Nitration of toluene: reaction equation

Let's talk about how nitration of toluene is carried out. Obtain a similar interaction of a large number of semi-finished products used in the manufacture of explosives, pharmaceuticals.

The importance of nitration

Benzene derivatives in the form of aromatic nitro compounds are produced in the modern chemical industry. Nitrobenzene is a semifinished product in anilinokrasochnom, perfumery, pharmaceutical production. It is an excellent solvent for many organic compounds, including cellulose nitrite, forming a gelatinous mass with it. In the oil industry it is used as a cleaner for lubricating oils. Nitration of toluene gives benzidine, aniline, aminosalicylic acid, phenylenediamine.

Characterization of nitration

Nitration is characterized by the introduction of the NO2 group into the molecule of the organic compound. Depending on the starting material, this process proceeds through a radical, nucleophilic, electrophilic mechanism. Nitronium cations, ions and NO2 radicals act as active particles. The nitration reaction of toluene refers to substitution. For other organic substances, substitution nitration is possible, as well as double bond attachment.

Nitration of toluene in the aromatic hydrocarbon molecule is carried out with a nitrating mixture (sulfuric and nitric acids). Catalytic properties are shown by sulfuric acid, which acts as a water-absorbing agent in this process.

The equation of the process

Nitration of toluene involves replacing one hydrogen atom with a nitro group. What does the scheme of the proceeding process look like?

In order to describe the nitration of toluene, the reaction equation can be represented in the following form:

ArH + HONO2 + = Ar-NO2 + H2O

It allows us to judge only the general course of interaction, but does not disclose all the features of this process. In fact, there is a reaction between aromatic hydrocarbons and nitric acid products.

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

In addition to sulfuric acid, acetic anhydride, polyphosphoric acids, and boron trifluoride are used as dehydrating agents. They make it possible to reduce the consumption of nitric acid, increase the effectiveness of interaction.

The nuances of the process

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

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

ArH + HNO3 + BF3 = Ar-NO2 + BF3 · H2 O

After the completion of the reaction, water is introduced, whereby the 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.

Specificity of nitration

There are some features of this process associated with the choice of reagents, the reaction substrate. Let's consider some of their options in more detail:

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

Nitration kinetics

Aromatic hydrocarbons interacting with a mixture of sulfuric and nitric acids are nitrated by the ionic mechanism. V. Marknovnikov managed to characterize the specifics of this interaction. The process proceeds 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 the water molecules are added to the mixture, the process slows down.

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

The resulting cation of nitronium is attached to the core of aromatic toluene, and an intermediate is formed. Further, the proton is separated, leading to the formation of nitrotoluene.

For a detailed description of the ongoing process, one can consider the formation of "sigma" and "pi" complexes. The formation of the "sigma" complex is the limiting stage of the interaction. The reaction rate will be directly related to the rate of attachment of the cation of nitronium to the carbon atom in the core of the aromatic compound. Cleavage of the proton from toluene is almost instantaneous.

Only in some situations, there may be some replacement problems associated with a significant primary kinetic isotopic effect. This is due to the acceleration of the reverse process in the presence of obstacles of different kinds.

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

Conclusion

Nitration of toluene produces nitrotoluene, which is a valuable product of the chemical industry. It is this substance that is an explosive compound, that is why it is claimed in blasting operations. Among the environmental problems associated with its industrial production, we note the use of a significant amount of concentrated sulfuric acid.

In order 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, easily regenerated media are used. Sulfuric acid has strong oxidizing properties, which negatively affects the corrosion of metals, poses an increased danger to living organisms. If all safety standards are observed, it is possible to cope with these problems, to obtain high quality nitro compounds.