Polyalkylated benzenes - production and uses

Petroleum- and coal-derived heavy gasoline fractions with a boiling range of around 160 to 220 °C contain polymethylated benzenes, such as trimethylbenzenes (pseudocumene, mesitylene and hemimellitene), together with the tetramethylated benzenes durene, isodurene and prehnitene. Indane and indene compounds, penta- and hexamethylbenzene and cumene, are also present in these heavy gasoline fraction. (Cumene is predominantly converted to phenol as described in Chapter 5.2). 

C9-aromatics from pyrolysis benzene from catalytic reformer 

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Polyalkylated benzenes - production and uses Table 8.1 Composition of C9-aromatics from pyrolysis gasoline and catalytic reforming (in percentage) ... 

Similar treating procedures can be used on more concentrated aromatic materials such as the polyalkylated benzenes which are formed as heavy by-products in the manufacture of cumene from propylene and benzene (18). 

This technique has been successfully used to characterize multispecies population in a laboratory-scale trickle bed bioreactor used for the biodegradation of a mixture of polyalkylated benzenes. Interestingly, the in situ hybridization results revealed that the aromatic-degrading cells constitute less than 10% while 60% of the cells were saprophytes and about 30% were inactive cells [119,120]. These saprophytes were believed to utilize intermediate compounds and cell lysis products. 

Apart from the fact that the photolytic bromine system is more applicable to deactivated, the cobalt system to activated, substrates, another important difference between the systems is their behaviour towards polyalkyl benzenes. For example, with /7-xylene, the photolytic system oxidises the methyl groups evenly, since //-abstraction from the benzyl bromide is more difficult than from the toluene (i.e. the main product of oxidation beyond the first stage is /7-bis-bromomethylbenzene). The cobalt system, on the other hand, gives /7-tolualdehyde and / -toluic acid before the second methyl group is oxidised, since the initially-formed alcohol is oxidised more rapidly than the toluene. Hence, it can be used to prepare 3,5-dimethylbenzoic acid from mesitylene. More recently, a system somewhat similar to the Co system but using cerium instead has been discovered [147],... 

The preferred route depends on such factors as convenience, expense, and the expected yield of the target molecule (the desired product). For example, the first route shown for the synthesis of 2-phenylethanol is the better procedure, because the second route has more steps, requires excess benzene to prevent polyalkylation, and uses a radical reaction that can produce unwanted side products. Moreover, the yield of the elimination reaction is not high (because some substitution product is formed as well). 

Studies by Kiersznicki and co-workers demonstrated that chlorosulfonic acid is an effective catalyst in the alkylation of arenes by reaction with alkenes. Benzene, toluene and ethylbenzene were alkylated by propene, elhene and 2-butene in the presence of chlorosulfonic acid which strongly catalysed the alkylations and inhibited polyalkylation. Increasing the concentration of the catalyst enhanced the proportion of /7-isomers in the products. Fluoro-, chloro-and bromobenzenes were similarly alkylated by reaction with C2-C4 alkenes using chlorosulfonic acid as catalyst. The optimum alkylation conditions were with a halobenzene alkene ratio of 1 0.25, a catalyst concentration of 0.33 mol mol" of fluorobenzene and 0.5 mol mol of the other halobenzenes, a temperature of 70 C and a reaction time of 2 hours. Alkylation with propene gave haloisopropylbenzenes the monoalkyl products were obtained as o-, m- and p- mixtures, the relative amounts depended on the quantity of catalyst used and the by-products were dialkyl derivatives, sulfonic acids and sulfones. In the reaction of benzene with propene, fluorosulfonic acid was a more potent alkylation catalyst than chlorosulfonic acid. ... 

Di- and polyalkylation can occur during alkylation with alkyl halides since the product alkylbenzenes are more reactive, although the reactivity difference with reactive alkylation systems is small. Toluene, for example, reacts only about 2-5 times faster in some benzylations than benzene.118,119 As alkylbenzenes, however, dissolve preferentially in the catalyst containing layer, heterogeneous systems can cause enhanced polysubstitution. The use of appropriate solvents and reaction conditions as well as of an excess of aromatics allow the preparation of monoalkyl-ated products in high yields. 

Ferrocene reacts with acetyl chloride and aluminum chloride to afford the acylated product (287) (Scheme 84). The Friedel-Crafts acylation of (284) is about 3.3 x 10 times faster than that of benzene. Use of these conditions it is difficult to avoid the formation of a disubstituted product unless only a stoichiometric amount of AlCft is used. Thus, while the acyl substituent present in (287) is somewhat deactivating, the relative rate of acylation of (287) is still rapid (1.9 x 10 faster than benzene). Formation of the diacylated product may be avoided by use of acetic anhydride and BF3-Et20. Electrophilic substitution of (284) under Vilsmeyer formylation, Maimich aminomethylation, or acetoxymercuration conditions gives (288), (289), and (290/291), respectively, in good yields. Racemic amine (289) (also available in two steps from (287)) is readily resolved, providing the classic entry to enantiomerically pure ferrocene derivatives that possess central chirality and/or planar chirality. Friedel Crafts alkylation of (284) proceeds with the formation of a mixture of mono- and polyalkyl-substituted ferrocenes. The reaction of (284) with other... 

A third limitation to the Friedel-Crafts alkylation is that it s often difficult to stop the reaction after a single substitution. Once the first alkyl group is on the ring, a second substitution reaction is facilitated for reasons we ll discuss in the nc.xt section. Thus, we often observe polyalkylation. Reaction of benzene with 1 mol equivalent of 2-chloro-2-inethylpropane, for example, yieldsp-di-A"t-butvlbenzene as the major product, along with small amounts of fc//-butyl-benzene and unreacted benzene. A high yield of monoalkylation product is obtained only when a large excess of benzene is used. 

Friedel-Crafts alkylation has the problem of cation rearrangement, but there is another problem with this reaction. When benzene reacts with 2-bromopro-pane and AICI3, it gives a mixture of 53 (1-methylethylbenzene or isopropylbenzene, also known as cumene), which is the expected product however, it also gives the disubstituted product 54. The latter may be the major product if an excess of 2-bromopropane is used. Therefore, the reaction with alkyl halides may lead to polyalkylation of the benzene ring, which is the second of the two problems noted for Friedel-Crafts alkylation. The only way to explain formation of 54 is via a Friedel-Crafts alkylation of the initially formed product 53 with 2-bromopropane. This result suggests that 53 must react more quickly with the carbocation derived from 2-bromopropane than does benzene. This point will be discussed in more detail later. 

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