The Friedel-Crafts Alkylation of Benzene

Friedel-Crafts alkylation substitutes an alkyl group for a hydrogen of benzene. 

The electrophile required for the reaction (a carbocation) is formed from the reaction of an alkyl halide with AICI3. Alkyl chlorides, alkyl bromides, and alkyl iodides can all be used. Vinylic halides and aryl halides caimot be used because their carbocations are 

A base ( B) in the reaction mixture removes the proton from the carbon that formed the bond with the electrophile. 

In Section 19.14, we will see that an alkyl-substituted benzene is more reactive than benzene. Therefore, a large excess of benzene is used in Friedel-Crafts alkylations to prevent the alkyl-substituted product from being alkylated instead of benzene. When benzene is in excess, the electrophile is more likely to encounter a molecule of benzene than a molecule of the more reactive alkyl-substituted benzene. 

We have seen that a carbocation will rearrange if rearrangement leads to a more stable carbocation (Section 6.7). When the carbocation employed in a Friedel-Crafts alkylation reaction rearranges, the major product will be the product with the rearranged alkyl group 

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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. 

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... 

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. 

Alkylbenzene sulfonates (R-C6H5-S03Na) are important surfactant compounds used in laundry detergents. Alkylbenzenes (made by the Friedel-Crafts alkylation of benzene using linear olefin molecules that have about twelve carbon atoms) are sulfonated, and the sulfonic acids are then neutralized with NaOH. 

In Section 5.2.5, we discussed the Friedel-Crafts alkylation of benzene with 2-chloropentane. This reaction includes a Wagner-Meerwein reaction in conjunction with other elementary reactions. The Lewis acid catalyst A1C13 first converts the chloride into the 2-pentyl cation A (Figure 11.3). Cation A then rearranges into the isomeric 3-pentyl cation B, in part or perhaps to the extent that the equilibrium ratio is reached. The new carbenium ion B is not significantly more stable than the original one (A),... 

Problem 18.7 Draw a stepwise mechanism for the Friedel-Crafts alkylation of benzene with CHsCHjCI and AICI3. 

The alkylation of arenes with alkenes such as ethylene and propene are of great commercial interest. Ethylbenzene and isopropylbenzene (cumene), products of the Friedel-Crafts alkylation of benzene with ethylene and propene, respectively, are two of the most important petrochemical raw materials. Roberts and Khalaf have follow the developments made in this vast field up to the early part of this decade. This is evident from the large number of references quoted, most of which describe efforts to evaluate conditions for optimal production in the presence of various catalyst systems. 

In the Xiao s laboratory [27], the pyridinium-based ionic liquids as the suitable media were used for the Friedel-Crafts alkylation of benzene, which was carried out at a better rate but at relatively lower temperature with high product conversion. They found that the [EtPyKCFjCOO] (i-ethyl-pyridinium trifluoroacetatel-FeClj was an enviromnental friendly, recyclable, and reusable solvent-catalyst system to replace the traditional aluminum catalysis system. During the process, the alkylation... 

The Friedel-Crafts alkylation of benzene with isopropyl bromide (8.10) was run by grinding the ingredients in an... 

Ideal kaolinites are ineffective as solid acid catalysts even after thermal and acid activation. The effect of impregnating ZnCU, FeCb, MnCl2, SnCL and AICI3 on the catalytic activity of a natural kaolinite and its activated form is examined. The process leads to catalysts with improved activity, the maximum being associated with FeCla when employed in the Friedel-Crafts alkylation of benzene with benzyl chloride. 1 g of supported FeCL catalyst (1 mmol/g clay) gave 86 mole% conversion to diphenylmethane with 100% selectivity when 2 mole benzerie is alkylated with 0.1 mole benzyl chloride. Supported AICI3 catalysts proved to be the least effective for the alkylation studied. 

The enhancement in catalytic activity of cations such as Zn(II) which has been achieved both through ion exchange as well as deposition of Zn(II) salts onto clay surfaces led to studies of the acidity and catalytic activity of such ions when incorporated directly into the lattice sites of synthetic clay minerals. Luca et al. showed that Lewis acid sites are generated on Zn2+-substituted fluoro-hectorite.27 The Zn2+-substituted fluorohectorite was synthesised by a sol-gel route. The sol was allowed to crystallise in a Parr autoclave at 250 °C for 24 hours. The Lewis acid sites were identified as Zn2+ at the edges of the fluorohectorite crystallites and were active towards the Friedel-Crafts alkylation of benzene with benzyl chloride. 

Conventional Method of Producing Cumene (Fig. 75). In the past, the Friedel -Crafts alkylation of benzene was carried out together with the conversion of the polyalkylated isopropylbenzenes in a single reactor (a). The catalyst was produced directly in the reaction mixture from aluminum chippings and HCl. It formed a second separate organic phase in the reactor. The water present in the propene was simply removed mechanically, whereas the benzene used was dried over. sodium hydroxide. 

Saponites were synthesised at 90°C and 1 atmosphere from a Si/Al-gel and a solution containing urea and M2+-nitrate (M2+= Zn, Mg, Ni and Co) in only a few hours. The products were characterised by XRD, TEM, BET, 27a1- and 29si-MASNMR. Furthermore, the catalytic properties of the synthetic saponites in the Friedel-Crafts alkylation of benzene with propylene to cumene were tested. Incorporation of Zn, Mg, Ni, Co, or a combination of Zn and Mg, in the octahedral layer, as well as controlling the Si/Al-ratio in the tetrahedral layer between 7.9 and 39 could easily be established. The specific surface areas and the pore volumes of the saponites are extremely high, viz., 100-750 m /g and 0.03-0.32 ml/g, respectively. Zn-saponite with A]3+ in the interlayer exhibited a higher catalytic activity as compared to a commercial SPA-catalyst (Solid Phosphoric Acid). 

Not only do the mechanistic details of the Friedel-Crafts alkylation depend on the allg l group (R), but even the gross outcome of the reaction depends on the structure of R. Although the alkyl halides we have mentioned so far all succeed in making the related alkylated benzenes, there are many that do not. Suppose we set out to make propylbenzene by the Friedel-Crafts alkylation of benzene using 1-chloropropane, benzene, and aluminum chloride. As we run this reaction, all looks well at first. The products formed are of the correct molecular weight, but a closer look uncovers a problem. Some propylbenzene is formed, but the major product is isopropylbenzene (Fig. 14.37). 

From Cumene Hydroperoxide. This process illustrates industrial chemistry at its best. Overall, it is a method for converting two relatively inexpensive organic compounds— benzene and propene—into two more valuable ones—phenol and acetone. The only other substance consumed in the process is oxygen from air. Most of the worldwide production of phenol is now based on this method. The synthesis begins with the Friedel-Crafts alkylation of benzene with propene to produce cumene (isopropylbenzene) ... 

In 1877, Friedel and Crafts discovered that a haloalkane reacts with benzene in the presence of an aluminum halide. The resulting products are the alkylbenzene and hydrogen halide. This reaction, which can be carried ont in the presence of other Lewis acid catalysts, is called the Friedel-Crafts alkylation of benzene. 

Write the structure of the product resulting from the Friedel-Crafts alkylation of benzene using chlorocyclohexane and aluminum trichloride. 

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