Nitration of Benzene and Toluene

Toluene, which is more reactive than benzene, may be nitrated with a more dilute mixed acid containing about 23% of water.97,98,100 Typical isomer compositions are 57-60% ortho, 37-40% para, and 3-4% meta isomers. Although 97-98% overall yields may be achieved with less than 0.5% of dinitrotoluenes, oxidative byproducts, mainly dinitrocresols, are formed.101 

Nitrobenzene and nitrotoluenes that are manufactured are converted to aniline, isocyanates, and other products used in the production of dyes and pharmaceuticals. 

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Burns, J. R., Ramshaw, C., A microreactor for the nitration of benzene and toluene, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, pp. 133-140 (5-9 March 2000), AlChE Topical Conf Proc., Atlanta, USA. 

No general experimental details for the preparation of nitro derivatives can be given, as the ease of nitration and the product formed frequently depend upon the exact experimental conditions. Moreover some organic compounds react violently so that nitrations should always be conducted on a small scale. Typical procedures for benzene hydrocarbons are illustrated by the following concise notes for the nitration of benzene and toluene to yield the solid dinitro compounds. (Full experimental details are given in Expts 6.17 and 6.18). 

From the chemical manufacturing industry, catalytic cracking and catalytic hydrogenation, gas absorption or scrubbing processes in which desired or waste products are removed from a waste stream, the nitration of benzene and toluene where the reactants have limited mutual solubility, and carbonylation processes using carbon monoxide. 

Fig 50. Heats of formation of nitro derivatives of benzene and toluene (a) and heats of nitration of benzene and toluene (b) in kcal/mole. (Garner and Abernethy [50]). 

The method of competitive nitration was extensively used by Ingold and coworkers [18] and by a number of other authors. Very spectacular are the results of the relative rates of nitration of benzene and toluene obtained by Cohn and co-workers [19]. The nitration was carried out with nitric acid in nitromethane. They gave relative reactivities of each position of toluene ... 

Industrial methods of mono-nitration of benzene and toluene Removal of phenolic by-produets Periodic nitration Continuous nitration... 

Industrial methods of mono-nitration of benzene and toluene are either periodical or continuous. More attention is now given to continuous methods safer and more economic when applied to a large, continuous production. Nevertheless. periodic nitration is still in use for smaller and less regular production. 

Matasft and Matasft (29) in their monograph collected technical data on the nitration of benzene and toluene (Table 26),... 

Several heterogeneous catalysts have been investigated for the liquid and vapor phase nitration processes. An early attempt by McKee and Wilhelm (3) used sihca gel as a solid catalyst for the nitration of benzene and toluene in the vapor phase. Various other heterogeneous catalysts systems have also been studied such as sulfonated polyorganosiloxanes (4), acidic resins (5), modified clays (6), zeolites (7,8), sulfated zirconia... 

Sulphuric acid acts as a solvent, increasing the aromatics concentration in the boundary acidic layer, where the reaction occurs. Both isothermal and adiabatic plants are used in industry for the continuous nitration of benzene and toluene. Despite conversions of about 98% or more in the case of benzene, the traditional "mixed-acid" process has some drawbacks. 

COMPETITIVE NITRATION OF BENZENE AND TOLUENE WITH N02 PFg" IN THE PRESENCE OF ALCOHOLS, ETHERS, AND THIOETHER IN CH N02 AT 25°... 

That, indeed, with highly reactive nitrating agents statistical substitution can take place was shown when carrying out free-radical nitration of benzene and toluene, for example, with... 

Whereas nitration of benzene and toluene with nltronlum salts Is generally too fast to allow noncompetitive rate measurement. In the case of nitrobenzene and nltrotoluenes. It Is possible to carry out noncompetitive rate studies. 

Figures 6 and 7 show the transformation between mass transfer cind kinetic control during the nitration of benzene and toluene for differing initial concentrations of nitric acid. Where the plots of log(Aw - At) cure linear, chemical kinetics are rate controlling, curved plots indicate the diffusion controlled and a treuisition region. If the initial concentration of nitric acid is low enough, the entire reaction is kinetically controlled.

In order to study this problem, Olah and Lin carried competitive studies of nitration of benzene and toluene with nitry chloride, catalyzed by Lewis acid halides. When carbon tetrachloride or excess aromatics were used as solvent, the data summarized in Table XII were obtained. The data show that the ortho para ratios are smaller than in nitrations with nitio-nium salts. The observed changes point to the fact that the nitrating agents arc the conesponding donor acceptor complexes and not the nitronium ion itself. The lower orthojpara ratios than those obtained in case of NOj, particularly point to bulkier nitrating agents. 

It is more fruitful to model the product distribution of fast competitive reactions using the engulfment micromixing mechanism . Thus for parallel reactions (e.g. simultaneous mono-nitration of benzene and toluene)... 

Traditional processes are still used for the nitration of benzene and toluene, and the hydrogenation of the nitro-derivatives to amines. Some aniline is also produced by amination of phenol. The production of polyurethanes, for foams or coatings, is a major outlet for aniline (c. 60%) and virtually the sole use for 2,4/2,6-toluenediamines. 

Nitration of aromatics has been studied by many workers (25-28) and complied by many workers (25). In general, all nitration reactions involve two steps formation of nitronium ion (NOj) and reaction between nitronium and the aromatic substance. Normally, the second step is the controlling one and the nitration of many aromatics, such as chlorobenzenes, is too slow to involve any diffusional resistance. On the other hand, depending on the conditions, the nitration of benzene and toluene may well fall into the fast reaction regime. Indeed, in the case of toluene nitration, at high concentrations of toluene, the reaction may be controlled by the formation of the nitronium ion, leading to pseudo zero order reaction. 

The characteristic kinetic feature of the case in which the formation of the electrophile is rate-determining is the absence of the concentration of the aromatic from the rate law. Nitration of benzene and toluene by nitric acid in nitromethane and of xylene and mesitylene in carbon tetrachloride are examples of nitration reactions that proceed at rates independent of the concentration of the aromatic substrate. With less reactive substrates, these nitrating systems exhibit rate laws including a term for the aromatic. Step 2 is slowed by the diminished reactivity on the part of the aromatic compound, and therefore becomes rate-determining. 

As might be expected, the type of feedstock chemical and industrial process play a key role in determining the presence of phenols in wastewater. For example, the nitration of benzene and toluene to produce nitrobenzene and dinitrotoluene also leads to the incidental formation of nitrophenol, dinitrophenol, and dinitro-o-cresol. Similarly, alkylphenols and methyl-phenols may be produced during alkylation and solvent extraction of toluene, xylene, and Cg-Q alkylphenols. Wise and Fahrenthold (1981) suggested that most industrial processes were not sources of priority pollutants because the processes do not involve critical precursor/process combinations. In addition, synthetic production methods generally lead to an increase in complexity of priority pollutant molecules. These in turn exhibit variable toxicity and persistance, which may be comparable to related priority pollutants. 

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