Dealkylation Friedel-Crafts reaction

One rather interesting aspect of the Friedel-Crafts reaction is that trialkylation of benzene often leads to a symmetrical 1,3,5-isomer rather than a 1,2,4-derivative as would be expected from the ortho-para orientation of the alkyl group introduced initially. Since alkylation by the Friedel-Crafts method has been shown to be a reversible reaction,68 this anomalous orientation has been explained on the basis that both alkylation and dealkylation occur readily in the ortho and para positions. Simultaneously, however, some alkylation occurs in the meta position but no dealkylation68 If the reaction mixture is allowed to stand in contact with aluminum chloride for some length of time, then principally meta trialkyl derivatives will be formed ... 

Tliis reaction is reversible. In a reversed Friedel-Crafts reaction or Friedel-Crafts dealkylation, alkyl groups can be removed in the presence of protons and a Lewis acid. 

Baleizao C, Pires N, Gigante B et al (2004) Friedel-Crafts reactions in ionic liquids the counter-ion effect on the dealkylation and acylation of methyl dehydroabietate. Tetrahedron Lett 45 4375-4377... 

PPA = polyphosphoric acid al = alkylation ac = acylation chlor = chlorination iso = isomerization halo = halogenation cycl = cyclization poly = polymerization for = formylation dehydr = dehydration nitr - nitration general = active in most Friedel-Crafts reactions deal = dealkylation rearr = rearrangement. 

In the first stage, a Friedel-Crafts reaction is commonly carried out by treating benzene with ethylene in the liquid phase at 90-100°C at slightly above atmospheric pressure. The catalyst is aluminium chloride (with ethyl chloride as catalyst promoter). A molar excess of benzene is used to reduce the formation of polyethylbenzenes the molar ratio of reactants is generally about 1 0.6. The reactants are fed continuously into the bottom of a reactor whilst crude product is removed from near the top. The product is cooled and allowed to separate into two layers the lower layer, which consists of an aluminium chloride-hydrocarbon complex, is removed and returned to the reactor. The remaining ethylbenzene is then separated by distillation from polyethylbenzenes and benzene, which are recycled (since dealkylation also occurs under the reaction conditions). In newer plants, ethylbenzene is produced in a gas phase process. An excess of benzene is treated with ethylene at about 420°C and 1.2-2 MPa (12-20 atmospheres) in the presence of a zeolite catalyst. 

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) ... 

Cross-linked polystyrene can be acylated with aliphatic and aromatic acyl halides in the presence of A1C13 (Friedel-Crafts acylation, Table 12.1). This reaction has mainly been used for the functionalization of polystyrene-based supports, and only rarely for the modification of support-bound substrates. Electron-rich arenes (Entry 3, Table 12.1) or heteroarenes, such as indoles (Entry 5, Table 15.7), undergo smooth Friedel-Crafts acylation without severe deterioration of the support. Suitable solvents for Friedel-Crafts acylations of cross-linked polystyrene are tetrachloroethene [1], DCE [2], CS2 [3,4], nitrobenzene [5,6], and CC14 [7]. As in the bromination of polystyrene, Friedel-Crafts acylations at high temperatures (e.g. DCE, 83 °C, 15 min [2]) can lead to partial dealkylation of phenyl groups and yield a soluble polymer. 

Friedel-Crafts alkylation reactions are, in general, accompanied by isomerization processes. Olah et a/. reported the results of the water-promoted, AlCb-catalyzed isomerization of o-, m- and p-di-f-bu-tylbenzene. No ortho isomer was present in the equilibrium mixture. The isomerization of o-di-r-bu-tylbenzene was very rapid largely due to relief of steric strain. In these and other related sterically hindered arenes, intramolecular isomerization and not dealkylation was observed. Isomerization of di-and mono-methylnaphthalenes, catalyzed by HF-BF3, was also reported. Isomerization of /i-alkyl-toluenes and -xylenes, catalyzed by AICI3 at room temperature, afforded chiefly /n-/i-alkyltoluenes and /n-/i-alkylxylenes, respectively. The process leading to the meta isomer has a lower energy than the other processes. 

Alkylation and Dealkylation Reactions. Alkylalioit is the addition of alkyl group to an organic compound. This type of reaction is commonly c tied out in the presence of the Friedel-Crafts catalysts. AlCb along wit trace of HCl. One such reaction is... 

Halogenation and dehalogenation [36], Friedel-Crafts-type alkylation [37], 0-and iV-dealkylation [38], carboxylation [39], and decarboxylation [40], isomerization [41], acyloin [42], and aldol reactions [43]. Even Michael... 

Reversible nature of Friedel-Crafts alkylation of benzene derivatives has been applied to remove ortho-tert-huty group, which is an essential substituent to isolate an atropisomeric anilide derivative as a stable compound at an ambient temperature. Simpkins and coworkers reported that during the AICI3 catalyzed trans-tert-butylation reaction of the particular enamide compound (12) in benzene, enamide function undergoes Friedel-Crafts alkylation to give phenylated product (13) in good yield (Scheme 6.12) [14]. Thus, in this case, Friedel-Crafts alkylation and dealkylation occur at the same time. 

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