Polyalkylation benzene derivatives

Oxidation is another side reaction noted especially with polycyclic hydrocarbons, or with polyalkylated benzene derivatives, especially at elevated temperatures and in the presence of catalytic materials such as mercury. 

The optimum molar ratio of ethylene to benzene is around 0.9 as ethylene concentration increases, so the level of polyalkylated benzene derivatives rises. In order to assure high selectivity with satisfactory conversion rates, the synthesis of ethylbenzene is carried out industrially with molar ratios of ethylene to benzene of only 0.35 to 0.55. 

Before explaining the previous data, it is important to understand why 54 is formed in the Friedel-Crafts alkylation reaction. This means that the reactivity of benzene derivatives must be addressed. If 54 is formed by a reaction of 53, then 53 must react with the intermediate carbocation more quickly than benzene. Why does 53 react more quickly than benzene In addition, this discussion must address the question of why polyalkylation is a problem but polyacylation is not. The answers to these questions will also explain the regioselectivity of the reaction. 

Concentrated sulphuric acid. The paraffin hydrocarbons, cych-paraffins, the less readily sulphonated aromatic hydrocarbons (benzene, toluene, xylenes, etc.) and their halogen derivatives, and the diaryl ethers are generally insoluble in cold concentrated sulphuric acid. Unsaturated hydrocarbons, certain polyalkylated aromatic hydrocarbons (such as mesitylene) and most oxygen-containing compounds are soluble in the cold acid. 

Cumene is manufactured by reacting benzene with propylene over a catalyst such as a phosphoric acid derivative at 175 to 250°C and 400 to 600 psi (Fig. 1). A refinery cut of mixed propylene-propane is frequently used instead of the more expensive pure propylene. Benzene is provided in substantial excess to avoid polyalkylation. The yield is near quantitative (in excess of 90 percent) based on propylene. 

V.D). When the electron density in the ring is high (as in polyalkyl phenols) and the ortho- and/or para position (with respect to the OH group) is vacant, the formation of ortho- or para-benzoquinone also occurs. Indeed, in the hydroxylation of phenol to catechol and hydroquinone, one of the major side products (and the main cause of the tar formation) is the formation of benzo-quinones and products derived from them. The benzoquinones of polyalkyl-benzenes are starting materials for many products in the photographic and fine chemicals industries. Trukhan et al. 234) reported the oxidation of 2,3,-... 

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... 

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. 

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. ... 

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