Nitration of nitrobenzenes

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

Our knowledge of the mechanism of the reaction in this medium comes from an investigation of the nitration of nitrobenzene, /)-chloronitro-benzene and i-nitroanthraquinone. These compounds underwent reaction according to the following rate law ... 

Nitration in aqueous solutions of nitric acid Added water retards nitration in concentrated nitric acid without disturbing the kinetic order of the reaction. The rate of nitration of nitrobenzene was depressed sixfold by the addition of 5 % of water, (c. 3 2 mol 1 ), but because of the complexity of the equilibria involving water, which exist in these media, no simple relationship could be found between the concentration of water and its effect on the rate. 

A simple kinetic order for the nitration of aromatic compounds was first established by Martinsen for nitration in sulphuric acid (Martin-sen also first observed the occurrence of a maximum in the rate of nitration, occurrii for nitration in sulphuric acid of 89-90 % concentration). The rate of nitration of nitrobenzene was found to obey a second-order rate law, first order in the concentration of the aromatic and of nitric acid. The same law certainly holds (and in many cases was explicitly demonstrated) for the compounds listed in table 2.3. 

If it be assumed that the ionising characteristics of nitric acid are similar to those of the organic indicators used to define the scales of acidity, then a correspondence between the acidity-dependence of nitration and would suggest the involvement of the nitronium ion, whereas a correspondence with Hq would support the h)rpothesis that the nitric acidium ion were active. The analogies with and Hg are expressed in the first and last pairs of the followii equations respectively. The symbol AQ represents anthraquinone, the indicator originally used in this way for comparison with the acidity dependence of the rate of nitration of nitrobenzene ... 

TABLE 4.4 The kinetics of nitration of nitrobenzene in various medial... 

Ridd - has reinterpreted the results concerning the anticatalysis of the first-order nitration of nitrobenzene in pure and in partly aqueous nitric acid brought about by the addition of dinitrogen tetroxide. In these media this solute is almost fully ionised to nitrosonium ion and nitrate ion. The latter is responsible for the anticatalysis, because it reduces the concentration of nitronium ion formed in the following equilibrium ... 

The nitration of nitrobenzene and of chlorobenzene are known to occur via the same mechanism the ring is initially attacked by NO2, yielding a cation intermediate for each isomer. When the nitration process is fully complete, the distribution of the various isomers of the final product varies greatly for the two compounds ... 

Compare energies for meta and para-dinitrobenzenium ions (intermediates in nitration of nitrobenzene). Is the ordering the same as those observed for intermediates in toluene and aniline nitration Examine electrostatic potential maps. What does your result suggest about the electron donor/aceptor properties of the nitro substituent ... 

In the production of TNT from the reaction between toluene and mixed acids (nitric/sulfuric), TeNMe forms in amounts between 0.2—0.4% of the total wt of TNT. This TeNMe has been held responsible for several expins which have occurred in TNT plants, causing fatal injuries to personnel and severe damage to facilities. These expins were attributed to the presence of TeNMe in the acid fume lines and the acid storage tanks. Mixts of TeNMe and readily oxidizable materials are known to form very powerful and sensitive expl mixts. Since TeNMe is also isolated from the nitration of Nitrobenzene (NB), the TeNMe formed in the nitration of toluene may arise from the oxidation of the aromatic ring and/or methyl group. In an effort to gain more informa-. tion on the origin of TeNMe from TNT production, radioactive carbon-14 (14C) was used as a tracer to determine the extent to which each of the carbon atoms in the toluene skeleton of the various nitro-substituted isomers contributes to... 

Kinetic studies on the nitration of nitrobenzene by nitronium borofhioride in the polar solvents sulphuric acid, methane-sulphuric acid, and acetonitrile show the reaction to be first-order in both nitronium salt and aromatic110. With the first two solvents, the rate coefficients are similar for nitration by nitric acid and by the nitronium salts, indicating a common nitrating entity. With acetonitrile the rate coefficients are very much lower, consistent with a much lower concentration of free nitronium ions in this medium and thus with the nitronium salts existing as ion pairs in organic solvents (see Table 25). 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

Exercise 22-24 Draw the structures of the intermediate cations for nitration of nitrobenzene in the 2, 3, and 4 positions. Use the structures to explain why the nitro group is meta-orienting with deactivation. Use the same kind of arguments to explain the orientation observed with —CF3, —CHO, —CH2Ci, and —NH2 groups in electrophilic aromatic substitution (Table 22-6),... 

Beilstein and Kuhlberg. .. in 1870. However, it may be that dinitrobenzene , m.p. 71°C obtained in 1841 by St. Claire-Deville. .. by the nitration of nitrobenzene prepared from light... 

Oxidants. Mixtures of nitrobenzene with sodium chlorate,9 nitric acid,10 and nitric acid and water10 are powerful explosives. Nitration of nitrobenzene with nitric and sulfuric acids may be explosive without adequate temperature control.10 A mixture of nitrobenzene and nitrogen dioxide forms a highly explosive liquid.11... 

Now let s consider an example where the substituent slows the reaction. The nitration of nitrobenzene occurs approximately 107 times more slowly than the nitration of benzene and gives predominantly the /wera-isomer of dinitrobenzene. 

Limit of nitration of nitrobenzene ------ Amounts of nitronium ion per 1000g of solution... 

Martinsen [83] observed already in 1905 that the rate of nitration of nitrobenzene with a mixture of nitric and sulphuric acids depends on the concentration of sulphuric acid, viz. it increases with the concentration up to 88.7% H2S04, after which further increase in sulphuric acid concentration results in a decreasing rate of nitration. 

Fig. 16. Variation of the rate of the nitration constant against the concentrations of sulphuric acid in the nitrating mixture I - Nitration of nitrobenzene at 25°C (Westheimer and Kharasch [89]). II - Nitration of nitrobenzene at 25°C (Martinsen [83]) III - Nitration of dinitromesitylene at 25°C (Westheimer and Kharasch [86]) ...

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