Alkylbenzene bromination

The comparative ease with which a benzylic hydrogen is abstracted leads to a regio-selective preference for substitution at the benzylic carbon in free-radical halogenations of alkylbenzenes. Bromination of alkylbenzenes using A/-bromosuccinimide (see Section 10.3) offers a convenient synthesis of benzylic bromides. 

It is worth noting here that the results of some other studies of aromatie substitutions, sueh as the Friedel-Crafts benzylation and iso-propylation of alkylbenzenes, and the bromination of alkylbenzenes with bromine, eatalysed by ferrie ehloride, are under suspicion as depending upon slow mixing. As regards halogenation eatalysed by Lewis aeids, positive evidenee to support this eritieism has been obtained. ... 

Kinetic data are available for the nitration of a series of p-alkylphenyl trimethylammonium ions over a range of acidities in sulphuric acid. - The following table shows how p-methyl and p-tert-h xty augment the reactivity of the position ortho to them. Comparison with table 9.1 shows how very much more powerfully both the methyl and the tert-butyl group assist substitution into these strongly deactivated cations than they do at the o-positions in toluene and ferf-butylbenzene. Analysis of these results, and comparison with those for chlorination and bromination, shows that even in these highly deactivated cations, as in the nitration of alkylbenzenes ( 9.1.1), the alkyl groups still release electrons in the inductive order. In view of the comparisons just... 

Halogenation (Section 11 12) Free radical halo genation of alkylbenzenes is highly selective for substitution at the benzylic position In the exam pie shown elemental bromine was used Alterna Lively N bromosuccinimide is a convenient re agent for benzylic bromination... 

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzyiic position). Explain, based on the bond dissociation energies in Table 5.3 on page 156. 

Side-chain bromination at the benzylic position occurs when an alkylbenzene is treated with /V-bromosuccinimide (NBS). For example, propylbenzene gives (l-bromopropyl)benzene in 97% yield on reaction tvith NBS in the presence of benzoyl peroxide, (PhC02)2f as a radical initiator. Bromination occurs exclusively in the benzylic position and does not give a mixture of products. 

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. 

The most extensive study of the effect of conditions upon the kinetics of bromination was made by Robertson et al.23l 279, who measured the rates of bromination of alkylbenzenes, acetanilide, aceto-p-toluidide, mesitylene, anisole and p-tolyl methyl ether in acetic acid at 24 °C. They found that at relatively high concentrations of bromine (A//40-M/100) the reaction is second-order in bromine, i.e. the rate equation is... 

The reaction of alkylbenzenes with copper(II) bromide is critically influenced by the presence of water in small quantities (ref. 10). With toluene, nuclear bromination predominates in a rigorously anhydrous system. When small amounts... 

Monohalogenated Benzenes and Naphthalenes Trends in aromatic toxicities (Table IV) are somewhat obscured in these two series by the toxicities of the individual substituents (2). Fluorine and chlorine substituents are reasonably inert however, the bromine and iodine atoms, particularly the latter, are extremely toxic. In the case of inert substituents (F, Cl) the results resemble those of the alkylbenzenes in terms of steric hindrance to adsorption. 

Alkylbenzenes and other activated aromatics, such as polynuclear compounds, may react without a catalyst. Halogenation, in this case, occurs with the involve-ment of molecular halogens. Polarization of chlorine and bromine molecules is brought about by their interaction with the aromatic ring. For example, bromination in the presence of acetic acid is suggested to take place through the transition state 5, leading to the formation of the a complex ... 

The zeolite can easily be regenerated by heating. Similar high para selectivity can be achieved in the case of toluene by use of tert-butyl hypobromite as reagent with zeolite HX in a solvent mixture (CCI4 and ether). ZnBr2 supported on mesoporous silica or acid-activated montmorillonite is a fast, effective, reusable catalyst for the para-bromination of alkylbenzenes.257... 

The bromination of aromatic hydrocarbons can occur either in a side chain or on the ring, depending on conditions. In the presence of sunlight alkylbenzenes are brominated predominately in the side chain (24). 

Light-induced, radical chlorination or bromination of alkylbenzenes with molecular chlorine or bromine was discussed previously (Section 14-3C). Under these conditions, methylbenzene reacts with chlorine to give successively phenylchloromethane, phenyldichloromethane, and phenyltrichloromethane ... 

Alkyl can be benzhydryl (89% yield), methoxymethyl (98% yield), inter alia, (b) Solvent effects in the bromination of alkylbenzenes.45... 

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

The in situ generation of bromine from H202 and HBr provides an extremely versatile indirect method for oxidizing toluenes and higher alkylbenzenes. It is... 

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

Recently, several selective bromination reagents for reactive aromatic amines have been developed, for example, 2,4,4,6-tetrabromocyclohexa-2,5-dienone (35), iV-bromosuccinimide-dimethylformamide (36), and hexabro-mocyclopentadiene (37). Although molecular bromine is too reactive to perform selective bromination (mono- versus polybromination), the combined used of bromine and zeolites X and Y has been reported to be applicable to the selective bromination of halobenzenes and alkylbenzenes (38). This zeolite method, however, was not successful in the selective bromination of highly active aromatic compounds. Bromine preadsorbed on zeolite 5A (Ca type) was found to monobrominate aniline in carbon tetrachloride with excellent regioselectivity (91-93% para selectivity) in the presence of organic base, pyridine or 2,6-lutidine (Table XII) (39). The preadsorption of bromine on zeolite 5A is necessary for selective bromination, because the inverse procedure of adding bromine to aniline that had been adsorbed on zeolite beforehand caused a nonselective reaction. 

An alkylbenzene with a side chain more complicated than methyl offers more than one position for attack, and so we must consider the likelihood of obtaining a mixture of isomers. Bromination of ethylbenzene, for example, could theoretically yield two products 1-bromo-l-phenylethane and 2-bromo-l-phenylethane. Despite... 

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