Everything about Benzene Ring totally explained
» For other uses, see Benzene (disambiguation).
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Benzene, or
benzol, is an
organic chemical compound and a known
carcinogen with the molecular formula
C6H6. It is sometimes abbreviated
Ph–H. Benzene is a
colorless and highly
flammable liquid with a sweet smell and a relatively high melting point. Because of this, its use as an additive in
gasoline is now limited, but it's 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's usually synthesized from other compounds present in petroleum. Benzene is an
aromatic hydrocarbon and the second [
n]-
annulene ([6]-annulene), a cyclic hydrocarbon with a continuous
pi bond.
History
Discovery
The word "benzene" derives historically from "gum benzoin", sometimes called "benjamin" (for example,
benzoin resin), an aromatic resin known to European pharmacists and perfumers since the 15th century as a product of southeast Asia. "Benzoin" is itself a corruption of the Arabic expression "luban jawi," or "frankincense of Java." An acidic material was derived from benzoin by sublimation, and named "flowers of benzoin," or benzoic acid. The hydrocarbon derived from benzoic acid thus acquired the name benzin, benzol, or benzene.
Benzene has been the subject of many studies by scientists ranging from
Michael Faraday to
Linus Pauling. Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name
bicarburet of hydrogen.
In 1833,
Eilhard Mitscherlich produced it via the
distillation of
benzoic acid (from
gum benzoin) and
lime. Mitscherlich gave the compound the name
benzin. In 1836 the French chemist
Auguste Laurent named the substance "phène"; this is the root of the word
phenol, which is hydroxylated benzene, and
phenyl, which is the radical formed by abstraction of a hydrogen atom from benzene.
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.
Gradually the sense developed among chemists that substances related to benzene formed a natural chemical family. In 1855
August Wilhelm Hofmann used the word "
aromatic" to designate this family relationship, after a characteristic property of many of its members.
Ring formula
The empirical formula for benzene was long known, but its highly
polyunsaturated structure was challenging to determine.
Archibald Scott Couper in 1858 and
Joseph Loschmidt in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its very early years, and too little evidence was then available to help chemists decide on any particular structure.
In 1865 the German chemist
Friedrich August Kekulé published a paper in French (for he was then teaching in Francophone Belgium) suggesting that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German on the same subject. Kekulé used evidence that had accumulated in the intervening years—namely, that there always appeared to be only one
isomer of any
monoderivative of benzene, and that there always appeared to be exactly three isomers of every diderivative—to argue in support of his proposed structure. Kekulé's symmetrical ring could explain these curious facts.
The new understanding of benzene, and hence of all aromatic compounds, proved to be so important for both pure and applied chemistry that in 1890 the German Chemical Society organized an elaborate appreciation in Kekulé's honor, celebrating the twenty-fifth anniversary of his first benzene paper. Here Kekulé spoke of the creation of the theory. He said that he'd discovered the ring shape of the benzene molecule after having a reverie or day-dream of a snake seizing its own tail (this is a common symbol in many ancient cultures known as the
Ouroboros). This vision, he said, came to him after years of studying the nature of carbon-carbon bonds. This was 20 years after he'd solved the problem of how carbon atoms could bond to up to four other atoms at the same time. It is curious that a similar humorous depiction of benzene had appeared in 1886 in the
Berichte der Durstigen Chemischen Gesellschaft (Journal of the Thirsty Chemical Society), a parody of the
Berichte der Deutschen Chemischen Gesellschaft, only the parody had monkeys seizing each other in a circle, rather than snakes as in Kekulé's anecdote. Some historians have suggested that the parody was a lampoon of the snake anecdote, possibly already well-known through oral transmission even if it hadn't yet appeared in print. in which these anecdotes appeared has been translated into English. If one takes the anecdote as the memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862.
The cyclic nature of benzene was finally confirmed by the crystallographer
Kathleen Lonsdale.
Structure
Benzene represents 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 (pm). The C–C
bond lengths are greater than a double bond (135pm) but shorter than a single bond (147pm). 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. Realising each carbon has 2p electrons, each carbon donates 1p electron into the delocalised ring above and below the benzene ring. It is the side-on overlap of p-orbitals that produces the pi clouds.
Benzene occurs sufficiently often as a component of organic molecules that there's a
Unicode symbol with the code U+232C (⌬) to represent it with three double bonds, and U+23E3 (⏣) for a delocalized version .
Substituted benzene derivatives
Many 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 as
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 can't 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 (or "coke-oven light oil") 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.
Four chemical processes contribute to industrial benzene production:
catalytic reforming,
toluene hydrodealkylation, toluene disproportionation, and
steam cracking. In the US, 50% of benzene comes from
catalytic reforming and 25% from
steam cracking. In Western Europe, 50% of benzene comes from
steam cracking and 25% from
catalytic reforming.
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 (or reformate) 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. The extraction step of aromatics from the reformate is designed to produce aromatics with lowest non-aromatic components. So-called "BTX (Benzene-Toluene-Xylenes)" process consists of such extraction and distillation steps.
Similarly to this catalytic reforming,
UOP and
BP commercialized a method from LPG (mainly propane and butane) to aromatics.
Toluene hydrodealkylation
Toluene hydrodealkylation converts
toluene to benzene. In this hydrogen-intensive 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 (at the similar reaction condition). Under these conditions, toluene undergoes dealkylation according to the
chemical equation:
» C6H5CH3 +
H2 → C
6H
6 +
CH4
This irreversible reaction is accompanied by an equilibrium side reaction that produces
biphenyl (aka diphenyl) at higher temperature:
2 C
6H
6 ↔ H
2 + C
12H
10
If the raw material stream contains much non-aromatic components (paraffins or naphthenes), those are likely decomposed to lower hydrocarbons such as methane, which increases the consumption of hydrogen.
A typical reaction yield exceeds 95%. Sometimes,
xylenes and heavier aromatics are used in place of toluene, with similar efficiency.
This is often called "on-purpose" methodology to produce benzene, compared to conventional BTX (benzene-toluene-xylene) processes. The hydrodealkylation process isn't economically feasible if the price gap between benzene and toluene is small (or the gap is smaller than about 15% of benzene price).
Toluene disproportionation
Where a chemical complex has similar demands for both benzene and
xylene, then toluene
disproportionation (
TDP) may be an attractive alternative to the toluene hydrodealkylation. 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 (
p-xylene) substantially exceeds demand for other xylene isomers, a refinement of the TDP process called
Selective TDP (STDP) may be used. In this process, the xylene stream exiting the TDP unit is approximately 90% paraxylene. In some current catalytic systems, even the benzene-to-xylenes ratio is decreased (more xylenes) when the demand of xylenes is higher.
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 (in BTX process) 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 was supplanted by other solvents, especially
toluene (methyl benzene), which has similar physical properties but isn't as carcinogenic.
In 1903, Ludwig Roselius popularized the use of benzene to decaffeinate coffee. This discovery led to the production of
Sanka (the letters "ka" in the brand name stand 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. 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 limits typically around 1%. European petrol specifications now contain the same 1% limit on benzene content. The US EPA has new regulations that will lower the benzene content in gasoline to 0.62% in 2011.
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 (via
cumene), 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,
napalm and
pesticides.
In both US and Europe, 50% of benzene is used in the production of
ethylbenzene / styrene, 20% is used in the production of cumene, and about 15% of benzene is used in the production of cyclohexane (eventually to
nylon).
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. It can also be found in
tobacco smoke.
Reactions of benzene
- Electrophilic aromatic substitution is a general method of derivatizing benzene. Benzene is sufficiently nucleophilic that it undergoes substitution by acylium ions or alkyl carbocations to give substituted derivatives.
- Hydrogenation(Reduction): Benzene and derivatives convert to cyclohexane and derivatives when treated with hydrogen at 450 K and 10 atm of pressure with a finely divided nickel catalyst.
- Benzene is an excellent ligand in the organometallic chemistry of low-valent metals. Important examples include the sandwich and half-sandwich complexes respectively Cr(C6H6)2 and [RuCl2(C6H6)]2.
Health effects
Benzene exposure has serious
health effects. Outdoor air may contain low levels of benzene from tobacco smoke, automobile service stations, exhaust from motor vehicles, and industrial emissions. Vapors from products that contain benzene, such as glues, paints, furniture wax, and detergents, can also be a source of exposure. Air around hazardous waste sites or gas stations will contain higher levels of benzene.
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, and death.
The major effects of benzene are
chronic (long-term) exposure through 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.
Human exposure to benzene is a global health problem. Benzene targets liver, kidney, lung, heart and the brain and can cause
DNA strand breaks,
chromosomal damage etc. Benzene causes
cancer in both animals and humans. Benzene was first reported to induce cancer in humans in the 1920's. It wasn't until 1979 that the cancer inducing properties were determined conclusively in humans. Industry exploited this "discrepancy" and tried to discredit animal studies which showed benzene caused cancer saying that they weren't relevant to humans. Benzene has been shown to cause cancer in both sexes of multiple species of laboratory animals exposed via various routes.
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 isn't 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.
Benzene has been connected to a rare form of kidney cancer in two separate studies, one involving tank truck drivers, and the other involving seamen on tanker vessels, both carrying benzene laden chemicals.
The
US Department of Health and Human Services (DHHS) 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 or
acute non-lymphocytic leukaemia (AML & ANLL) may be caused by benzene.
Several tests can determine exposure 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, such as
trans,trans-muconic acid can be measured in the
urine. However, this test must be done shortly after exposure and isn't a reliable indicator of benzene exposure, since the same metabolites may be present in urine from other sources.
The
United States 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 (4.5 kg) or more of benzene be reported to the EPA.
The US
Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 0.5 part of benzene per million parts of air (.5 ppm) in the workplace during an 8-hour workday, 40-hour workweek. The short term exposure limit for airborne benzene is 5 ppm for 15 minutes.
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
anilide-contaminated
rapeseed oil.
Chronic Fatigue Syndrome has also been correlated with people who eat "denatured" food that use solvents to remove fat or contain
benzoic acid but causality is unproven.
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 isn't 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 (CNPC) factory in the city of Jilin on
13 November.
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. See
benzene in soft drinks
Biological oxidation and carcinogenic activity
One way of understanding the carcinogenic effects of benzene is by examining the products of biological oxidation. Pure benzene, for example, oxidizes in the body to produce an epoxide, benzene oxide, which isn't excreted readily and can interact with DNA to produce harmful mutations.
Further Information
Get more info on 'Benzene Ring'.
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