Benzene is a colorless, odorless and volatile liquid that burns with a yellow flame along with soot. And in industrial production, a group of materials such as polystyrene, synthetic rubber and nylon are used. This liquid is also used in the preparation of detergents and paints.
All benzene derivatives can be considered as aromatic organic substances.
Benzene belongs to the family of hydrocarbons, each molecule of which has 6 carbon atoms and 6 hydrogen atoms that form a ring arrangement. This arrangement is called a benzene ring, which is present in many compounds, including aspirin and the explosive trinitrotoluene. Benzene is toxic and carcinogenic.
Adding benzene to gasoline increases the octane number of gasoline and reduces the possibility of engine knocking. For this reason, until the 1950s, most gasoline had several percentages of benzene. But then tetraethyl lead became more common than benzene. The obsolescence of leaded gasoline has caused the return of benzene to the gasoline of some countries. But due to the negative effects of this material on health, strict regulations have been established regarding the amount of benzene in gasoline. which usually limits its amount to less than one percent.
History
Benzene was discovered by Michael Faraday in 1825. Faraday was the first to isolate benzene from a compressed luminous gas produced by the pyrolysis of Wall oil and named it hydrogen bicarb for the first time. Then, in 1834, a person named Eilhart Micherlich synthesized benzene by heating benzoic acid with calcium oxide and by measuring its vapor density, he showed that benzene has the molecular formula C6H6. In the past, benzene was obtained by heating coal tar and then turning its vapor into a liquid. But today, benzene is extracted in large quantities from crude oil.
molecular structure
According to the molecular formula of benzene, different structures are imagined for this molecule. Scientists of organic chemistry in the 19th century had different guesses about the structure of this molecule. Some of them were rejected due to the number of isomeric products derived from benzene, and others were accepted until the original and real structure of benzene was identified by Friedrich August Kekule in 1865.
Based on Kekule’s description, benzene is a cyclic molecule in which six carbon atoms form a regular hexagonal structure and each one is connected to a hydrogen atom. He stated that the covalent bonds between carbon atoms are single, simple and double. Of course, he believed that the places of single and double bonds change quickly and this causes the length of carbon-carbon bonds to be the same. (In fact, he described something similar to today’s concept of resonance for benzene).
Orbital description
Based on the description of orbitals caused by quantum mechanics, each carbon can be considered as having sp2 hybridization, and each of these orbitals are used to form a covalent sigma bond with the hydrogen atom and two neighboring carbon atoms. Finally, there remains a p orbital that is perpendicular to the plane of the molecule and has the ability to overlap laterally with similar orbitals on the adjacent carbons; Therefore, the pi electron cloud (∏) is formed due to the overlapping of six p orbitals on the six carbon atoms of the ring.
Attributes
Colorless liquid, aromatic and perhaps with a good smell, evaporates quickly and has extremely high flammability. This material is available in plastic materials, resin, nylon, lubricating oils, dyes, cleaners, pesticides, etc.
The permissible amount of benzene in gasoline
According to EN 228 and ASTM D 1319, the maximum allowed amount of benzene in gasoline is 1 percent by volume. For 2011, the United States has reduced the permissible amount of benzene in gasoline to 0.62%. In Iran, although using Euro 4 gasoline, the amount of benzene in gasoline has decreased. However, there is a difference between the responsible organizations regarding compliance with the standard.
The European standard limit for the airborne benzene pollutant is five milligrams per cubic meter (PPB). In Japan, it is three milligrams per cubic meter. While in the five polluted areas of Tehran, the highest level of benzene in the air is 150 milligrams per cubic meter and the lowest level is 50 milligrams per cubic meter. Therefore, the amount of benzene pollutant in the air of the capital of Iran in the late 1980s was at least 12 times higher than the international standards, and sometimes it reached 300 PPB and in some areas 1000 PPB. With the distribution of Euro 4 gasoline in metropolitan cities, the amount of benzene has reached 30 PPB in the mid-1390s.
The dangers of benzene
Benzene is known as a carcinogenic compound. All procedures involving benzene should be done under the hood and laboratory gloves should be used.
Benzene has been used as a common solvent in laboratories in the past. But after scientists discovered its carcinogenic identity. Its use as a solvent was very limited and it was tried to use similar solvents such as acetone and…
Long-term contact with benzene has destructive effects on the tissues that make up blood cells, especially bone marrow cells. Complications of chronic contact with benzene, reduction of body hematopoiesis, inability of the body’s immune system, as well as blood cancer, leukemia, disorder in the respiratory system, chronic forgetfulness, delay in the ossification of the human fetus, damage to the human reproductive system, infertility, production of glandular tumors. Lymph and damage to the liver. Several institutions, including the World Cancer Research Association, the American Environmental Protection Association, and the American Health Service Administration, have identified benzene as a cause of leukemia (leukemia) and a carcinogenic grade of 1. The latent period of leukemia usually occurs 5 to 15 years after the first contact.
Physical and chemical properties
| chemical formula | C6H6 |
| Molar mass | 78.114 g·mol−1 |
| Appearance | Colorless and aromatic liquid |
| density | 0.8765(20) g/cm3 |
| melting point | 5.53 °C (41.95 °F; 278.68 K) |
| boiling point | 80.1 °C (176.2 °F; 353.2 K) |
| Solubility in water | 1.53 g/L (0 °C) 1.81 g/L (9 °C) 1.79 g/L (15 °C) 1.84 g/L (30 °C) 2.26 g/L (61 °C) 3.94 g/L (100 °C) 21.7 g/kg (200 °C, 6.5 MPa) 17.8 g/kg (200 °C, 40 MPa) |
| Solubility in other solvents | Soluble in alcohol, diethyl ether, acetone, acetic acid, CHCl3, CCl4 |
| Solubility of benzene in ethanol alcohol | 20 °C, solution in water: 1.2 mL/L (20% v/v) |
| Chemical structure |  |
Is benzene polar?
It is a planar molecule shaped like a regular hexagon. It can be said that benzene is non-polar because it is a hydrocarbon containing only C-C bonds and C-H bonds and hydrocarbons are non-polar. But carbon is slightly more negative than H (0.35 units), so the C-H bond is slightly polar and has a weak dipole moment.
consumer market
The consumption market of this substance has grown significantly in the past years, and it is expected to have a significant growth trend in the coming years, especially in the countries of Asia and the Pacific, which are the most important consumers of this substance. . One of the biggest consumers of this substance is China with a compound annual growth rate of 4.8% from 2013 to 2018.
What is benzene in?
This substance is formed from both natural processes and human activities.
Its natural sources include volcanoes and forest fires. Benzene is also a natural component of crude oil, gasoline, and cigarette smoke.
This substance is widely used in the United States. It is among the top 20 chemicals in terms of production volume.
Some industries use this substance to make other chemicals that are used to make plastics, resins, nylons, and synthetic fibers. It is also used to make some types of lubricants, rubbers, paints, detergents, medicines and pesticides.
Production Process
The most important method of producing this substance is using steam cracking or LP catalytic reforming, especially naphtha, while benzene is also recovered from the aromatic stream.
applications
1) It is used as a raw material for the production of other chemicals, including alkylbenzene used in detergents, maleic anhydride and chlorobenzenes.
2) in the production of ethylbenzene, cyclohexane, cumene and nitrobenzene
3) in the printing industry
This material is also used in printing and paint industries. The ink that is used in printing equipment also contains amounts of this rheumatic substance. The reason for using benzol in printing inks is to maintain the liquid state of the ink until it is needed to use them in printing machines.
4) Tire and rubber production industries
This substance is used as a solvent in the rubber and tire industry. This material is also used as an adhesive for gluing the soles of shoes.
5) Repairs
In some industrial equipment, lubricants that contain this solvent are used to clean hydraulic equipment and fuel systems, because the solvents that contain this substance dissolve industrial greases without harming metals and equipment. hit
Other applications
Other uses of this material include lubricants, paint cleaners, lotions, and solvents to remove grease and clean industrial metals.
Health and environment
This chemical in the environment can react with various substances and produce smoke. On the other hand, this material may be broken down naturally and turned into other materials. But in fact, it is attached to raindrops or snow and in this way it contaminates water and soil. In aquatic environments where creatures such as fish live, the resulting pollution changes the appearance of these creatures and reduces their lifespan. On the other hand, due to the fact that humans also eat seafood, pollution may also enter the human body. The growth of plants in soil contaminated with C6H6 is greatly reduced.
