The benzene is the most stable molecule and the most cancerigenous one.

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Benzene
From Wikipedia, the free encyclopedia
This article is about the chemical compound. For other uses, see Benzene
(disambiguation).
Not to be confused with Benzine.
Benzene
Skeletal formula detail of benzene.
Geometry
Benzene ball-and-stick model
Ball and stick model
Benzene molecule
Space-filling model
Names
Preferred IUPAC name
Benzene
Other names
Benzol (historic/German)
[6]annulene (not recommended[1])
Identifiers
CAS Number
71-43-2 Yes
3D model (JSmol)
Interactive image
ChEBI
CHEBI:16716 Yes
ChemSpider
236 Yes
ECHA InfoCard 100.000.685
EC Number 200-753-7
KEGG
C01407 Yes
PubChem CID
241
RTECS number CY1400000
UNII
J64922108F Yes
InChI[show]
SMILES[show]
Properties
Chemical formula
C6H6
Molar mass 78.11 gmol-1
Appearance Colorless liquid
Odor Aromatic, gasoline-like
Density 0.8765(20) g/cm3[2]
Melting point 5.53 C (41.95 F; 278.68 K)
Boiling point 80.1 C (176.2 F; 353.2 K)
Solubility in water
1.53 g/L (0 C)
1.81 g/L (9 C)
1.79 g/L (15 C)[3][4][5]
1.84 g/L (30 C)
2.26 g/L (61 C)
3.94 g/L (100 C)
21.7 g/kg (200 C, 6.5 MPa)
17.8 g/kg (200 C, 40 MPa)[6]
Solubility Soluble in alcohol, CHCl3, CCl4, diethyl ether, acetone, acetic acid[6]
Solubility in ethanediol 5.83 g/100 g (20 C)
6.61 g/100 g (40 C)
7.61 g/100 g (60 C)[6]
Solubility in ethanol 20 C, solution in water:
1.2 mL/L (20% v/v)[7]
Solubility in acetone 20 C, solution in water:
7.69 mL/L (38.46% v/v)
49.4 mL/L (62.5% v/v)[7]
Solubility in diethylene glycol 52 g/100 g (20 C)[6]
log P 2.13
Vapor pressure 12.7 kPa (25 C)
24.4 kPa (40 C)
181 kPa (100 C)[8]
UV-vis (?max) 255 nm
Magnetic susceptibility (?)
-54.810-6 cm3/mol
Refractive index (nD)
1.5011 (20 C)
1.4948 (30 C)[6]
Viscosity 0.7528 cP (10 C)
0.6076 cP (25 C)
0.4965 cP (40 C)
0.3075 cP (80 C)
Structure
Molecular shape
Trigonal planar
Dipole moment
0 D
Thermochemistry
Specific
heat capacity (C)
134.8 J/molK
Std molar
entropy (So298)
173.26 J/molK[8]
Std enthalpy of
formation (?fHo298)
48.7 kJ/mol
Std enthalpy of
combustion (?cHo298)
3267.6 kJ/mol[8]
Hazards
Main hazards potential occupational carcinogen, flammable
Safety data sheet See: data page
HMDB
GHS pictograms The flame pictogram in the Globally Harmonized System of
Classification and Labelling of Chemicals (GHS)The exclamation-mark pictogram in
the Globally Harmonized System of Classification and Labelling of Chemicals
(GHS)The health hazard pictogram in the Globally Harmonized System of
Classification and Labelling of Chemicals (GHS)[9]
GHS signal word Danger
GHS hazard statements
H225, H304, H315, H319, H340, H350, H372[9]
GHS precautionary statements
P201, P210, P301+310, P305+351+338, P308+313, P331[9]
NFPA 704
NFPA 704 four-colored diamond
320
Flash point -11.63 C (11.07 F; 261.52 K)
Autoignition
temperature
497.78 C (928.00 F; 770.93 K)
Explosive limits 1.27.8%
Lethal dose or concentration (LD, LC):
LD50 (median dose)
930 mg/kg (rat, oral)
LCLo (lowest published)
44,000 ppm (rabbit, 30 min)
44,923 ppm (dog)
52,308 ppm (cat)
20,000 ppm (human, 5 min)[11]
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 1 ppm, ST 5 ppm[10]
REL (Recommended)
Ca TWA 0.1 ppm ST 1 ppm[10]
IDLH (Immediate danger)
500 ppm[10]
Related compounds
Related compounds
Toluene
Borazine
Supplementary data page
Structure and
properties
Refractive index (n),
Dielectric constant (er), etc.
Thermodynamic
data
Phase behaviour
solidliquidgas
Spectral data
UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state
(at 25 C [77 F], 100 kPa).
Yes verify (what is Yes ?)
Infobox references
Benzene is an important organic chemical compound with the chemical formula C6H6.
The benzene molecule is composed of 6 carbon atoms joined in a ring with 1 hydrogen
atom attached to each. As it contains only carbon and hydrogen atoms, benzene is
classed as a hydrocarbon.
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 ([6]-
annulene). It is sometimes abbreviated PhH. 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.
Contents [hide]
1 History
1.1 Discovery
1.2 Ring formula
1.3 Nomenclature
1.4 Early applications
1.5 Occurrence
2 Structure
3 Benzene derivatives
4 Production
4.1 Catalytic reforming
4.2 Toluene hydrodealkylation
4.3 Toluene disproportionation
4.4 Steam cracking
4.5 Other methods
5 Uses
5.1 Component of gasoline
6 Reactions
6.1 Sulfonation, chlorination, nitration
6.2 Hydrogenation
6.3 Metal complexes
7 Health effects
8 Exposure to benzene
8.1 Benzene exposure limits
8.2 Toxicology
8.2.1 Biomarkers of exposure
8.2.2 Biotransformations
8.2.3 Molecular toxicology
8.2.4 Biological oxidation and carcinogenic activity
8.3 Routes of exposure
8.3.1 Inhalation
8.3.2 Exposure from soft drinks
8.3.3 Contamination of water supply
9 See also
10 References
11 External links
History[edit]
Discovery[edit]
The word "benzene" derives historically from "gum benzoin" (benzoin resin), an
aromatic resin known to European pharmacists and perfumers since the 15th century
as a product of southeast Asia.[12] 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.[13]
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.[14][15] In 1833, Eilhard Mitscherlich produced it by distilling benzoic
acid (from gum benzoin) and lime. He gave the compound the name benzin.[16] In
1836, the French chemist Auguste Laurent named the substance "phne";[17] 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.

Kekul's 1872 modification of his 1865 theory, illustrating rapid alternation of

double bonds[18]
.
In 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated
benzene from coal tar.[19] Four years later, Mansfield began the first industrial-
scale production of benzene, based on the coal-tar method.[20][21] 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.[22] In 1997, benzene was detected in deep space.[23]
Ring formula[edit]

Historic benzene structures (from left to right) by Claus (1867),[24] Dewar (1867),
[25] Ladenburg (1869),[26] Armstrong (1887),[27] Thiele (1899)[28] and Kekul
(1865). Dewar benzene and prismane are different chemicals that have Dewar's and
Ladenburg's structures. Thiele and Kekul'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 in 1858 and Joseph Loschmidt in 1861[29]
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 structure.
In 1865, the German chemist Friedrich August Kekul 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.[30][31]
Kekul used evidence that had accumulated in the intervening yearsnamely, 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
derivativenow understood to correspond to the ortho, meta, and para patterns of
arene substitutionto argue in support of his proposed structure.[32] Kekul's
symmetrical ring could explain these curious facts, as well as benzene's 1:1
carbon-hydrogen ratio.
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 Kekul's honor, celebrating the
twenty-fifth anniversary of his first benzene paper. Here Kekul 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).[33] 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 Kekul's anecdote.[34] 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.[13] Kekul's 1890 speech[35]
in which this anecdote appeared has been translated into English.[36] If the
anecdote is the memory of a real event, circumstances mentioned in the story
suggest that it must have happened early in 1862.[37]
The cyclic nature of benzene was finally confirmed by the crystallographer Kathleen
Lonsdale in 1929.[38][39]
Nomenclature[edit]
The German chemist Wilhelm Krner 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.[40] It was the German chemist Karl Grbe 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).[41] In 1870, the
German chemist Viktor Meyer first applied Grbe's nomenclature to benzene.[42]
Early applications[edit]
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.
In 1903, 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 needed]
Occurrence[edit]
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 from coal.[43]
Structure[edit]
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 CC 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 CC bonding are
distributed equally between each of the six carbon atoms. Benzene has 6 hydrogen
atoms fewer than the corresponding parent alkane, hexane. The molecule is planar.
[44] The MO description involves the formation of three delocalized p orbitals
spanning all six carbon atoms, while the VB description involves a superposition of
resonance structures.[45][46][47][48] 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 carbon atoms.
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,[49] and
U+23E3 (?) for a delocalized version.[50]
Benzene derivatives[edit]
Main articles: 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, C6H5C6H5. 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
pyrazine.[51]
Production[edit]
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 4450% of the total U.S
benzene production.[43]
Catalytic reforming[edit]
In catalytic reforming, a mixture of hydrocarbons with boiling points between
60200 C is blended with hydrogen gas and then exposed to a bifunctional platinum
chloride or rhenium chloride catalyst at 500525 C and pressures ranging from 850
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 aromatics.
Toluene hydrodealkylation[edit]
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 500600 C and 4060 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:
2 C
6H
6 ? H
2 + C
6H
5C
6H
5
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) extraction processes.
Toluene disproportionation[edit]
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[edit]
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).
Other methods[edit]
Although of no commercial significance, many other routes to benzene exist. 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 benzene. Trimerization of acetylene gives
benzene.
Uses[edit]
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 stagnating.[52]
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.[53]

Major commodity chemicals and polymers derived from benzene. Clicking on the image
loads the appropriate article
Component of gasoline[edit]
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%.[54] 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
0.62%.[55]
Reactions[edit]
The most common reactions of benzene involve substitution of a proton by other
groups.[56] 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 of benzene

The most widely practiced example of this reaction is the ethylation of benzene.
EtC6H5route.png
Approximately 24,700,000 tons were produced in 1999.[57] 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. Similarly, the 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[edit]
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.
Hydrogenation[edit]
Via hydrogenation, benzene and its derivatives convert to cyclohexane and
derivatives. This reaction is achieved by the use of high pressures of hydrogen at
high temperatures in the presence of a finely divided nickel, which serves as a
catalyst. In the absence of the catalyst, benzene is impervious to hydrogen. This
reaction is practiced on a very large scale industrially.
Metal complexes[edit]
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[edit]

A bottle of benzene. The warnings show benzene is a toxic and flammable liquid.
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.[58][59][60] 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).[61]
The American Petroleum Institute (API) stated in 1948 that "it is generally
considered that the only absolutely safe concentration for benzene is zero".[62]
There is no safe exposure level; even tiny amounts can cause harm.[63] 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.[64] 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 various routes.[65][66]
Exposure to benzene[edit]
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.[67]
OSHA regulates levels of benzene in the workplace.[68] 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.[69]
Benzene exposure limits[edit]
The 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 Drinking Water Regulations.[70] 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.
The U.S. 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.[71] 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.[72]
The U.S. National Institute for Occupational Safety and Health (NIOSH) revised the
Immediately Dangerous to Life and Health (IDLH) concentration for benzene to 500
ppm. The current NIOSH definition for an 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[73]].[74] 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.[75] 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.
Toxicology[edit]
Biomarkers of exposure[edit]
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 g/g creatinine for muconic acid and 25 g/g creatinine for
phenylmercapturic acid in an end-of-shift urine specimen.[76][77][78][79]
Biotransformations[edit]
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 NADH to catechol. The catechol is then metabolized to acetyl CoA and
succinyl CoA, used by organisms mainly in the Krebs Cycle for energy production.
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, inducing
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.[61][80]
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.[81]
Molecular toxicology[edit]
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 and effect.[82] 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.[83] 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 single-stranded breaks.[84]
Biological oxidation and carcinogenic activity[edit]
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 harmful mutations.
Routes of exposure[edit]
Inhalation[edit]
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.[85] About 50% of the entire nationwide (United States)
exposure to benzene results from smoking tobacco or from exposure to tobacco smoke.
[86] 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.[87]
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.[88]
Exposure from soft drinks[edit]
Main article: 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.[89]
Contamination of water supply[edit]
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 2005.
See also[edit]
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 Petroleum Institute
BTEX
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Soleo L, Tranfo G, Manno M (2010). "Low air levels of benzene: Correlation between
biomarkers of exposure and genotoxic effects". Toxicol Lett. 192 (1): 228.
doi:10.1016/j.toxlet.2009.04.028. PMID 19427373.
Jump up ^ Eastmond, D.A.; Rupa, DS; Hasegawa, LS (2000). "Detection of
hyperdiploidy and chromosome breakage in interphase human lymphocytes following
exposure to the benzene metabolite hydroquinone using multicolor fluorescence in
situ hybridization with DNA probes". Mutat Res. 322 (1): 920. PMID 7517507.
Jump up ^ Garte, S; Taioli, E; Popov, T; Bolognesi, C; Farmer, P; Merlo, F (2000).
"Genetic susceptibility to benzene toxicity in humans". J Toxicol Environ Health A.
71 (22): 14821489. doi:10.1080/15287390802349974. PMID 18836923.
Jump up ^ ToxFAQs for Benzene, Agency for Toxic Substances and Disease Registry,
Department of Health and Human Services[dead link]
Jump up ^ ToxGuide for Benzene, Agency for Toxic Substances and Disease Registry,
Department of Health and Human Services
Jump up ^ Public Health Statement. Benzene, Division of Toxicology and
Environmental Medicine, August 2007
Jump up ^ Benzene, CASRN: 71-43-2. Hazardous Substances Data Bank, U.S. National
Library of Medicine. National Institutes of Health.
Jump up ^ "FDA: Too Much Benzene In Some Drinks", CBS News, May 19, 2006. Retrieved
July 11, 2006.
External links[edit]
Wikimedia Commons has media related to Benzene.
Wikimedia Commons has media related to Benzene.
Look up benzene in Wiktionary, the free dictionary.
Wikiquote has quotations related to: Benzene
Benzene at The Periodic Table of Videos (University of Nottingham)
International Chemical Safety Card 0015
USEPA Summary of Benzene Toxicity
NIOSH Pocket Guide to Chemical Hazards
CID {{{1}}} from PubChemEdit this at Wikidata
Dept. of Health and Human Services: TR-289: Toxicology and Carcinogenesis Studies
of Benzene
Video Podcast of Sir John Cadogan giving a lecture on Benzene since Faraday, in
1991
Substance profile
Benzene in the ChemIDplus database
NLM Hazardous Substances Databank Benzene
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