Mechanisms Friedel-Crafts alkylation

The Friedel-Crafts alkylation mechanism involves the generation of an electrophile by adding an alkyl halide to the Lewis acid aluminium trichloride, which results in the formation of an organometallic complex. In this complex the carbon attached to the chlorine has a great deal of positive charge character (in fact, for practical purposes it is considered as a carbocation). 

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

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. 

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. 

Mechanism of the Friedel-Crafts alkylation reaction of benzene with 2-chloropropane to yield isopropylbenzene (cumene). 

In this section, the reactivities of organosilicon compounds for the Friedel-Crafts alkylation of aromatic compounds in the presence of aluminum chloride catalyst and the mechanism of the alkylation reactions will be discus.sed, along with the orientation and isomer distribution in the products and associated problems such as the decomposition of chloroalkylsilanes to chlorosilanes.. Side reactions such as transalkylation and reorientation of alkylated products will also be mentioned, and the insertion reaction of allylsilylation and other related reactions will be explained. 

The mechanism for the production of 9-((chlorosilyl)alkyl)(luorenes from the Friedel-Crafts alkylation reaction of biphenyl with (l,2-dichloroethyl)silane in the presence of aluminum chloride as catalyst is outlined in Scheme 4. At the beginning stage of the reaction, one of two C—Cl bondsof (1,2-dichloroethyl)silane (CICH2—CICH—SiXi) interacts with aluminum chloride catalyst to give intermediate 1 (a polar +C-CI - ( +C-C1—Al CI3) or a carbocation C AICU ... 

However, one of the most common mechanisms is undoubtedly proton transfer but whereas in alkene polymerizations this reaction leaves a terminal double bond, in arylene polymerizations these are generally not found. Instead the terminal group is usually a substituted indane formed by an internal Friedel-Crafts alkylation [8, 21, 23], e.g., for a-methyl styrene ... 

Note This reaction involves a polar acidic mechanism, not a free-radical mechanism It is a Friedel-Crafts alkylation, with the slight variation that the requisite carbocation is made by protonation of an alkene instead of ionization of an alkyl halide. Protonation of C4 gives a C3 carbocation. Addition to Cl and fragmentation gives the product. 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

Here is the general mechanism of a Friedel-Crafts alkylation ... 

Scheme 6 Loss of stereoinformation during the Bi(OTf)3-catalyzed Friedel-Crafts-alkylation implies a carbocationic intermediate. Mechanism A TfOH generated in situ from Bi(OTf)3 is thought to be the catalytic active species. Mechanism B Bismuth(III) acts as a Lewis acid. TfOH only regenerates Bi(OTf)3 from its less reactive monohydroxide...

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 ... 

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 ... 

HF calculations with the 6-31G(d) basis set were used to study the mechanism of the Michael addition (or Friedel-Crafts alkylation) reaction of indole with dimethyl alkylidenemalonate. This reaction proceeds through two transition states, TSi and TS2 in the first step, assumed to be rate determining, the new C-C bond is formed, whereas in the second step, proton transfer from indole to malonate occurs with the formation of the new C-H bond. The calculations show that the transfer and interaction of the 7r-electrons in the reactant molecules may play an important role in the cleavage of the original C=C bond and the formation of the new bonds (C-C and C-H) the electron transfer is believed to be the driving force for the reaction to occur. 

The nature of the reagent in the Friedel-Crafts alkylation is less easily defined. The most satisfactory mechanism involves nucleophilic attack of the aromatic species on a complex of the alkyl halide and... 

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