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Benzene

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The chemical compound benzene (C6H6) is a colorless, flammable, aromatic hydrocarbon, that is a known carcinogen. It boils at 80.1°C and solidifies at 5.5°C. Benzene has a heat of vaporization of 44.3 kJ/mol and a heat of fusion of 9.84 kJ/mol. Produced by hydrogen reduction of some allotropes of carbon, or from petroleum, it is used in the creation of drugs, plastics, gasoline, synthetic rubber, and dyes.

Other aromatic compounds created by the replacement of hydrogen atoms with methyl (CH3) groups are called the benzene series. If one hydrogen is replaced, the new chemical is called toluene, (C6H5CH3), from which trinitrotoluene (TNT) is derived. If two hydrogens are replaced it becomes xylene, (C6H4(CH3)2).

Replacement of the hydrogen atoms with other functional groups produces additional derivaties. A hydroxyl group (OH) produces phenol (C6H5OH), and additional nitration produces picric acid, or trinitrophenol. Replacement with an amino group (-NH2) produces aniline[?] (C6H5(NH2))

Two or more rings may be joined together, as in naphthalene, anthracene[?], and phenanthrene[?]. Other atoms, such as nitrogen, may be substituted for carbon atoms in the ring, as in pyridine (C5H5N) and pyrimidine (C4H4N2): in this case the ring is said to be a heterocyclic ring.

Structure

The formula of benzene (C6H6), caused a mystery for some time after its discovery, as no proposed structure could take account of all the bonds (Carbon usually forms four single bonds and hydrogen one). Legend has it that the chemist Kekulé dreamt of a snake eating its own tail, and upon waking was inspired to deduce the ring structure of benzene. Benzene presents a problem, as to account for all the bonds, there must be alternating double carbon bonds:

However, all of the carbon-carbon bonds in benzene are of the same length, and it is known that a single bond is longer than a double bond. In addition, the bond length (the distance between the two bonded atoms) in benzene is greater than a double bond, but shorter than a single bond. There seems in effect to be a bond and a half between each carbon.

This is explained by electron delocalization[?]. In order to picture this, we must consider the position of electrons in the bonds of benzene. The single bonds are formed with electrons orbiting in paths in line with this page. The double bonds consist of a single bond and another bond. This second bond has electrons orbiting in paths above and below the plane of this page at each bonded carbon atom. In the diagram below we take a side-view of this occurring:

@ @
C-C
@ @

The '-' denotes the single bond, and the '@'s denote the orbitals of the electrons forming the double bonds.

Being out of the plane of the atoms, these electrons can interact with each other freely, and become delocalised. This means that instead of being tied to one atom of carbon, they are shared by all six in the ring. Thus there are not enough to form double bonds on all the carbon atoms, but the atoms do strengthen all of the bonds on the ring equally.

What in effect is happening is that the structure exists as a superposition of the above forms, rather than either form individually. This type of structure is called a resonance hybrid.

To reflect the delocalised nature of the bonding, benzene is usually depicted as a circle inside a hexagon in chemical structure diagrams:

Benzene occurs sufficiently often as a component of organic molecules that there is a Unicode symbol to represent it; code 232C, ⌬

Industrial uses

Benzene is widely used in the United States; it ranks in the top 20 chemicals for production volume. Some industries use benzene to make other chemicals which are used to make plastics, resins, and nylon and synthetic fibers. Benzene is also used to make some types of rubbers, lubricants, dyes, detergents, drugs, and pesticides. Natural sources of benzene include volcanoes and forest fires. Benzene is also a natural part of crude oil, gasoline, and cigarette smoke.

Health effects

Breathing very high levels of benzene can result in death, while high levels can cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, and unconsciousness. Eating or drinking foods containing high levels of benzene can cause vomiting, irritation of the stomach, dizziness, sleepiness, convulsions, rapid heart rate, and death.

The major effect of benzene from long-term (365 days or longer) exposure is on the blood. Benzene causes harmful effects on the bone marrow and can cause a decrease in red blood cells leading to anemia. It can also cause excessive bleeding and can affect the immune system, increasing the chance for infection.

Some women who breathed high levels of benzene for many months had irregular menstrual periods and a decrease in the size of their ovaries. It is not known whether benzene exposure affects the developing fetus in pregnant women or fertility in men.

Animal studies have shown low birth weights, delayed bone formation, and bone marrow damage when pregnant animals breathed benzene.

The US Department of Health and Human Services[?] (DHHS) has determined that benzene is a known human carcinogen. Long-term exposure to high levels of benzene in the air can cause leukemia, cancer of the blood-forming organs.

Several tests can show if you have been exposed to benzene. There is test for measuring benzene in the breath; this test must be done shortly after exposure. Benzene can also be measured in the blood, however, since benzene disappears rapidly from the blood, measurements are accurate only for recent exposures.

In the body, benzene is converted to products called metabolites. Certain metabolites can be measured in the urine. However, this test must be done shortly after exposure and is not a reliable indicator of how much benzene you have been exposed to, since the metabolites may be present in urine from other sources.

The US Environmental Protection Agency has set the maximum permissible level of benzene in drinking water at 0.005 milligrams per liter (0.005 mg/L). The EPA requires that spills or accidental releases into the environment of 10 pounds or more of benzene be reported to the EPA.

The US Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 1 part of benzene per million parts of air (1 ppm) in the workplace during an 8-hour workday, 40-hour workweek.



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