Nitration of aromatic hydrocarbons. Nitration of toluene: reaction equation Selected synthesis route - analysis, description of methods for performing experimental stages

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Unpurified TNT can only be used for the manufacture of explosive mixtures intended for rapid use, for example, for blasting operations. TNT used to equip ammunition that is subject to long-term storage must be subjected to purification.
TECHNOLOGY FOR OBTAINING TNT
Regularities of the nitration process
Nitration of toluene to mononitrotoluene. When toluene is nitrated to mononitrotoluene, three isomers are obtained: ortho-, para- and meta, with the ortho-mer predominant. The meta-isomer is formed in a relatively small amount, but since its further nitration produces unsymmetrical trinitrotoluenes, its formation is undesirable.
The main work on studying the laws of the reaction of nitration of toluene to mononitrotoluene was aimed mainly at identifying the conditions for the lowest yield of the let isomer. When continuous nitration processes became dominant and the question of rational designs of nitrators arose, the study of nitration kinetics began, mainly in hetero-
Rice. 17. Solubility of toluene in sulfuric acid of various concentrations at 55 °C.
65 70 75 80 85 90 Concentration HzS047%
95
gene environment, which corresponds to industrial conditions. At the same time, the solubility of toluene in sulfuric-nitric acid mixtures, the distribution of components between layers, the effect of stirring and the ratio of the volumes of organic and acidic layers on the reaction rate were studied. The solubility of toluene in sulfuric acid increases with increasing acid concentration; up to an acid concentration of 80%, the solubility of toluene is very low (Fig. 17).
The rate of toluene nitration under heterogeneous conditions depends sharply on the intensity of stirring and the bath modulus (the ratio of the volumes of the mineral and organic layers) (Fig. 18).
The distribution coefficient of nitric acid between the toluene and sulfuric acid layers is 0.066. This indicates that nitric acid during the heterogeneous nitration of toluene passes into the organic layer only to a small extent and therefore the proportion of the reaction occurring there is practically zero.
The low solubility of toluene in sulfuric acid of moderate concentrations, the absence of transition of nitric acid into the organic layer, as well as the sharp dependence of the reaction rate of toluene nitration on the intensity of mixing and the volume fraction of the mineral layer suggest that the nitration reaction of toluene under heterogeneous conditions occurs at the interface of the layers. The reaction rate in this case depends on the concentration of the reacting components^ on this surface, which in turn is determined by the rate of diffusion of the reacting components from the depth of the layer to the interface and the rate of departure of the reaction products from it. All this, as well as the state of the reacting components, depends on temperature (Fig. 19, c), the concentration of the acid mixture (Fig. 19, b) and the intensity of stirring (Fig. 18), and the rate of toluene nitration in heterogeneous conditions is lower than in homogeneous conditions (Fig. 19.6).
The formation of 5-6% l-nitrotoluene during the nitration of toluene further leads to the formation of 5-6% unsymmetrical trinitrotoluenes that contaminate TNT. Solidification temperature
Rice. 18. Influence of stirring intensity (a) and yanna modulus (6") on the degree of nitration of toluene.
The concentration of TNT containing asymmetrical isomers decreases according to the following dependence:
Г3 = (80.80 - 0.465С)
where C is the content of l-nitrotoluene in the original mononitrotoluene, %.
The isomeric composition of mononitrotoluene is significantly influenced by the nitration temperature of toluene (see page 85). The study of the rate of nitration of toluene at 0 and 30 °C and the determination of the isomeric composition made it possible to calculate in the Arrhenius equation (see page 54) the coefficient B for the entry of the nitro group into various positions relative to the CH3 group: B0 = 2.90 V, Bn = 2, 70 Vm\ activation energy for the corresponding positions is equal to: E„ - E0 = 3.83 kJ/(mol-°C), E„ - ?„ = 4.61 kJ/(mol-°C). From these data, the following rule can be formulated: “a decrease in the nitration temperature helps to increase the yield of p-nitrotoluene and decrease the yield of o- and m-nitrotoluenes.
The nitrating activity of the acid mixture has a significant influence on the yield of l-nitrotoluene. An increase in F from 68 to 82% during the nitration of toluene with sulfur-nitrogen acid mixtures at 55 °C reduces the yield of the let-isomer by 2.4 times. In Fig. Figure 20 shows the effect of temperature and the nitrating activity factor of the acid mixture on the yield of l-nitrotoluene.
The use of dilute nitric acid (70%) for the nitration of toluene leads to the formation of oxidation products, mainly benzoic acid. The use of an even more dilute acid (32%) at elevated temperature (105 ° C) causes nitration of the side chain, phenyl-nitromethane is formed, which is also obtained by nitration of toluene with nitrogen dioxide.
-l-I-a,; ,„i, I_I I-1_¦ \ ¦_I
25 zo 40 60 No. 70 in 64 her her yu 72 74
Tvmperashura, °С F
Rice. 19. Dependence of the rate of toluene nitration on temperature (a) and the nitrating activity factor F (6*):

It is not possible to introduce more than three nitro groups into benzene and toluene molecules by nitration.

Amino-2-nitrotoluene l-Nitro-p-toluidine sn L h/ 1 mn 0, and Toluene Nitration- reduction with soda sulphide

M. S. Bykhovskaya describes a method for the separate determination of benzene and toluene. By nitration, benzene is converted into dinitrobenzene, and toluene into trinitrotoluene 77

When fractionating the liquid part of the catalyzate, 14.5 g of hydrocarbon per bp was isolated. -IG (756 mai) n 1.4967 df 0.8665. A comparison of the constants of this fraction with literature data for toluene shows that it consists of toluene.

Nitration of this fraction yielded a nitro product with mp. 69° C. A mixed melting test with 2,4-dinitrotoluene did not produce depression.

Xylyl production. As we saw from the works of Martinsen, with the introduction of the 2nd methyl group into the nucleus, the rate of the nitration reaction increases significantly. The speed of the nitration reaction of ti-xi-lol is several times greater than that of toluene.

Therefore, mixtures for nitration of xylene can be significantly weaker than for nitration of toluene. This is also the reason why xylene can be nitrated into trinitroxylene even under factory conditions. one phase, while for toluene nitration V one phase has never been produced anywhere, due to the fact that this is associated with a huge consumption of acids, and therefore uneconomical, not to mention other difficulties and disadvantages of the nitration of toluene into trinitrotoluene in one phase.

Formation of significant amounts of zheta-substituted products when alkylation of toluene and other monosubstituted benzenes can be explained by the high reactivity of the attacking reagent. Since bromination is an example of a fairly mild substitution reaction, in this case there are strong differences between benzene and toluene, as well as between the meta and para positions in toluene.

Nitration is less selective than bromination; isopropylation is much less selective than nitration. At alkylation of toluene 30% of the summer isomer is formed. Moreover, with this reaction the differences between toluene and benzene become negligible.

A review of substitution reactions in the aromatic series allows us to draw a parallel between the selectivity of reactions with benzene and toluene, on the one hand, and between the meta- and para-positions in toluene, on the other. In both cases, selectivity decreases with increasing reaction ability of attacking agent 2. Table data. 4 illustrate these points.70

When considering the methods for determining toluene given in the literature, it seemed possible to use the photometric method. The latter has found wide application for the determination of small amounts of toluene, in particular in wastewater and in the air of industrial enterprises. Many colorimetric determinations of toluene are based on nitration and subsequent interaction of the resulting nitro compound with alkali or ammonia in various solvents: acetone, alcohol, methyl ethyl ketone, butanol, alcohol-ether mixture and others.
Depending on the intensity of cooling and stirring, the addition of toluene lasts 4-8 hours. The nitration temperature is maintained at about 50° and after the pouring is completed, it is heated for another period. % hour at 80-90°. Longer and stronger heating is impractical, since after the end of the infusion toluene nitration unlikely to continue.

For example, when heating is continued at 90° for 2 hours. The solidification temperature of dinitrotoluene increased in one case from 33.7° to only 35.6°. Upon completion of nitration, the contents of the apparatus are cooled to a temperature slightly higher than the temperature. melting of the dinitro product, and press the contents of the reactor using compressed air into one of the leaded separators (1,2), mixing the reaction mixture with such an amount of water that the dilution reaches approximately 16%. In this case, at first there is a very strong release of nitrogen oxides, for the removal of which a sufficiently powerful exhaust pipe is required. After standing for several hours at a temperature slightly higher than the melting point of the nitro product, the acid separates from the product forming the upper layer.377

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Let's talk about how toluene nitration is carried out. Through such interaction, a huge number of semi-finished products used in the manufacture of explosives and pharmaceuticals are obtained.

Importance of nitration

Benzene derivatives in the form of aromatic nitro compounds are produced in the modern chemical industry. Nitrobenzene is an intermediate product in aniline dye, perfume, and pharmaceutical production. It is an excellent solvent for many organic compounds, including cellulose nitrite, forming a gelatinous mass with it. In the petroleum industry it is used as a purifier for lubricating oils. The nitration of toluene produces benzidine, aniline, and phenylenediamine.

Nitration Characteristics

Nitration is characterized by the introduction of a NO2 group into the molecule of an organic compound. Depending on the starting substance, this process occurs according to a radical, nucleophilic, or electrophilic mechanism. Nitronium cations, NO2 ions and radicals act as active particles. The nitration reaction of toluene is a substitution reaction. For other organic substances, substitutive nitration is possible, as well as addition at a double bond.

Nitration of toluene in an aromatic hydrocarbon molecule is carried out using a nitrating mixture (sulfuric and nitric acids). It acts as a water-removing agent in this process and exhibits catalytic properties.

Process equation

Nitration of toluene involves the replacement of one hydrogen atom with a nitro group. What does the flow diagram of the process look like?

To describe the nitration of toluene, the reaction equation can be represented as follows:

ArH + HONO2+ = Ar-NO2 +H2O

It allows one to judge only the general course of interaction, but does not reveal all the features of this process. What actually occurs is a reaction between aromatic hydrocarbons and nitric acid products.

Considering that the products contain water molecules, this leads to a decrease in the concentration of nitric acid, so the nitration of toluene slows down. In order to avoid this problem, this process is carried out at low temperatures, using nitric acid in excess quantities.

In addition to sulfuric acid, polyphosphoric acids and boron trifluoride are used as water-removing agents. They make it possible to reduce the consumption of nitric acid and increase the efficiency of interaction.

Process nuances

Nitration of toluene was described at the end of the nineteenth century by V. Markovnikov. He was able to establish a connection between the presence in the reaction mixture and the rate of the process. In modern production of nitrotoluene, anhydrous nitric acid is used, taken in some excess.

In addition, the sulfonation and nitration of toluene involves the use of the available water-removing component boron fluoride. Its introduction into the reaction process makes it possible to reduce the cost of the resulting product, which makes the nitration of toluene accessible. The equation for the ongoing process is presented in general form below:

ArH + HNO3 + BF3= Ar-NO2 + BF3 ·H2 O

After completion of the reaction, water is introduced, due to which boron fluoride monohydrate forms a dihydrate. It is distilled off in a vacuum, then calcium fluoride is added, returning the compound to its original form.

Specifics of nitration

There are some features of this process related to the choice of reagents and reaction substrate. Let's look at some of their options in more detail:

• 60-65 percent nitric acid mixed with 96 percent sulfuric acid;
• a mixture of 98% nitric acid and concentrated sulfuric acid is suitable for slightly reactive organic substances;
• Potassium or ammonium nitrate with concentrated sulfuric acid is an excellent choice for the production of polymer nitro compounds.

Nitration kinetics

Reacting with a mixture of sulfuric and nitric acids, they are nitrated by an ionic mechanism. V. Markovnikov managed to characterize the specifics of this interaction. The process takes place in several stages. First, nitrosulfuric acid is formed, which undergoes dissociation in an aqueous solution. Nitronium ions react with toluene, forming nitrotoluene as a product. When water molecules are added to the mixture, the process slows down.

In solvents with an organic nature - nitromethane, acetonitrile, sulfolane - the formation of this cation allows you to increase the rate of nitration.

The resulting nitronium cation attaches to the aromatic toluene core, forming an intermediate. Next, proton abstraction occurs, leading to the formation of nitrotoluene.

For a detailed description of the ongoing process, we can consider the formation of “sigma” and “pi” complexes. The formation of the “sigma” complex is the limiting stage of the interaction. will be directly related to the rate of addition of the nitronium cation to the carbon atom in the nucleus of the aromatic compound. The removal of a proton from toluene occurs almost instantly.

Only in certain situations may there be any substitution problems associated with a significant primary kinetic isotope effect. This is due to the acceleration of the reverse process in the presence of various types of obstacles.

When choosing concentrated sulfuric acid as a catalyst and water-removing agent, a shift in the equilibrium of the process towards the formation of reaction products is observed.

Conclusion

The nitration of toluene produces nitrotoluene, which is a valuable product of the chemical industry. This substance is an explosive compound, therefore it is in demand in blasting operations. Among the environmental problems associated with its industrial production, we note the use of a significant amount of concentrated sulfuric acid.

To cope with this problem, chemists are looking for ways to reduce the sulfuric acid waste produced after the nitration process. For example, the process is carried out at low temperatures and easily regenerated media are used. Sulfuric acid has strong oxidizing properties, which negatively affects the corrosion of metals and poses an increased danger to living organisms. If you comply with all safety standards, you can overcome these problems and obtain high-quality nitro compounds.

Author L.A. Tsvetkov

Nitration of benzene can be carried out with small amounts of starting materials without isolating a pure product. To obtain nitrobenzene using the equation:

C 6 H 6 + HNO 3 à C 6 H 5 NO 2+ H 2 O

concentrated nitric acid (specific gravity 1.4) is required. The reaction mixture should not be heated above 50-60°C. When dilute acid is used, the nitration reaction does not occur; with increasing temperature, a noticeable formation of dinitrobenzene begins.

It follows from the equation that the reaction requires equimolecular amounts of the starting materials. However, in this case, the reaction will not reach completion, since the released water will dilute the nitric acid, and it will lose its nitrating property. Consequently, in order to complete the reaction, it is necessary to take more nitric acid than should be according to theory. But to prevent the reaction from becoming too violent, nitric acid must be dissolved in concentrated sulfuric acid, which does not deprive nitric acid of its nitrating effect and binds the water released during the reaction.

To prevent the possibility of an increase in temperature during the reaction, do not mix all the substances at once, but gradually add benzene to the mixture of acids. 8 ml of concentrated sulfuric acid and 5 ml of concentrated nitric acid are poured into a small flask. Cool the mixture under running water. Then 4 ml of benzene is added to the cooled mixture in small portions, constantly shaking the flask to achieve greater mixing of liquids that do not dissolve in each other (the mixture of acids makes up the bottom layer, benzene makes up the top layer). After adding all the benzene to achieve completeness of the reaction, the flask is closed with a stopper with a vertical tube (benzene vapor is volatile) and heated in a water bath preheated to 60°C

Shake the flask from time to time to better mix the liquids.

The duration of heating may be determined not so much by the need to achieve completeness of the reaction, but by the availability of time in the lesson. When working in a mug, heating should continue for 30-40 minutes. In the lesson, it is possible to demonstrate the formation of nitrobenzene after heating for 10 minutes and even without additional heating at all, if the reaction went well when benzene was added to a mixture of acids.

Nitrobenzene is placed in a layer on top of the acid mixture. Pour the contents of the flask into a glass with plenty of water. In this case, the acids dissolve in water, while nitrobenzene collects at the bottom of the glass in the form of a heavy yellowish liquid. If time permits, drain some of the liquid from the nitrobenzene and separate it using a separating funnel.

When significant quantities of nitrobenzene are obtained and it is necessary to purify it, nitrobenzene is washed with water, a diluted (5 percent) alkali solution, then again with water, each time separating the liquids using a separating funnel. The nitrobenzene is then dehydrated by heating it with granular calcium chloride until the liquid becomes clear. Heating is necessary in order to reduce the viscosity of nitrobenzene and thus achieve more complete contact with calcium chloride. Finally, nitrobenzene can be distilled from a small flask with an air cooler at a temperature of 204-207°C. To avoid decomposition of dinitrobenzene residues, dry distillation is not recommended.