Friedel-Crafts alkylation of benzene

Alkyl hahdes react with benzene in the presence of aluminum chloride to yield alkyl-benzenes. 

Step 1 The bromine-iron(III) bromide complex is the active electrophile that attacks benzene. 

Two of the n electrons of benzene are used to form a bond to bromine and give a cyclohexadienyl cation intermediate. 

Alkylation of benzene with alkyl halides in the presence of aluminum chloride was discovered by Charles Friedel and James M. Crafts in 1877. Crafts, who later became president of the Massachusetts Instimte of Technology, collaborated with Friedel at the Sor-bonne in Paris, and together they developed what we now call the Friedel-Crafts reaction into one of the most useful synthetic methods in organic chemistry. 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene. Aluminum chloride serves as a Lewis acid catalyst to enhance the elec-trophilicity of the alkylating agent. With tertiary and secondary alkyl halides, the addition of aluminum chloride leads to the formation of carbocations, which then attack the aromatic ring. 

Step 2 Loss of a proton from the cyclohexadienyl cation yields bromobenzene. 

Mechanism 12.4 illustrates the reaction of benzene with tert-h xiy cation (step 1) followed by formation of r butylbenzene by abstraction of a proton from the cyclohexa-dienyl cation intermediate (step 2). 

Secondary alkyl halides react by a similar mechanism involving a secondary carbocation. Methyl and ethyl halides do not form carbocations when treated with aluminum chloride, but do alkylate benzene under Friedel-Crafts conditions. The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds, and these complexes are the electrophilic species that react with benzene. 

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One drawback to Fnedel-Crafts alkylation is that rearrangements can occur espe cially when primary alkyl halides are used For example Friedel-Crafts alkylation of benzene with isobutyl chloride (a primary alkyl halide) yields only tert butylbenzene... 

Isopropylbenzene is prepared by the Friedel-Crafts alkylation of benzene y using isopropyl chloride and aluminum chloride (Section 12 6) j... 

Catalysts used in the polymerization of C-5 diolefins and olefins, and monovinyl aromatic monomers, foUow closely with the systems used in the synthesis of aHphatic resins. Typical catalyst systems are AlCl, AIBr., AlCl —HCl—o-xylene complexes and sludges obtained from the Friedel-Crafts alkylation of benzene. Boron trifluoride and its complexes, as weU as TiCl and SnCl, have been found to result in lower yields and higher oligomer content in C-5 and aromatic modified C-5 polymerizations. 

Cumene. Cumene (qv) is produced by Friedel-Crafts alkylation of benzene by propylene (103,104). The main appHcation of cumene is the production of phenol (qv) and by-product acetone (qv). Minor amounts are used in gasoline blending (105). 

Because acylation of an aromatic ring can be accomplished without reanangement, it is frequently used as the first step in a procedure for the alkylation of aromatic compounds by acylation-reduction. As we saw in Section 12.6, Friedel-Crafts alkylation of benzene with primary alkyl halides nonrrally yields products having rearranged alkyl groups as substituents. When a compound of the type ArCFl2R is desued, a two-step sequence is used in which the first step is a Friedel-Crafts acylation. 

Vinylchlorosilanes react with aromatic compounds in the presence of Lewis acid to give the alkylation products 2-(chlorosilyl)ethylarenes. In the Friedel-Crafts alkylation of aromatic compounds, the reactivity of vinylchlorosilanes is slightly lower than that of allylchlorosilanes.Friedel-Crafts alkylation of benzene derivatives with vinylsilanes to give 2-(chlorosilyl)ethylarenes was first reported by the Andrianov group (Eq. (5))." The reactivity of vinylsilanes in the... 

Reaction type (d) also complicates the Friedel-Crafts alkylation of benzene (a type c/b reaction, p. 141) by 1-bromopropane, MeCH2-CH2Br, in the presence of gallium bromide, GaBr3, as Lewis acid... 

Friedel-Crafts alkylation of benzene or toluene by allyl chloride in presence of ethylaluminium chlorides is vigorous even at — 70°C, and explosions have occurred. See Lewis acids, etc., next below... 

The Friedel-Crafts alkylation of benzene with l-chloropro-pane in the presence of aluminium trichloride gives, after chromatography, the isomeric products A (76%) and B (24%). Predict the products of the fragmentation of both A and B and, by comparing them with the mass spectra given in Figures 5.22 and 5.23, deduce which is the major. A, and the minor, B, product of this reaction. 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

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