Toluene bromine reaction

Free-radical reactions written in the simplest way imply no separation of charge. The case of toluene bromination can be used to illustrate this point ... 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Draw resonance structures for the benzyl radical, C6H5CH2-, the intermediate produced in the NBS bromination reaction of toluene (Problem 10.27). 

In our previous studies on chlorination of toluene we had found that solvent had an important effect on the selectivity. In particular, the use of diethyl ether as a cosolvent was advantageous for the production of a high proportion of the para-isomer (ref. 9). An experiment in which the amount of ether in a tetrachloromethane/diethyl ether solvent mixture was varied under otherwise identical reaction conditions (Ih reaction at 18°C with 1.04 molar equivalent of tert-butyl hypobromite) demonstrated that diethyl ether also had a marked influence on the selectivity of the bromination reaction (Fig. 6). There was also an effect on the yield of the reaction as performed under these standard conditions. As the... 

Zeolite Beta is also highly effective in the bromination of toluene 255 Reactions of monoalkylbenzenes with bromine in the presence of stoichiometric amounts of zeolite NaY proceed in high yield and with high selectivity to give the corresponding para-bromo products 256... 

The relative rates of reaction of ethane toluene and ethylbenzene with bromine atoms have been measured The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one Arrange these hydrocarbons in order of decreasing reactivity... 

These reactions occur on the benzylic hydrogens because these hydrogens are much more reactive. Competition experiments show, for example, that at 40°C a benzylic hydrogen of toluene is 3.3 times as reactive toward bromine atoms as the tertiary hydrogen of an alkane and nearly 100 million times as reactive as a hydrogen of methane. 

Molecular bromine is believed to be the reactive brominating agent in uncatalyzed brominations. The brominations of benzene and toluene are first-order in both bromine and the aromatic substrate in trifluoroacetic acid solution, but the rate expressions become more complicated when these reactions take place in the presence of water. " The bromination of benzene in aqueous acetic acid exhibits a first-order dependence on bromine concentration when bromide ion is present. The observed rate is dependent on bromide ion concentration, decreasing with increasing bromide ion concentration. The detailed kinetics are consistent with a rate-determining formation of the n-complex when bromide ion concentration is low, but with a shift to reversible formation of the n-complex... 

Bromination is catalyzed by Lewis acids, and a study of the kinetics of bromination of benzene and toluene in the presence of aluminum chloride has been reported. Toluene is found to be about 35 times more reactive than benzene under these conditions. The catalyzed reaction thus shows a good deal less substrate selectivity than the uncatalyzed reaction, as would be expected on the basis of the greater reactivity of the aluminum chloride-bromine complex. 

The classification of hydrocar bons as aliphatic or ar omatic took place in the 1860s when it was aheady apparent that there was something special about benzene, toluene, and their- derivatives. Their molecular- for-mulas (benzene is CgHg, toluene is CyKj ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and alkynes, however, benzene proved to be inert. Benzene does react with Br-2 in the presence of iron(III) bromide as a catalyst, but even then addition isn t observed. Substitution occurs instead ... 

The ortho, meta, para system of nomenclature is also useful when discussing reactions. For example, we might describe the reaction of bromine with toluene by saying, "Reaction occurs atthe para position"—in other words, at the position para to the methyl group already present on the ring. 

What is an immediate precursor of toluene " Benzene, which could be methylated in a Friedel-Crafts reaction. Alternatively, "What is an immediate precursor of bromobenzene " Benzene, which could be brominated. 

Derbyshire and Waters192 measured the rates of bromination of sodium toluene-m-sulphonate (in water) and of benzoic acid (in aqueous acetic acid) by hypobromous acid with sulphuric or perchloric acids as catalysts, all at 21.5 °C. No bromination occurred in the absence of mineral acid and the reaction was strictly first-order in aromatic and in hypobromous acid. The function of the catalyst was considered to be the formation of a positive brominating species, according to the equilibrium... 

Subsequently, rate coefficients were determined for the zinc chloride-catalysed bromination of benzene, toluene, i-propyl-benzene, r-butylbenzene, xylenes, p-di-f-butylbenzene, mesitylene, 1,2,4-trimethyl-, sym-triethyl-, sym-tri-f-butyl-, 1,2,3,5-and 1,2,4,5-tetramethyl- and pentamethylbenzenes, all at 25.4 °C and in acetic acid, and it was shown that the reaction was inhibited by HBr.ZnCl2 which accumulates during the bromination and was considered to cause the first step of the reaction (formation of ArHBr2) to reverse320. The second-order coefficients for bromination of o-xylene at 25.0 °C were shown to be inversely dependent upon the hydrogen bromide concentration and the reversal of equilibrium (155)... 

Iodine acetate would seem to be unambiguously present in the iodination of pentamethylbenzene in acetic acid by iodine and mercuric acetate, since the latter components form an equilibrium mixture of iodine acetate and acetoxy-mercuric iodide and mercuric acetate speeds up the iodination332. Second-order rate coefficients of 0.078 (25 °C) and 0.299 (45 °C) were obtained, and these values are intermediate between those obtained for the reaction of bromine acetate with benzene (2.5 xlO-3) and toluene (1.2) at 25 °C, indicating that bromine acetate is the stronger electrophile. 

For instance, bromination of toluene in carbon tetrachloride did not proceed at reflux, even though pentamethylbenzene was brominated at 30°C to give bromopentamethylbenzene quantitatively. Toluene and copper(II) bromide reacted at reflux for 72 h. to give benzyl bromide as the main product. In a similar reaction with alumina-supported copper(II) bromide, bromotoluene (o/p = l) was obtained in good yield and no side-chain-brominated compounds were detected. 

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... 

Avramoff et al. have already reported that the reaction of hydrocarbons such as toluene with tetramethylammonium tribromide (TMA Br3) in benzene, in the presence of benzoyl peroxide at room temperature gave benzylic bromination products (ref. 21). However, TMA Br3 is not easy to handle in comparison with the stable BTMA Br3 because of its hydroscopic character. Furthermore, as shown in their literature, a large excess of TMA Br3 is necessary to brominate arenes. 

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