Toluene radical bromination

Lewis acid catalyst is normally required when ammonium polyhalides are used, although recourse does not have to be made to strong acids, such as aluminium trichloride. Bromination and iodination reactions are normally conducted in acetic acid in the presence of zinc chloride [32], but chlorination using the ammonium tetrachloroiodate in acetic acid does not require the additional presence of a Lewis acid [33]. Radical chlorination of toluenes by benzyltrimethylammonium tetrachloroiodate in the presence of AIBN gives mixtures of the mono-and dichloromethylbenzenes [34], Photo-catalysed side-chain chlorination is less successful [35], Radical bromination using the tribromide with AIBN or benzoyl peroxide has also been reported [36, 37],... 

Exercise 14-6 a. Write the initiation and propagation steps involved in the radical bromination of methylbenzene (toluene) with bromine. Write the low-energy valence-bond structures of the intermediate phenylmethyl radical. 

In 1994, the free-radical bromination of toluene (and other alkylaro-matics) in sc C02 was reported (Tanko and Blackert, 1994 Tanko et al., 1994). Product yields were similar to those obtained with conventional solvents. The relative reactivity of the secondary hydrogens of ethylbenzene versus the primary hydrogens of toluene on a per hydrogen basis, r(2°/l°), were assessed via competition experiments and (a) did not vary with pressure, (b) were nearly identical to what is observed in conventional solvents. 

Free-radical bromination of toluene is selective for the benzylic position. Benzyl bromide cannot undergo elimination, and so nucleophilic substitution of bromide by hydroxide will work well. 

These mechanistic descriptions, in spite of their obvious oversimplification, may provide us with important clues about the conditions necessary to avoid completely the formation of 4-bromotoluene 8, if our efforts are aimed at the preparation of benzyl bromide 7, and vice versa. In order to prepare benzyl bromide 7 cleanly, toluene and bromine must not contain impurities capable of generating Br (for example, it is not a good idea to use toluene stored in a rusty barrel without purification). The ionic bromination of toluene to form 4-bromotoluene 8 must be carried out in the dark to prevent the light-induced formation of the radical bromine species responsible for the formation of benzyl bromide. 

Acyl bromides. Heating aldehydes with Br3COOEt and (PhCOO)2 in toluene accomplishes radical bromination. ... 

In 1994, Tanko and Blackett reported that the free radical bromination of alkyl-aromatics (e.g. toluene) could be carried out in scCOa [57]. This reaction is photo-initiated, and proceeds via the chain process outlined in Scheme 4.4-7 [58]. 

The usual free radical bromination of toluene with N-bromosuccinimide gives benzyl bromide. This bromination is carried out in a solvent, e.g., CCl. It has been found that free radical bromination of toluene with NBS in supercritical carbon dioxide gave 100% yield of benzyl bromide (Scheme 15). 

Benzyl bromide can be made from toluene via radical bromination at the benzylic position. 

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). 

Fig. 6 High p,T operation for the radical side-chain bromination of m-nitro toluene in a micro-mixer-reactor setup. The large increase in operational temperature increases conversion at good selectivities, which tend to decline slightly with temperature. The two-fold substituted product, m-nitro toluene benzal bromide, is formed in larger amounts at temperatures above 200°C (IMM, unpublished results)...

A benzylic radical is generated if a compound like toluene reacts with bromine or chlorine atoms. Hydrogen abstraction occurs from the side-chain methyl, producing a resonance-stabilized radical. The... 

Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The observed selectivity in SC-CO2 is similar to that observed in conventional organic solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are observed. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for free-radical reactions (Tanko and Blackert, 1994). 

Bromine selectively suhstitutes for a henzylic hydrogen in toluene in a radical suhstitution reaction to produce hromomethylhenzene or henzylhro-mide. A-hromosuccinimide is used to carry out henzylic hromination of toluene. 

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

Benzoic Acid. The industrial-scale oxidation of toluene to benzoic acid is carried out with cobalt catalysts.973,978,982 983 The process, a free-radical autoxidation, is significantly promoted by bromide ions.984 Under these conditions bromine atoms rather than alkylperoxy radicals serve as a regenerable source of chain-initiating free radicals. Substantial rate increase can be achieved by the addition of manga-nese(II) ions.984... 

The stability of free radicals is reflected in their ease of formation. Toluene, which forms a benzyl radical, reacts with bromine 64,000 times faster than does ethane, which forms a primary alkyl radical. Ethylbenzene, which forms a secondary benzylic radical, reacts 1 million times faster than ethane. 

Urry (13) studied hydrogen abstraction from toluene a-d versus toluene by chlorine, bromine and CH COCV radicals and found that the attack on toluene a-d. is slower than that on toluene. The isotope effect was found to be in the range of 2.2 to 9.9. 

Catalysis of these reactions by potassium bromide has been observed.232,236 Aralkyl bromides are formed as intermediates that undergo acetolysis to the corresponding acetates under the reaction conditions. Results236 were consistent with a free radical mechanism, even with reactive toluenes, in contrast to the electron transfer mechanism observed in the absence of potassium bromide. Relative reactivities236 corresponded closely to those observed in photochemical brominations, suggesting that bromine atoms, formed by electron transfer oxida-... 

The selectivity of free-radical side-chain bromination of toluene derivatives using A-bromosuccinimide and leading to mono- and dibromo-substituted toluenes has been studied in different solvents [577]. Surprisingly, yields and seleetivities are much better in solvents such as methyl formate and dichloromethane than in the more commonly used tetrachloromethane. 

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