZnCl2) and for less reactive halides, a highly active catalyst (e.g. AlCl3) is used. Commonly used solvents are nitrobenzene, ether, carbon disulphide etc.
(i)Polyalkylation: As we know alkyl group activates the ring towards electrophilic substitution, it is clear that product of Friedel-Crafts alkyation will be more reactive and can undergo further alkylation. So, we get monoalkylated as well as polyalkylated products. However, polyalkylation may be minimized by taking aromatic compound in large excess.
(ii)Rearrangement: As we know, carbocations can undergo rearrangements easily to more stable carbocations, sometime the carbocation produced during Friedel-Crafts alkylation undergo rearrangement before attack on aromatic ring. This leads to the formation of product, substituted by rearranged carbocation.
(iii)Effect of already present substituent on Aromatic Rings: Electron-withdrawing substituents inhibit the reaction while highly activating groups (like - OH, -NH2, -NHR) reacts with the catalyst to retard the reaction. Hence aromatic compounds having strong activating and strong deactivating groups give poor yield. Naphthalene and heterocylic compounds are also very reactive and react with the catalyst, thereby giving very poor yield of substituted product.
(iv)Isomerisation and Disproportionation: In presence of excess catalyst and at high temperature, isomerisation and disproportionation can also take place.
(v) Aryl and vinylic halides cannot be used as the halide component because they do not form carbocations readily.
List of Name Reaction of Organic Chemistry consist of detail Reaction Mechanism of all name reactions of Organic Chemistry.