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Friedel Crafts Alkylation

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Introduction of an alkyl group onto an aromatic substrate by treating the substrate with an alkylating agent such as alkyl halide, alkene, alkyne and alcohol in the presence of a Lewis acid. In 1877, Charles Friedal and James Crafts reported that benzene and 2-chloro-propane reacted together in the presence of AlCl3 to form 2-phenylpropane. This type of reaction is now generally known as Friedal-Crafts alkylation.

The Friedal crafts alkylation of phenols gives ortho and para alkylphenols, the regioselectivity being dependent on the catalyst used. The alkylations can be initiated by a wide variety of substrates, such as alcohols, alkyl halides and alkenes.

Reaction

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Carbocations are perhaps the most important electrophiles capable of substituting onto aromatic rings, because this substitution forms a new carbon-carbon bond. Reactions of carbocations with aromatic compounds were first studied in 1877 by the French alkaloid chemist Charles Friedal and his American partner, James Crafts. In the presence of Lewis acid catalysts such as aluminum chloride (AlCl3) or ferric chloride (FeCl3), alkyl halides were found to alkylate benzene to give alkyl benzenes. This useful reaction is called the Friedal-Crafts alkylation.

Friedal Crafts Alkylation

For example, aluminum chloride catalyzes the alkylation of benzene by t-butyl chloride. HCl gas is evolved. 

Friedal Crafts Alkylation Example

This alkylation is a typical electrophilic aromatic substitution, with the t-butyl cation acting as the electrophile. Friedal crafts alkylations are used with a wide variety of primary, secondary and tertiary alkyl halides.

Mechanism

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Among the most useful electrophilic aromatic substitution reactions in the laboratory is alkylation - the introduction os an alkyl group onto the benzene ring. The reaction is carried out by treating the aromatic compound with an alkyl chloride, RCl in the presence of AlCl3 to generate a carbocation electrophile. Aluminum chloride catalyzes the reaction by helping the alkyl halide to dissociate in much the same way that FeBr3 catalyzes aromatic brominations by polarizing Br2. Loss of H+ then completes the reaction. 

Friedal Crafts Alkylation Mechanism

Friedal Crafts Alkylation of Benzene

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Reaction of benzene with chloro or bromoalkanes (RCl or RBr) and a Lewis acid catalyst produces alkylbenzenes in Friedel-crafts alkylation reaction. The mechanism of alkylation follows the general mechanism for electropilic susbtitution reactions of benzene, but includes an initial step in which the chloro or bromoalkane reacts with the Lewis acid to form a strong electrophile.
  • The primary chloroalkane reacts with AlCl3 to form a coordination complex. By forming a coordination complex with the Lewis acid, the primary chloroalkane is converted into a stronger electrophile because the electrons in the C-Cl bond of the coordination complex is strongly attracted to the positively charged chlorine.
  • Benzene attacks the partially positive carbon atom in the complex to produce a nonaromatic carbocation together with [AlCl4]-.
  • [AlCl4]- removes a proton from the nonaromatic carbocation to give an alkylbenzene together with HCl and AlCl3. Since AlCl3 is generated, it acts as a catalyst for alkylation of benzene. 

Friedal Crafts Alkylation of Benzene

Limitations 

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Several limitation occur with the Friedal Crafts alkylation reaction. 
  1. First, the alkyl group that is added to the ring is an activating group. This causes the alkylated product to be more reactive than the starting aromatic compound. Therefore a significant amount of product where two or more alkyl groups have been added is commonly formed.
  2. A second limitation is that aromatic compounds substituted with moderately or strongly deactivating group cannot be alkylated. The deactivated ring is just too poor a nucleophile to react with the unstable carbocation electrophile before other reactions occur that destroy it.
  3. The final limitation is one that plagues all carbocation reactions: rearrangements. Because the aromatic compound is a weak nucleophile, the cabocation has a lifetime that is longer than in the case in most of the other reactions invovling this intermediate, allowing ample time for rearrangements to occur.