Nitrotoluenes bromination

Often the directing effects of substituents reinforce each other. Bromination of p-nitrotoluene, for exfflnple, takes place at the position that is ortho to the ortho, paradirecting methyl group and rneta to the meta-directing nitro group. 

The bromination of meta-nitrotoluene is an example for a high-temperature, high-pressure (high p,T) side-chain bromination of alkylaromatics [25]. 

Molar ratios of bromine to m-nitrotoluene ranging from 0.25 to 1.00 were applied. The reactants were contacted in an interdigital micro mixer followed by a capillary reactor. At temperatures of about 200°C nearly complete conversion is achieved (see Fig. 6). The selectivity to the target product benzyl bromide is reasonably high (at best being 85% at 200°C and higher being 80%). The main sideproduct formed is the nitro-substituted benzal bromide, i.e. the two-fold brominated side-chain product. 

N-Bromosuccinimide in bromination of o-nitrotoluene to form u-nitro-benzyl bromide, 46, 81 -Bromotoluene, conversion to Grignard reagent, 47,108... 

Method I.—5 gms. pure p-nitrotoluene, 2 c.cs. of bromine, and a crystal of iodine are placed in a sealed tube. The tube is placed in a bomb furnace and gradually heated up during 40 minutes to 130°, at which temperature it is maintained for 160 minutes. After cooling, the tube is opened and the product extracted with about 60 c.cs. of hot alcohol. From the resulting solution crystals separate on cooling, which are filtered off a second crop is obtained after concentrating and cooling the mother liquor. Water is added to the final mother liquor to precipitate a small quantity of the nitrobenzyl bromide, which is filtered off, dried, and purified by recrystallisation from petroleum ether. The first and second crops should be washed with cold petroleum ether. 

Method II.—10 gms. of p-nitrotoluene and a crystal of iodine dissolved in 100 c.cs. of carbon tetrachloride are placed in a silica flask provided with a reflux condenser. The solution is covered with water (about 50 c.cs.) and heated to gentle boiling, while situated about 15 cms. from a mercury vapour lamp (Fig. 46). A solution of 15 gms. bromine in 50 c.cs. carbon tetrachloride is then run in drop by drop from a dropping funnel at the top of the condenser. When all the bromine is in, boiling is continued until the solution becomes almost colourless. The contents of the flask are cooled, transferred to a separating funnel, and the lower carbon tetrachloride layer run into a distilling flask. Carbon tetrachloride is distilled off over a water bath, and the residue of p-nitrobenzyl bromide recrystallised from alcohol or petroleum ether. 

It is worthy of note that o-nitrotoluene is not brominated by this method even at temperatures up to 200°—an interesting example of steric hindrance. The m-compound, on the other hand, gives a 20% yield of the bromo-compound. 

Side-chain halogenation is illustrated also by the bromination of p-nitrotoluene (Expt 6.28). The radical mechanism with molecular bromine is similar to that of chlorination above N-bromosuccinimide has also found use as a side-chain brominating reagent and its application is described in Expts 6.119 and 6.152. 

The reaction may not even need light just refluxing p-nitrotoluene II with bromine in petrol ether gives a moderate yield of the benzylic bromide 12. Benzyl chloride 14 can be... 

This oxidation is used in situ for aromatic and side-chain brominations with bromine or hydrogen bromide302,303. Typical industrial examples are brominations of bisphenol A (equation 43)304 305, diphenyl ether306 and nitrotoluene (equation 44)307,308. 

Bromine adds to the alkene but substitutes on the aryl ether, evolving gaseous HBr. 17.10. Strong acid is used for nitration, and the amino group of aniline is protonated to a deactivating — NH3+ group. 17.12. (a) 2,4- and 2,6-dinitrotoluene (b) 3-chloro-4-nitrotoluene and 5-chloro-2-nitrotoluene (c) 3- and 5-nitro-2-bromobenzoic acid ... 

---