Transalkylation Friedel-Crafts reaction

The only way to functionalize the upper rim of 4-f-butylcalix[4]arene is to remove the f-butyl group and replace it with something more useful. Two methods have been reported by which this de-ferf-butylation, sometimes referred to by the more evocative term of neutering , may be achieved. The first method is probably the more conventional of the two and involves the use of aluminium trichloride and phenol. It is in essence a Friedel-Crafts reaction where a f-butyl group is attached to either benzene or phenol the innovation is that the source of the f-butyl substituent comes from the calixarene [1]. This retro-Friedel-Crafts reaction is thus an effective method for transalkylation that removes an unwanted alkyl group from calix[4]arenes as shown in Figures 3.10 and 3.11. Moreover, it is possible to subtly alter the reaction conditions and partially de-ferf-butylate... 

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. 

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) ... 

Silica-occluded H3PW12O40 can be used as an insoluble solid-acid catalyst in several liquid-phase reactions such as ester hydrolysis [15c], esterification [15dj, hydration [15d], alkylation of phenol [15d] in polar media, and Friedel-Crafts type alkylation and transalkylation [16] in non polar media. 

The use of traditional Friedel-Crafts catalysts usually leads to complex product mixtures. During alkylation several reactions can take place concurrently, e. g. polymerization, polyalkylation, isomerization, transalkylation, cracking, etc., and complexation of the reactants with the catalyst often occurs. One of the most important drawbacks of these catalysts is probably increasing environmental concern. 

In fact, the reaction of coal under Friedel-Crafts conditions is not a simple alkylation but involves a series of complex reactions. The molecnlar dynamics of coal molecules under such conditions and the mineral matter in coal may interfere with or catalyze these reactions. Extraction can be used as a measure of the effectiveness of reactions snch as degradation, disintegration, and transalkylation of coal. In fact, the reaction of coal with alkylating agents under Friedel-Crafts conditions may be a complicated series reaction involving alkylation-transalkylation-dehydrogenation-degradation-dissociation (Sharma and Mishra, 1992). 

In transalkylation, one of the alkyl groups is transferred from one alkylaromatic molecule to another aromatic molecule. The mechanism of transalkylation was studied extensively in Friedel-Crafts chemistry. Though the reaction conditions are quite different from those of Friedel - Crafts catalysts, it seems quite probable that an essentially same mechanism is operative also in transalkylation with solid-acid catalysts. Thus, Kaeding et al. proposed the following mechanism for disproportionation of toluene over zeolites. ... 

This can be done by Friedel and Crafts catalysts, and indeed classical ones as AlClj and BFj-HF were primarily used (172). The isomerization is accompanied by a transalkylation reaction giving toluene and trimethylbenzenes ... 

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