View source for Benzene

{{Other uses}}
{{pp-move-vandalism|small=yes}}
{{chembox
| Watchedfields = changed
| verifiedrevid = 325612702
| Name = Benzene
| ImageFileL1 = Benzene-2D-full.svg
| ImageSizeL1 = 120px
| ImageNameL1 = Full structural formula
| ImageFileR1 = Benzene-aromatic-3D-balls.png
| ImageSizeR1 = 120px
| ImageNameR1 = Ball-and-stick model
| ImageFileL2 = Benzene circle.svg
| ImageSizeL2 = 80px
| ImageNameL2 = Simplified skeletal formula
| ImageFileR2 = Benzene-3D-vdW.png
| ImageSizeR2 = 100px
| ImageNameR2 = Space-filling model
| ImageName = Benzene
| IUPACName = Benzene
| OtherNames = Benzol<br />cyclohexa-1,3,5-triene
| Section1 = {{Chembox Identifiers
| SMILES = c1ccccc1
| InChIKey = UHOVQNZJYSORNB-UHFFFAOYAH
| CASNo = 71-43-2
| CASNo_Ref = {{cascite}}
| ChEBI = 16716
| ChemSpiderID = 236
| InChI = 1/C6H6/c1-2-4-6-5-3-1/h1-6H
| PubChem = 241
| RTECS = CY1400000
 }}
| Section2 = {{Chembox Properties
|C=6|H=6
| Appearance = Colorless liquid
| Density = 0.8765(20) g/cm<sup>3</sup> <ref name=handbook13>{{cite journal|author= David R. Lide, ed.|title= "Physical Constants of Organic Compounds", in CRC Handbook of Chemistry and Physics, Internet Version 2005, <http://www.hbcpnetbase.com>|journal=CRC Press}}</ref>
| Solubility = 1.8 g/L (15 °C) <ref>{{cite journal|last1=Arnold|first1=D.|last2=Plank|first2=C.|last3=Erickson|first3=E.|last4=Pike|first4=F.|title=Solubility of Benzene in Water.|journal=Industrial & Engineering Chemistry Chemical & Engineering Data Series|volume=3|pages=253|year=1958|doi=10.1021/i460004a016}}</ref><!--1st ref is most comprehensive but is paywalled---><ref>{{cite journal|doi=10.1073/pnas.87.1.167|last1=Breslow|first1=R|last2=Guo|first2=T|title=Surface tension measurements show that chaotropic salting-in denaturants are not just water-structure breakers.|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=87|issue=1|pages=167–9|year=1990|pmid=2153285|pmc=53221}}</ref><ref>{{cite book|url=http://books.google.com/books?id=N8RcH8juG_YC&pg=PA114|page=114|title=Ludwig's applied process design for chemical and petrochemical plants, Volume 1|author=A. Kayode Coker, Ernest E. Ludwig|publisher=Elsevier|year=2007|isbn=075067766X}}</ref>
| MeltingPtC = 5.5
| BoilingPtC = 80.1
| Viscosity = 0.652 [[Poise|cP]] at 20 °C
| Dipole = 0 [[Debye|D]]
 }}
| Section7 = {{Chembox Hazards
| FlashPt = −11 °C
| EUClass = Flammable ('''F''')<br/>[[Carcinogen|Carc. Cat. 1]]<br/>[[Mutagen|Muta. Cat. 2]]<br/>Toxic ('''T''')
| NFPA-H = 2
| NFPA-F = 3
| NFPA-R = 0
| RPhrases = {{R45}}, {{R46}}, {{R11}}, {{R36/38}},{{R48/23/24/25}}, {{R65}}
| SPhrases = {{S53}}, {{S45}}
 }}
| Section8 = {{Chembox Related
| OtherCpds = [[toluene]]<br/>[[borazine]]
 }}
}}
'''Benzene''' is an [[organic compound|organic]] [[chemical compound]] with the molecular formula [[Carbon|C]]<sub>6</sub>[[Hydrogen|H]]<sub>6</sub>. It is sometimes abbreviated [[Phenyl|Ph]]–H. Benzene is a [[color]]less and highly [[flammable]] liquid with a sweet smell and a relatively high melting point. Because it is a known [[carcinogen]], its use as an additive in [[gasoline]] is now limited, but it is an important industrial [[solvent]] and [[precursor]] in the production of [[medication|drugs]], [[plastic]]s, synthetic [[rubber]], and [[dye]]s. Benzene is a natural constituent of [[Petroleum|crude oil]], and may be [[Chemical synthesis|synthesized]] from other compounds present in petroleum. Benzene is an [[aromatic hydrocarbon]] and the second [''n'']-[[annulene]] ([6]-annulene), a cyclic hydrocarbon with a continuous [[pi bond]]. It is also related to the functional group [[arene]] which is a generalized structure of benzene.
==History==
===Discovery===
The word "benzene" derives historically from "gum benzoin", sometimes called "benjamin" (i.e., [[benzoin resin]]), an aromatic resin known to European pharmacists and perfumers since the 15th century as a product of southeast Asia. "Benzoin" is itself a corruption of the Arabic expression "luban jawi," or "[[frankincense]] of [[Java]]." An acidic material was derived from benzoin by sublimation, and named "flowers of benzoin," or benzoic acid. The hydrocarbon derived from benzoic acid thus acquired the name benzin, benzol, or benzene.<ref name=rocke>{{cite journal|author= A. J. Rocke|title=Hypothesis and Experiment in the Early Development of Kekule's Benzene Theory|journal=[[Annals of Science]]|volume=42|year=1985|pages=355–81|doi =10.1080/00033798500200411}}</ref>

[[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''.<ref>{{cite journal|author= [[Michael Faraday|M. Faraday]]|title=On New Compounds of Carbon and Hydrogen, and on Certain Other Products Obtained during the Decomposition of Oil by Heat
|journal=[[Philosophical Transactions of the Royal Society of London]]|volume=115|year=1825|pages=440–466| url=http://links.jstor.org/sici?sici=0261-0523%281825%29115%3C440%3AONCOCA%3E2.0.CO%3B2-B|doi=10.1098/rstl.1825.0022}}</ref><ref>{{cite journal|author= R. Kaiser|title=Bicarburet of Hydrogen. Reappraisal of the Discovery of Benzene in 1825 with the Analytical Methods of 1968|journal=Angewandte Chemie International Edition in English|volume=7|issue=5|year=1968|pages=345–350|doi=10.1002/anie.196803451}}</ref>
In 1833, [[Eilhard Mitscherlich]] produced it via the [[distillation]] of [[benzoic acid]] (from [[gum benzoin]]) and [[calcium oxide|lime]]. Mitscherlich gave the compound the name ''benzin''.<ref>{{cite journal|author= E. Mitscherlich|title=Ueber das Benzol und die Säuren der Oel- und Talgarten |journal=[[Liebigs Annalen|Annalen der Pharmacie]]|volume=9|issue=1|year=1834|pages=39–48| doi=10.1002/jlac.18340090103 }}</ref> In 1836, the French chemist [[Auguste Laurent]] named the substance "phène"; this is the root of the word [[phenol]], which is hydroxylated benzene, and [[phenyl]], which is the radical formed by abstraction of a hydrogen atom (free radical H*) from benzene.

[[File:Historic Benzene Formulae Kekulé (original).png|thumb|left|300px|Historic benzene formulae as proposed by Kekulé.<ref name="August Kekulé">August Kekulé: ''Ueber einige Condensationsproducte des Aldehyds'', Liebigs Ann. Chem. '''1872''', ''162'' (1), 77–124, [[DOI:10.1002/jlac.18721620110]].</ref>]]

In 1845, Charles Mansfield, working under [[August Wilhelm von Hofmann]], isolated benzene from [[coal tar]]. Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.

Gradually the sense developed among chemists that substances related to benzene formed a natural chemical family. In 1855 [[August Wilhelm Hofmann]] used the word "[[aromatic]]" to designate this family relationship, after a characteristic property of many of its members.

===Ring formula===
The empirical formula for benzene was long known, but its highly [[Saturation (chemistry)|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 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its very early years, and too little evidence was then available to help chemists decide on any particular structure.

In 1865, the German chemist [[Friedrich August Kekulé von Stradonitz|Friedrich August Kekulé]] published a paper in French (for he was then teaching in Francophone Belgium) suggesting that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German on the same subject.<ref>{{cite journal|author=[[Friedrich August Kekulé von Stradonitz|F. A. Kekulé]]|title=Sur la constitution des substances aromatiques|journal=Bulletin de la Societe Chimique de Paris|volume=3|year=1865|pages= 98–110}}</ref><ref>{{cite journal|author=F. A. Kekulé|title=Untersuchungen uber aromatische Verbindungen|doi = 10.1002/jlac.18661370202|journal=Liebigs Annalen der Chemie|volume=137 |year=1866|pages=129–36}}</ref> Kekulé used evidence that had accumulated in the intervening years—namely, that there always appeared to be only one [[isomer]] of any [[Derivative (chemistry)|monoderivative]]
of benzene, and that there always appeared to be exactly three isomers of every diderivative—now understood to correspond to the ortho, meta, and para patterns of [[Arene substitution patterns|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 carbon-hydrogen ratio.

[[Image:Historic Benzene Formulae V.3.svg|thumb|left|600px|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 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. It is curious that a similar, humorous depiction of benzene had appeared in 1886 in the ''Berichte der Durstigen Chemischen Gesellschaft'' (Journal of the Thirsty Chemical Society), a parody of the ''Berichte der Deutschen Chemischen Gesellschaft'', only the parody had monkeys seizing each other in a circle, rather than snakes as in Kekulé's anecdote.<ref>Translated into English by D. Wilcox and F. Greenbaum, ''Journal of Chemical Education'', '''42''' (1965), 266–67.</ref> 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.<ref name=rocke/> Others have speculated that Kekulé's story in 1890 was a re-parody of the monkey spoof, and was a mere invention rather than a recollection of an event in his life. Kekulé's 1890 speech<ref>{{cite journal| author=F. A. Kekulé| title= Benzolfest: Rede|journal =Berichte der Deutschen Chemischen Gesellschaft|volume = 23| year=1890| pages=1302–11| url=http://gallica.bnf.fr/ark:/12148/bpt6k90720c/f1304.chemindefer| doi= 10.1002/cber.189002301204}}</ref> in which these anecdotes appeared has been translated into English.<ref>O. T.
Benfey, "August Kekulé and the Birth of the Structural Theory of Organic Chemistry in 1858," ''Journal of Chemical Education,'' '''35''' (1958), 21–23</ref> If one takes the anecdote as the memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862.<ref>Jean Gillis, "Auguste Kekulé et son oeuvre, realisee a Gand de 1858 a 1867," ''Memoires de l'Academie Royale de Belgique'', '''37''':1 (1866), 1–40.</ref>

The cyclic nature of benzene was finally confirmed by the crystallographer [[Kathleen Lonsdale]] in 1929.<ref>{{cite journal |author= [[Kathleen Lonsdale|K. Lonsdale]] |title=The Structure of the Benzene Ring in Hexamethylbenzene |journal=[[Proceedings of the Royal Society]] |volume=123A |pages=494 |year=1929}}</ref><ref>{{cite journal |author=K. Lonsdale |title=An X-Ray Analysis of the Structure of Hexachlorobenzene, Using the Fourier Method |journal=[[Proceedings of the Royal Society]] |volume=133A |pages=536–553 |year=1931 |url=http://gallica.bnf.fr/ark:/12148/bpt6k56226p/f558.table}}</ref>

==Structure==
{{Main|Aromaticity}}
Benzene represents a special problem in that, to account for all the bonds, there must be alternating [[double bond|double]] carbon bonds:<ref>[http://www.chemguide.co.uk/basicorg/bonding/benzene1.html bonding in benzene - the Kekulé structure]</ref>

[[File:Benzene Representations.svg|thumb|center|720px|The various representations of benzene|500px|centre]]

Using [[X-ray diffraction]], researchers discovered that all of the carbon-carbon bonds in benzene are of the same length of 140 [[picometre]]s (pm). The C–C [[bond length]]s are greater than a double bond (135pm) but shorter than a single bond (147pm).<!--147pm is the sp2–sp2 single bond length without conjugation --> This intermediate distance is explained by electron [[Delocalized electron|delocalization]]: the electrons for C–C bonding are distributed equally between each of the six carbon atoms. The molecule is planar (ignoring quantum/thermal vibrations), although many calculations
predict otherwise.<ref>{{ cite journal|doi = 10.1021/ja0630285|title = Popular Theoretical Methods Predict Benzene and Arenes To Be Nonplanar|pmid = 16848464|year = 2006|last1 = Moran|first1 = Damian|last2 = Simmonett|first2 = Andrew C.|last3 = Leach|first3 = Franklin E.|last4 = Allen|first4 = Wesley D.|last5 = Schleyer|first5 = Paul v. R.|last6 = Schaefer|first6 = Henry F.|journal = Journal of the American Chemical Society|volume = 128|issue = 29|pages = 9342}}</ref> One representation is that the structure exists as a superposition of so-called [[resonance structure]]s, rather than either form individually. The delocalization of electrons is one explanation for the thermodynamic stability of benzene and related ringed molecules. It is likely that it is this stability that contributes to the peculiar molecular and chemical properties known as [[aromaticity]]. To reflect the delocalized nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of carbon atoms:

The delocalized picture of benzene has been contested by Copper, Gerratt and Raimondi in their article published in 1986 in the journal [[Nature (journal)|Nature]]. They showed that the electrons in benzene are almost certainly localized, and the aromatic properties of benzene originate from spin coupling rather than electron delocalization.<ref>{{cite journal|last1=Copper|first1=David L.|last2=Gerratt|first2=Joseph|last3=Raimondi|first3=Mario|title=The electronic structure of the benzene molecule|journal=Nature|volume=323|pages=699|year=1986|doi=10.1038/323699a0}}</ref> This view has been supported in the next-year Nature issue,<ref>{{cite journal|last1=Pauling|first1=Linus|title=Electronic structure of the benzene molecule|journal=Nature|volume=325|pages=396|year=1987|doi=10.1038/325396d0}}</ref><ref>{{cite journal|last1=Messmer|first1=Richard P.|last2=Schultz|first2=Peter A.|title=The electronic structure of the benzene molecule|journal=Nature|volume=329|pages=492|year=1987|doi=10.1038/329492a0}}</ref><ref>{{cite journal|last1=Harcourt|first1=Richard D.|title=The electronic structure of the benzene molecule|journal=Nature|volume=329|pages=491|year=1987|doi=10.1038/329491b0}}</ref> but it has been slow to permeate the general chemistry community.

As is common in organic chemistry, the carbon atoms in the diagram above have been left unlabeled. Realizing each carbon 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.

Benzene occurs sufficiently often as a component of organic molecules that there is a [[Unicode]] symbol in the [[Miscellaneous Technical (Unicode)|Miscellaneous Technical]] block with the code U+232C (⌬) to represent it with three double bonds,<ref>{{cite web |url=http://www.fileformat.info/info/unicode/char/232c/index.htm |title=Unicode Character 'BENZENE RING' (U+232C) |accessdate=2009-01-16}}</ref> and U+23E3 (⏣) for a delocalized version.<ref>{{cite web |url=http://www.fileformat.info/info/unicode/char/23e3/index.htm |title=Unicode Character 'BENZENE RING WITH CIRCLE' (U+23E3) |accessdate=2009-01-16}}</ref>

==Substituted benzene derivatives==
{{Main|Aromatic hydrocarbons}}

Many important chemicals 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 PhNH<sub>2</sub>, respectively. Linking benzene rings gives [[biphenyl]], C<sub>6</sub>H<sub>5</sub>–C<sub>6</sub>H<sub>5</sub>. Further loss of hydrogen gives "fused" aromatic hydrocarbons, such as [[naphthalene]] and [[anthracene]]. The limit of the fusion process is the hydrogen-free material [[graphite]].

In [[heterocyclic chemistry|heterocycles]], carbon atoms in the benzene ring are replaced with other elements. The most important derivatives are the rings containing [[nitrogen]]. Replacing one CH with N gives the compound [[pyridine]], C<sub>5</sub>H<sub>5</sub>N. Although benzene and pyridine are ''structurally'' related, benzene cannot be converted into pyridine. Replacement of a second CH bond with N gives, depending on the location of the second N, [[pyridazine]], [[pyrimidine]], and [[pyrazine]].

==Production==
Trace amounts of benzene may result whenever [[carbon]]-rich materials undergo incomplete [[combustion]]. It is produced in [[volcano]]es and [[forest fire]]s, and is also a component of [[cigarette]] smoke. Benzene is a principal component 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 (fuel)|coke]] production (or "coke-oven light oil") in the [[steel]] industry. However, in the 1950s, increased demand for benzene, especially from the growing [[plastic]]s industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the [[petrochemical]] industry, with only a small fraction being produced from coal.

Four chemical processes contribute to industrial benzene production: [[catalytic reforming]], [[toluene]] hydrodealkylation, toluene disproportionation, and [[steam cracking]]. In the US, 50% of benzene comes from [[catalytic reforming]] and 25% from [[steam cracking]]. In Western Europe, 50% of benzene comes from [[steam cracking]] and 25% from [[catalytic reforming]].{{Citation needed|date=August 2010}}

===Catalytic reforming===
In catalytic reforming, a mixture of [[hydrocarbon]]s 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 [[Liquid-liquid extraction|extraction]] with any one of a number of [[solvent]]s, including [[diethylene glycol]] or [[sulfolane]], and benzene is then separated from the other aromatics by distillation. The extraction step of aromatics from the reformate is designed to produce aromatics with lowest non-aromatic components. So-called BTX (benzene-toluene-xylene) process consists of such extraction and distillation steps. One such widely used process from [[UOP LLC|UOP]] was licensed to producers and called the Udex process.

Similarly to this catalytic reforming, [[UOP LLC|UOP]] and [[BP]] commercialized a method from LPG (mainly propane and butane) to aromatics.

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

:C<sub>6</sub>H<sub>5</sub>CH<sub>3</sub> + H<sub>2</sub> → C<sub>6</sub>H<sub>6</sub> + CH<sub>4</sub>

This irreversible reaction is accompanied by an equilibrium side reaction that produces
[[biphenyl]] (aka diphenyl) at higher temperature:

:2 {{chem|C|6|H|6}} {{eqm}} {{chem|H|2}} + {{chem|C|6|H|5|–C|6|H|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, [[xylene]]s and heavier aromatics are used in place of toluene, with similar efficiency.

This is often called "on-purpose" methodology to produce benzene, compared to conventional BTX (benzene-toluene-xylene) processes.

===Toluene disproportionation===
Where a chemical complex has similar demands for both benzene and [[xylene]], then toluene [[disproportionation]] ('''TDP''') may be an attractive alternative to the toluene hydrodealkylation. Broadly speaking 2 toluene molecules are reacted and the methyl groups rearranged from one toluene molecule to the other, yielding one benzene molecule and one xylene molecule.

Given that demand for ''para''-xylene ([[P-Xylene|''p''-xylene]]) substantially exceeds demand for other xylene isomers, a refinement of the TDP process called '''Selective TDP''' (STDP) may be used. In this process, the xylene stream exiting the TDP unit is approximately 90% paraxylene. In some current catalytic systems, even the benzene-to-xylenes ratio is decreased (more xylenes) when the demand of xylenes is higher.

===Steam cracking===
[[Steam cracking]] is the process for producing [[ethylene]] and other [[alkene]]s from [[Aliphatic compound|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 distilled (in BTX process) to separate it into its components, including benzene.

==Uses==
===Early uses===
In the 19th and early-20th centuries, benzene was used as an after-shave lotion because of its pleasant smell. Prior to the 1920s, benzene was frequently used as an industrial solvent, especially for degreasing metal. As its toxicity became obvious, benzene was supplanted by other solvents, especially [[toluene]] (methyl benzene), which has similar physical properties but is not as carcinogenic.

In 1903, [[Ludwig Roselius]] popularized the use of benzene to [[caffeine|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 significant contaminant until the late 1970s when leukemia deaths were found associated with Goodyear's Pliofilm production operations in Ohio. Until the late 1970s, many hardware stores, paint stores, and other retail outlets sold benzene in small cans, such as quart size, for general-purpose use. Many students were exposed to benzene in school and university courses while performing laboratory experiments with little or no ventilation in many cases. This very dangerous practice has been almost totally eliminated.

As a [[gasoline]] (petrol) additive, benzene increases the [[octane rating]] and reduces [[Engine knocking|knocking]]. Consequently, 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 entering the [[groundwater]] have led to stringent regulation of gasoline's benzene content, with limits typically around 1%.<ref>Kolmetz, Gentry, Guidelines for BTX Revamps, AIChE 2007 Spring Conference</ref> European petrol specifications now contain the same 1% limit on benzene content. The [[United States Environmental Protection Agency‎]] has new regulations that will lower the benzene content in gasoline to 0.62% in 2011.<ref>{{cite web |url=http://www.epa.gov/EPA-AIR/2006/March/Day-29/a2315b.htm |title=Control of Hazardous Air Pollutants From Mobile Sources |publisher=U.S. Environmental Protection Agency |pages=15853 |accessdate=2008-06-27 |date=2006-03-29}}</ref>

===Current uses===

Today benzene is mainly used as an intermediate to make other chemicals. Its most widely-produced derivatives include [[styrene]], which is used to make polymers and plastics, [[phenol]] for resins and adhesives (via [[cumene]]), and [[cyclohexane]], which is used in the manufacture of Nylon. Smaller amounts of benzene are used to make some types of [[rubber]]s, [[lubricant]]s, [[dye]]s, [[detergent]]s, [[drug]]s, [[explosive]]s, [[napalm]] and [[pesticide]]s.

In both the US and Europe, 50% of benzene is used in the production of [[ethylbenzene]] / styrene, 20% is used in the production of [[cumene]], and about 15% of benzene is used in the production of [[cyclohexane]] (eventually to [[nylon]]).

In laboratory research, [[toluene]] is now often used as a substitute for benzene. The solvent-properties of the two are similar but toluene is less toxic and has a wider liquid range.

Benzene has been used as a basic research tool in a variety of experiments including analysis of a [[two-dimensional gas]].

<imagemap>
File:Benzene_uses.png|center|Major commodity chemicals and polymers derived from benzene.  Clicking on the image loads the appropriate article|600px|thumb
rect 39 660 435 807 [[Benzene]] 
rect 665 60 1062 207 [[Ethylbenzene]]
rect 665 426 1062 579 [[Cumene]]
rect 665 795 1062 942 [[Cyclohexane]]
rect 665 1161 1062 1317 [[Aniline]]
rect 665 1533 1062 1686 [[Chlorobenzene]]
rect 1215 345 1614 495 [[Acetone]]
rect 1215 636 1614 783 [[Phenol]]
rect 1764 57 2163 210 [[Styrene]]
rect 1764 432 2163 585 [[Bisphenol A]]
rect 1764 1083 2163 1233 [[Adipic acid]]
rect 1764 1332 2163 1482 [[Caprolactam]]
rect 2313 57 2712 207 [[Polystyrene]]
rect 2313 315 2712 462 [[Polycarbonate]]
rect 2313 570 2712 717 [[Epoxy resin]]
rect 2313 822 2712 975 [[Phenolic resin]]
rect 2313 1083 2712 1233 [[Nylon 6-6]]
rect 2313 1335 2712 1485 [[Nylon 6]]
desc bottom-left
</imagemap>

==Reactions==

*[[Electrophilic aromatic substitution]] is a general method of derivatizing benzene. Benzene is sufficiently [[nucleophile|nucleophilic]] that it undergoes substitution by [[acylium]] ions or alkyl [[carbocation]]s to give substituted derivatives.

[[Image:OChem-Mech-ElectrophilicAromaticSubstitution-General.png|thumb|center|450px|Electrophilic aromatic substitution of benzene]]

:*The [[Friedel-Crafts acylation]] is a specific 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 chloride]] which act as a halogen carrier.

[[Image:Friedel-Crafts acylation of benzene by ethanol chloride.png|thumb|center|300px|Friedel-Crafts acylation of benzene by acetyl chloride]]

:*Like the Friedel-Crafts acylation, the [[Friedel-Crafts alkylation]] involves the [[alkylation]] of benzene (and many other aromatic rings) using an [[alkyl halide]] in the presence of a strong Lewis acid catalyst.

[[Image:Friedel-craft-alk.png|thumb|center|400px|Friedel-Crafts alkylation of benzene with methyl chloride]]

:*[[Aromatic sulfonation|Sulfonation]]. The most common method involves mixing sulfuric acid with sulfate, a mixture called fuming sulfuric acid. The sulfuric acid protonates the sulfate, giving the sulfur atom a permanent, rather than resonance stabilized positive formal charge. This molecule is very electrophillic and Electrophillic Aromatic Substitution then occurs.
:*[[Nitration]]: Benzene undergoes nitration with nitronium ions (NO<sub>2</sub><sup>+</sup>) as the electrophile. Thus, warming benzene at 50–55 °C, with a combination of concentrated sulfuric and nitric acid to produce the electrophile, gives nitrobenzene.
*[[Hydrogenation]](Reduction): Benzene and derivatives convert to cyclohexane and derivatives when treated with hydrogen at 450 K and 10 atm of pressure with a finely divided [[nickel]] [[catalyst]].
*Benzene is an excellent [[ligand]] in the [[complex (chemistry)|organometallic]] chemistry of low-valent metals. Important examples include the sandwich and half-sandwich complexes respectively [[Bis(benzene)chromium|Cr(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>]] and [RuCl<sub>2</sub>(C<sub>6</sub>H<sub>6</sub>)]<sub>2</sub>.

==Environmental transformation==
Even if it is not a common substrate for the metabolism of organisms, benzene could be oxidized by both [[bacteria]] and [[eukaryote]]s.

In bacteria, [[dioxygenase]] enzyme can add an [[oxygen]] molecule 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.

==Mapping of industrial releases in the United States==
One tool that maps releases of benzene [http://toxmap.nlm.nih.gov/toxmap/tri/mapIt.do?chemicalName=benzene] to particular locations in the United States<ref>{{cite web|url=http://toxmap.nlm.nih.gov/toxmap/tri/mapIt.do?chemicalName=benzene|title=TRI Releases Map |publisher=Toxmap.nlm.nih.gov |accessdate=2010-03-23}}</ref> and also provides additional information about such releases is [[TOXMAP]]. TOXMAP is a Geographic Information System (GIS) from the Division of Specialized Information Services of the [[United States National Library of Medicine]] (NLM) that uses maps of the United States to help users visually explore data from the [[United States Environmental Protection Agency]]'s (EPA) [[Toxics Release Inventory]] and [[Superfund Basic Research Program]]s. TOXMAP is a resource funded by the US Federal Government. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network (TOXNET)<ref>[http://toxnet.nlm.nih.gov TOXNET - Databases on toxicology, hazardous chemicals, environmental health, and toxic releases]</ref> and [[PubMed]], and from other authoritative sources.

==Health effects==
[[File:Benzol.JPG|A bottle of benzene. The warnings show benzene is a toxic and flammable liquid.|thumb]]
Benzene exposure has serious health effects. 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.<ref>[http://www.atsdr.cdc.gov/tfacts3.html#bookmark04 ToxFAQs for Benzene], Agency for Toxic Substances and Disease Registry, Department of Health and Human Services</ref> 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 benzene content. Air around hazardous waste sites or gas stations may contain higher levels of benzene. About 50% of the entire nationwide (United States) exposure to benzene results from smoking tobacco or from exposure to tobacco smoke.<ref>[http://www.atsdr.cdc.gov/toxguides/toxguide-3.pdf?id=39&tid=14 ToxGuide for Benzene], Agency for Toxic Substances and Disease Registry, Department of Health and Human Services</ref>

The short term breathing of high levels of benzene can result in [[death]], while low levels can cause drowsiness, dizziness, rapid heart rate, [[headache]]s, [[tremors]], confusion, and unconsciousness. Eating or drinking foods containing high levels of benzene can cause vomiting, irritation of the [[stomach]], dizziness, sleepiness, convulsions, and death.

The major effects of benzene are manifested via [[Chronic (medicine)|chronic]] (long-term) exposure through the [[blood]]. Benzene damages the [[bone marrow]] and can cause a decrease in red blood cells, leading to [[anemia]]. It can also cause excessive bleeding and depress the [[immune system]], increasing the chance of [[infection]]. Benzene causes leukemia and is associated with other 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 and can cause [[DNA]] strand breaks, [[chromosomal]] damage etc. Benzene causes [[cancer]] in both animals and humans. Benzene was first reported to induce cancer in humans in the 1920s. The chemical industry claims it wasn't until 1979 that the cancer-inducing properties were determined "conclusively" in humans, despite many references to this fact in the medical literature. Industry exploited this "discrepancy" and tried to discredit animal studies which showed benzene caused cancer, saying that they weren't relevant to humans. Benzene has been shown to cause cancer in both sexes of multiple species of laboratory animals exposed via various routes.<ref>{{cite journal| journal =Int J Occup Environ Health| year =2007| month =Apr-Jun| volume =13| issue =2| pages =213–21| title =Benzene-induced cancers: abridged history and occupational health impact| author =Huff J| pmid
=17718179}}</ref><ref>{{cite journal| journal =J Environ Biol|year =2005| month =April|volume =26|issue =2|pages =157–68|title =Biochemical toxicity of benzene|author =Rana SV|coauthors = Verma Y|pmid =16161967}}</ref>

Some women who breathed high levels of benzene for many months had irregular [[menstruation|menstrual]] periods and a decrease in the size of their [[ovaries]]. It is not known whether benzene exposure affects the developing [[fetus]] in pregnant women or fertility in men.

Animal studies have shown low birth weights, delayed bone formation, and bone marrow damage when pregnant animals breathed benzene.

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

The [[US Department of Health and Human Services]] (DHHS) classifies benzene as a human [[carcinogen]]. Long-term exposure to excessive levels of benzene in the air causes [[leukemia]], a potentially fatal [[cancer]] of the blood-forming organs, in susceptible individuals. In particular, [[Acute myeloid leukemia]] or [[acute non-lymphocytic leukaemia]] (AML & ANLL) is not disputed to be caused by benzene.

The [[United States Environmental Protection Agency]] has set the maximum permissible level of benzene in drinking water at 0.005&nbsp;mg/L. The EPA requires that spills or accidental releases into the environment of 10 pounds (4.5&nbsp;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 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.

Workers in various industries that make or use benzene may be at risk for being exposed to high levels of this carcinogenic chemical. Industries that involve the use of benzene include the [[rubber]] industry, oil refineries, chemical plants, shoe manufacturers, and [[gasoline]]-related industries. In 1987, [[Occupational Safety and Health Administration|OSHA]] estimated that about 237,000 workers in the United States were potentially exposed to benzene, but it is not known if this number has substantially changed since then.

Water and [[soil contamination]] are important pathways of concern for transmission of benzene contact. In the US alone there are approximately 100,000 different sites which have benzene soil or groundwater contamination. In 2005, the water supply to the city of [[Harbin]] in China with a population of almost nine million people, was cut off because of a [[Jilin chemical plant explosions 2005|major benzene exposure]]. Benzene leaked into the [[Songhua River]], which supplies drinking water to the city, after an explosion at a China National Petroleum Corporation (CNPC) factory in the city of Jilin on 13 November.

In March 2006, the official [[Food Standards Agency]] in [[United Kingdom|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.<ref>[http://www.cbsnews.com/stories/2006/05/19/health/main1638170.shtml "FDA: Too Much Benzene In Some Drinks"], ''CBS News'', May 19, 2006, retrieved July 11, 2006</ref> (See also [[benzene in soft drinks]]).

===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 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 [[Hydroxyquinol|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 specimen.<ref>{{cite journal|last1=Ashley|pmid=8013127|first1=DL|year=1994|pages=1401–4|issue=7 Pt 2|last2=Bonin|volume=40|journal=Clinical chemistry|first2=MA|last3=Cardinali|first3=FL|last4=McCraw|first4=JM|last5=Wooten|first5=JV|title=Blood concentrations of volatile organic compounds in a nonoccupationally exposed US population and in groups with suspected exposure.|url=http://www.clinchem.org/cgi/reprint/40/7/1401.pdf}}</ref><ref>{{cite journal|last1=Fustinoni|first1=S|last2=Buratti|first2=M|last3=Campo|first3=L|last4=Colombi|first4=A|last5=Consonni|first5=D|last6=Pesatori|first6=AC|last7=Bonzini|first7=M|last8=Farmer|first8=P|last9=Garte|first9=S|title=Urinary t,t-muconic acid, S-phenylmercapturic acid and benzene as biomarkers of low benzene exposure.|journal=Chemico-biological interactions|volume=153-154|pages=253–6|year=2005|pmid=15935823|doi=10.1016/j.cbi.2005.03.031}}</ref><ref>ACGIH. ''2009 TLVs and BEIs''. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, 2009.</ref><ref>R. Baselt, ''Disposition of Toxic Drugs and Chemicals in Man'', 8th edition, Biomedical Publications, Foster City, CA, 2008, pp. 144-148.</ref>

===Molecular toxicology===
The paradigm of toxicological assessment of benzene is slowly 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.<ref name=r3>{{Cite journal|last=Fracasso|first=M.E.|last2=Doria|year=2009|pages=22–8|first2=D|issue=1|last3=Bartolucci|first3=GB|last4=Carrieri|first4=M|last5=Lovreglio|first5=P|last6=Ballini|first6=A|last7=Soleo|first7=L|last8=Tranfo|first8=G|last9=Manno|first9=M|volume=192|title=Low air levels of benzene: Correlation between biomarkers of exposure and genotoxic effects|journal=Toxicol Lett|pmid=19427373|doi=10.1016/j.toxlet.2009.04.028}}</ref> 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.<ref name=r1>{{Cite journal|last=Eastmond|first=D.A.|year=2000|last2=Rupa|first2=DS|last3=Hasegawa|first3=LS|title=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|journal=Mutat Res|issue=1|volume=322| pages=9–20|pmid=7517507}}</ref> 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.<ref name=r2>{{Cite journal|doi=10.1080/15287390802349974|last=Garte|first=S|last2=Taioli|year=2000|first2=E|last3=Popov|first3=T|last4=Bolognesi|first4=C|last5=Farmer|first5=P|last6=Merlo|first6=F|title=Genetic susceptibility to benzene toxicity in humans|journal=J Toxicol Environ Health A|issue=22|volume=71|pages=1482–1489|pmid=18836923}}</ref>

===Summary===
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 most common route of benzene exposure is through inhalation of air emissions from tobacco smoke and motor vehicle exhaust; 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 into by-products called metabolites.<ref>[http://www.atsdr.cdc.gov/mhmi/mmg3handout.pdf Agency for Toxic Substances and Disease Registry. (2007). Benzene: Patient information sheet.]</ref>

====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 harmful mutations.

==See also==
*[[Industrial Union Department v. American Petroleum Institute]]
*[[Benzene in soft drinks]]

==References==
{{Reflist|colwidth=35em}}

==External links==
{{commons|Benzene|Benzene}}
{{Wiktionary|benzene}}
*[http://www.eco-usa.net/toxics/chemicals/benzene.shtml Benzene (www.eco-usa.net)]
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc00/icsc0015.htm International Chemical Safety Card 0015]
*[http://www.epa.gov/iris/subst/0276.htm USEPA Summary of Benzene Toxicity]
*[http://www.cdc.gov/niosh/npg/npgd0049.html NIOSH Pocket Guide to Chemical Hazards]
*{{PubChemLink|241}}
*[http://ntp.niehs.nih.gov/index.cfm?objectid=0707525C-0F07-05BF-A16CAC7B0ECC97B5 Dept. of Health and Human Services: TR-289: Toxicology and Carcinogenesis Studies of Benzene]
*[http://www.ch.ic.ac.uk/video/faraday_l.m4v Video Podcast] of Sir John Cadogan giving a lecture on Benzene since Faraday, in 1991
*[http://ntp.niehs.nih.gov/ntp/roc/eleventh/profiles/s019benz.pdf Substance profile]
*[http://chem.sis.nlm.nih.gov/chemidplus/direct.jsp?name=Benzene&result=advanced U.S. National Library of Medicine: ChemIDplus - Benzene]
*[http://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs+hsdb:@term+@DOCNO+35 NLM Hazardous Substances Databank – Benzene]
{{Functional Groups}}

[[Category:Annulenes]]
[[Category:Simple aromatic rings]]
[[Category:IARC Group 1 carcinogens]]
[[Category:Soil contamination]]
[[Category:Hydrocarbon solvents]]
[[Category:Hazardous air pollutants]]
[[Category:Immunotoxins]]
[[Category:Occupational safety and health]]

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