Let's talk about how toluene is nitrated. A huge amount of semi-finished products used in the manufacture of explosives, pharmaceuticals are obtained by such an interaction.
Significance of nitration
Derivatives of benzene in the form of aromatic nitro compounds are produced in the modern chemical industry. Nitrobenzene is an intermediate in aniline-paint, 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. When nitration of toluene get benzidine, aniline, aminosalicylic acid, phenylenediamine.
Nitration characteristic
Nitration is characterized by the introduction of a NO2 group into an organic compound molecule. Depending on the starting material, this process proceeds according to a radical, nucleophilic, electrophilic mechanism. Nitronium cations, ions and NO2 radicals act as active particles. The toluene nitration reaction refers to substitution. For other organic substances, substitution 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). The catalytic properties are manifested by sulfuric acid, which acts as a water-withdrawing agent in this process.
Process equation
Nitration of toluene involves the replacement of one hydrogen atom with a nitro group. What does the flow 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 one to judge only the general course of interaction, but does not reveal all the features of this process. In fact, a reaction occurs 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 toluene nitration is slowed down. In order to avoid such a problem, carry out this process at low temperatures, using nitric acid in excess.
In addition to sulfuric acid, acetic anhydride, polyphosphoric acids, boron trifluoride are used as water-stripping agents. They make it possible to reduce the consumption of nitric acid, increase the efficiency of interaction.
The nuances of the process
Toluene nitration 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 the modern production of nitrotoluene, anhydrous nitric acid, taken in some excess, is used.
In addition, toluene sulfonation and nitration is associated with the use of the available water-borne boron fluoride component. Its introduction into the reaction process allows to reduce the cost of the resulting product, which makes available the nitration of toluene. The equation of the process in general is presented below:
ArH + HNO3 + BF3 = Ar-NO2 + BF3 · H2 O
After the completion of the interaction, water is introduced, due to which boron fluoride monohydrate forms a dihydrate. It is distilled off in vacuo, then calcium fluoride is added, returning the compound to its original form.
Nitration specificity
There are some features of this process associated with the choice of reagents, substrate of the reaction. Let's consider 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.
Kinetics of nitration
Aromatic hydrocarbons interacting with a mixture of sulfuric and nitric acids are nitrated by the ionic mechanism. V. Markovnikov was able to characterize the specifics of this interaction. The process proceeds in several stages. First, nitrosulphuric acid is formed, which undergoes dissociation in an aqueous solution. Nitronium ions interact 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 is attached to the core of aromatic toluene, and an intermediate compound is formed. Then the proton is separated, 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 a “sigma" complex is a limiting stage of interaction. The reaction rate will be directly related to the rate of attachment of the nitronium cation to the carbon atom in the nucleus of the aromatic compound. The proton is cleaved from toluene almost instantly.
Only in some situations can there be any problems with substitution associated with a significant primary kinetic isotopic effect. This is due to the acceleration of the reverse process in the presence of obstacles of various kinds.
When choosing concentrated sulfuric acid as a catalyst and a water-taking agent, a shift in the equilibrium of the process towards the formation of reaction products is observed.
Conclusion
Nitrotoluene is formed during toluene nitration, which is a valuable chemical product. It is this substance that 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.
In order to cope with this problem, chemists are looking for ways to reduce the sulfuric acid waste obtained 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, is an increased danger to living organisms. If all safety standards are observed, you can cope with these problems and get high quality nitro compounds.