Bromination of p-nitrotoluene

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. 

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

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. 

The reactants were contacted in a high-pressure interdigital micromixer followed by a capillary reactor, all immersed in an oil bath [45,46]. Pure bromine and pure aromatic were fed, mixed and reacted. By a high-temperature, high-pressure (high-p,T) route, the side-chain bromination of meta-nitrotoluene was achieved. This allows one to extend the operational range much beyond the boiling point, which is the technical limit of many reactions carried out batchwise. 

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. 

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... 

In a i-L three-necked flask is placed 300 g. (2.19 moles) of technical -nitrotoluene (m.p. 51-52°). The flask is fitted with a liquid-sealed stirrer, reflux condenser, and a separatory funnel arranged so that the stem reaches nearly to the bottom of the flask. The condenser is provided with a tube for disposing of the hydrogen bromide evolved (Org. Syn. 14, 2). The flask is heated in oil bath at 145-150° (Note i), and 368 g. (118 cc., 2.3 moles) of bromine is added dropwise over a period of two hours (Note 2). 

In addition to the reaction schemes described earlier, there are many other types of systems that are quite common. In one of these, a single reactant may be converted into several different products simultaneously. There are numerous examples of such reactions in organic chemistry. For example, the reaction of toluene with bromine in the presence of iron at 25°C produces 65% p-bromotoluene and 35% o—bromotoluene. Similarly, the nitration of toluene imder different conditions can lead to different amounts of o—nitrotoluene and p-nitrotoluene, but a mixture of these products is obtained in any event. Tailoring the conditions of a reaction to obtain the most favorable distribution of products is a common practice in synthetic chemistry. We will now illustrate the mathematical analysis of the kinetics of such reactions. 

An ab initio MO study of the mechanism of chlorination of benzene by chlorine in the presence of aluminium chloride has appeared. The rate-limiting stage leads to the formation of the ion-pair complex CeHeCl AlCU" without the explicit formation of Cl, the formation of C-Cl and Al-Cl bonds occurring synchronously with the breaking of the Cl-Cl bond. The introduction of a 3-chloro substituent into benzoyl chloride deactivates the system to further chlorination at the 5-position by a factor of about 6. ° Benzene, toluene, the xylenes, durene, mesitylene, nitrobenzene, and p-nitrotoluene are selectively brominated at 50-65 °C in an HBr-HN03-H2S04-H20 mixture with the participation of oxygen. The small catalytic amount of nitric acid oxidizes HBr to Br+, which reacts with the aromatic compound, and the nitrous acid formed reacts... 

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