Mechanism of Friedel-Crafts alkylation

FIGURE 12 7 The mechanism of Friedel-Crafts alkylation The molecular model depicts the cyclohexadienyl cation intermediate formed in step 1... 

The mechanism of Friedel-Crafts alkylation with alkyl halides involves initial formation of the active alkylating agent, which then reacts with the aromatic ring. Depending on the catalyst, the solvent, the reaction conditions, and the alkyl halide, the formation of a polarized donor-acceptor complex or real carbocations (as either an ion pair or a free entity) may take place ... 

FIGURE 12.5 The mechanism of Friedel-Crafts alkylation. An electrostatic potential map of tert-butyl cation can be viewed on Learning By Modeling. 

In the mechanism of Friedel-Crafts alkylation, the Lewis acid functions to generate an alkyl carbonium ion. 

The various steps involved in the mechanism of Friedel-Crafts alkylation are (i) Generation of electrophile... 

The mechanism of Friedel-Crafts alkylation is discussed in Section... 

Mechanism of Friedel-Crafts Alkylation with Primary Haloalkanes Step 1. Haloalkane activation... 

It should be noted that Scheme 5.1-44 shows idealized Friedel-Crafts allcylation reactions. In practice, there are a number of problems associated with the reaction. These include polyalkylation reactions, since the products of a Friedel-Crafts alkylation reaction are often more reactive than the starting material. Also, isomerization and rearrangement reactions can occur, and can result in a large number of products [74, 75]. The mechanism of Friedel-Crafts reactions is not straightforward, and it is possible to propose two or more different mechanisms for a given reaction. Examples of the typical processes occurring in a Friedel-Crafts alkylation reaction are given in Scheme 5.1-45 for the reaction between 1-chloropropane and benzene. 

The mechanism of the chloromethylation reaction is related to that of Friedel-Crafts alkylation and acylation and probably involves an incipient chloro-methyl cation, CH2C1 ... 

Mechanism 17-6 Friedel-Crafts Acylation 782 Summary Comparison of Friedel-Crafts Alkylation and Acylation 784... 

The mechanism of Friedel-Crafts acylation (shown next) resembles that for alkylation, except that the electrophile is a resonance-stabilized acylium ion. The acylium ion reacts with benzene or an activated benzene derivative via an electrophilic aromatic substitution to form an acylbenzene. 

On the other hand, there is additional evidence (of a kind we cannot go into here) that makes it very likely that there is a second mechanism for Friedel-Crafts alkylation. In this mechanism, the electrophile is not an alkyl carbonium ion, but an acid-base complex of alkyl halide and Lewis acid, from which the alkyl group is transferred in one step from halogen to the aromatic ring. 

The reaction mechanism is similar to that of Friedel-Crafts Alkylation. 

The mechanism for Friedel-Crafts alkylation, like that for halogenation, nitration, and sulfonation, involves the attack of the aromatic ring onto a strong electrophile, in this case, a carbocation formed by reaction between the alkyl halide and the Lewis acid catalyst. 

Mechanism ofthe Friedel-Crafts alkylation reaction. The electrophile is a carbocation, generated byAICIj-assisted dissociation of an alkyl halide. 

As an example involving Lewis acid-base catalysis, we can cite the case of Friedel-Crafts alkylation of benzene hydrocarbons catalyzed by aluminum chloride, which is a Lewis acid. A four-step mechanism reports the results ... 

The mechanisms of Friedel-Crafts reactions have been studied extensively. The alkylations involve the use of olefins, alkyl halides, alcohols, ethers, and esters. The acylations make use of acids, esters, acid halides, and acid anhydrides. This type of reaction is acid-catalyzed, using such compounds as boron, aluminum, iron, tin, and other metallic halides, as well as sulfuric acid, phosphorus pentoxide, orthophosphoric acid, and hydrogen fluoride. These act as acids in the catalytic activity described because they all have a strong tendency to accept a share in an electron pair as the first step in the reaction. 

All these kinetic results can be accommodated by a general mechanism that incorporates the following fundamental components (1) complexation of the alkylating agent and the Lewis acid (2) electrophilic attack on the aromatic substrate to form the a-complex and (3) deprotonation. In many systems, there m be an ionization of the complex to yield a discrete carbocation. This step accounts for the fact that rearrangement of the alkyl group is frequently observed during Friedel-Crafts alkylation. 

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