Alkylbenzenes side-chain alkylation

The industrial alkylation of aromatics with olefins is one of the major examples of development of environmentally friendly processes with solid acid catalysts [221, 222]. The principal products obtained are ethylbenzene (EB), cumene (CUM), p-diethylbenzene, p-diisopropylbenzene, Cio-Ci4linear alkylbenzenes (LAB) and cymene. Figure 2.28 summarizes several aromatic alkylations industrially applied for the preparation of important chemical intermediates [222]. These reactions include the most important aromatic substrates, benzene, toluene and xylene, and different olefins. They also include two different kinds of alkylation electrophilic alkylation on the aromatic ring catalyzed by acids and side-chain alkylation catalyzed by bases. In terms of production volume, add-catalyzed alkylations are by far the most... 

A solid superbase whose basicity (H ) is higher than 37 has been prepared by the successive treatment of y -alumina with alkali metal hydroxide and alkali metal at 200-600t under nitrogen. The solid base has been found to be highly effective in the catalytic isomerization of olefinic double bonds and in the side-chain alkylation of alkylbenzenes with olefins. 

In comparison to intense activities in the field of the solid superacids, not much work has been done on solid superbase catalysis[l]. Here we report first the preparation of a solid superbase which exhibits remarkably strong basicity, and then its synthetic application to the olefin isomerizations[2] and side-chain alkylations of alkylbenzenes with olefins[3]. The reactions proceed smoothly under mild reaction conditions to give the products quantitatively. 

However, another type of reactions of benzene derivatives was studied by in-situ IR spectroscopy as well, viz. the side-chain alkylation of alkylbenzenes, for instance of toluene, over basic zeolite catalysts such as M -X zeolites (M=Na, K, Rb, Cs) [901,902]. The intermediate conversion of methanol to formaldehyde turned out to be crucial for the side-chain alkylation as well as a strong polarization of the methyl group of toluene, the preferential adsorption of toluene, and a sufficient basicity, i.e., base strength of the catalyst. Related to these IR studies of side-chain alkylation of toluene were in-situ IR spectroscopic investigations of the decomposition of methanol over basic zeolites (M+-X, M =Na+, K+, Rb, Cs+ Na-ZSM-5 and Cs-ZSM-5 [903]). It was shown that over weakly basic zeolites (Na-ZSM-5, Cs-ZSM-5) dimethyl ether was formed from methanol, whereas over more strongly basic X-type zeolites formaldehyde was produced, which is an indispensable intermediate for the side-chain reaction (vide supra). 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Alkyl-substituted benzenes are oxidized both on the benzene ring and on the side chain. Additionally, some dimerization occurs.36 Alkylbenzenes containing linear alkyl groups are oxidized preferentially at the side chain33 nearest the benzene ring for example, ethylbenzene oxidizes first to 1-phenyl ethanol and then to acetophenone.36... 

Many side-chain halogen compounds can be synthesized by reactions that also are applicable to alkyl halides (see Table 14-5), but there are other methods especially useful for the preparation of arylmethyl halides. The most important of these are the chloromethylation of aromatic compounds (to be discussed later in this section) and radical halogenation of alkylbenzenes. 

Alkyl side chain of p-alkylbenzene sulfonate as counteranion of Bzl-V2+. bAl= values at /.max indicated in parentheses, A50= after 50 cycles of irradiation and subsequent thermal bleaching (70°C for lOmin). 

It is evident from the data for monoalkylbenzenes on a-CD that the retention of n-alkylbenzenes is affected by the high stabilities of the complexes formed, caused by the location of the alkyl group in the cyclodextrin cavity. Branching of the side chain leads to a pronounced decrease in the retention. These results are similar those obtained for the interactions of n-alkanes and branched alkanes with a-CD (12.13). ... 

A survey of the literature reveals that the majority of the nonbiodegradable detergents are alkylbenzene sulfonates (ABS). Theoretically, there are over 80,000 isomeric alkylbenzenes in the range of C10 to C15 for the alkyl side chain. Costs, however, generally favor the use of dodecene (propylene tetramer) as the starting material for ABS. 

Beactiun occurs with P and 2 but not 3. alkyl side- chains. 6. Benzylic bromination of alkylbenzenes (Section 16.19)... 

The simplest of the alkylbenzenes, methylbenzenc, is given the special name of toluene. Compounds containing longer side chains are named by prefixing the name of the alkyl group to the word -benzene, as, for example, in ethylbenzene, n-propylbenzene, and isobutylbenzene. 

There are frequently available aromatic compounds containing aliphatic side chains that are not simple alkyl groups. An alkylbenzene can be prepared from one of these compounds by converting the side chain into an alkyl group. Although there is an aromatic ring in the molecule, this conversion is essentially the preparation of an alkane from some other aliphatic compound. The methods used are those that we have already learned for the preparation of alkanes hydrogenation of a carbon carbon double bond in a side chain, for example. Many problems of the alkylbenzenes are solved by a consideration of simple alkane chemistry. 

From their studies on dealkylation of alkylbenzenes with AICI3 at 100 C, Roberts et al. found that, in general, the catalyst and experimental conditions required to produce dealkylation are forceful enough to effect several other reactions, such as disproportionation, isomerization, rearrangement of the side chain and fragmentation. The latter refers to a process in which the alkylbenzene affords alkanes and alkylbenzenes with fewer carbons than those in the original alkyl side chain. Formation of alkanes in the dealkylation process can take place even in the absence of an added hydride donor agent. In this case, a second molecule of the alkylbenzene acts as the hydride donor. 

Ammoxidation of alkylbenzenes with side-chains of different length leads to reaction at the position a to the benzene ring [38]. Reactivity increases with increasing length of the alkyl group (toluene < ethyl benzene < isopropyl benzene [58,59]), but in all cases only benzonitrile is formed. 

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