Encyclopedia
Benzene, also known as
benzol, is an
organic chemical compound with the formula
C6H6. It is sometimes abbreviated
PhH. Benzene is a
colorless and
flammable liquid with a sweet smell and a relatively high melting point. It is carcinogenic and is no longer used as an additive in
gasoline, but it is an important industrial solvent and precursor in the production of
drugs,
plastics, synthetic
rubber, and
dyes. Benzene is a natural constituent of
crude oil, but it is usually synthesized from other compounds present in petroleum. Benzene is an
aromatic hydrocarbon and the second [
n]-annulene .
History
Benzene has been the subject of many studies by many famous scientists ranging from
Michael Faraday to
Linus Pauling. In 1825 Faraday reported its isolation from oil gas and gave it the name
bicarburet of hydrogen. In 1833,
Eilhard Mitscherlich produced it via the
distillation of
benzoic acid and
lime. Mitscherlich gave the compound the name
benzin. In 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.
Structure
- Main article: aromaticity
The formula of benzene mystified scientists who could not figure out its structure. Friedrich August Kekulé von Stradonitz was the first to deduce the ring structure of benzene. An often-repeated story claims that after years of studying carbon bonding, benzene and related molecules, he dreamt one night of the
Ouroboros, a
snake eating its own tail, and that upon waking he was inspired to deduce the ring structure of benzene. However, the story first appeared in the
Berichte der Durstigen Chemischen Gesellschaft , a
parody of the
Berichte der Deutschen Chemischen Gesellschaft, which appeared annually in the late-19th century on the occasion of the congress of German chemists; as such, it is probably to be treated with circumspection.
While his claims were well-publicized and accepted, by the early-1920s Kekulé's biographer came to the conclusion that Kekulé's understanding of the tetravalent nature of carbon bonding depended on the previous research of Archibald Scott Couper ; furthermore, Josef Loschmidt had earlier posited a cyclic structure for benzene as early as 1861. The cyclic nature of benzene was finally confirmed by the eminent crystallographer Kathleen Lonsdale.
Benzene presents a special problem in that, to account for all the bonds, there must be alternating
double carbon bonds:
Using
X-ray diffraction, researchers discovered that all of the carbon-carbon bonds in benzene are of the same length of 140 picometres . The C-C
bond lengths are greater than a double bond but shorter than a single bond . This intermediate distance is explained by electron
delocalization: the electrons for C-C bonding are distributed equally between each of the six carbon atoms. One representation is that the structure exists as a superposition of so-called
resonance structures, rather than either form individually. This delocalisation of electrons is known as
aromaticity, and gives benzene great stability. This enhanced stability is the fundamental property of aromatic molecules that differentiates them from molecules that are non-aromatic. To reflect the delocalised nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of carbon atoms:
As is common in organic chemistry, the carbon atoms in the diagram above have been left unlabeled.
Benzene occurs sufficiently often as a component of organic molecules that there is a
Unicode symbol with the code 232C to represent it:
?.
Many fonts do not have this Unicode character, so a browser may not be able to display it correctly.
Substituted benzene derivatives
Main article:
Aromatic hydrocarbonsMany important chemicals are derived from benzene, wherein with one or more of the hydrogen atoms is replaced with another
functional group. Examples of simple benzene derivatives are
phenol,
toluene, and
aniline, abbreviated PhOH,PhMe, and PhNH
2, respectively. Linking benzene rings gives biphenyl, C
6H
5-C
6H
5. Further loss of hydrogen gives "fused" aromatic hydrocarbons, such
naphthalene and
anthracene. The limit of the fusion process is the hydrogen-free material
graphite.
In
heterocycles, carbon atoms in the benzene ring are replaced with other elements. The most important derivatives are the rings containing
nitrogen. Replacing one CH with N gives the compound
pyridine, C
5H
5N. Although benzene and pyridine are
structurally related, benzene cannot be converted into pyridine. Replacement of a second CH bond with N gives, depending on the location of the second N,
pyridazine,
pyrimidine, and
pyrazine.
Production
Trace amounts of benzene may result whenever
carbon-rich materials undergo incomplete combustion. It is produced in
volcanoes and
forest fires, and is also a component of
cigarette smoke.
Up until
World War II, most benzene was produced as a byproduct of coke production in the
steel industry. However, in the 1950s, increased demand for benzene, especially from the growing
plastics industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal.
Three chemical processes contribute equally to industrial benzene production: catalytic reforming,
toluene hydrodealkylation, and
steam cracking.
Catalytic reforming
In catalytic reforming, a mixture of
hydrocarbons with boiling points between 60-200 °C is blended with
hydrogen gas and then exposed to a bifunctional
platinum chloride or rhenium chloride
catalyst at 500-525 °C and pressures ranging from 8-50 atm. Under these conditions, aliphatic hydrocarbons form rings and lose hydrogen to become aromatic hydrocarbons. The aromatic products of the reaction are then separated from the reaction mixture by extraction with any one of a number of solvents, including
diethylene glycol or
sulfolane, and benzene is then separated from the other aromatics by distillation.
Toluene hydrodealkylation
Toluene hydrodealkylation converts
toluene to benzene. In this process, toluene is mixed with hydrogen, then passed over a
chromium, molybdenum, or
platinum oxide catalyst at 500-600 °C and 40-60 atm pressure. Sometimes, higher temperatures are used instead of a catalyst. Under these conditions, toluene undergoes dealkylation according to the chemical equation:
- C6H5CH3 + H2
| Triple point [i] || 13.8033 K, 7.042 kPa
...
? C
6H
6 +
CH4Typical reaction yields exceed 95%. Sometimes,
xylene and heavier aromatics are used in place of toluene, with similar efficiency.
Toluene disproportionation
Where a chemical complex has similar demands for both benzene and
xylene, then toluene disproportionation may be an attractive alternative. Broadly speaking 2 toluene molecules are reacted and the methyl groups rearranged from one toluene molecule to the other, yielding one benzene molecule and one xylene molecule.
Given that demand for
para-xylene substantially exceeds demand for other xylene isomers, a refinement of the TDP process called
Selective TDP may be used. In this process, the xylene stream exiting the TDP unit is approximately 90% paraxylene.
Steam cracking
Steam cracking is the process for producing
ethylene and other
olefins from aliphatic hydrocarbons. Depending on the feedstock used to produce the olefins, steam cracking can produce a benzene-rich liquid byproduct called
pyrolysis gasoline. Pyrolysis gasoline can be blended with other hydrocarbons as a gasoline additive, or distilled to separate it into its components, including benzene.
Uses
Early uses
In the 19th and early-20th centuries, benzene was used as an after-shave lotion because of its pleasant smell. Prior to the 1920s, benzene was frequently used as an industrial solvent, especially for degreasing metal. As its toxicity became obvious, benzene has been supplanted by other solvents.
In 1903, Lugwig Roselius popularized the use of benzene to decaffeinate coffee. This discovery lead to the production of Sanka, -ka for kaffein, This process was later discontinued.
As a petrol additive, benzene increases the
octane rating and reduces knocking. Consequently, petrol often contained several percent benzene before the 1950s, when
tetraethyl lead replaced it as the most widely-used antiknock additive. However, with the global phaseout of leaded petrol, benzene has made a comeback as a gasoline additive in some nations. In the
United States, concern over its negative health effects and the possibility of benzene entering the
groundwater have led to stringent regulation of petrol's benzene content, with values around 1% typical. European petrol specifications now contain the same 1% limit on benzene content.
Current uses of benzene
Today benzene is mainly used as an intermediate to make other chemicals. Its most widely-produced derivatives include
styrene, which is used to make polymers and plastics,
phenol for resins and adhesives , and
cyclohexane, which is used in the manufacture of Nylon. Smaller amounts of benzene are used to make some types of
rubbers, lubricants,
dyes,
detergents,
drugs,
explosives and
pesticides.
In laboratory research,
toluene is now often used as a substitute for benzene. The solvent-properties of the two are similar but toluene is less toxic and has a wider liquid range.
Benzene has been used as a basic research tool in a variety of experiments including analysis of a
two-dimensional gas.
Reactions of benzene
- sulfonation.
- Nitration: Benzene undergoes nitration with nitronioum ions as the electrophile. Thus, warming benzene with a combination of concentrated sulphuric and nitric acid gives nitrobenzene.
- Hydrogenation: Benzene and derivatives convert to cyclohexane and derivatives when treated with hydrogen at high hydrogen pressures.
- Benzene is an excellent ligand in the organometallic chemistry of low-valent metals. Important examples include the sandwich and half-sandwich complexes respectively Cr2 and [RuCl2]2.
Health effects
Benzene exposure has serious health effects. Breathing high levels of benzene can result in
death, while low 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 chronic exposure is to the
blood. Benzene damages the
bone marrow and can cause a decrease in red blood cells, leading to anemia. It can also cause excessive bleeding and depress the
immune system, increasing the chance of 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 classifies benzene as a human carcinogen. Long-term exposure to high levels of benzene in the air can cause leukemia, a potentially fatal
cancer of the blood-forming organs. In particular,
Acute Myeloid Leukemia may be caused by benzene.
Several tests can show if you have been exposed to benzene. There is a test for measuring benzene in the breath; this test must be done shortly after exposure. Benzene can also be measured in the blood; however, because benzene disappears rapidly from the blood, measurements are accurate only for recent exposures.
In the body, benzene is
metabolized. 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 same 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 . 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 has set a permissible exposure limit of 1 part of benzene per million parts of air in the workplace during an 8-hour workday, 40-hour workweek.
In March 2006, the official Food Standards Agency in
Britain conducted a survey of 150 brands of soft drinks. It found that four contained benzene levels above
World Health Organization limits. The affected batches were removed from sale.
In recent history there have been many examples of the harmful health effects of benzene and its derivatives. Toxic Oil Syndrome caused localised immune-suppression in
Madrid in 1981 from people ingesting benzene-contaminated olive-oil. Chronic Fatigue Syndrome has also been highly correlated with people who eat "denatured" food that use solvents to remove fat or contain
benzoic acid.
Workers in various industries that make or use benzene may be at risk for being exposed to high levels of this carcinogenic chemical. Industries that involve the use of benzene include the
rubber industry, oil refineries, chemical plants, shoe manufacturers, and
gasoline related industries. In 1987, OSHA estimated that about 237,000 workers in the United States were potentially exposed to benzene, and it is not known if this number has substantially changed since then.
Water and
soil contamination are important pathways of concern for transmission of benzene contact. In the U.S. alone there are approximately 100,000 different sites which have benzene soil or groundwater contamination. In 2005, the water supply to the city of
Harbin in China with a population of almost nine million people, was cut off because of a
major benzene exposure. Benzene leaked into the
Songhua River, which supplies drinking water to the city, after an explosion at a China National Petroleum Corporation factory in the city of Jilin on 13 November.
- Main article: benzene in soft drinks
References
- Archibald Scott Couper, On a New Chemical Theory, Philosophical Magazine 16, 104-116
- Josef Loschmidt, Chemische Studien I, Carl Gerold's Sohn, Vienna ,
- Josef Loschmidt, Chemische Studien I, Aldrich Chemical Co, Milwaukee , catalog no. Z-18576-0, and catalog no. Z-18577-9
- Kathleen Lonsdale, "The Structure of the Benzene Ring in Hexamethylbenzene," Proceedings of the Royal Society 123A: 494 .
- Kathleen Lonsdale, "An X-Ray Analysis of the Structure of Hexachlorobenzene, Using the Fourier Method," Proceedings of the Royal Society 133A: 536 .
- , CBS News, May 19, 2006, retrieved July 11, 2006
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