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, these are bright yellow crystals or an oily liquid. Has an almond smell. Toxic
The structural formula of nitrobenzene
Nitro group is a very strong electron density acceptor. Therefore, the nitrobenzene molecule has a negative inductive and negative mesomeric effect. The nitro group quite strongly attracts the electron density of the aromatic nucleus, deactivating it. Electrophilic reagents are not so strongly attracted to the core, and therefore nitrobenzene is not so active in such reactions. To directly attach another nitro group to nitrobenzene, very stringent conditions are needed, much more stringent than in the synthesis of mononitrobenzene. The same goes for halogens, sulfo groups, etc.
It can be seen from the structural formula of nitrobenzene 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.
Nitrobenzene production in industry
Nitrobenzene is an important intermediate in the synthesis of many substances. Therefore, it is produced on an industrial scale. The main method for producing nitrobenzene is 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 type. In 1995, nitrobenzene production in the United States 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.
Getting nitrobenzene in the laboratory
There is another way to obtain nitrobenzene. Aniline (aminobenzene), which is oxidized with peroxy compounds, is used here as a raw material. Due to this, the amino group is replaced by a nitro group. But in the course of 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 the smell of bitter almonds. At 5.8 ° C, it solidifies, turning into yellow crystals. At 211 ° C, nitrobenzene boils, and at 482 ° C it spontaneously ignites. This substance, like almost any aromatic compound, is insoluble in water, but it is readily soluble in organic compounds, especially benzene. It can also be steam distilled.
Electrophilic substitution
For nitrobenzene, as well as for any arena, electrophilic substitution reactions to the nucleus are characteristic, although they are somewhat more difficult in comparison with benzene due to the influence of the nitro group. Thus, dinitrobenzene can be obtained from nitrobenzene by further nitration with a mixture of nitric and sulfuric acids at elevated temperatures. The resulting product will mainly (93%) consist of meta-dinitrobenzene. It is even possible to obtain trinitrobenzene directly. 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 sulfonating reagent - oleum (solution of sulfur oxide VI in sulfuric acid). The temperature of the reaction mixture should be at least 80 ° C. Another electrophilic substitution reaction is direct halogenation. Strong Lewis acids (aluminum chloride, boron trifluoride, etc.), as well as elevated temperature, are used as catalysts.
Nucleophilic substitution
As can be seen from the structural formula, nitrobenzene can react with strong electron-donating compounds. This is possible due to the influence of the nitro group. An example of such a reaction is the interaction with concentrated or solid alkali metal hydroxides. But sodium nitrobenzene is not formed in this reaction. The chemical formula of nitrobenzene suggests rather the addition of a hydroxyl group to the nucleus, i.e., the formation of nitrophenol. But this happens only under fairly harsh conditions.
A similar reaction proceeds 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 an amino group. Nucleophilic substitution reactions are simpler if there are several nitro groups, since they will pull the electron density of the nucleus even harder.
Recovery reaction
As you know, nitro compounds can be reduced to amines. Nitrobenzene, the formula of which suggests the possibility of this reaction, is no exception. It is often used in industry for the synthesis of aniline.
But nitrobenzene can give a lot of other recovery products. Most often, atomic hydrogen reduction is used at the moment of its evolution, i.e., an acid-metal reaction is carried out in the reaction mixture , and the hydrogen released reacts with nitrobenzene. Typically, this reaction produces aniline.
If nitrobenzene is affected by zinc dust in a solution of ammonium chloride, then the reaction product will be N-phenylhydroxylamine. This compound can quite easily be reduced in the standard way to aniline, or it can be oxidized back to nitrobenzene with a strong oxidizing agent.
Reduction can also be carried out in the gas phase by molecular hydrogen in the presence of platinum, palladium or nickel. In this case, aniline is also obtained, but there is a possibility of restoration of the benzene core itself, which is often undesirable. Sometimes a catalyst such as Raney nickel is also used. It is a porous nickel saturated with hydrogen and containing 15% aluminum.
When nitrobenzene is reduced by potassium or sodium alkoxides, azoxybenzene is formed. If you use stronger reducing agents in an alkaline environment, then azobenzene is obtained. This reaction is also quite important, since some dyes are synthesized with its help. Azobenzene can be further reduced in an alkaline medium to form hydrazobenzene.
Initially, the reduction of nitrobenzene was carried out with ammonium sulfide. This method was proposed by N. N. Zinin in 1842, 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 very rarely used, only as a selective solvent (for example, for cellulose ethers) or a mild oxidizing agent. Sometimes it is added to polishing solutions for metals.
Almost all nitrobenzene produced is used for the synthesis of other useful substances (for example, aniline), which, in turn, are 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 the third level of health hazard out of four according to the NFPA 704 standard. Besides the fact that it can enter the body through breathing or through the mucous membranes, it is also absorbed through the skin. In case of poisoning with a high 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 a high infection rate. Symptoms appear very quickly: reflexes are broken, the blood acquires a dark brown color due to the formation of methemoglobin in it. Sometimes skin rashes may be present. The concentration sufficient for administration is very small, although there is no exact data on the lethal dose. In the literature there is often information that 1-2 drops of nitrobenzene are enough to kill a person.
Treatment
In case of nitrobenzene poisoning, the victim must be immediately removed from the toxic zone and rid of contaminated clothing. The body is washed with warm water and soap to remove nitrobenzene from the skin. Every 15 minutes, the patient is inhaled with carbogen. For mild poisoning, it is necessary to take cystamine, pyridoxine or lipoic acid. In more severe cases, it is recommended to use methylene blue or chromosmon intravenously. In case of poisoning with nitrobenzene 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.