Nitration of toluene

Nitration of toluene is the only important reaction that involves the aromatic ring rather than the aliphatic methyl group. The nitration reaction occurs with an electrophilic substitution hy the nitronium ion. The reaction conditions are milder than those for henzene due to the activation of the ring hy the methyl substituent. A mixture of nitrotoluenes results. The two important monosubstituted nitrotoluenes are o- and p-nitrotoluenes  

Mononitrotoluenes are usually reduced to corresponding toluidines, which make dyes and ruhher chemicals  

Dinitrotoluenes are produced hy nitration of toluene with a mixture of concentrated nitric and sulfuric acid at approximately 80°C. The main products are 2,4- and 2,6-dinitrotoluenes  

The dinitrotoluenes are important precursors for toluene diisocyanates (TDI), monomers used to produce polyurethanes. 

The TDI mixture is synthesized from dinitrotoluenes hy a frrst-step hydrogenation to the corresponding diamines. The diamines are then treated with phosgene to form TDI. The yield from toluene is approximately 85%  

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It is prepared by the direct nitration of toluene as a 50-60% component of the mixture of isomers. Used for the preparation of o-toluidine. 

It is prepared by the direct nitration of toluene with a mixture of nitric and sulphuric acids. TNT is a very stable, violent and powerful high explosive, but less sensitive to shock and friction than picric acid. It is widely used as a filling for shells, bombs, etc. often mixed with ammonium nitrate and other high explosives. The lower grades of TNT may contain isomers which under hot storage conditions may give rise to exudation. 

The addition of water depresses zeroth-order rates of nitration, although the effect is very weak compared with that of nitrate ions concentrations of 6x io mol 1 of water, and 4X io mol 1 of potassium nitrate halve the rates of reaction under similar conditions. In moderate concentrations water anticatalyses nitration under zeroth-order conditions without changing the kinetic form. This effect is shown below (table 3.5) for the nitration of toluene in nitromethane. More strikingly, the addition of larger proportions of water modifies the kinetic... 

TABLE 3.5. The ejfect of added water on the zeroth-order rates of nitration of toluene (faromatic = o-og mol l ) in a solution at —10 °C of nitric acid - yo mol l ) in nitromethane... 

It has been necessary to comment upon these various studies because Olah and his co-workers have suggested that whilst nitrations, like those with nitronium salts, which give a relative rate of reaction of toluene with respect to benzene not much greater than unity involve the nitronium ion as the electrophile, this is not so in other cases. It is important to consider these opinions closely. In the earlier of the two relevant papers it is agreed that since nitrations of toluene with nitronium tetrafluoroborate in sulpholan show no abnormal o -ratio there... 

TABLE 4.5 Competitive nitrations of toluene and benzene with 1 -nitropyridinium tetrafluoroborates in acetonitrile at 25 °... 

The isomer proportions for the nitration of the chlorotoluenes, to be expected from the additivity principle, have been calculated from the partial rate factors for the nitration of toluene and chlorobenzene and compared with experimental results for nitration with nitric acid at o °C. The calculated values are indicated in brackets beside the experimental values on the following structural formulae. In general, it can be... 

Three products are possible from nitration of toluene o mtrotoluene m nitro toluene and p mtrotoluene All are formed but not m equal amounts Together the ortho and para substituted isomers make up 97% of the product mixture the meta only 3%... 

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and ortho para directing... 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

Dinitrotoluene can be prepared by the nitration of -nitrotoluene with yields of ca 96% or it can be obtained from the direct nitration of toluene. 

In some processes the reactant bases are too weak to be protonated significantly except in the presence of very strong acids such as fuming sulfuric acid or a mixture of concentrated sulfuric and nitric acids, ie, mixed acid. Nitration of toluene, for example, requires such solutions two Hquid phases are present in the reactor. 

A here y is the number of equivalent positions. A partial rate factor calculation for nitration of toluene is given in Example 10.1. 

Example 10.1. The nitration of toluene is 23 times faster than nitration of benzene in nitric acid-acetic anhydride. The product ratio is 63% ortho, 34% para, and 3% meta. Calculate the partial rate factors. 

Draw a Lewis structure (or a series of Lewis structures) for nitrobenzenium ion. Where is the positive charge Examine the electrostatic potential map for nitrobenzenium ion. Where would you expect electron-donor substituents to have the greatest stabilizing effect (consider meta and para positions only) Which is the more stable, meta or para-nitrotoluenium ion (intermediates in nitration of toluene) Compare electrostatic potential maps to that for nitrobenzenium ion. Does your result suggest that methyl acts as an electron donor ... 

Predict the products of electrophilic nitration of toluene, aniline and nitrobenzene. 

Trinitrotoluene (TNT) is made by nitration of toluene. Display electrostatic potential maps for toluene, 4-nitrotoluene (the first nitration product) and 2,4-dinitrotoluene (the second nitration product). Are the second and third nitration steps likely to be easier or more difficult than the initial nitration of toluene Explain. 

Toluene (methylbenzene) is similar to benzene as a mononuclear aromatic, but it is more active due to presence of tbe electron-donating metbyl group. However, toluene is much less useful than benzene because it produces more polysubstituted products. Most of tbe toluene extracted for cbemical use is converted to benzene via dealkylation or disproportionation. Tbe rest is used to produce a limited number of petro-cbemicals. Tbe main reactions related to tbe cbemical use of toluene (other than conversion to benzene) are the oxidation of the methyl substituent and the hydrogenation of the phenyl group. Electrophilic substitution is limited to the nitration of toluene for producing mono-nitrotoluene and dinitrotoluenes. These compounds are important synthetic intermediates. 

Figure 16.13 Carbocation intermediates in the nitration of toluene. Ortho and para intermediates are more stable than the meta intermediate because the positive charge is on a tertiary carbon rather than a secondary carbon.

Influence of the Rate of Stirring on the Rate of Nitration of Toluene. 

The best yield, 95%, was obtained with the K salt (Ref 38). It is formed in small amts during the nitration of toluene to prepare TNT (Refs 37 38), and has been held responsible for exp Ins which have occurred in TNT mfg plants. 

The appropriate mixed acid compns for the nitration processes that produce militarily and industrially important expls will be described in Sections V VI. Typical MA compns for aromatic nitrations contain 110 to 200% nitric acid over the stoichiometric requirement. For the nitration of toluene to MNT DNT, a typical MA compn in round figures is 30% HN03,60% H2S04, 10% H20. For the nitration of DNT to TNT the MA contains no water and is approx 20% HN03, 80% H2S04... 

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