Let's talk about how toluene is nitrated. A huge number of semi-finished products used in the manufacture of explosives, pharmaceuticals are obtained by such interaction.
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, perfumery, 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 lubricant cleaner. Nitration of toluene gives benzidine, aniline, aminosalicylic acid, phenylenediamine.
Nitration characteristic
Nitration is characterized by the introduction of the NO2 group into the molecule of an organic compound. Depending on the starting substance, this process proceeds according to 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 substancessubstitutional nitration is possible, as well as double bond addition.
Nitration of toluene in an aromatic hydrocarbon molecule is carried out using a nitrating mixture (sulfuric and nitric acids). Catalytic properties are exhibited by sulfuric acid, which acts as a dewatering agent in this process.
Process equation
Nitration of toluene involves the replacement of one hydrogen atom with a nitro group. What does the process diagram look like?
In order to describe the nitration of toluene, the reaction equation can be represented as follows:
ArH + HONO2+=Ar-NO2 +H2 O
It allows us to judge only the general course of interaction, but does not reveal all the features of this process. What is actually happening is a reaction between aromatic hydrocarbons and nitric acid products.
Given that there are water molecules in the products, this leads to a decrease in the concentration of nitric acid, so the nitration of toluene slows down. To avoid this problem, this process is carried out at low temperatures, using excess nitric acid.
In addition to sulfuric acid, acetic anhydride, polyphosphoric acids, boron trifluoride are used as water-removing agents. They make it possible to reduce the consumption of nitric acid, increase the efficiency of interaction.
Nuances of the process
Nitration of toluene was described at the end of the nineteenth century by V. Markovnikov. He managed to establish a connection between the presence of concentrated sulfuric acid 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 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 resulting product, which makes the nitration of toluene available. The equation of the current process in general form is presented below:
ArH + HNO3 + BF3=Ar-NO2 + BF3 H2 O
After completion of the interaction, 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.
Nitration specifics
There are some features of this process related to the choice of reagents, reaction substrate. Consider some of their options in more detail:
- 60-65% nitric acid mixed with 96% sulfuric acid;
- mixture of 98% nitric acid and concentrated sulfuric acid is suitable for slightly reactive organics;
- potassium or ammonium nitrate with concentrated sulfuric acid is an excellent choice for the production of polymeric nitro compounds.
Nitration kinetics
Aromatic hydrocarbons interacting with a mixture of sulfuric andnitric acids are nitrated by the ionic mechanism. V. Markovnikov 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 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 you to increase the rate of nitration.
The resulting nitronium cation is attached to the core of aromatic toluene, and an intermediate compound is formed. Next, a proton is detached, 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. The reaction rate 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 elimination of a proton from toluene is almost instantaneous.
Only in some situations can 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 concentrated sulfuric acid is chosen as a catalyst and dewatering 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, therefore it is in demand in blasting. Among the environmental problems associated with its industrial production, we note the use of a significant amount of concentrated sulfuric acid.
To deal with this problem, chemists are looking for ways to reduce the sulfuric acid waste generated from 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 and poses an increased danger to living organisms. Subject to all safety standards, you can cope with these problems, get high quality nitro compounds.
