Electrophilic aromatic substitutions alkylbenzenes

Two of the reactions that are used in the industrial preparation of detergents are electrophilic aromatic substitution reactions. First, a large hydrocarbon group is attached to a benzene ring by a Friedel-Crafts alkylation reaction employing tetrapropene as the source of the carbocation electrophile. The resulting alkylbenzene is then sulfonated by reaction with sulfuric acid. Deprotonation of the sulfonic acid with sodium hydroxide produces the detergent. 

Electron-donating groups have a dual effect rate enhancement owing to electronic factors, as expected for electrophilic aromatic substitution, and a rate deerease as a result of steric and transport restrictions. Even the size of the methyl group in toluene is sufficient to compensate for the increased electron density on the aromatie nucleus, as shown by competition kinetics (benzene/toluene 1 1.3 for TS-I/H2O2, 1 10 for trifluoroperacetic acid) [2,27]. The nuclear reactivity trend of alkylbenzenes was in the sequence toluene > /7-xylene > ethylbenzene > /7-methylethylbenzene, in accordance with increasing molecular size... 

In 1877, Charles Friedel and James M. Crafts discovered new methods for the preparation of alkylbenzenes, known as Friedel-Crafts alkylation reactions. The mechanism includes an electrophilic aromatic substitution whereby a carbocation is generated as the electrophile in the presence of a Lewis acid catalyst. The general scheme of F-C alkylation reaction is (16) as follows ... 

The alkyl groups of alkylbenzenes release electrons in electrophilic aromatic substitution so that they activate the ring and direct ortho and para. Hence, when nitric acid and sulfuric acid are mixed to generate a nitronium ion (+NO2) for a nitration reaction, the ortho and para products are to be expected as shown ... 

Representative Electrophilic Aromatic Substitution Reactions of Benzene 457 Mechanistic Principles of Electrophilic Aromatic Substitution 458 Nitration of Benzene 459 Sulfonation of Benzene 461 Halogenation of Benzene 462 Biosynthetic Halogenation 464 Friedel-Crafts Alkylation of Benzene Friedel-Crafts Acylation of Benzene Synthesis of Alkylbenzenes by Acylation-Reduction 469 Rate and Regioselectivity in Electrophilic Aromatic Substitution 470 Rate and Regioselectivity in the Nitration ofToluene 472... 

If acetoxylation were a conventional electrophilic substitution it is hard to understand why it is not more generally observed in nitration in acetic anhydride. The acetoxylating species is supposed to be very much more selective than the nitrating species, and therefore compared with the situation in (say) toluene in which the ratio of acetoxylation to nitration is small, the introduction of activating substituents into the aromatic nucleus should lead to an increase in the importance of acetoxylation relative to nitration. This is, in fact, observed in the limited range of the alkylbenzenes, although the apparently severe steric requirement of the acetoxylation species is a complicating feature. The failure to observe acetoxylation in the reactions of compounds more reactive than 2-xylene has been attributed to the incursion of another mechan-104... 

Aromatics. The application of solid acid catalysts provides excellent possibilities to carry out aromatic electrophilic substitutions in an environmentally friendly way. Various zeolites were found by Smith and coworkers to exhibit high activities and selectivities.250 Acetyl nitrate generated in situ from acetic anhydride and HNO3 transforms alkylbenzenes to the corresponding para-nitro derivatives in high yield (92-99%) and with excellent selectivity (79-92%) when applied in the presence of large-pore H-Beta zeolites.251 Lattice flexibility and the coordination of acetyl... 

The cracking of alkylbenzenes can be treated as a case of aromatic electrophilic substitution (for recent views on this type of reaction see ref. 241) where the attacking agent is either a proton from a surface Br0nsted site or a coordinatively unsaturated surface cation acting as a Lewis site (cf. ref. 238)... 

Toluene, an alkylbenzene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Carbocations are perhaps the most important electrophiles capable of substituting onto aromatic rings, because this substitution forms a new carbon-carbon bond. Reactions of carbocations with aromatic compounds were first studied in 1877 by the French alkaloid chemist Charles Friedel and his American partner, James Crafts. In the presence of Lewis acid catalysts such as aluminum chloride (A1C13) or ferric chloride (FeCl3), alkyl halides were found to alkylate benzene to give alkylbenzenes. This useful reaction is called the Friedel-Crafts alkylation. 

Solution N-propylbenzene is a compound made up of aromatic and alkane units. Hence, it belongs in the category of alkylbenzenes, which, in turn, are part of the group of compounds known as arenes (compounds that contain both aliphatic and aromatic units). Such compounds show two sets of chemical properties. The ring undergoes the electrophilic substitution characteristic of benzene, whereas the side chain undergoes the free-radical substitution characteristic of alkanes. Each should modify the other. Except for oxidation and hydrogenation these are the reactions to be expected for arenes. 

Nevertheless, such reactions catalyzed by zeolites have been discussed in the review of 2001 (1) isomerization (double-bond shift, isomerization of tricyclic molecules, like synthesis of adamantane, isomerization of terpenes, diverse rearrangements, conversion of aldehydes into ketones), (2) electrophilic substitution in arenes (alkylation of aromatics, including the synthesis of linear alkylbenzenes, alkylation and acylation of phenols, heteroarenes and amines, aromatics nitration and halogenation), (3) cyclization, including the formation of heterocycles, Diels-Alder reaction, (4) nucleophilic substitution and addition,... 

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