Benzene 1

Benzene

Benzene

Identifiers

CAS number 71-43-2 [1] 

PubChem 241 [2] 

ChemSpider 236 [3]

UNII J64922108F [4] 

ChEBI 16716 [5]

RTECS number CY1400000

SMILES

InChI

InChI key

Properties

Molecular formula C6H6

Molar mass 78.11 g mol−1

Appearance Colorless liquid

Density 0.8765(20) g/cm3 [6]

Melting point 5.5 °C, 279 K, 42 °F

Boiling point 80.1 °C, 353 K, 176 °F

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Solubility in water 1.8 g/L (15 °C) [7] [8] [9]

Viscosity 0.652 cP at 20 °C

Dipole moment 0 D

Hazards

EU classification Flammable (F)Carc. Cat. 1Muta. Cat. 2Toxic (T)

R-phrases R45, R46, R11, R36/38,R48/23/24/25, R65

S-phrases S53, S45

NFPA 704

Flash point −11 °C

Related compounds

Related compounds tolueneborazine

 (what is this?)   (verify) [10]

Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Infobox references

Benzene is an organic chemical compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H.Benzene is a colorless and highly flammable liquid with a sweet smell. Because it is a known carcinogen, its use asan additive in gasoline is now limited, but it is an important industrial solvent and precursor in the production ofdrugs, plastics, synthetic rubber, and dyes. Benzene is a natural constituent of crude oil, and may be synthesizedfrom 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. It is also related to the functional group arene whichis a generalized structure of benzene.

History

DiscoveryThe word "benzene" derives historically from "gum benzoin", sometimes called "benjamin" (i.e., benzoin resin), anaromatic 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 materialwas derived from benzoin by sublimation, and named "flowers of benzoin," or benzoic acid. The hydrocarbonderived from benzoic acid thus acquired the name benzin, benzol, or benzene.[11]

Michael Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production ofilluminating gas, giving it the name bicarburet of hydrogen.[12] [13] In 1833, Eilhard Mitscherlich produced it via thedistillation of benzoic acid (from gum benzoin) and lime. Mitscherlich gave the compound the name benzin.[14] In1836, the French chemist Auguste Laurent named the substance "phène"; this is the root of the word phenol, which ishydroxylated benzene, and phenyl, which is the radical formed by abstraction of a hydrogen atom (free radical H*)from benzene.

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Historic benzene formulae as proposed by Kekulé.[15]

In 1845, Charles Mansfield, working underAugust Wilhelm von Hofmann, isolatedbenzene from coal tar. Four years later,Mansfield began the first industrial-scaleproduction of benzene, based on the coal-tarmethod.

Gradually the sense developed amongchemists that substances related to benzeneformed a natural chemical family. In 1855

August Wilhelm Hofmann used the word "aromatic" to designate this family relationship, after a characteristicproperty of many of its members.

Ring formulaThe empirical formula for benzene was long known, but its highly polyunsaturated structure, with just one hydrogenatom for each carbon atom, was challenging to determine. Archibald Scott Couper in 1858 and Joseph Loschmidt in1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromaticcompounds was in its very early years, and too little evidence was then available to help chemists decide on anyparticular structure.In 1865, the German chemist Friedrich August Kekulé published a paper in French (for he was then teaching inFrancophone Belgium) suggesting that the structure contained a six-membered ring of carbon atoms with alternatingsingle and double bonds. The next year he published a much longer paper in German on the same subject.[16] [17]

Kekulé used evidence that had accumulated in the intervening years—namely, that there always appeared to be onlyone isomer of any monoderivative of benzene, and that there always appeared to be exactly three isomers of everydiderivative—now understood to correspond to the ortho, meta, and para patterns of arene substitution—to argue insupport of his proposed structure. Kekulé's symmetrical ring could explain these curious facts, as well as benzene's1:1 carbon-hydrogen ratio.

Historic benzene formulae (from left to right) by Claus (1867), Dewar (1867), Ladenburg(1869), Armstrong (1887), Thiele (1899) and Kekulé (1865).

The new understanding of benzene,and hence of all aromatic compounds,proved to be so important for both pureand applied chemistry that in 1890 theGerman Chemical Society organizedan elaborate appreciation in Kekulé's

honor, celebrating the twenty-fifth anniversary of his first benzene paper. Here Kekulé spoke of the creation of thetheory. He said that he had discovered the ring shape of the benzene molecule after having a reverie or day-dream ofa snake seizing its own tail (this is a common symbol in many ancient cultures known as the Ouroboros or Endlessknot). This vision, he said, came to him after years of studying the nature of carbon-carbon bonds. This was 7 yearsafter he had solved the problem of how carbon atoms could bond to up to four other atoms at the same time. It iscurious that a similar, humorous depiction of benzene had appeared in 1886 in the Berichte der DurstigenChemischen Gesellschaft (Journal of the Thirsty Chemical Society), a parody of the Berichte der DeutschenChemischen Gesellschaft, only the parody had monkeys seizing each other in a circle, rather than snakes as inKekulé's anecdote.[18] 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.[11] Others havespeculated that Kekulé's story in 1890 was a re-parody of the monkey spoof, and was a mere invention rather than arecollection of an event in his life. Kekulé's 1890 speech[19] in which these anecdotes appeared has been translated

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into English.[20] If one takes the anecdote as the memory of a real event, circumstances mentioned in the storysuggest that it must have happened early in 1862.[21]

The cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale in 1929.[22] [23]

StructureBenzene represents a special problem in that, to account for all the bonds, there must be alternating double carbonbonds:[24]

The various representations of benzene

Using X-ray diffraction, researchers discovered that all of the carbon-carbon bonds in benzene are of the same lengthof 140 picometres (pm). The C–C bond lengths are greater than a double bond (135pm) but shorter than a singlebond (147pm). This intermediate distance is explained by electron delocalization: the electrons for C–C bonding aredistributed equally between each of the six carbon atoms. The molecule is planar (ignoring quantum/thermalvibrations), although many calculations predict otherwise.[25] One representation is that the structure exists as asuperposition of so-called resonance structures, rather than either form individually. The delocalization of electronsis one explanation for the thermodynamic stability of benzene and related ringed molecules. It is likely that it is thisstability that contributes to the peculiar molecular and chemical properties known as aromaticity. To reflect thedelocalized nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of carbonatoms:The delocalized picture of benzene has been contested by Cooper, Gerratt and Raimondi in their article published in1986 in the journal Nature. They showed that the electrons in benzene are almost certainly localized, and thearomatic properties of benzene originate from spin coupling rather than electron delocalization.[26] This view hasbeen supported in the next-year Nature issue,[27] [28] [29] but it has been slow to permeate the general chemistrycommunity.As is common in organic chemistry, the carbon atoms in the diagram above have been left unlabeled. Realizing eachcarbon has 2p electrons, each carbon donates an electron into the delocalized ring above and below the benzene ring.It is the side-on overlap of p-orbitals that produces the pi clouds.

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Benzene occurs sufficiently often as a component of organic molecules that there is a Unicode symbol in theMiscellaneous Technical block with the code U+232C (⌬) to represent it with three double bonds,[30] and U+23E3(⏣) for a delocalized version.[31]

Substituted benzene derivativesMany important chemicals are derived from benzene by replacing one or more of its hydrogen atoms with anotherfunctional 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 thehydrogen-free material graphite.In heterocycles, carbon atoms in the benzene ring are replaced with other elements. The most important derivativesare the rings containing nitrogen. Replacing one CH with N gives the compound pyridine, C5H5N. Althoughbenzene and pyridine are structurally related, benzene cannot be converted into pyridine. Replacement of a secondCH bond with N gives, depending on the location of the second N, pyridazine, pyrimidine, and pyrazine.

ProductionTrace amounts of benzene may result whenever carbon-rich materials undergo incomplete combustion. It isproduced in volcanoes and forest fires, and is also a component of cigarette smoke. Benzene is a principalcomponent of combustion products produced by the burning of PVC (polyvinyl chloride).Until World War II, most benzene was produced as a by-product of coke production (or "coke-oven light oil") in thesteel 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 petrochemicalindustry, 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 catalyticreforming.

Catalytic reformingIn catalytic reforming, a mixture of hydrocarbons with boiling points between 60–200 °C is blended with hydrogengas and then exposed to a bifunctional platinum chloride or rhenium chloride catalyst at 500–525 °C and pressuresranging from 8–50 atm. Under these conditions, aliphatic hydrocarbons form rings and lose hydrogen to becomearomatic hydrocarbons. The aromatic products of the reaction are then separated from the reaction mixture (orreformate) by extraction with any one of a number of solvents, including diethylene glycol or sulfolane, and benzeneis then separated from the other aromatics by distillation. The extraction step of aromatics from the reformate isdesigned to produce aromatics with lowest non-aromatic components. So-called BTX (benzene-toluene-xylene)process consists of such extraction and distillation steps. One such widely used process from UOP was licensed toproducers and called the Udex process.Similarly to this catalytic reforming, UOP and BP commercialized a method from LPG (mainly propane and butane)to aromatics.

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Toluene hydrodealkylationToluene hydrodealkylation converts toluene to benzene. In this hydrogen-intensive process, toluene is mixed withhydrogen, then passed over a chromium, molybdenum, or platinum oxide catalyst at 500–600 °C and 40–60 atmpressure. Sometimes, higher temperatures are used instead of a catalyst (at the similar reaction condition). Underthese conditions, toluene undergoes dealkylation to benzene and methane:

C6H5CH3 + H2 → C6H6 + CH4This irreversible reaction is accompanied by an equilibrium side reaction that produces biphenyl (aka diphenyl) athigher temperature:

2 C6H6 H2 + C6H5–C6H5If the raw material stream contains much non-aromatic components (paraffins or naphthenes), those are likelydecomposed 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, withsimilar efficiency.This is often called "on-purpose" methodology to produce benzene, compared to conventional BTX(benzene-toluene-xylene) processes.

Toluene disproportionationWhere 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 reactedand the methyl groups rearranged from one toluene molecule to the other, yielding one benzene molecule and onexylene molecule.Given that demand for para-xylene (p-xylene) substantially exceeds demand for other xylene isomers, a refinementof the TDP process called Selective TDP (STDP) may be used. In this process, the xylene stream exiting the TDPunit is approximately 90% paraxylene. In some current catalytic systems, even the benzene-to-xylenes ratio isdecreased (more xylenes) when the demand of xylenes is higher.

Steam crackingSteam cracking is the process for producing ethylene and other alkenes from aliphatic hydrocarbons. Depending onthe feedstock used to produce the olefins, steam cracking can produce a benzene-rich liquid by-product calledpyrolysis gasoline. Pyrolysis gasoline can be blended with other hydrocarbons as a gasoline additive, or distilled (inBTX process) to separate it into its components, including benzene.

Uses

Early usesIn the 19th and early-20th centuries, benzene was used as an after-shave lotion because of its pleasant smell. Prior tothe 1920s, benzene was frequently used as an industrial solvent, especially for degreasing metal. As its toxicitybecame obvious, benzene was supplanted by other solvents, especially toluene (methyl benzene), which has similarphysical 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 (the letters "ka" in the brand name stand for kaffein). This process was later discontinued. Benzene was historically found as a significant component in many consumer products such as Liquid Wrench, several paint strippers, rubber cements, spot removers and other hydrocarbon-containing products. Some ceased manufacture of their benzene-containing formulations in about 1950, while others continued to use benzene as a component or

Benzene 7

significant contaminant until the late 1970s when leukemia deaths were found associated with Goodyear's Pliofilmproduction operations in Ohio. Until the late 1970s, many hardware stores, paint stores, and other retail outlets soldbenzene in small cans, such as quart size, for general-purpose use. Many students were exposed to benzene in schooland university courses while performing laboratory experiments with little or no ventilation in many cases. This verydangerous practice has been almost totally eliminated.As a gasoline (petrol) additive, benzene increases the octane rating and reduces knocking. Consequently, gasolineoften contained several percent benzene before the 1950s, when tetraethyl lead replaced it as the most widely-usedantiknock additive. With the global phaseout of leaded gasoline, benzene has made a comeback as a gasolineadditive in some nations. In the United States, concern over its negative health effects and the possibility of benzeneentering the groundwater have led to stringent regulation of gasoline's benzene content, with limits typically around1%.[32] European petrol specifications now contain the same 1% limit on benzene content. The United StatesEnvironmental Protection Agency has new regulations that will lower the benzene content in gasoline to 0.62% in2011.[33]

Current usesToday benzene is mainly used as an intermediate to make other chemicals. Its most widely-produced derivativesinclude styrene, which is used to make polymers and plastics, phenol for resins and adhesives (via cumene), andcyclohexane, which is used in the manufacture of Nylon. Smaller amounts of benzene are used to make some typesof rubbers, lubricants, dyes, detergents, drugs, explosives, napalm and pesticides.In both the US and Europe, 50% of benzene is used in the production of ethylbenzene / styrene, 20% is used in theproduction 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 aresimilar 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-dimensionalgas.

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Major commodity chemicals and polymers derived from benzene. Clicking on the image loads the appropriate article

Reactions• 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.

Electrophilic aromatic substitution of benzene

• The Friedel-Crafts acylation is a specific example of electrophilic aromatic substitution. The reaction involvesthe acylation of benzene (or many other aromatic rings) with an acyl chloride using a strong Lewis acidcatalyst such as aluminium chloride or iron chloride which act as a halogen carrier.

Friedel-Crafts acylation of benzene by acetyl chloride

• Like the Friedel-Crafts acylation, the Friedel-Crafts alkylation involves the alkylation of benzene (and manyother aromatic rings) using an alkyl halide in the presence of a strong Lewis acid catalyst.

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Friedel-Crafts alkylation of benzene with methyl chloride

• Sulfonation. The most common method involves mixing sulfuric acid with sulfate, a mixture called fumingsulfuric acid. The sulfuric acid protonates the sulfate, giving the sulfur atom a permanent, rather thanresonance stabilized positive formal charge. This molecule is very electrophillic and Electrophillic AromaticSubstitution then occurs.

• Nitration: Benzene undergoes nitration with nitronium ions (NO2+) as the electrophile. Thus, warming benzene

at 50–55 °C, with a combination of concentrated sulfuric and nitric acid to produce the electrophile, givesnitrobenzene.

• Hydrogenation (reduction): Benzene and derivatives convert to cyclohexane and derivatives when treated withhydrogen 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 includethe sandwich and half-sandwich complexes respectively Cr(C6H6)2 and [RuCl2(C6H6)]2.

Environmental transformationEven if it is not a common substrate for the metabolism of organisms, benzene could be oxidized by both bacteriaand eukaryotes.In bacteria, dioxygenase enzyme can add an oxygen molecule to the ring, and the unstable product is immediatelyreduced (by NADH) to a cyclic diol with two double bonds, breaking the aromaticity. Next, the diol is newlyreduced by NADH to catechol.The catechol is then metabolized to acetyl CoA and succinyl CoA, used by organisms mainly in the Krebs Cycle forenergy production.

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Health effects

A bottle of benzene. The warnings show benzeneis a toxic and flammable liquid.

Benzene exposure has serious health effects. The American PetroleumInstitute (API) stated in 1948 that "it is generally considered that theonly absolutely safe concentration for benzene is zero."[34] The USDepartment of Health and Human Services (DHHS) classifies benzeneas a human carcinogen. Long-term exposure to excessive levels ofbenzene in the air causes leukemia, a potentially fatal cancer of theblood-forming organs, in susceptible individuals. In particular, Acutemyeloid leukemia or acute non-lymphocytic leukaemia (AML &ANLL) is not disputed to be caused by benzene.[35] IARC ratedbenzene as "known to be carcinogenic to humans" (Group 1).

Outdoor air may contain low levels of benzene from tobacco smoke,wood smoke, automobile service stations, the transfer of gasoline,exhaust from motor vehicles, and industrial emissions.[36] About 50%of the entire nationwide (United States) exposure to benzene resultsfrom smoking tobacco or from exposure to tobacco smoke.[37]

Vapors from products that contain benzene, such as glues, paints,furniture wax, and detergents, can also be a source of exposure,although many of these have been modified or reformulated since the late 1970s to eliminate or reduce the benzenecontent. Air around hazardous waste sites or gas stations may contain higher levels of benzene. Because petroleumhydrocarbon products are complex mixtures of chemicals, risk assessments for these products generally focus onspecific toxic constituents. The petroleum constituents of primary interest to human health have been the aromatichydrocarbons (i.e., benzene, ethylbenzene, toluene, and xylenes). OSHA requires that a mixture "shall be assumed topresent a carcinogenic hazard if it contains a component in concentrations of 0.1% or greater, which is considered tobe a carcinogen.[38] [39]

The short term breathing of 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 containinghigh levels of benzene can cause vomiting, irritation of the stomach, dizziness, sleepiness, convulsions, and death.The major effects of benzene are manifested via chronic (long-term) exposure through the blood. Benzene damagesthe bone marrow and can cause a decrease in red blood cells, leading to anemia. It can also cause excessive bleedingand depress the immune system, increasing the chance of infection. Benzene causes leukemia and is associated withother blood cancers and pre-cancers of the blood.Human exposure to benzene is a global health problem. Benzene targets liver, kidney, lung, heart and the brain andcan 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 1920s. The chemical industry claims it wasn't until1979 that the cancer-inducing properties were determined "conclusively" in humans, despite many references to thisfact in the medical literature. Industry exploited this "discrepancy" and tried to discredit animal studies whichshowed benzene caused cancer, saying that they weren't relevant to humans. Benzene has been shown to causecancer in both sexes of multiple species of laboratory animals exposed via various routes.[40] [41]

Some women who breathed high levels of benzene for many months had irregular menstrual periods and a decreasein the size of their ovaries. It is not known whether benzene exposure affects the developing fetus in pregnantwomen or fertility in men.Animal studies have shown low birth weights, delayed bone formation, and bone marrow damage when pregnantanimals breathed benzene.

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Benzene has been connected to a rare form of kidney cancer in two separate studies, one involving tank truckdrivers, and the other involving seamen on tanker vessels, both carrying benzene-laden chemicals.

Exposure to benzeneWorkers in various industries that make or use benzene may be at risk for being exposed to high levels of thiscarcinogenic chemical. Industries that involve the use of benzene include the rubber industry, oil refineries, coke andchemical plants, shoe manufacturers, and gasoline-related industries. Downstream petroleum industry operationsinclude the following categories: refinery, pipeline, marine, rail, bulk terminals and trucks, service stations,underground storage tanks, tank cleaning, and site characterization and remediation.In 1987, OSHA estimated that about 237,000 workers in the United States were potentially exposed to benzene, butit is not known if this number has substantially changed since then.Water and soil contamination are important pathways of concern for transmission of benzene contact. In the USalone there are approximately 100,000 different sites which have benzene soil or groundwater contamination. In2005, the water supply to the city of Harbin in China with a population of almost nine million people, was cut offbecause of a major benzene exposure. Benzene leaked into the Songhua River, which supplies drinking water to thecity, after an explosion at a China National Petroleum Corporation (CNPC) factory in the city of Jilin on 13November.In March 2006, the official Food Standards Agency in Britain conducted a survey of 150 brands of soft drinks. Itfound that four contained benzene levels above World Health Organization limits. The affected batches wereremoved from sale.[42] (See also benzene in soft drinks).

Benzene exposure limitsThe United States Environmental Protection Agency has set a maximum contaminant level (MCL) for benzene indrinking water at 0.005 mg/L (5 ppb), as promulgated via the National Primary Drinking Water Regulations.[43] Thisregulation is based on preventing benzene leukemogenesis. The maximum contaminant level goal (MCLG), anonenforceable health goal that would allow an adequate margin of safety for the prevention of adverse effects, iszero benzene concentration in drinking water. The EPA requires that spills or accidental releases into theenvironment 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 1 part ofbenzene per million parts of air (1 ppm) in the workplace during an 8-hour workday, 40-hour workweek. The shortterm exposure limit for airborne benzene is 5 ppm for 15 minutes.[44] These legal limits were based on studiesdemonstrating compelling evidence of health risk to workers exposed to benzene. The risk from exposure to 1 ppmfor a working lifetime has been estimated as 5 excess leukemia deaths per 1,000 employees exposed. (This estimateassumes no threshold for benzene's carcinogenic effects.) OSHA has also established an action level of 0.5 ppm toencourage even lower exposures in the workplace.[45]

The National Institute for Occupational Safety and Health (NIOSH) revised the Immediately Dangerous to Life or 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[46] ].[47] In September 1995, NIOSH issued a new policy for developing recommended exposure limits (RELs) for substances, including carcinogens. Because benzene can cause cancer, NIOSH recommends that all workers wear special breathing equipment when they are likely to be exposed to

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benzene at levels exceeding the REL (10-hour) of 0.1 ppm.[48] The NIOSH STEL (95 min) is 1 ppm.American Conference of Governmental Industrial Hygienists (ACGIH) adopted Threshold Limit Values (TLVs) forbenzene at 0.5 ppm TWA and 2.5 ppm STEL.Under New Jersey's Right-to-Know law, respiratory protection for benzene is discussed.[49] As stated, improper useof respirators is dangerous. Respirators should only be used when there is a written respiratory program in place asdescribed in the OSHA Respiratory Protection Standard (29 CFR 1910.134).[50] The employer is to develop andimplement a written respiratory protection program with required worksite-specific procedures and elements forrequired respirator use. This program must be administered by a suitably trained program administrator. Employersmust use the assigned protection factors (APF) listed in Table 1 of 29 CFR 1910.134 to select a respirator that meetsor exceeds the required level of employee protection. Applicable to benzene:• If there is a potential for exposure to 0.1 ppm, a NIOSH-approved half face respirator must be worn with an

organic vapor cartridge (APF 10).• If there is a potential for exposure to 0.5 ppm, a NIOSH-approved full face respirator must be worn with an

organic vapor cartridge (APF 50).• Where the potential exists for exposure to over 5 ppm, a NIOSH-approved air-supplied respirator with a full

facepiece operated under pressure-demand or other positive-pressure mode must be used (APF 1,000).

Exposure monitoringAirborne exposure monitoring for benzene must be conducted in order to properly assess personal exposures andeffectiveness of engineering controls. Initial exposure monitoring should be conducted by an industrial hygienist orperson specifically trained and experienced in sampling techniques. Contact an AIHA Accredited Laboratory foradvice on sampling methods.[51]

Each employer who has a place of employment where occupational exposures to benzene occur shall monitor eachof these workplaces and work operations to determine accurately the airborne concentrations of benzene to whichemployees may be exposed.[52] Representative 8-hour TWA employee exposures need to be determined on the basisof one sample or samples representing the full shift exposure for each job classification in each work area. Unless airsamples are taken frequently, the employer does not know the concentration and would not know how much of aprotection factor is needed.[53]

In providing consultation on work safety during oil clean-up operations following the Deepwater Horizon accident,OSHA has worked with an number of other government agencies to protect Gulf cleanup workers. OSHA partneredwith the NIOSH to issue "Interim Guidance for Protecting Deepwater Horizon Response Workers and Volunteers"and recommend measures that should be taken to protect workers from a variety of different health hazards that theseworkers face.[54] OSHA conceded that it recognizes that most of its PELs are outdated and inadequate measures ofworker safety. In characterizing worker exposure OSHA instead relies on more up-to-date recommended protectivelimits set by organizations such as NIOSH, the ACGIH, and the American Industrial Hygiene Association (AIHA),and not on the older, less protective PELS. Results of air monitoring are compared to the lowest known OccupationalExposure Limit for the listed contaminant for purposes of risk assessment and protective equipmentrecommendations. [55]

Biomarkers of exposureSeveral 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 persons 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

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accumulate in the urine in proportion to the extent and duration of exposure, and they may still be present for somedays after exposure has ceased. The current ACGIH biological exposure limits for occupational exposure are 500μg/g creatinine for muconic acid and 25 μg/g creatinine for phenylmercapturic acid in an end-of-shift urinespecimen.[56] [57] [58] [59]

Molecular toxicologyThe paradigm of toxicological assessment of benzene is slowly shifting towards the domain of molecular toxicologyas it allows understanding of fundamental biological mechanisms in a better way. Glutathione seems to play animportant role by protecting against benzene induced DNA breaks and it is being identified as a new biomarker forexposure and effect.[60] Benzene causes chromosomal aberrations in the peripheral blood leukocytes and bonemarrow explaining the higher incidence of leukemia and multiple myeloma caused by chronic exposure. Theseaberrations can be monitored using fluorescent in situ hybridization (FISH) with DNA probes to assess the effects ofbenzene along with the hematological tests as markers of hematotoxicity.[61] Benzene metabolism involves enzymescoded for by polymorphic genes. Studies have shown that genotype at these loci may influence susceptibility to thetoxic 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 greaterfrequency of DNA single-stranded breaks.[62]

SummaryAccording to the Agency for Toxic Substances and Disease Registry (ATSDR) (2007), benzene is both ananthropogenically produced and naturally occurring chemical from processes that include: volcanic eruptions, wildfires, 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 servicestations, exhaust from motor vehicles, and industrial emissions; however, ingestion and dermal absorption ofbenzene can also occur through contact with contaminated water. Benzene is hepatically metabolized and excreted inthe urine. Measurement of air and water levels of benzene is accomplished through collection via activated charcoaltubes, which are then analyzed with a gas chromatograph. The measurement of benzene in humans can beaccomplished via urine, blood, and breath tests; however, all of these have their limitations because benzene israpidly metabolized in the human body into by-products called metabolites.[63]

OSHA regulates levels of benzene in the workplace.[64] The maximum allowable amount of benzene in workroomair during an 8-hour workday, 40-hour workweek is 1 ppm. Because benzene can cause cancer, NIOSH recommendsthat all workers wear special breathing equipment when they are likely to be exposed to benzene at levels exceedingthe recommended (8-hour) exposure limit of 0.1 ppm.[65]

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 readilyand can interact with DNA to produce harmful mutations.

See also• Industrial Union Department v. American Petroleum Institute• Benzene in soft drinks

Benzene 14

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Benzene 15

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Benzene 16

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External links• Benzene (www.eco-usa.net) (http:/ / www. eco-usa. net/ toxics/ chemicals/ benzene. shtml)• International Chemical Safety Card 0015 (http:/ / www. ilo. org/ public/ english/ protection/ safework/ cis/

products/ icsc/ dtasht/ _icsc00/ icsc0015. htm)• USEPA Summary of Benzene Toxicity (http:/ / www. epa. gov/ iris/ subst/ 0276. htm)• NIOSH Pocket Guide to Chemical Hazards (http:/ / www. cdc. gov/ niosh/ npg/ npgd0049. html)• CID 241 (http:/ / pubchem. ncbi. nlm. nih. gov/ summary/ summary. cgi?cid=241) from PubChem• Dept. of Health and Human Services: TR-289: Toxicology and Carcinogenesis Studies of Benzene (http:/ / ntp.

niehs. nih. gov/ index. cfm?objectid=0707525C-0F07-05BF-A16CAC7B0ECC97B5)• Video Podcast (http:/ / www. ch. ic. ac. uk/ video/ faraday_l. m4v) of Sir John Cadogan giving a lecture on

Benzene since Faraday, in 1991• Substance profile (http:/ / ntp. niehs. nih. gov/ ntp/ roc/ eleventh/ profiles/ s019benz. pdf)• U.S. National Library of Medicine: ChemIDplus - Benzene (http:/ / chem. sis. nlm. nih. gov/ chemidplus/ direct.

jsp?name=Benzene& result=advanced)• NLM Hazardous Substances Databank – Benzene (http:/ / toxnet. nlm. nih. gov/ cgi-bin/ sis/ search/ a?dbs+

hsdb:@term+ @DOCNO+ 35)

Article Sources and Contributors 17

Article Sources and ContributorsBenzene  Source: http://en.wikipedia.org/w/index.php?oldid=395938069  Contributors: 08prl, 16@r, 7&6=thirteen, ABCD, AManWithNoPlan, Acdx, Acewolf359, Aechman, Ajrocke, Al.Glitch,AlB1337, Alex723723, Alfio, American Eagle, Amyvel315, AndrewRA, Andux, Anlace, Annwhent2, Antandrus, Antije, ArnoGourdol, Artichoker, Arvindn, Atemperman, Aushulz, AussieAlchemist, Ayeroxor, Aznlilbadboy, Bakilas, BalazsH, Banus, Barticus88, Baseball 292992, Bazzargh, Beamertate, Beetstra, Benjah-bmm27, Bensaccount, Bichologo, Bkell, Blahaccountblah,Bobblewik, Boing! said Zebedee, Bomac, Borb, BorzouBarzegar, Bowenpan, Brane.Blokar, Brighterorange, Bryan Derksen, C6541, Cacycle, Camp3rstrik3r, CanadianLinuxUser, Canderson7,Cargoking, Casforty, Cassan, Charlesdrakew, Chase me ladies, I'm the Cavalry, Chem-awb, ChemNerd, Chempedia, Chimpman, Chiu frederick, Chris Capoccia, ChrisTek, Chriswiki, ChuunenBaka, Ciaccona, Col tom, Common Man, Contango, Conversion script, Courcelles, Craigy144, Cyberevil, Cyde, Cygfrydd Llewellyn, DD 8630, DFS454, DMacks, DO11.10, DVD R W, DanielCase, Davidruben, Dbiel, DeadEyeArrow, Dekergnp, Deli nk, Deviator13, DocWatson42, DocteurCosmos, DragonflySixtyseven, Dratman, Dravick, Dtsang, Dysprosia, Dzordzm, ESkog,Ebaksa, EconoPhysicist, Ed Poor, EdH, Edgar181, Edivorce, Edsanville, Eeekster, Eequor, Egonw, Either way, ElBenevolente, Element16, ElliottEnterprise, Emijrp, Eoscariii, Epbr123,Ephemeronium, Epop fr, Ergzay, Esurnir, Euryalus, EvaK, Ewen, Exabyte, FTGHSmith, Fcueto, FelixP, Fornaeffe, Fred Bauder, Fttguitarist, Gaius Cornelius, Gavin sidhu, Gazjo, Gentgeen,Giftlite, Gilliam, Gmaxwell, GraemeL, GregorB, Ground Zero, GroupWizard, Grunt, Gruzd, Gundwane, H Padleckas, Hadal, Halmstad, Hayabusa future, HazyM, Hellbus, Heron, Heteren,Hobartimus, Ian01, Ilesn, Imasleepviking, InfoCan, Iridium77, IslandHopper973, Isopropyl, Itub, Ivarmcdonald, J.Steinbock, JForget, JLaTondre, JSpung, Jachin, Jack B108, JamesChemFindIt,Jameswilson, Jamieicis, Jauerback, Jbworks0917, Jeeves, Jeffq, JeramieHicks, Jessicap123, Joanjoc, Jon Harald Søby, Jons63, Jrtayloriv, Jü, Kandar, Karlhahn, Kattlguard, KenFehling, Kenyon,Kjkolb, Kkolmetz, Kku, Klawehtgod, Kozuch, Kpengboy, Ktsquare, Kupos, Kymacpherson, La goutte de pluie, LarryMorseDCOhio, Lchiarav, Ling.Nut, Literaturegeek, Littlealien182,LizardWizard, Llort, Logical2u, Logophile, Luna Santin, M.mural.lee, MarcK, Marek69, Martinman11, MastCell, Materialscientist, Mav, Mdhowe, Mervyn, Metalloid, Metamagician3000,MichaK, Michael Frind, Michall, Mikespedia, Mjmcb1, Montrealais, Moreau1, Mph2010, Mr3641, Muchie11791, NCFCQ, Nagy, NawlinWiki, Nenad Seguljev, Neparis, Nerguy, Nick,Nickeudaly, Nickptar, Nonagonal Spider, Northgrove, Nuberger13, Nufy8, Nuggetboy, Nunquam Dormio, Onco p53, Opelio, Ortonmc, Paaerduag, Parable1991, Paul Drye, Paul-L, Pcbene, Peak,Pelirojopajaro, Persian Poet Gal, PeterSymonds, Phasechange, Phgao, Physchim62, Pisanidavid, Pit, Polonium, Prari, Prmacn, Pádraic MacUidhir, Quidam65, Qxz, R9tgokunks, RK, RMFan1,Rabid Lemur, RaseaC, Rbaselt, Renamed vandal 86, Retired username, Rice.brendan, Rich Farmbrough, Rifleman 82, Rje, Rjwilmsi, Rocastelo, Root4(one), Rossheth, RuudVisser, Ryulong,Rzepa, Sabbut, Sandshinobi23, Sciencen3rd, Sciurinæ, SemperBlotto, Semperf, Seraphimblade, Shaddack, Shefgodeep, Shellreef, Shimmin, Shinjiman, Shyamal, Silverbutterflies, Simoes,SimonP, Sitek, Sleeping123, Smk99, Smokefoot, Snezzy, Snowolf, Solipsist, Spiffy sperry, SpikeTorontoRCP, Srnec, SteinAlive, Stemonitis, Stephenb, Steveholland87, Steviedpeele, Stone,Subsolar, Superfly2005, Sverdrup, Synergy, TSLIII, Tarquin, Taweetham, Taxman, Tetracube, The Thing That Should Not Be, Thekrazykool809, Thisisshorty, Timwi, Tisdalepardi, Tom-,Townview science, Tpk5010, Traceymarr, TreyHarris, Unconcerned, Unitknow, Unyoyega, User A1, V8rik, Vladsinger, Vonfraginoff, Vuo, W2raphael, Walkerma, Wavelength, Weiteck,Wervo, Wik, Wiki alf, Wikislemur, WilfriedC, William Avery, Wimvandorst, Wk muriithi, Wkltan, Wmahan, WriterHound, XJamRastafire, Xchrisblackx, Xcomradex, Xeno, Yahia.barie,Yakobbokay, Zxmaster, ~K, 602 ,طبلا يلع نسح anonymous edits

Image Sources, Licenses and ContributorsFile:Benzene-2D-full.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Benzene-2D-full.svg  License: Public Domain  Contributors: User:EphemeroniumFile:Benzene-aromatic-3D-balls.png  Source: http://en.wikipedia.org/w/index.php?title=File:Benzene-aromatic-3D-balls.png  License: Public Domain  Contributors: Benjah-bmm27File:Benzene circle.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Benzene_circle.svg  License: Public Domain  Contributors: User:Bryan DerksenFile:Benzene-3D-vdW.png  Source: http://en.wikipedia.org/w/index.php?title=File:Benzene-3D-vdW.png  License: Public Domain  Contributors: Benjah-bmm27File:Yes check.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Yes_check.svg  License: Public Domain  Contributors: User:Gmaxwell, User:WarXFile:Historic Benzene Formulae Kekulé (original).png  Source: http://en.wikipedia.org/w/index.php?title=File:Historic_Benzene_Formulae_Kekulé_(original).png  License: Public Domain Contributors: Friedrich August Kekulé von Stradonitz (1829–1896)Image:Historic Benzene Formulae V.3.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Historic_Benzene_Formulae_V.3.svg  License: Public Domain  Contributors: User:JüFile:Benzene Representations.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Benzene_Representations.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:VladsingerImage:Equilibrium.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Equilibrium.svg  License: Public Domain  Contributors: User:L'AquatiqueFile:Benzene_uses.png  Source: http://en.wikipedia.org/w/index.php?title=File:Benzene_uses.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:ItubImage:OChem-Mech-ElectrophilicAromaticSubstitution-General.png  Source: http://en.wikipedia.org/w/index.php?title=File:OChem-Mech-ElectrophilicAromaticSubstitution-General.png License: Public Domain  Contributors: Conscious, David Berardan, Leyo, ShizhaoImage:Friedel-Crafts acylation of benzene by ethanol chloride.png  Source: http://en.wikipedia.org/w/index.php?title=File:Friedel-Crafts_acylation_of_benzene_by_ethanol_chloride.png License: GNU Free Documentation License  Contributors: User:DiberriImage:Friedel-craft-alk.png  Source: http://en.wikipedia.org/w/index.php?title=File:Friedel-craft-alk.png  License: Public Domain  Contributors: User:YikrazuulFile:Benzol.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Benzol.JPG  License: unknown  Contributors: . Original uploader was DFS454 at en.wikipedia

LicenseCreative Commons Attribution-Share Alike 3.0 Unportedhttp:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/