What is nitrobenzene? This is an organic compound that is an aromatic nucleus and a nitro group attached to it. In appearance, depending on the temperature, they are bright yellow crystals or an oily liquid. Has an almond scent. Toxic.
Structural formula of nitrobenzene
The nitro group is a very strong electron density acceptor. Therefore, the nitrobenzene molecule has a negative inductive and negative mesomeric effect. The nitro group rather strongly attracts the electron density of the aromatic nucleus, deactivating it. Electrophilic reagents are no longer attracted to the nucleus so strongly, and therefore nitrobenzene is not so actively involved in such reactions. In order to directly add another nitro group to nitrobenzene, very harsh conditions are needed, much more stringent than in the synthesis of mononitrobenzene. The same applies to halogens, sulfo groups, etc.
From the structural formula of nitrobenzene it can be seen that one bond of nitrogen with oxygen is single, and the other is double. But in fact, due to the mesomeric effect, they are both equivalent and have the same length of 0.123 nm.
Obtaining nitrobenzene in industry
Nitrobenzene is an important intermediate in the synthesis of many substances. Therefore, it is produced on an industrial scale. The main way to obtain nitrobenzene is the nitration of benzene. Usually, a nitrating mixture (a mixture of concentrated sulfuric and nitric acid) is used for this. The reaction is carried out for 45 minutes at a temperature of about 50 °C. The yield of nitrobenzene is 98%. That is why this method is mainly used in industry. For its implementation, there are special installations of both periodic and continuous types. In 1995, US nitrobenzene production was 748,000 tons per year.
Nitration of benzene can also be carried out simply with concentrated nitric acid, but in this case the yield of the product will be lower.
Obtaining nitrobenzene in the laboratory
There is another way to get nitrobenzene. Aniline (aminobenzene) is used here as a raw material, which is oxidized with peroxy compounds. Due to this, the amino group is replaced by a nitro group. But during this reaction, several by-products are formed, which prevents the effective use of this method in industry. Moreover, nitrobenzene is mainly used for the synthesis of aniline, so it makes no sense to use aniline for the production of nitrobenzene.
Physical properties
At room temperature, nitrobenzene is a colorless oily liquid with a bitter almond odor. At a temperature of 5.8 °C, ithardens into yellow crystals. Nitrobenzene boils at 211°C, and ignites spontaneously at 482°C. This substance, almost like any aromatic compound, is insoluble in water, but it is highly soluble in organic compounds, especially in benzene. It can also be steam distilled.
Electrophilic substitution
For nitrobenzene, as for any arene, reactions of electrophilic substitution into the nucleus are characteristic, although they are somewhat difficult compared to benzene due to the influence of the nitro group. So, dinitrobenzene can be obtained from nitrobenzene by further nitration with a mixture of nitric and sulfuric acids at an elevated temperature. The resulting product will predominantly (93%) consist of meta-dinitrobenzene. It is even possible to obtain trinitrobenzene in a direct way. But for this it is necessary to use even more stringent conditions, as well as boron trifluoride.
In the same way, nitrobenzene can be sulfonated. To do this, use a very strong sulphurizing agent - oleum (solution of sulfur oxide VI in sulfuric acid). The temperature of the reaction mixture must be at least 80 °C. Another electrophilic substitution reaction is direct halogenation. Strong Lewis acids (aluminum chloride, boron trifluoride, etc.) and elevated temperatures are used as catalysts.
Nucleophilic substitution
As can be seen from the structural formula, nitrobenzene can react with strong electron-donor compounds. This ispossibly due to the influence of the nitro group. An example of such a reaction is the interaction with concentrated or solid hydroxides of alkali metals. But this reaction does not form sodium nitrobenzene. The chemical formula of nitrobenzene rather suggests the addition of a hydroxyl group to the core, i.e., the formation of nitrophenol. But this happens only under fairly harsh conditions.
A similar reaction occurs with organomagnesium compounds. The hydrocarbon radical is attached to the nucleus in the ortho or para position to the nitro group. A side process in this case is the reduction of the nitro group to the amino group. Nucleophilic substitution reactions are easier if there are several nitro groups, since they will pull the electron density of the nucleus even more strongly.
Recovery reaction
As you know, nitro compounds can be reduced to amines. Nitrobenzene is no exception, the formula of which suggests the possibility of this reaction. It is often used in industry for the synthesis of aniline.
But nitrobenzene can give a lot of other recovery products. Most often, reduction with atomic hydrogen is used at the time of its release, i.e., an acid-metal reaction is carried out in the reaction mixture, and the released hydrogen reacts with nitrobenzene. Usually, this interaction produces aniline.
If nitrobenzene is treated with zinc dust in an ammonium chloride solution, the reaction product will be N-phenylhydroxylamine. This compound can be quite easily reduced to aniline in the standard way, or oxidized back to nitrobenzene with a strong oxidizing agent.
Reduction can also be carried out in the gas phase with molecular hydrogen in the presence of platinum, palladium or nickel. In this case, aniline is also obtained, but there is a possibility of reduction of the benzene ring itself, which is often undesirable. Sometimes a catalyst such as Raney nickel is also used. It is porous nickel, saturated with hydrogen and containing 15% aluminum.
When nitrobenzene is reduced by potassium or sodium alcoholates, azoxybenzene is formed. If you use stronger reducing agents in an alkaline environment, you get azobenzene. This reaction is also quite important, since some dyes are synthesized with its help. Azobenzene can be subjected to further reduction in an alkaline medium to form hydrazobenzene.
Initially, the reduction of nitrobenzene was carried out with ammonium sulfide. This method was proposed in 1842 by N. N. Zinin, so the reaction bears his name. But at the moment it is rarely used in practice due to the low yield.
Application
Nitrobenzene itself is used very rarely, only as a selective solvent (for example, for cellulose ethers) or a mild oxidizing agent. It is sometimes added to metal polishing solutions.
Almost all of the produced nitrobenzene is used to synthesize other useful substances (for example, aniline), which, in turn,used for the synthesis of drugs, dyes, polymers, explosives, etc.
Danger
Due to its physical and chemical properties, nitrobenzene is a very dangerous compound. It has a he alth hazard level three out of four under NFPA 704. In addition to being inhaled or through mucous membranes, it is also absorbed through the skin. When poisoned by a large concentration of nitrobenzene, a person can lose consciousness and die. At low concentrations, symptoms of poisoning include malaise, dizziness, tinnitus, nausea and vomiting. A feature of nitrobenzene poisoning is the high rate of infection. Symptoms appear very quickly: reflexes are disturbed, the blood becomes dark brown due to the formation of methemoglobin in it. Sometimes there may be rashes on the skin. The concentration sufficient for administration is very low, although there are no exact data on the lethal dose. Information is often found in specialized literature that 1-2 drops of nitrobenzene is enough to kill a person.
Treatment
In case of nitrobenzene poisoning, the victim must be immediately removed from the toxic area and rid of contaminated clothing. The body is washed with warm soapy water to remove nitrobenzene from the skin. Every 15 minutes, the victim is inhaled with carbogen. For mild poisoning, cystamine, pyridoxine, or lipoic acid should be taken. In more severe cases, intravenous methylene blue or chromosmon is recommended. Atpoisoning with nitrobeznol through the mouth, it is necessary to immediately induce vomiting and rinse the stomach with warm water. It is contraindicated to take any fat, including milk.
