Electrophilic substitution, mechanism Friedel-Crafts reaction

Aromatic compounds react mainly by electrophilic aromatic substitution, in which one or more ring hydrogens are replaced by various electrophiles. Typical reactions are chlorination, bromination, nitration, sulfonation, alkylation, and acylation (the last two are Friedel-Crafts reactions). The mechanism involves two steps addition of the electrophile to a ring carbon, to produce an intermediate benzenonium ion, followed by proton loss to again achieve the (now substituted) aromatic system. 

The effect of solvent on the solubility of the product can be explained on the basis of the general mechanism of a typical Friedel-Crafts reaction. Thus, asphaltene reacts with AlCl3 to form intermediate carbonium ions, which then undergo electrophilic substitution. If substitution occurs within the asphaltene molecule new bonds are formed and, depending on the size of initial fragments, the molecule may grow bigger and, thus, less soluble. 

The successful use of neopentyl bromide as electrophile raled out the possibility that the reaction occurs via a Friedel-Crafts reaction or a simple nucleophibc substitution. In addition, when 1-hexene was employed instead of the alkyl halide, only traces of the desired product were obtained, suggesting that a mechanism involving initial -elimination of HX from the alkyl halide, along with a subsequent ruthenium-catalyzed hydroarylation (part 4), is not operative. 

PROBLEM 14.9 The synthesis of toluene by the aluminum chloride-catalyzed Friedel-Crafts alkylation of benzene with methyl chloride is badly complicated by the formation of di-, tri-, and polymethylated benzenes. It appears that the initial product of the reaction, toluene, is more reactive in the Friedel-Crafts reaction than is benzene. Analyze the mechanism of electrophilic aromatic substitution to see why toluene is more reactive than benzene. Hint. Look carefully at substitution in the position directly across the ring from the methyl group (the para position) for toluene, and look for differences from the reaction with benzene. 

The mechanism of the Nicholas reaction is best described as an SnI process. Protonation of the alcohol in 4 followed by loss of water from cation 8 yields cobalt-stabilized carbocation 5. Friedel-Crafts reaction of this electrophile with anisole provides resonance-stabilized carbocation 9 which, upon removal of a proton, furnishes the substitution product 6. In addition to electron rich aromatics like anisole, a variety of neutral carbo- and heterocyclic nucleophiles react successfully with the carbocation... 

The main mechanism for arene reactions is electrophilic substitution. This enables arenes to retain their delocalised n electrons. Hydrogen atoms on the benzene ring may be replaced by a variety of other atoms or groups, including halogen atoms and nitro (—NO2) groups, as well as alkyl or acyl groups in Friedel-Crafts reactions. 

The reaction presented in this problem is known as a Friedel-Crafts acylation. Technically, this example belongs to a class of reactions referred to as electrophilic aromatic substitutions. Furthermore, the actual mechanism associated with this reaction, utilizing Lewis acid reagents as catalysts, proceeds through initial formation of an electrophilic acyl cation followed by reaction with an aromatic ring acting as a nucleophile. This mechanism, shown below, reflects distinct parallels to standard addition-elimination reaction mechanisms warranting introduction at this time. 

Please note that while the Friedel-Crafts acylation reaction is presented in discussions of addition-elimination reaction mechanisms, this reaction is actually an electrophilic aromatic substitution reaction. The correct mechanisms for a Freidel-Crafts acylation was presented in the solution for Problem 6 (h) from Chapter 7. 

Exactly the same sort of mechanism accounts for the reactions of aryl silanes with electrophiles under Friedel-Crafts conditions. Instead of the usual rules governing ortho, meta, and para substitution using the directing effects of the substituents, there is just one rule the silyl group is replaced by the electrophile at the same atom on the ring—this is known as ipso substitution. Actually, this selectivity comes from the same principles as those used for ordinary aromatic substitution (Chapter 22) the electrophile reacts to produce the most stable cation—in this case (3 to silicon. Cleavage of the weakened C-Si bond by any nucleophile leads directly to the ipso product. 

Most of the support for this mechanism comes from evidence about the nature of the attacking particle in each of these reactions evidence, that is, that substitution is electrophilic. This evidence, in turn, comes largely from kinetics, augmented by various other observations the nitrating power of preformed nitronium salts (Sec. 11.8), for example, or carbonium ion-like rearrangements in some Friedel-Crafts alkylations (Problem 11.3 above). The electrophilic nature of these reactions is supported in a very broad way by the fact that other reactions which show the same reactivity and orientation features also fit into the same mechanistic pattern. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

An attractive route to compatibilize thermoplastic blends, hke polyolehn-PS, is the Friedel-Crafts ahcylahon (F-C). By this reaction, a hydrocarbon chain can be chemically bonded to the PS benzene ring through an aromatic electrophilic substitution. The graft copolymer formed (polyolefin-g-PS), situated at the interphase, will behave as an in situ compatibilizer. The present work discusses the binary (PE/PS, PP/PS) and ternary (PE/PP/PS) blends compatibihzation by F-C reactions. Detailed smdies were performed on the F-C and side reaction characterization, blend morphological aspects, and final mechanical properties. 

In 1877, Charles Friedel and James M. Crafts discovered new methods for the preparation of alkylbenzenes, known as Friedel-Crafts alkylation reactions. The mechanism includes an electrophilic aromatic substitution whereby a carbocation is generated as the electrophile in the presence of a Lewis acid catalyst. The general scheme of F-C alkylation reaction is (16) as follows ... 

Carbon monoxide, hydrogen cyanide, and nitriles also react with aromatic compounds in the presence of strong acids or other Friedel-Crafts catalysts. These reactions are quite useful for synthetic purposes, since the products are formyl- or acyl-substituted aromatics. These electron-withdrawing groups retard any further electrophilic substitutions. Detailed mechanistic studies have not been carried out, but the general outlines of the mechanism of these reactions are given below ... 

The mechanism of each of the reactions in the synthesis of phenol from benzene and propene via cumene hydroperoxide requires some comment. The first reaction is a familiar one. The isopropyl cation generated by the reaction of propene with the acid (H3PO4) alkylates benzene in a typical Friedel-Crafts electrophilic aromatic substitution ... 

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