Nitrotoluene, nitration

Smith and coworkers have screened the solid catalysts for aromatic nitration, and found that zeolite (3 gives the best result. Simple aromatic compounds such as benzene, alkylbenzenes, halogenobenzenes, and certain disubstituted benzenes are nitrated in excellent yields with high regioselectivity under mild conditions using zeolite (3 as a catalyst and a stoichiometric quantity of nitric acid and acetic anhydride.11 For example, nitration of toluene gives a quantitative yield of mononitrotoluenes, of which 79% is 4-nitrotoluene. Nitration of fluorobenzene under the same conditions gives p-fluoronitrobenzene exclusively (Eqs. 2.1 and 2.2)... 

The same sort of situation is encountered in the nitration of the nitrotoluenes. The following diagrams record the observed partial rate... 

Mixtures of HNO, H2SO4, and SO also result in high concentrations of NO/, and toluene can be readily nitrated at —40 to — 10°C as a result (6). At these low temperatures, the formation of the meta-isomer of mononitrotoluene (MNT) is greatiy reduced. Such a reduction is highly desired in the production both of dinitrotoluenes (DNTs) employed to produce intermediates for polyurethane production and of trinitrotoluene (TNT), which is a high explosive. > -MNT results in the production of undesired DNT and TNT isomers (see Nitrobenzene and nitrotoluenes). 

The mononitration of toluene results in the formation of a mixture of the ortho, meta, and para isomers of nitrotoluene. The presence of the methyl group on the aromatic ring facilitates the nitration, but it also increases the ease of oxidation. 

If pure isomers are required, the ortho and meta compounds can be prepared by indirect methods. o-Nitrotoluene can be obtained by treating 2,4-dinitrotoluene with ammonium sulfide followed by diazotization and boiling with ethanol. / -Nitrotoluene can be prepared from -toluidine by acetylation, nitration deacetylation, diazotization, and boiling with ethanol. A fairly pure -nitrotoluene, which has been isolated from the isomeric mixture, can be purified further by repeated crystallization. 

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. 

Processes. Toluene is nitrated ia two stages. Mononitration occurs ia mixed acid, 30% HNO and 55% H2SO4, at 30—70°C ia a series of continuous stirred-tank reactors. Heat is Hberated and must be removed. The isomer distribution is approximately 58% o-nitrotoluene 38% -nitrotoluene, and 4% y -nitrotoluene (Fig. 1). 

The nitration step produces two isomers, 2,4-dinitrotoluene and 2,6-dinitrotoluene, the former predorninating. Mixtures of the two isomers are frequendy used, but if single isomers are desired, particulady the 2,4-dinitrotoluene, nitration is stopped at the mono stage and pure i ra-nitrotoluene is obtained by crysta11i2ation. Subsequent nitration of this material yields only 2,4-dinitrotoluene for conversion to the diisocyanate. 

The TDI mixtures are produced by a series of reactions starting from toluene. The first stage is the nitration of the toluene to yield 2-nitrotoluene and 4-nitrotoluene in roughly equal proportions. These can be further nitrated, the 4-isomer yielding only 2,4-dinitrotoluene and 2-nitrotoluene both the 2,4- and the 2,6-dinitro compound Figure 27.1). 

If toluene is dinitrated without separation a mixture of about 80% 2,4-dinitrotoluene and 20% 2,6-dinitrotoluene is obtained. Nitration of separated 2-nitrotoluene will yield a mixture of approximately 65% of the 2,4- and 35% of... 

Three products are possible from nitration of toluene o-nitrotoluene, rw-nitro-toluene, and p-nitrotoluene. All ar e formed, but not in equal amounts. Together, the ortho-and para-substituted isomers make up 97% of the product mixture the rneta only 3%. 

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. 

While the direct halogenation of toluene gives a mixture of isomers that is difficult to separate into the pure isomers, the isomeric o- and /r-nitrotoluenes 6a and 6b, formed by nitration, are easy to separate from each other. Thus reduction of the single o- or /j-nitrotoluene 6 to the o- or /j-toluidine 7a or 7b respectively, followed by conversion into the corresponding diazonium salt 8 and a subsequent Sandmeyer reaction leads to the pure o- or /j-halotoluene 9. 

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. 

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 ... 

Influence of the Intensity of Stirring on the Rate of Nitration of p-Nitrotoluene. . . ... 

Schwarzenbeig, Berlin (1960), 768—84. Nitrobenzene, Nitrotoluene and Nitroxylene L.F. Albright, Ed, Kirk-Othmer Encyclopedia of Chemical Technology 13, 2nd Ed, Interscience, NY (1967), 784—96 (Nitration)... 

An important aspect of nitrating toluene to nitrotoluene is the very low solubility of toluene and nitroioluenes in nitrating mix Is. Therefore the nitration proceeds in a two-phase system and the rate of nitration depends greatly on dispersion, which in turn depends on keeping the two phases efficiently stirred. This is emphasized in Fig 4, which gives the yield of MNT for a 11/64/25% nitric acid/sulfuric acid/water MA and a nitration time of 30 minutes as a function of stirring speed... 

Fig 7a Influence of temperature on the yield of DNT, Nitration of o- and p-nitrotoluenes in nitrating mixtures with various concentrations of sulfuric acid (Kobe, Skinner and Prindle, Ref 45)... 

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