Benzene. Colorless liquid with a sweet odor. It evaporates quickly into the air and is slightly soluble in water . It is highly flammable and is formed both by natural processes and by human activities.
Brief history
Benzene was discovered in 1825 by the English scientist Michael Faraday , C6H6, as one of the components of an oil he collected from lighting gas pipes . Its composition was C6H6, given with modern symbols, and it had a melting point of 5 °C and a boiling point of 80 °C.
Hoffmann, in 1845 , discovered it in coal tar, which is still the main source of benzene and its derivatives. It was soon observed that benzene behaves like the compound from which a huge number of compounds were derived, which had been isolated in nature or had been prepared in the laboratory, with the famous tar dyes.
One tonne of coal transformed into coke in a furnace produces about 7.6 litres of benzene. Large quantities of benzene are currently obtained from petroleum, either by extracting it directly from certain types of crude oil or by chemical treatment of the oil.
The structure of the benzene molecule is of great importance in organic chemistry. The resonance ring structure theory described above was first formulated by the German chemist August Kekulé Von Stradonitz in 1865. For various reasons, 20th-century scientists had difficulty grasping this idea, and instead developed an orbital molecular description of electrons orbiting the entire molecule rather than the carbon atoms.
Benzene is used extensively in the United States ; it ranks 20th on the list of chemicals by largest production volume. Some industries use benzene to manufacture other chemicals used to make plastics , resins , nylon , and other synthetic fibers. Benzene is also used to make certain types of rubber , lubricants , dyes , detergents , medications, and pesticides . Volcanoes and forest fires are natural sources of benzene. Benzene is also a natural component of petroleum , gasoline , and cigarette smoke.
Structure of benzene
Analysis and determination of molecular weight show that the molecular formula of benzene is C6R6. Since the number of hydrogen atoms in benzene is much smaller than in the corresponding paraffinic hydrocarbon, hexane, C6R14, it is expected that benzene will undergo unsaturation reactions. This occurs, for example, in the following reactions: a) Benzene adds halogens up to a maximum of six atoms. b) Benzene can be catalytically hydrogenated to cyclohexane, being able to add a maximum of six hydrogen atoms.
These reactions indicate that benzene has three double bonds. However, they show that these bonds behave in a special way compared to aliphatic compounds, for example:
- Alkaline permanganate does not react with benzene in cold conditions, but prolonged boiling transforms it into CO2 and H20.
- In the absence of sunlight (and preferably in the presence of halogen transporters), benzene reacts with halogens by substitution.
Hydrogen halides do not add to benzene.
Physical properties
Benzene is a colorless liquid with a characteristic odor, indolucent in water, but soluble in alcohol, rubber, etc. It dissolves iodine, phosphorus, sulfur, camphor, fatty substances, rubber, etc. It is less dense than water, it boils at 80 c.
Chemical Properties
Aromatic substitution can follow three pathways; electrophilic, nucleophilic and free radical. The most common aromatic substitution reactions are those originated by electrophilic reagents. Their capacity to act as an electron donor is due to the polarization of the benzene nucleus. Typical reactions of benzene are substitution reactions. The most frequently used substitution agents are chlorine, bromine, nitric acid and hot concentrated sulfuric acid.
Benzene and the environment
Industrial processes are the main source of benzene in the environment. Benzene can get into the air from soil and water. In the air it reacts with other chemicals and breaks down within a few days. Benzene in the air can be washed onto the ground by rain or snow. It breaks down more slowly in water and soil, and can get into groundwater through the soil. Benzene does not accumulate in plants or animals.
Exposure to benzene
Low levels of benzene are found outdoors in tobacco smoke, gas stations, motor vehicle exhaust, and industrial emissions. Vapors (or gases) from products containing benzene, such as glues, paints, furniture polish, and detergents, can also be sources of benzene. The air around hazardous waste sites or gas stations may have higher levels of benzene. Working in industries that use or manufacture benzene.
Benzene and health
Inhaling very high levels of benzene can be fatal. Inhaling high levels can cause drowsiness, dizziness, rapid heartbeat, headache, tremors, confusion, and loss of consciousness. Swallowing food or drink containing high levels of benzene can cause vomiting, stomach irritation, dizziness, drowsiness, seizures, rapid heartbeat, and death.
The main effect of prolonged exposure to benzene is on the blood. Benzene causes changes in bone marrow and can lead to a decrease in the number of red blood cells, which can cause anemia. It can also cause bleeding and can affect the immune system, increasing the likelihood of contracting infections.
Some women who breathed high levels of benzene for several months experienced irregular menstruation and shrinkage of their ovaries, but it is not known for certain whether benzene caused these effects. It is not known whether benzene affects fertility in men.
Long-term exposure to high levels of benzene in the air can cause leukemia, especially acute myeloid leukemia, often referred to as AML. This is a cancer of the organs that make blood cells. The Department of Health and Human Services (DHHS) has determined that benzene is a known carcinogen. The International Agency for Research on Cancer (IARC) and the EPA have determined that benzene is carcinogenic to humans.
Benzene affects children
Benzene may affect children in the same way as adults. We do not know whether children are more susceptible to benzene toxicity than adults.
Benzene can pass from the mother’s blood to the fetus. Low birth weight, delayed bone formation, and bone marrow damage have been reported in offspring of animals exposed to benzene during pregnancy.
Benzene and risk of exposure
Exposure to benzene can be reduced by limiting contact with gasoline and cigarette smoke. Families should not smoke indoors or around children.
There are several tests that can determine if you have been exposed to benzene. There is a test to measure the amount of benzene in your breath; this test should be done shortly after exposure. Benzene can also be measured in your blood; however, because benzene disappears quickly from the blood, this test can only check for recent exposures.
In the body, benzene is broken down into products called metabolites. Some metabolites can be measured in urine. The metabolite S-phenylmercapturic acid in urine is a very sensitive index of benzene exposure. However, this test must be performed shortly after exposure and does not indicate with certainty how much benzene you have been exposed to because metabolites may have been generated from other sources.