Benzene is an important organic chemical compound with the chemical
formula C6H6. The benzene molecule is composed of six carbon atoms
joined in a ring with one hydrogen atom attached to each. As it
contains only carbon and hydrogen atoms, benzene is classed as a
Benzene is a natural constituent of crude oil and is one of the
elementary petrochemicals. Due to the cyclic continuous pi bond
between the carbon atoms, benzene is classed as an aromatic
hydrocarbon, the second [n]-annulene (-annulene). It is sometimes
Benzene is a colorless and highly flammable liquid
with a sweet smell, and is responsible for the aroma around petrol
stations. It is used primarily as a precursor to the manufacture of
chemicals with more complex structure, such as ethylbenzene and
cumene, of which billions of kilograms are produced. As benzene has a
high octane number, it is an important component of gasoline.
As benzene is a human carcinogen, most non-industrial applications
have been limited.
1.2 Ring formula
1.4 Early applications
4.1 Catalytic reforming
4.4 Steam cracking
4.5 Other methods
5.1 Component of gasoline
6.1 Sulfonation, chlorination, nitration
6.3 Metal complexes
7 Health effects
8 Exposure to benzene
Benzene exposure limits
8.2.1 Biomarkers of exposure
8.2.3 Molecular toxicology
8.2.4 Biological oxidation and carcinogenic activity
8.3 Routes of exposure
8.3.2 Exposure from soft drinks
8.3.3 Contamination of water supply
9 See also
11 External links
The word "benzene" derives from "gum benzoin" (benzoin resin), an
aromatic resin known to European pharmacists and perfumers since the
15th century as a product of southeast Asia. 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. Michael 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 by distilling benzoic acid (from gum benzoin) and lime. He gave the
compound the name benzin. In 1836, the French chemist Auguste
Laurent named the substance "phène"; this word has become the
root of the English word "phenol", which is hydroxylated benzene, and
"phenyl", the radical formed by abstraction of a hydrogen atom (free
radical H•) from benzene.
Kekulé's 1872 modification of his 1865 theory, illustrating rapid
alternation of double bonds
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 a number of substances were chemically related to benzene,
comprising a diverse chemical family. In 1855, Hofmann used the word
"aromatic" to designate this family relationship, after a
characteristic property of many of its members. In 1997, benzene
was detected in deep space.
Historic benzene structures (from left to right) by Claus (1867),
Dewar (1867), Ladenburg (1869), Armstrong (1887), Thiele
(1899) and Kekulé (1865).
Dewar benzene and prismane are
different chemicals that have Dewar's and Ladenburg's structures.
Thiele and Kekulé's structures are used today.
The empirical formula for benzene was long known, but its highly
polyunsaturated structure, with just one hydrogen atom for each carbon
atom, was challenging to determine.
Archibald Scott Couper
Archibald Scott Couper in 1858 and
Joseph Loschmidt in 1861 suggested possible structures that
contained multiple double bonds or multiple rings, but too little
evidence was then available to help chemists decide on any particular
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 ring of six 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 disubstituted derivative—now understood to correspond to the
ortho, meta, and para patterns of arene substitution—to argue in
support of his proposed structure. Kekulé's symmetrical ring
could explain these curious facts, as well as benzene's 1:1
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 had 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 or Endless knot). This vision, he said, came to him
after years of studying the nature of carbon-carbon bonds. This was 7
years after he had solved the problem of how carbon atoms could bond
to up to four other atoms at the same time. Curiously, a similar,
humorous depiction of benzene had appeared in 1886 in a pamphlet
entitled 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 had not yet appeared in print. Kekulé's 1890
speech in which this anecdote appeared has been translated into
English. If the anecdote is 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
Kathleen Lonsdale in 1929.
The German chemist
Wilhelm Körner suggested the prefixes ortho-,
meta-, para- to distinguish di-substituted benzene derivatives in
1867; however, he did not use the prefixes to distinguish the relative
positions of the substituents on a benzene ring. It was the German
chemist Karl Gräbe who, in 1869, first used the prefixes ortho-,
meta-, para- to denote specific relative locations of the substituents
on a di-substituted aromatic ring (viz, naphthalene). In 1870, the
Viktor Meyer first applied Gräbe's nomenclature to
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 (methylbenzene),
which has similar physical properties but is not as carcinogenic.
Ludwig Roselius popularized the use of benzene to
decaffeinate coffee. This discovery led to the production of Sanka.
This process was later discontinued.
Benzene was historically used as
a significant component in many consumer products such as Liquid
Wrench, several paint strippers, rubber cements, spot removers, and
other products. Manufacture of some of these benzene-containing
formulations ceased in about 1950, although Liquid Wrench continued to
contain significant amounts of benzene until the late 1970s.[citation
Trace amounts of benzene are found in petroleum and coal. It is a
byproduct of the incomplete combustion of many materials. For
commercial use, until World War II, most benzene was obtained as a
by-product of coke production (or "coke-oven light oil") for the steel
industry. However, in the 1950s, increased demand for benzene,
especially from the growing polymers industry, necessitated the
production of benzene from petroleum. Today, most benzene comes from
the petrochemical industry, with only a small fraction being produced
Main article: Aromaticity
The various representations of benzene
X-ray diffraction shows that all six carbon-carbon bonds in benzene
are of the same length, at 140 picometres (pm). The
C–C bond lengths are greater than a double bond (135 pm) but shorter
than a single bond (147 pm). This intermediate distance is consistent
with electron delocalization: the electrons for C–C bonding are
distributed equally between each of the six carbon atoms.
6 hydrogen atoms – fewer than the corresponding parent alkane,
hexane. The molecule is planar. The MO description involves the
formation of three delocalized π orbitals spanning all six carbon
atoms, while the VB description involves a superposition of resonance
structures. It is likely that this stability
contributes to the peculiar molecular and chemical properties known as
aromaticity. To accurately reflect the nature of the bonding, benzene
is often depicted with a circle inside a hexagonal arrangement of
Derivatives of benzene occur sufficiently often as a component of
organic molecules that the
Unicode Consortium has allocated a symbol
in the Miscellaneous Technical block with the code U+232C (⌬) to
represent it with three double bonds, and U+23E3 (⏣) for a
Aromatic hydrocarbons and Alkylbenzenes
Many important chemical compounds are derived from benzene by
replacing one or more of its hydrogen atoms with another functional
group. Examples of simple benzene derivatives are phenol, toluene, and
aniline, abbreviated PhOH, PhMe, and PhNH2, respectively. Linking
benzene rings gives biphenyl, C6H5–C6H5. Further loss of hydrogen
gives "fused" aromatic hydrocarbons, such as naphthalene and
anthracene. The limit of the fusion process is the hydrogen-free
allotrope of carbon, graphite.
In heterocycles, carbon atoms in the benzene ring are replaced with
other elements. The most important variations contain nitrogen.
Replacing one CH with N gives the compound pyridine, C5H5N. 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
Four chemical processes contribute to industrial benzene production:
catalytic reforming, toluene hydrodealkylation, toluene
disproportionation, and steam cracking. According to the ATSDR
Toxicological Profile for benzene, between 1978 and 1981, catalytic
reformats accounted for approximately 44–50% of the total U.S
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. Recovery of the aromatics, commonly referred
to as BTX (benzene, toluene and xylene isomers), involves such
extraction and distillation steps. There are a good many licensed
processes available for extraction of the aromatics.
In similar fashion to this catalytic reforming, UOP and BP
commercialized a method from LPG (mainly propane and butane) to
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 to
benzene and methane:
C6H5CH3 + H2 → C6H6 + CH4
This irreversible reaction is accompanied by an equilibrium side
reaction that produces biphenyl (aka diphenyl) at higher temperature:
6 ⇌ H
2 + C
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
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) extraction
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. In the broad sense, 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 is the process for producing ethylene and other alkenes
from aliphatic hydrocarbons. Depending on the feedstock used to
produce the olefins, steam cracking can produce a benzene-rich liquid
by-product called pyrolysis gasoline.
Pyrolysis gasoline can be
blended with other hydrocarbons as a gasoline additive, or routed
through an extraction process to recover BTX aromatics (benzene,
toluene and xylenes).
Although of no commercial significance, many other routes to benzene
Phenol and halobenzenes can be reduced with metals. Benzoic
acid and its salts undergo decarboxylation to benzene. Via the
reaction the diazonium compound with hypophosphorus acid aniline gives
Trimerization of acetylene gives benzene.
Benzene is used mainly as an intermediate to make other chemicals,
above all ethylbenzene, cumene, cyclohexane, nitrobenzene, and
alkylbenzene. More than half of the entire benzene production is
processed into ethylbenzene, a precursor to styrene, which is used to
make polymers and plastics like polystyrene and EPS. Some 20% of the
benzene production is used to manufacture cumene, which is needed to
produce phenol and acetone for resins and adhesives. Cyclohexane
consumes ca. 10% of the world's benzene production; it is primarily
used in the manufacture of nylon fibers, which are processed into
textiles and engineering plastics. Smaller amounts of benzene are used
to make some types of rubbers, lubricants, dyes, detergents, drugs,
explosives, and pesticides. In 2013, the biggest consumer country of
benzene was China, followed by the USA.
Benzene production is
currently expanding in the Middle East and in Africa, whereas
production capacities in Western Europe and North America are
Toluene is now often used as a substitute for benzene, for instance as
a fuel additive. The solvent-properties of the two are similar, but
toluene is less toxic and has a wider liquid range.
Toluene is also
processed into benzene.
Major commodity chemicals and polymers derived from benzene. Clicking
on the image loads the appropriate article
Component of gasoline
As a gasoline (petrol) additive, benzene increases the octane rating
and reduces knocking. As a consequence, gasoline 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 gasoline, 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's entering
the groundwater have led to stringent regulation of gasoline's benzene
content, with limits typically around 1%. European petrol
specifications now contain the same 1% limit on benzene content. The
United States Environmental Protection Agency introduced new
regulations in 2011 that lowered the benzene content in gasoline to
The most common reactions of benzene involve substitution of a proton
by other groups.
Electrophilic aromatic substitution
Electrophilic aromatic substitution is a general
method of derivatizing benzene.
Benzene is sufficiently nucleophilic
that it undergoes substitution by acylium ions and alkyl carbocations
to give substituted derivatives.
Electrophilic aromatic substitution
Electrophilic aromatic substitution of benzene
The most widely practiced example of this reaction is the ethylation
Approximately 24,700,000 tons were produced in 1999. Highly
instructive but of far less industrial significance is the
Friedel-Crafts alkylation of benzene (and many other aromatic rings)
using an alkyl halide in the presence of a strong
Lewis acid catalyst.
Friedel-Crafts acylation is a related example of
electrophilic aromatic substitution. The reaction involves the
acylation of benzene (or many other aromatic rings) with an acyl
chloride using a strong
Lewis acid catalyst such as aluminium chloride
or Iron(III) chloride.
Friedel-Crafts acylation of benzene by acetyl chloride
Sulfonation, chlorination, nitration
Using electrophilic aromatic substitution, many functional groups are
introduced onto the benzene framework. Sulfonation of benzene involves
the use of oleum, a mixture of sulfuric acid with sulfur trioxide.
Sulfonated benzene derivatives are useful detergents. In nitration,
benzene reacts with nitronium ions (NO2+), which is a strong
electrophile produced by combining sulfuric and nitric acids.
Nitrobenzene is the precursor to aniline. Chlorination is achieved
with chlorine to give chlorobenzene in the presence of a catalyst such
as aluminium tri-chloride.
Via hydrogenation, benzene and its derivatives convert to cyclohexane
and derivatives. This reaction is achieved by the use of high
pressures of hydrogen in the presence of heterogeneous catalysts, such
as finely divided nickel. Whereas alkenes can be hydrogenated near
room temperatures, benzene and related compounds are more reluctant
substrates, requiring temperatures >100 °C. This reaction is
practiced on a large scale industrially. In the absence of the
catalyst, benzene is impervious to hydrogen.
Hydrogenation cannot be
stopped to give cyclohexene or cyclohexadienes as these are superior
substrates. Birch reduction, a non catalytic process, however
selectively hydrogenates benzene to the diene.
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.
A bottle of benzene. The warnings show benzene is a toxic and
Benzene increases the risk of cancer and other illnesses, and is also
a notorious cause of bone marrow failure. Substantial quantities of
epidemiologic, clinical, and laboratory data link benzene to aplastic
anemia, acute leukemia, bone marrow abnormalities and cardiovascular
disease. The specific hematologic malignancies that
benzene is associated with include: acute myeloid leukemia (AML),
aplastic anemia, myelodysplastic syndrome (MDS), acute lymphoblastic
leukemia (ALL), and chronic myeloid leukemia (CML).
Petroleum Institute (API) stated in 1948 that "it is
generally considered that the only absolutely safe concentration for
benzene is zero". There is no safe exposure level; even tiny
amounts can cause harm. The US Department of Health and Human
Services (DHHS) classifies benzene as a human carcinogen. Long-term
exposure to excessive levels of benzene in the air causes leukemia, a
potentially fatal cancer of the blood-forming organs. In particular,
acute myeloid leukemia or acute nonlymphocytic leukemia (AML &
ANLL) is not disputed to be caused by benzene. IARC rated benzene
as "known to be carcinogenic to humans" (Group 1).
As benzene is ubiquitous in gasoline and hydrocarbon fuels are in use
everywhere, 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
animals including humans.
Benzene has been shown to cause cancer in
both sexes of multiple species of laboratory animals exposed via
Exposure to benzene
According to the Agency for Toxic Substances and Disease Registry
(ATSDR) (2007), benzene is both an anthropogenically produced and
naturally occurring chemical from processes that include: volcanic
eruptions, wild fires, synthesis of chemicals such as phenol,
production of synthetic fibers, and fabrication of rubbers,
lubricants, pesticides, medications, and dyes. The major sources of
benzene exposure are tobacco smoke, automobile service stations,
exhaust from motor vehicles, and industrial emissions; however,
ingestion and dermal absorption of benzene can also occur through
contact with contaminated water.
Benzene is hepatically metabolized
and excreted in the urine. Measurement of air and water levels of
benzene is accomplished through collection via activated charcoal
tubes, which are then analyzed with a gas chromatograph. The
measurement of benzene in humans can be accomplished via urine, blood,
and breath tests; however, all of these have their limitations because
benzene is rapidly metabolized in the human body.
OSHA regulates levels of benzene in the workplace. The maximum
allowable amount of benzene in workroom air during an 8-hour workday,
40-hour workweek is 1 ppm. As benzene can cause cancer, NIOSH
recommends that all workers wear special breathing equipment when they
are likely to be exposed to benzene at levels exceeding the
recommended (8-hour) exposure limit of 0.1 ppm.
Benzene exposure limits
United States Environmental Protection Agency has set a maximum
contaminant level (MCL) for benzene in drinking water at
0.005 mg/L (5 ppb), as promulgated via the U.S. National Primary
Water Regulations. This regulation is based on preventing
benzene leukemogenesis. The maximum contaminant level goal (MCLG), a
nonenforceable health goal that would allow an adequate margin of
safety for the prevention of adverse effects, is zero benzene
concentration in drinking water. The EPA requires that spills or
accidental releases into the environment of 10 pounds (4.5 kg) or
more of benzene be reported.
Occupational Safety and Health Administration
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. The short term exposure limit for airborne benzene is 5 ppm
for 15 minutes. These legal limits were based on studies
demonstrating compelling evidence of health risk to workers exposed to
benzene. The risk from exposure to 1 ppm for a working lifetime has
been estimated as 5 excess leukemia deaths per 1,000 employees
exposed. (This estimate assumes no threshold for benzene's
carcinogenic effects.) OSHA has also established an action level of
0.5 ppm to encourage even lower exposures in the workplace.
National Institute for Occupational Safety and Health
National Institute for Occupational Safety and Health (NIOSH)
Immediately Dangerous to Life and Health
Immediately Dangerous to Life and Health (IDLH)
concentration for benzene to 500 ppm. The current NIOSH definition for
IDLH condition, as given in the NIOSH Respirator Selection Logic,
is one that poses a threat of exposure to airborne contaminants when
that exposure is likely to cause death or immediate or delayed
permanent adverse health effects or prevent escape from such an
environment [NIOSH 2004]. The purpose of establishing an
IDLH value is
(1) to ensure that the worker can escape from a given contaminated
environment in the event of failure of the respiratory protection
equipment and (2) is considered a maximum level above which only a
highly reliable breathing apparatus providing maximum worker
protection is permitted [NIOSH 2004]. In September 1995, NIOSH
issued a new policy for developing recommended exposure limits (RELs)
for substances, including carcinogens. As benzene can cause cancer,
NIOSH recommends that all workers wear special breathing equipment
when they are likely to be exposed to benzene at levels exceeding the
REL (10-hour) of 0.1 ppm. The NIOSH short-term exposure limit
(STEL – 15 min) is 1 ppm.
American Conference of Governmental Industrial Hygienists (ACGIH)
adopted Threshold Limit Values (TLVs) for benzene at 0.5 ppm TWA and
2.5 ppm STEL.
Biomarkers of exposure
Several tests can determine exposure to benzene.
Benzene itself can be
measured in breath, blood or urine, but such testing is usually
limited to the first 24 hours post-exposure due to the relatively
rapid removal of the chemical by exhalation or biotransformation. Most
people in developed countries have measureable baseline levels of
benzene and other aromatic petroleum hydrocarbons in their blood. In
the body, benzene is enzymatically converted to a series of oxidation
products including muconic acid, phenylmercapturic acid, phenol,
catechol, hydroquinone and 1,2,4-trihydroxybenzene. Most of these
metabolites have some value as biomarkers of human exposure, since
they accumulate in the urine in proportion to the extent and duration
of exposure, and they may still be present for some days after
exposure has ceased. The current ACGIH biological exposure limits for
occupational exposure are 500 μg/g creatinine for muconic acid and 25
μg/g creatinine for phenylmercapturic acid in an end-of-shift urine
Even if it is not a common substrate for metabolism, benzene can be
oxidized by both bacteria and eukaryotes. In bacteria, dioxygenase
enzyme can add an oxygen to the ring, and the unstable product is
immediately reduced (by NADH) to a cyclic diol with two double bonds,
breaking the aromaticity. Next, the diol is newly reduced by
catechol. The catechol is then metabolized to acetyl CoA and succinyl
CoA, used by organisms mainly in the
Krebs Cycle for energy
The pathway for the metabolism of benzene is complex and begins in the
liver. Several enzymes are involved. These include cytochrome P450 2E1
(CYP2E1), quinine oxidoreductase (NQ01), GSH, and myeloperoxidase
(MPO). CYP2E1 is involved at multiple steps: converting benzene to
oxepin (benzene oxide), phenol to hydroquinone, and hydroquinone to
both benzenetriol and catechol. Hydroquinone, benzenetriol and
catechol are converted to polyphenols. In the bone marrow, MPO
converts these polyphenols to benzoquinones. These intermediates and
metabolites induce genotoxicity by multiple mechanisms including
inhibition of topoisomerase II (which maintains chromosome structure),
disruption of microtubules (which maintains cellular structure and
organization), generation of oxygen free radicals (unstable species)
that may lead to point mutations, increasing oxidative stress,
DNA strand breaks, and altering
DNA methylation (which can
affect gene expression). NQ01 and GSH shift metabolism away from
toxicity. NQ01 metabolizes benzoquinone toward polyphenols
(counteracting the effect of MPO). GSH is involved with the formation
of phenylmercapturic acid.
Genetic polymorphisms in these enzymes may induce loss of function or
gain of function. For example, mutations in CYP2E1 increase activity
and result in increased generation of toxic metabolites. NQ01
mutations result in loss of function and may result in decreased
detoxification. Myeloperoxidase mutations result in loss of function
and may result in decreased generation of toxic metabolites. GSH
mutations or deletions result in loss of function and result in
decreased detoxification. These genes may be targets for genetic
screening for susceptibility to benzene toxicity.
The paradigm of toxicological assessment of benzene is shifting
towards the domain of molecular toxicology as it allows understanding
of fundamental biological mechanisms in a better way. Glutathione
seems to play an important role by protecting against benzene-induced
DNA breaks and it is being identified as a new biomarker for exposure
Benzene causes chromosomal aberrations in the
peripheral blood leukocytes and bone marrow explaining the higher
incidence of leukemia and multiple myeloma caused by chronic exposure.
These aberrations can be monitored using fluorescent in situ
hybridization (FISH) with
DNA probes to assess the effects of benzene
along with the hematological tests as markers of hematotoxicity.
Benzene metabolism involves enzymes coded for by polymorphic genes.
Studies have shown that genotype at these loci may influence
susceptibility to the toxic effects of benzene exposure. Individuals
carrying variant of NAD(P)H:quinone oxidoreductase 1 (NQO1),
microsomal epoxide hydrolase (EPHX) and deletion of the glutathione
S-transferase T1 (GSTT1) showed a greater frequency of DNA
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 is not excreted readily and can interact with
DNA to produce
Routes of exposure
Outdoor air may contain low levels of benzene from automobile service
stations, wood smoke, tobacco smoke, the transfer of gasoline, exhaust
from motor vehicles, and industrial emissions. About 50% of the
entire nationwide (United States) exposure to benzene results from
smoking tobacco or from exposure to tobacco smoke. After smoking
32 cigarettes per day, the smoker would take in about 1.8 milligrams
(mg) of benzene. This amount is about 10 times the average daily
intake of benzene by nonsmokers.
Inhaled benzene is primarily expelled unchanged through exhalation. In
a human study 16.4 to 41.6% of retained benzene was eliminated through
the lungs within five to seven hours after a two- to three-hour
exposure to 47 to 110 ppm and only 0.07 to 0.2% of the remaining
benzene was excreted unchanged in the urine. After exposure to 63 to
405 mg/m3 of benzene for 1 to 5 hours, 51 to 87% was excreted in
the urine as phenol over a period of 23 to 50 hours. In another human
study, 30% of absorbed dermally applied benzene, which is primarily
metabolized in the liver, was excreted as phenol in the urine.
Exposure from soft drinks
Benzene in soft drinks
Under specific conditions and in the presence of other chemicals
benzoic acid (a preservative) and ascorbic acid (Vitamin C) may
interact to produce benzene. 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.
Similar problems were reported by the FDA in the United States.
Contamination of water supply
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
Petroleum Corporation (CNPC) factory in the city of Jilin on
13 November 2005.
6-membered aromatic rings with one carbon replaced by another group:
borabenzene, benzene, silabenzene, germabenzene, stannabenzene,
pyridine, phosphorine, arsabenzene, pyrylium salt
Industrial Union Department v. American
^ Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th
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^ Arnold, D.; Plank, C.; Erickson, E.; Pike, F. (1958). "
Benzene in Water". Industrial & Engineering Chemistry Chemical
& Engineering Data Series. 3 (2): 253–256.
^ Breslow, R.; Guo, T. (1990). "Surface tension measurements show that
chaotropic salting-in denaturants are not just water-structure
breakers". Proceedings of the National Academy of Sciences of the
United States of America. 87 (1): 167–9.
PMC 53221 . PMID 2153285.
^ Coker, A. Kayode; Ludwig, Ernest E. (2007). Ludwig's Applied Process
Design for Chemical And
Petrochemical Plants. 1. Elsevier.
p. 114. ISBN 0-7506-7766-X. Retrieved 2012-05-31.
^ a b c d e
^ a b Atherton Seidell; William F. Linke (1952). Solubilities of
Inorganic and Organic Compounds: A Compilation of
Solubility Data from
the Periodical Literature. Supplement. Van Nostrand.
^ a b c
Benzene in Linstrom, Peter J.; Mallard, William G.
(eds.); NIST Chemistry WebBook, NIST Standard Reference Database
Number 69, National Institute of Standards and Technology,
Gaithersburg (MD), http://webbook.nist.gov (retrieved 2014-05-29)
^ a b c
Sigma-Aldrich Co., Benzene. Retrieved on 2014-05-29.
^ a b c "NIOSH Pocket Guide to Chemical Hazards #0049". National
Institute for Occupational Safety and Health (NIOSH).
Immediately Dangerous to Life and Health
Immediately Dangerous to Life and Health Concentrations
(IDLH). National Institute for Occupational Safety and Health
^ The word "benzoin" is derived from the Arabic expression "luban
jawi", or "frankincense of Java". Morris, Edwin T. (1984). Fragrance:
The Story of Perfume from Cleopatra to Chanel. Charles Scribner's
Sons. p. 101. ISBN 0684181959.
^ a b Rocke, A. J. (1985). "Hypothesis and Experiment in the Early
Development of Kekule's
Benzene Theory". Annals of Science. 42 (4):
^ Faraday, M. (1825). "On new compounds of carbon and hydrogen, and on
certain other products obtained during the decomposition of oil by
heat". Philosophical Transactions of the Royal Society. 115:
440–466. doi:10.1098/rstl.1825.0022. JSTOR 107752. On
pages 443–450, Faraday discusses "bicarburet of hydrogen" (benzene).
On pages 449–450, he shows that benzene's empirical formula is C6H6,
although he doesn't realize it because he (like most chemists at that
time) used the wrong atomic mass for carbon (6 instead of 12).
^ Kaiser, R. (1968). "Bicarburet of Hydrogen. Reappraisal of the
Benzene in 1825 with the Analytical Methods of 1968".
Angewandte Chemie International Edition in English. 7 (5): 345–350.
^ Mitscherlich, E. (1834). "Über das Benzol und die Säuren der Oel-
und Talgarten" [On benzol and oily and fatty types of acids]. Annalen
der Pharmacie. 9 (1): 39–48. doi:10.1002/jlac.18340090103. In
a footnote on page 43, Liebig, the journal's editor, suggested
changing Mitscherlich's original name for benzene (namely, "benzin")
to "benzol", because the suffix "-in" suggested that it was an
alkaloid (e.g., Chinin (quinine)), which benzene isn't, whereas the
suffix "-ol" suggested that it was oily, which benzene is. Thus on
page 44, Mitscherlich states: "Da diese Flüssigkeit aus der
Benzoësäure gewonnen wird, und wahrscheinlich mit den
Benzoylverbindungen im Zusammenhang steht, so gibt man ihr am besten
den Namen Benzol, da der Name Benzoïn schon für die mit dem
Bittermandelöl isomerische Verbindung von Liebig und Wöhler gewählt
worden ist." (Since this liquid [benzene] is obtained from benzoic
acid and probably is related to benzoyl compounds, the best name for
it is "benzol", since the name "benzoïn" has already been chosen, by
Liebig and Wöhler, for the compound that's isomeric with the oil of
bitter almonds [benzaldehyde].)
^ Laurent, Auguste (1836) "Sur la chlorophénise et les acides
chlorophénisique et chlorophénèsique," Annales de Chemie et de
Physique, vol. 63, pp. 27–45, see p. 44: "Je donne le nom de phène
au radical fondamental des acides précédens (φαινω,
j'éclaire), puisque la benzine se trouve dans le gaz de
l'éclairage." (I give the name of "phène" (φαινω, I illuminate)
to the fundamental radical of the preceding acids, because benzene is
found in illuminating gas.)
^ Critics pointed out a problem with Kekulé's original (1865)
structure for benzene: Whenever benzene underwent substitution at the
ortho position, two distinguishable isomers should have resulted,
depending on whether a double bond or a single bond existed between
the carbon atoms to which the substituents were attached; however, no
such isomers were observed. In 1872, Kekulé suggested that benzene
had two complementary structures and that these forms rapidly
interconverted, so that if there were a double bond between any pair
of carbon atoms at one instant, that double bond would become a single
bond at the next instant (and vice versa). To provide a mechanism for
the conversion process, Kekulé proposed that the valency of an atom
is determined by the frequency with which it collided with its
neighbors in a molecule. As the carbon atoms in the benzene ring
collided with each other, each carbon atom would collide twice with
one neighbor during a given interval and then twice with its other
neighbor during the next interval. Thus, a double bond would exist
with one neighbor during the first interval and the other neighbor
during the next interval. See pages 86–89 of Auguste Kekulé (1872)
"Ueber einige Condensationsprodukte des Aldehyds" (On some
condensation products of aldehydes), Liebig's Annalen der Chemie und
Pharmacie, 162(1): 77–124, 309–320.
^ Hofmann, A. W. (1845) "Ueber eine sichere Reaction auf Benzol" (On a
reliable test for benzene), Annalen der Chemie und Pharmacie, vol. 55,
pp. 200–205; on pp. 204–205, Hofmann found benzene in coal tar
^ Mansfield Charles Blachford (1849). "Untersuchung des
Steinkohlentheers". Annalen der Chemie und Pharmacie. 69: 162–180.
^ Charles Mansfield filed for (November 11, 1847) and received (May
1848) a patent (no. 11,960) for the fractional distillation of coal
^ Hoffman, Augustus W. (1856). "On insolinic acid". Proceedings of the
Royal Society. 8: 1–3. doi:10.1098/rspl.1856.0002. The existence and
mode of formation of insolinic acid prove that to the series of
monobasic aromatic acids, Cn2Hn2-8O4, the lowest known term of which
is benzoic acid, … . [Note: The empirical formulas of organic
compounds that appear in Hofmann's article (p. 3) are based upon an
atomic mass of carbon of 6 (instead of 12) and an atomic mass of
oxygen of 8 (instead of 16).]
^ Cernicharo, José; et al. (1997), "Infrared Space Observatory's
Discovery of C4H2, C6H2, and
Benzene in CRL 618", Astrophysical
Journal Letters, 546 (2): L123–L126, Bibcode:2001ApJ...546L.123C,
^ Claus, Adolph K.L. (1867) "Theoretische Betrachtungen und deren
Anwendungen zur Systematik der organischen Chemie" (Theoretical
considerations and their applications to the classification scheme of
organic chemistry), Berichte über die Verhandlungen der
Naturforschenden Gesellschaft zu Freiburg im Breisgau (Reports of the
Proceedings of the Scientific Society of Freiburg in Breisgau),
4 : 116-381. In the section Aromatischen Verbindungen (aromatic
compounds), pp. 315-347, Claus presents Kekulé's hypothetical
structure for benzene (p. 317), presents objections to it, presents an
alternative geometry (p. 320), and concludes that his alternative is
correct (p.326). See also figures on p. 354 or p. 379.
^ Dewar, James (1867) "On the oxidation of phenyl alcohol, and a
mechanical arrangement adapted to illustrate structure in the
Proceedings of the Royal Society of
Edinburgh 6: 82–86.
^ Ladenburg, Albert (1869) "Bemerkungen zur aromatischen Theorie"
(Observations on the aromatic theory), Berichte der Deutschen
Chemischen Gesellschaft 2: 140–142.
^ Armstrong, Henry E. (1887) "An explanation of the laws which govern
substitution in the case of benzenoid compounds," Journal of the
Chemical Society, 51, 258–268; see p. 264.
^ Thiele, Johannes (1899) "Zur Kenntnis der ungesättigten
Verbindungen" (On our knowledge of unsaturated compounds), Justus
Liebig’s Annalen der Chemie,306: 87–142; see: "VIII. Die
aromatischen Verbindungen. Das Benzol." (VIII. The aromatic compounds.
Benzene.), pp. 125–129. See further: Thiele (1901) "Zur Kenntnis der
ungesättigen Verbindungen," Justus Liebig’s Annalen der Chemie,
^ In his 1890 paper, Armstrong represented benzene nuclei within
polycyclic benzenoids by placing inside the benzene nuclei a letter
"C", an abbreviation of the word "centric". Centric affinities (i.e.,
bonds) acted within a designated cycle of carbon atoms. From p. 102: "
… benzene, according to this view, may be represented by a double
ring, in fact." See:
Armstrong, H.E. (1890). "The structure of cycloid hydrocarbons".
Proceedings of the Chemical Society. 6: 101–105.
The use of a circle to denote a benzene nucleus first appeared in:
Armit, James Wilson; Robinson, Robert (1925). "Polynuclear
heterocyclic aromatic types. Part II. Some anhydronium bases". Journal
of the Chemical Society, Transactions. 127: 1604–1618.
A history of the determination of benzene's structure is recounted in:
Balaban, Alexandru T.; Schleyer, Paul v. R.; Rzepa, Henry S. (2005).
"Crocker, Not Armit and Robinson, Begat the Six
Chemical Reviews. 105 (10): 3436–3447. doi:10.1021/cr0300946.
^ J. Loschmidt, Chemische Studien (Vienna, Austria-Hungary: Carl
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^ Kekulé, F. A. (1865). "Sur la constitution des substances
aromatiques". Bulletin de la Societe Chimique de Paris. 3:
98–110. On p. 100, Kekulé suggests that the carbon atoms of
benzene could form a "chaîne fermée" (a closed chain, a loop).
^ Kekulé, F. A. (1866). "Untersuchungen über aromatische
Verbindungen (Investigations of aromatic compounds)". Liebigs Annalen
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^ Rocke, A. J. (2010) Image and Reality: Kekule, Kopp, and the
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^ English translation Wilcox, David H.; Greenbaum, Frederick R.
(1965). "Kekule's benzene ring theory: A subject for lighthearted
banter". Journal of Chemical Education. 42 (5): 266–67.
^ Kekulé, F. A. (1890). "Benzolfest: Rede". Berichte der Deutschen
Chemischen Gesellschaft. 23: 1302–11.
^ Benfey O. T. (1958). "
August Kekulé and the Birth of the Structural
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Benzene Ring in
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Wilhelm Körner (1867) "Faits pour servir à la détermination du lieu
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chemical location in the aromatic series), Bulletins de l'Académie
royale des sciences, des lettres et des beaux-arts de Belgique, 2nd
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dérivés bihydroxyliques ont leurs terms correspondants, par les
préfixes ortho-, para- et mêta-." (One easily distinguishes these
three series – in which the dihydroxy derivatives have their
corresponding terms – by the prefixes ortho-, para- and meta-.)
Hermann von Fehling, ed., Neues Handwörterbuch der Chemie [New
concise dictionary of chemistry] (Braunschweig, Germany: Friedrich
Vieweg und Sohn, 1874), vol. 1, p. 1142.
^ Graebe (1869) "Ueber die Constitution des Naphthalins" (On the
structure of naphthalene), Annalen der Chemie und Pharmacie,
149 : 20–28 ; see especially p. 26.
^ Victor Meyer (1870) "Untersuchungen über die Constitution der
zweifach-substituirten Benzole" (Investigations into the structure of
di-substituted benzenes), Annalen der Chemie und Pharmacie, 156 :
265–301 ; see especially pp. 299–300.
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Compounds in italics are aromatic
CnH2n + 2
CnH2n − 2
Alkene (Allyl, Vinyl)
Alkyl (Methyl, Ethyl, Propyl, Butyl, Pentyl)
C, H, O
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Sulfonylalkanes (e.g., sulfonmethane (sulfonal), tetronal, trional)
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