Hydrocarbons Friedel-Crafts alkylation

Aluminium chloride Resin manufacture by polymerization of low molecular-weight hydrocarbons Friedel-Crafts reactions to manufacture detergent alkylate, agrochemicals, drugs Irritation due to formation of HCI with moisture... 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

Friedel-Crafts alkylation can occur intramolecularly to form a fused ring. Intramolecular Friedel-Crafts reactions provide an important method for constructing polycyclic hydrocarbon frameworks. It is somewhat easier to form six-membered than five-membered rings in such reactions. Thus, whereas 4-phenyl-1-butanol gives a 50% yield of a cyclized product in phosphoric acid, 3-phenyl-1-propanol is mainly dehydrated to alkenes.43... 

Bromoadamantane and 1-bromoadamantane are reduced to adamantane in yields of 84% and 79%, respectively, when treated with triethylsilane and catalytic amounts of aluminum chloride.186 Similar treatment of benzhydryl chloride and exo-2-bromonorbomane gives the related hydrocarbons in yields of 100% and 96%, respectively.186 In contrast, 2-bromo-l-phenylpropane gives only a 43% yield of 1-phenylpropane the remainder consists of Friedel-Crafts alkylation products.186 Some alkyl halides resist reduction by this method, even when forcing conditions are employed. These include p-nitrobenzyl bromide, 3-bromopropanenitrile, and 5-bromopentanenitrile.186... 

Non-chlorinated Lewis acids, such as scandium triflate, were found to be good catalysts for Friedel-Crafts alkylation reactions (167). Although no aromatic hydrocarbon alkylation occurred in CH2CI2, [BMIMJPFg, Sc(OTf)3 catalyzed the alkylation of benzene with high yields of the monoalkylated product. The lower acidity of the ionic liquid led to fewer byproducts and therefore higher yields. The products were separated by simple decantation and the catalyst was reused. 

Preparative use could be made also from the perturbed redox catalysis (Eqs. (84)—(86), (93)-(95). For example, alkylated aromatic hydrocarbons that are difficult to obtain by Friedel-Crafts alkylation could be synthesized in this way. The most suitable alkylating agent is tert-butyl chloride or bromide -ass) alkyla-... 

The importance of aluminum trichloride (and AlBr3 which is more soluble in hydrocarbons) as a catalyst, particularly for Friedel-Crafts alkylation and acylation of aromatic compounds,... 

Transalkylation and Dealkylation. In addition to isomerizations (side-chain rearrangement and positional isomerization), transalkylation (disproportionation) [Eq. (5.56)] and dealkylation [Eq. (5.57)] are side reactions during Friedel-Crafts alkylation however, they can be brought about as significant selective hydrocarbon transformations under appropriate conditions. Transalkylation (disproportionation) is of great practical importance in the manufacture of benzene and xylenes (see Section 5.5.4) ... 

Satisfactory results were obtained in the Nafion-H-catalyzed gas-phase alkylation of aromatic hydrocarbons with alkyl halides235 [Eq. (5.88)]. Alkylhalides are reactive Friedel-Crafts alkylating agents and give high conversions when alkylating benzene in the gas phase over Nafion-H catalyst. For example, in the alkylation of benzene with isopropyl chloride, conversions as high as 87% were achieved (Table 5.17, run 11). Conversions, however, were temperature and contact time dependent (Table 5.17). 

Aryl alkyl ketones are readily prepared by the Friedel-Crafts acylation process (see Section 6.11.1, p. 1006) and their Clemmensen reduction constitutes a more efficient procedure for the preparation of monoalkylbenzenes than the alternative direct Friedel-Crafts alkylation reaction (see below). Alternatively aldehydes and ketones may be reduced to the corresponding hydrocarbon by the Wolff-Kishner method which involves heating the corresponding hydrazone or semicarbazone with potassium hydroxide or with sodium ethoxide solution. 

The Friedel-Crafts alkylation reaction. An alkyl halide reacts with an aromatic hydrocarbon in the presence of aluminium chloride to yield in the first instance a hydrocarbon, thus ... 

Two of the reactions that are used in the industrial preparation of detergents are electrophilic aromatic substitution reactions. First, a large hydrocarbon group is attached to a benzene ring by a Friedel-Crafts alkylation reaction employing tetrapropene as the source of the carbocation electrophile. The resulting alkylbenzene is then sulfonated by reaction with sulfuric acid. Deprotonation of the sulfonic acid with sodium hydroxide produces the detergent. 

The product is a substituted phenol, whose -OH group directs the orientation of the -C(CH3)3 groups. The precursor to MON-0585 is synthesized by a Friedel-Crafts alkylation of phenol by the appropriate hydrocarbon halide. This compound is synthesized by NBS bromination of the product of alkylation of benzene with 2-chloropropane. 

Apart from the generalised solvating power of the medium, there exist more specific interactions between solvent and other components of a polymerisation system vriiich must sometimes be taken into account. We have already mentioned examples of catalyst-solvent interactions of a detrimental nature. In other situations the solvent can play a cocatalytic role, or have peculiar beneficial properties, as in the case of liquid SO2, a compound of modest polarity (e = 12.4 at 22 °C), but very conductive towards cationic polymerisation and related reactions The active species can also give specific reactions with some solvents, as in the case of aromatic hydrocarbons which can suffer Friedel-Crafts alkylations under certain conditions. These transfer and termination reactions are however outside the scope of the present work. 

Boron halides can also act as halide ion acceptors when they serve as catalysts— for example, in the Friedel-Crafts alkylation of aromatic hydrocarbons ... 

Friedel-Crafts alkylation is an example of electrophilic substitution in aromatic compounds. The electrophile is formed in the reaction of an alkylhalide with a Lewis acid. The Lewis acid polarizes the alkylhalide molecule, making the hydrocarbon part of it bear a positive charge and thus become more electrophilic. 

Another important application of Friedel-Crafts alkylation of aromatic hydrocarbons includes the production of ethylbenzene via alkylation of benzene with ethylene. 

The answer is D. This reaction is a Friedel-Crafts alkylation reaction. This rules out Choice A. In an alkylation reaction, there is a possibility of rearrangement. The primary alkyl halide ( -propyl chloride) can form secondary carbocations by rearrangement. To avoid this type of rearrangement, chemists often use Friedel-Crafts acylation reactions which do not involve rearrangement. The carbonyl group in the acylation product can be easily reduced to get the desired hydrocarbon. 

The boron trihalides, BXj, are Lewis acids (Chapter 6). These compounds are monomeric and planar—unlike diborane, B2H5, and the aluminum halides, AI2X5 (Section 3.1.4). As Lewis acids, boron trihalides can accept an electron pair from a halide to form tetrahalobo-rate ions, BX4. Boron halide catalysts act as halide ion acceptors, as in the Friedel-Crafts alkylation of aromatic hydrocarbons (in margin). 

Black, K.D., andF.D. Gunstone, Friedel-Crafts Alkylation of Benzene and Toluene with Olefinic Cg Hydrocarbons and Esters, Chem. Phys. Lipids 79 79-86 (1996). 

This reaction has been widely used to convert a carbonyl group into a methylene group, with important applications in the preparation of polycyclic aromatics and aromatics containing unbranched side hydrocarbon chains,the latter is not attainable from the Friedel-Crafts Alkylation. 

Although a mechanism involving a radical cation has been proposed for the Scholl reaction, as indicated by the paramagnetic properties of many polycyclic aromatic hydrocarbons (PAHs) when they are treated with Lewis acids or concentrated sulfuric acid, it is assumed that the Scholl reaction occurs in a manner similar to the Friedel-Crafts Alkylation, involving an arenium cation instead of a radical cation. In detail, the Scholl reaction of hexaphenylbenzene involves the complexation between a Lewis acid and aromatic nucleus, electrophilic addition, and deprotonation,as illustrated here. In the presence of NaCl or HCl, chloride is beneficial for the elimination of aryl hydrogen by the formation of hydrogen chloride, as indicated by the bold chloride. 

Chloroalkanes are also used, although less commonly than 1-alkenes, in Friedel-Crafts alkylation of benzenoid hydrocarbons. 

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