Bromination of methylbenzene

Exercise 4-17 The peroxide-induced bromination of methylbenzene with bromo-trichloromethane gives bromomethylbenzene and trichloromethane ... 

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. 

Exercise 22-25 The product distribution in the bromination of methylbenzene (toluene) depends on the nature of the brominating agent. Pertinent information follows ... 

Electrophilic bromination of methylbenzene (toluene) is considerably faster than the bromi-nation of benzene itself. The reaction is also regioselective It results mainly in para (60%) and ortho (40%) substitutions, with virtually no meta product... 

Why are the two favored products, ortho and para, not formed in equal amounts Frequently, the answer is steric effects. Thus, attack ortho to an existing substituent, especially when it is bulky, by an electrophile (again, especially when bulky) is sterically more encumbered than para attack. Therefore, para products often predominate over their ortho isomers. In the bromination of methylbenzene (toluene), this predominance is small. However, similar halogenation of 1,1-dimethylethyl (tert-butyl) benzene results in a much larger para ortho ratio (—10 1). 

A kinetic isotope effect, kH/kD = 1.4, has been observed in the bromination of 3-bromo-l,2,4,5-tetramethylbenzene and its 6-deuterated isomer by bromine in nitromethane at 30 °C, and this has been attributed to steric hindrance to the electrophile causing kLx to become significant relative to k 2 (see p. 8)268. A more extensive subsequent investigation304 of the isotope effects obtained for reaction in acetic acid and in nitromethane (in parentheses) revealed the following values mesitylene, 1.1 pentamethylbenzene 1.2 3-methoxy-1,2,4,5-tetramethyl-benzene 1.5 5-t-butyl-1,2,3-trimethylbenzene 1.6 (2.7) 3-bromo-1,2,4,5-tetra-methylbenzene 1.4 and for 1,3,5-tri-f-butylbenzene in acetic acid-dioxan, with silver ion catalyst, kH/kD = 3.6. All of these isotope effects are obtained with hindered compounds, and the larger the steric hindrance, the greater the isotope... 

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 ... 

The additivity treatment also allows one to evaluate the influence of substituents which are otherwise obtainable only with difficulty. The study of the non-catalytic bromination of the halo-substituted poly-methylbenzenes by Illuminati and Marino (1956) allowed the evaluation of the partial rate factors for the highly deactivating m- and p-halogens. These data for the slow, highly selective bromination are inaccessible by other techniques. Analysis of the relative rates is made by application of the additivity equations (5) and (6) as described in Section I. An important aspect of the chemistry of the substituted polymethyl-benzenes, in contrast to the monosubstituted benzenes, is the large difference in p for bromination. The partial rate factors derived for each reaction are correlated with good precision by the tr4 -constants (Figs. 11 and 19). Yet the susceptibility of the reactions to the influence of substituents is altered by more than 25%. As already noted, this aspect of the problem is not well defined and is worthy of additional attention. 

The rate constant for the bromination of toluene (methylbenzene) is about 4000 times that for benzene (this may sound like a lot, but the fate constant for N,N-dimethylaniline is 1014 times greater). The methyl group also directs electrophiles mostly into the ortho and para positions. These two observations together suggest that alkyl groups may also increase the electron density in the 7t system of the benzene ring, specifically in the ortho and para positions, rather like a weakened version of an OR group. 

Figure 15.5 Calculated electrostatic potential maps for the arenium ions from electrophilic addition of bromine to (a) methylbenzene (toluene) and (b) trifluoromethylbenzene. The positive charge in the arenium ion ring of methylbenzene (a) is delocalized by the electron-releasing ability of the methyl group, whereas the positive charge in the arenium ion of trifluoromethylbenzene (b) is enhanced by the electron-withdrawing effect of the trifluoromethyl group. (The electrostatic potential maps for the two structures use the same color scale with respect to potential so that they can be directly compared.)...

Treatment of methylbenzenes containing electron-withdrawing substituents (COOH, NO2, Br) with ammonium hexanitratocerate(IV) in a mixture of acetic acid and potassium bromide at 80 °C allows bromination of the methyl group (Baciocchi et al., 1984b) (scheme 27). Benzyl bromides were also prepared by reaction of /7<3ra-substituted toluenes with ammonium hexanitratocerate(IV) in aqueous solutions containing trifluoroacetic acid and sodium bromide (Maknon kov et al., 1986). 

Electrophilic bromination of an equimolar mixture of methylbenzene (toluene) and (trifluoromethyl)benzene with one equivalent of bromine gives only l-bromo-2-methylbenzene and l-bromo-4-methylbenzene. Explain. 

In contrast, heat or light allows attack by chlorine or bromine on methylbenzene (toluene) even in the absence of a catalyst. Analysis of the products shows that reaction takes place at the methyl group, not at the aromatic ring, and that excess halogen leads to multiple substitution. 

In the presence of bromide the oxidation is thought to be mediated by the very reactive bromine atoms, without any direct interaction between cobalt(III) and the methylbenzene (Equations 3 X is a generic anionic ligand) ... 

When methylbenzene (toluene) reacts with W-bromosuccinimide (NBS) in the presence of light, for example, the major product is benzyl bromide. W-Bromosuccinimide furnishes a low concentration of Br2, and the reaction is analogous to that for allylic bromination that we studied in Section 10.8B. 

Cyclohexene (161), for example, reacts with NBS in the presence of light to give 3-bromocyclohexene (162) in 45% yield by the radical substitution mechanism presented in Section 11.9.1, once diatomic bromine is generated. The reaction of NBS and toluene (methylbenzene PhMe) gives benzyl bromide. The mechanism that involves succinimide will not be elaborated. Both NBS and NCS react as if bromine or chlorine were being added to the reaction, but they are readily available and easy to handle. 

With respect to bromination, in the presence of iron(III) salts or iodine (F) as catalyst and in the dark the addition of bromine (Br2) to methylbenzene (toluene, C6H5CH3) occurs almost exclusively ortho- and para- to the methyl group. 2-Bromo-l-methylbenzene (2-bromotoluene) and 4-bromo-l-methylbenzene (4-bromotoluene) (Equation 6.109 compare Equation 6.107) are produced. However, in the sunlight (or with NBS) the outcome is different (Equation 6.110). Under these different conditions bromomethylbenzene (benzyl bromide, C6HsCH2Br) results. In contrast to the ionic electrophihc substitution, the reaction in the presence of sunlight (or NBS) apparently involves free radicals and the intermediacy of the phenyl-methyl (benzyl) radical [Ci sCli2 vide infra). 

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