View source for Benzene

{{Short description|Hydrocarbon compound (C6H6)}}
{{About|the chemical compound}}
{{Distinguish|Petroleum benzine}}
{{chembox
| Watchedfields  = changed
| verifiedrevid  = 464365865
| Name           = Benzene
| ImageFileL1_Ref = {{chemboximage|correct|??}}
| ImageFileL1    = Benzene geometrie (2).svg
| ImageNameL1    = Skeletal formula detail of benzene.
| ImageSizeL1    = 130px
| ImageClassL1   = skin-invert-image
| ImageCaptionL1 = Geometry
| ImageFileR1    = Benzene-aromatic-3D-balls.png
| ImageNameR1    = Benzene ball-and-stick model
| ImageSizeR1    = 125px
| ImageCaptionR1 = Ball and stick model
| ImageClassR1   = bg-transparent
| ImageFile1     = Benzene-3D-vdW.png
| ImageName1     = Benzene molecule
| ImageSize1     = 150px
| ImageClass1     = bg-transparent
| ImageCaption1  = Space-filling model
| ImageFile2     = Benzene sample.jpg
| ImageName2     = Sample of benzene
| ImageSize2     = 200px
| ImageCaption2  = Benzene at room temperature
| PIN            = Benzene<ref name=iupac2013>{{cite book | title =  Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Blue Book) | publisher = The [[Royal Society of Chemistry]] | date = 2014 | location = Cambridge | pages = 10, 22, 204, 494, 577 | doi = 10.1039/9781849733069 | isbn = 978-0-85404-182-4| last1 = Favre | first1 = Henri A. | last2 = Powell | first2 = Warren H. }}</ref>
| OtherNames     = Benzol ''(historic/German)''<br />Cyclohexa-1,3,5-triene; 1,3,5-Cyclohexatriene (theoretical resonance isomers)<br>[6]Annulene (not recommended<ref name=iupac2013 />)<br />Phene ''(historic)''
| Section1       = {{Chembox Identifiers
|CASNo = 71-43-2
|CASNo_Ref = {{cascite|correct|CAS}}
|PubChem = 241
|ChemSpiderID = 236
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|UNII = J64922108F
|UNII_Ref = {{fdacite|correct|FDA}}
|KEGG = C01407
|KEGG_Ref = {{keggcite|correct|kegg}}
|ChEBI = 16716
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEMBL = 277500
|ChEMBL_Ref = {{ebicite|correct|EBI}}
|RTECS = CY1400000
|SMILES = c1ccccc1
|EINECS = 200-753-7
|StdInChI = 1S/C6H6/c1-2-4-6-5-3-1/h1-6H
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = UHOVQNZJYSORNB-UHFFFAOYSA-N
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
}}
| Section2       = {{Chembox Properties
|C=6 | H=6
|Appearance = Colorless liquid
|Odour = sweet aromatic
|Density = 0.8765(20) g/cm<sup>3</sup><ref name="handbook13">{{RubberBible86th}}</ref>
|Solubility = 1.53 g/L (0 °C)<br> 1.81 g/L (9 °C)<br> 1.79 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 |year=1958 |journal=[[Industrial & Engineering Chemistry Chemical & Engineering Data Series]] |volume=3 |issue=2 |pages=253–256 |doi=10.1021/i460004a016}}</ref><!--1st ref is most comprehensive but is paywalled---><ref>{{cite journal |last1=Breslow |first1=R. |last2=Guo |first2=T. |title=Surface tension measurements show that chaotropic salting-in denaturants are not just water-structure breakers |year=1990 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=87 |issue=1 |pages=167–9 |pmid=2153285 |pmc=53221 |doi=10.1073/pnas.87.1.167|bibcode = 1990PNAS...87..167B |doi-access=free }}</ref><ref>{{cite book |last1=Coker |first1=A. Kayode |last2=Ludwig |first2=Ernest E. |title=Ludwig's Applied Process Design for Chemical And Petrochemical Plants |url=https://books.google.com/books?id=N8RcH8juG_YC&pg=PA114 |access-date=2012-05-31 |volume=1 |year=2007 |publisher=Elsevier |isbn=978-0-7506-7766-0 |page=114 |archive-date=2016-03-12 |archive-url=https://web.archive.org/web/20160312120850/https://books.google.com/books?id=N8RcH8juG_YC&pg=PA114 |url-status=live}}</ref><br> 1.84 g/L (30 °C)<br> 2.26 g/L (61 °C)<br> 3.94 g/L (100 °C)<br> 21.7 g/kg (200 °C, 6.5 MPa)<br> 17.8 g/kg (200 °C, 40 MPa)<ref name=chemister>{{Cite web | url=http://chemister.ru/Database/properties-en.php?dbid=1&id=644 | title=Benzol | access-date=2014-05-29 | archive-date=2014-05-29 | archive-url=https://web.archive.org/web/20140529232137/http://chemister.ru/Database/properties-en.php?dbid=1&id=644 | url-status=live }}</ref>
|SolubleOther = Soluble in [[Alcohol (chemistry)|alcohol]], [[Chloroform|CHCl<sub>3</sub>]], [[Carbon tetrachloride|CCl<sub>4</sub>]], [[diethyl ether]], [[acetone]], [[acetic acid]]<ref name=chemister />
|Solubility1 = 5.83 g/100 g (20 °C)<br> 6.61 g/100 g (40 °C)<br> 7.61 g/100 g (60 °C)<ref name=chemister />
|Solvent1 = ethanediol
|Solubility4 = 52 g/100 g (20 °C)<ref name=chemister />
|Solvent4 = diethylene glycol
|MeltingPtC = 5.53
|BoilingPtC = 80.1
|Viscosity = 0.7528 [[Poise (unit)|cP]] (10 °C)<br> 0.6076 cP (25 °C)<br> 0.4965 cP (40 °C)<br> 0.3075 cP (80 °C)
|LambdaMax = 255 nm
|RefractIndex = 1.5011 (20 °C)<br> 1.4948 (30 °C)<ref name=chemister />
|ConjugateAcid = [[Benzenium]]<ref>{{Cite web|publisher=[[PubChem]]|title=Benzenium (CID 12533897|url=https://pubchem.ncbi.nlm.nih.gov/compound/12533897|access-date=September 18, 2022|date=February 8, 2007}}</ref>
|ConjugateBase = [[Arenide ion|Benzenide]]<ref>{{Cite web|publisher=[[PubChem]]|date=June 24, 2005|title=Benzenide (CID 5150480)|url=https://pubchem.ncbi.nlm.nih.gov/compound/5150480|access-date=September 18, 2022}}</ref>
|LogP = 2.13
|MagSus = −54.8·10<sup>−6</sup> cm<sup>3</sup>/mol
|VaporPressure = 12.7 kPa (25 °C)<br> 24.4 kPa (40 °C)<br> 181 kPa (100 °C)<ref name=nist>{{nist|name=Benzene|id=C71432|accessdate=2014-05-29|mask=FFFF|units=SI}}</ref>
}}
| Section3       = {{Chembox Structure
|MolShape = [[Trigonal planar molecular geometry|Trigonal planar]]
|Dipole = 0 [[Debye|D]]
}}
| Section4       = {{Chembox Thermochemistry
|DeltaHf = 48.7 kJ/mol
|HeatCapacity = 134.8 J/mol·K
|Entropy = 173.26 J/mol·K<ref name=nist />
|DeltaHc = −3267.6 kJ/mol<ref name=nist />
}}
| Section7       = {{Chembox Hazards
|ExternalSDS = [http://www.hmdb.ca/system/metabolites/msds/000/001/365/original/HMDB01505.pdf?1358893540 HMDB]
|GHSPictograms = {{GHS02}}{{GHS06}}{{GHS07}}{{GHS08}}{{GHS environment}}<ref name="sigma">[[Sigma-Aldrich]] Co., [http://www.sigmaaldrich.com/catalog/product/sial/401765 Benzene] {{Webarchive|url=https://web.archive.org/web/20161201060259/http://www.hmdb.ca/system/metabolites/msds/000/001/365/original/HMDB01505.pdf?1358893540 |date=2016-12-01 }}. Retrieved on 2014-05-29.</ref>
|GHSSignalWord = Danger
|HPhrases = {{H-phrases|225|302|304|305|315|319|340|350|372|410}}<ref name="sigma" />
|PPhrases = {{P-phrases|201|210|301+310|305+351+338|308+313|331}}<ref name="sigma" />
|FlashPtC = −11.63
|AutoignitionPtC = 497.78
|ExploLimits = 1.2–7.8%
|NFPA-H = 2
|NFPA-F = 3
|NFPA-R = 0
|LD50 = 930 mg/kg (rat, oral)<ref>[https://fscimage.fishersci.com/msds/02610.htm MSDS]</ref>
|PEL = TWA 1 ppm, ST 5 ppm<ref name=PGCH>{{PGCH|0049}}</ref>
|REL = Ca TWA 0.1 ppm ST 1 ppm<ref name=PGCH/>
|IDLH = 500 ppm<ref name=PGCH/>
|MainHazards = potential occupational carcinogen, flammable
|LCLo = 44,000 ppm (rabbit, 30 min)<br />44,923 ppm (dog)<br />52,308 ppm (cat)<br />20,000 ppm (human, 5 min)<ref>{{IDLH|71432|Benzene}}</ref>
}}
| Section8       = {{Chembox Related
|OtherCompounds = [[Toluene]]<br> [[Borazine]]
}}
}}

'''Benzene''' is an [[Organic compound|organic]] [[chemical compound]] with the [[Chemical formula#Molecular formula|molecular formula]] C<sub>6</sub>H<sub>6</sub>. The benzene [[molecule]] is composed of six [[carbon]] atoms joined in a planar hexagonal [[Ring (chemistry)|ring]] with one [[hydrogen]] atom attached to each. Because it contains only [[carbon]] and hydrogen atoms, benzene is classed as a [[hydrocarbon]].

Benzene is a natural constituent of [[petroleum]] and is one of the elementary [[petrochemical]]s. Due to the cyclic continuous [[pi bond]]s between the carbon atoms, benzene is classed as an [[aromatic hydrocarbon]]. Benzene is a colorless and highly [[Combustibility and flammability|flammable]] liquid with a sweet smell, and is partially responsible for the aroma of [[gasoline]]. It is used primarily as a [[Precursor (chemistry)|precursor]] to the manufacture of chemicals with more complex structures, such as [[ethylbenzene]] and [[cumene]], of which billions of kilograms are produced annually. Although benzene is a major [[Chemical industry|industrial chemical]], it finds limited use in consumer items because of its toxicity. Benzene is a [[volatile organic compound]].<ref>{{cite web | title=Benzene fact sheet | website=CDC | date=2 September 2021 | url= https://www.cdc.gov/biomonitoring/Benzene_FactSheet.html | access-date=16 Aug 2023}}</ref>

Benzene is classified as a [[carcinogen]]. Its particular effects on [[human health]], such as the long-term results of accidental exposure, have been reported on by news organizations such as ''[[The New York Times]]''. For instance, a 2022 article stated that benzene contamination in the [[Boston metropolitan area]] caused hazardous conditions in multiple places, with the publication noting that the compound may eventually cause [[leukemia]] in some individuals.<ref>{{cite news | url=https://www.nytimes.com/2022/06/28/climate/natural-gas-home-toxic-chemicals.html | title=Gas Piped into Homes Contains Benzene and Other Risky Chemicals, Study Finds | work=The New York Times | date=28 June 2022 | last1=Shao | first1=Elena }}</ref>

==History==
===Discovery===
The word "''benzene''" derives from "''gum benzoin''" ([[Benzoin (resin)|benzoin resin]]), an aromatic resin known since ancient times in Southeast Asia, and later to European pharmacists and perfumers in the 16th century via trade routes.<ref>The word "benzoin" is derived from the Arabic expression "''luban jawi''", or "[[frankincense]] of [[Java]]". {{cite book|last=Morris|first=Edwin T.|title=Fragrance: The Story of Perfume from Cleopatra to Chanel|year=1984|publisher=Charles Scribner's Sons|isbn=978-0684181950|page=101}}</ref> An [[acid]]ic material was derived from benzoin by [[Sublimation (phase transition)|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= Rocke, A. J. |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|issue= 4}}</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=Faraday, M.|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]]|volume=115|year=1825|pages=440–466|url=http://gallica.bnf.fr/ark:/12148/bpt6k559209/f473.image|doi=10.1098/rstl.1825.0022|jstor=107752|author-link=Michael Faraday|doi-access=free|access-date=2012-01-15|archive-date=2020-11-21|archive-url=https://web.archive.org/web/20201121235017/http://gallica.bnf.fr/ark:/12148/bpt6k559209/f473.image|url-status=live|url-access=subscription}}  On pages 443–450, Faraday discusses "bicarburet of hydrogen" (benzene).  On pages 449–450, he shows that benzene's empirical formula is C<sub>6</sub>H<sub>6</sub>, although he doesn't realize it because he (like most chemists at that time) used the wrong atomic mass for carbon (6 instead of 12).</ref><ref>{{cite journal|author= Kaiser, R. |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 by [[Distillation|distilling]] [[benzoic acid]] (from [[Benzoin (resin)|gum benzoin]]) and [[Calcium oxide|lime]]. He gave the compound the name ''benzin''.<ref>{{cite journal|author=Mitscherlich, E.|title=Über das Benzol und die Säuren der Oel- und Talgarten|url=https://books.google.com/books?id=JEs9AAAAcAAJ&pg=PA39|trans-title=On benzol and oily and fatty types of acids|journal=[[Liebigs Annalen|Annalen der Pharmacie]]|volume=9|issue=1|year=1834|pages=39–48|doi=10.1002/jlac.18340090103|access-date=2015-06-27|archive-date=2015-11-23|archive-url=https://web.archive.org/web/20151123133406/https://books.google.com/books?id=JEs9AAAAcAAJ&pg=PA39|url-status=live}}  In a footnote on page 43, Liebig, the journal's editor, suggested changing Mitscherlich's original name for benzene (namely, "benzin") to "benzol", because the suffix "-in" suggested that it was an alkaloid (e.g., Chinin (quinine)), which benzene isn't, whereas the suffix "-ol" suggested that it was oily, which benzene is. Thus on page 44, Mitscherlich states: ''"Da diese Flüssigkeit aus der Benzoësäure gewonnen wird, und wahrscheinlich mit den Benzoylverbindungen im Zusammenhang steht, so gibt man ihr am besten den Namen Benzol, da der Name Benzoïn schon für die mit dem Bittermandelöl isomerische Verbindung von Liebig und Wöhler gewählt worden ist."'' (Since this liquid [benzene] is obtained from benzoic acid and probably is related to benzoyl compounds, the best name for it is "benzol", since the name "benzoïn" has already been chosen, by Liebig and Wöhler, for the compound that's isomeric with the oil of bitter almonds [benzaldehyde].)</ref> In 1836, the French chemist [[Auguste Laurent]] named the substance "phène";<ref>Laurent, (1836) "Sur la chlorophénise et les acides chlorophénisique et chlorophénèsique," ''Annales de Chemie et de Physique'', vol. 63, pp. 27–45, see [https://books.google.com/books?id=Lx0AAAAAMAAJ&pg=PA44 p. 44] {{Webarchive|url=https://web.archive.org/web/20150320073432/http://books.google.com/books?id=Lx0AAAAAMAAJ&pg=PA44 |date=2015-03-20}}: ''"Je donne le nom de phène au radical fondamental des acides précédens (φαινω, j'éclaire), puisque la benzine se trouve dans le gaz de l'éclairage."'' (I give the name of "phène" (φαινω, I illuminate) to the fundamental radical of the preceding acids, because benzene is found in illuminating gas.)</ref> this word has become the root of the English word "[[phenol]]", which is [[Hydroxylation|hydroxylated]] benzene, and "[[Phenyl group|phenyl]]", the radical formed by abstraction of a hydrogen atom from benzene.

In 1845, [[Charles Blachford Mansfield]], working under [[August Wilhelm von Hofmann]], isolated benzene from [[coal tar]].<ref>{{cite journal |last = Hofmann|first = A. W. |date =1845|url =https://books.google.com/books?id=E-1AAAAAYAAJ&pg=PA200 |title = Ueber eine sichere Reaction auf Benzol|archive-url=https://web.archive.org/web/20151122124839/https://books.google.com/books?id=E-1AAAAAYAAJ&pg=PA200 |url-status = live|archive-date=2015-11-22 |trans-title = On a reliable test for benzene|journal =Annalen der Chemie und Pharmacie|volume = 55 |issue = 2 |language = de|pages = 200–205, especially 204–205|doi = 10.1002/jlac.18450550205 }}</ref> Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.<ref>{{cite journal | author = Mansfield Charles Blachford | year = 1849 | title = Untersuchung des Steinkohlentheers | url = https://books.google.com/books?id=kD4aAQAAMAAJ&pg=PA162 | journal = Annalen der Chemie und Pharmacie | volume = 69 | issue = 2 | pages = 162–180 | doi = 10.1002/jlac.18490690203 | access-date = 2015-06-27 | archive-date = 2015-10-27 | archive-url = https://web.archive.org/web/20151027022228/https://books.google.com/books?id=kD4aAQAAMAAJ&pg=PA162 | url-status = live }}</ref><ref>Charles Mansfield filed for (November 11, 1847) and received (May 1848) a patent (no. 11,960) for the fractional distillation of coal tar.</ref> Gradually, the sense developed among chemists that a number of substances were chemically related to benzene, comprising a diverse chemical family. In 1855, Hofmann was the first to apply the word "[[Aromaticity|aromatic]]" to designate this family relationship, after a characteristic property of many of its members.<ref>{{cite journal|author=Hoffman, Augustus W. |year=1856|doi=10.1098/rspl.1856.0002 |title= On insolinic acid|journal=[[Proceedings of the Royal Society]]|volume=8| pages =1–3|s2cid=97105342 |quote= The existence and mode of formation of insolinic acid prove that to the series of monobasic aromatic acids, C<sub>n2</sub>H<sub>n2-8</sub>O<sub>4</sub>, the lowest known term of which is benzoic acid, ... .|doi-access= }} [Note: The empirical formulas of organic compounds that appear in Hofmann's article (p. 3) are based upon an atomic mass of carbon of 6 (instead of 12) and an atomic mass of oxygen of 8 (instead of 16).]</ref> In 1997, benzene was [[List of interstellar and circumstellar molecules|detected in deep space]].<ref name="Cernicharo_546">{{citation | display-authors=1 | last1=Cernicharo | first1=José | last2=Heras | first2=Ana M. | last3=Tielens | first3= A. G. G. M. | last4=Pardo | first4=Juan R. | last5=Herpin | first5=Fabrice | last6=Guélin | first6=Michel | last7=Waters | first7=L. B. F. M. | title=Infrared Space Observatory's Discovery of C<sub>4</sub>H<sub>2</sub>, C<sub>6</sub>H<sub>2</sub>, and Benzene in CRL 618 | journal=Astrophysical Journal Letters | year=2001 | volume=546 | issue=2 | pages=L123–L126| doi=10.1086/318871 | bibcode=2001ApJ...546L.123C| doi-access=free}}</ref>

===Ring formula===
{| class="wikitable" style="text-align:center; font-size:90%; margin-left:20px;" width="50%"
|-
!colspan="6" | Historic proposals of benzene structures
|- style="background-color:white"
| [[File:Historic Benzene Formula Claus 1867 (original)2.png|120px]]
| [[File:Historic Benzene Formula Dewar 1869 (original Dewar benzene).png|91px]]
| [[File:Historic Benzene Formula Ladenburg 1869 (original).png|80px]]
|- align="center" style="vertical-align:top"
| By [[Adolf Karl Ludwig Claus]] (1867)<ref>Claus, Adolph K.L. (1867) "Theoretische Betrachtungen und deren Anwendungen zur Systematik der organischen Chemie" (Theoretical considerations and their applications to the classification scheme of organic chemistry), ''Berichte über die Verhandlungen der Naturforschenden Gesellschaft zu Freiburg im Breisgau'' ([[Reports of the Proceedings of the Scientific Society of Freiburg]] in Breisgau), '''4''' : 116–381. In the section ''Aromatischen Verbindungen'' (aromatic compounds), pp. 315–347, Claus presents Kekulé's hypothetical structure for benzene ([http://gdz.sub.uni-goettingen.de/en/dms/loader/img/?PID=PPN623920514_0004%7CLOG_0021&physid=PHYS_0339 p. 317]), presents objections to it, presents an alternative geometry ([http://gdz.sub.uni-goettingen.de/en/dms/loader/img/?PID=PPN623920514_0004%7CLOG_0021&physid=PHYS_0342 p. 320]), and concludes that his alternative is correct ([http://gdz.sub.uni-goettingen.de/en/dms/loader/img/?PID=PPN623920514_0004%7CLOG_0021&physid=PHYS_0348 p. 326]). See also figures on [http://gdz.sub.uni-goettingen.de/en/dms/loader/img/?PID=PPN623920514_0004%7CLOG_0021&physid=PHYS_0376 p. 354] or [http://gdz.sub.uni-goettingen.de/en/dms/loader/img/?PID=PPN623920514_0004%7CLOG_0023&physid=PHYS_0401 p. 379.]</ref>
| Listed, but not supported,<ref group="note"> Dewar included this isomer in a list of possible C6H6 structures in 1869. However, he did not propose it for benzene, and in fact he supported the correct structure proposed by Kekulé. See [[Dewar benzene]] </ref> by [[James Dewar]] (1869)<ref>{{cite journal | author = Dewar James | year = 1869 | title = On the oxidation of phenyl alcohol, and a mechanical arrangement adapted to illustrate structure in the non-saturated hydrocarbons | url = https://books.google.com/books?id=PmlUAAAAIAAJ&pg=PA82 | journal = [[Proceedings of the Royal Society of Edinburgh]] | volume = 6 | pages = 82–86 | doi = 10.1017/S0370164600045387 | access-date = 2015-06-27 | archive-date = 2015-11-24 | archive-url = https://web.archive.org/web/20151124162938/https://books.google.com/books?id=PmlUAAAAIAAJ&pg=PA82 | url-status = live }}</ref>
| By [[Albert Ladenburg]] (1869)<ref>{{cite journal | author = Ladenburg Albert | year = 1869 | title = Bemerkungen zur aromatischen Theorie | trans-title = Observations on the aromatic theory | url = https://books.google.com/books?id=Epg8AAAAIAAJ&pg=PA140 | journal = Berichte der Deutschen Chemischen Gesellschaft | volume = 2 | pages = 140–142 | doi = 10.1002/cber.18690020171 | access-date = 2015-06-27 | archive-date = 2015-11-22 | archive-url = https://web.archive.org/web/20151122175439/https://books.google.com/books?id=Epg8AAAAIAAJ&pg=PA140 | url-status = live }}</ref>
|-
|colspan="3" style="background:#FFFFFF"| [[File:Historic Benzene Formulae Kekulé (original).png|300px]]
|- align="center" style="vertical-align:top"
|colspan="3"| By [[August Kekulé]] (1865/1872)<ref name="Kekulé1865" /><ref>{{citation|surname1=Aug. Kekulé|periodical=[[Annalen der Chemie und Pharmacie]]|title=Ueber einige Condensationsproducte des Aldehyds|volume=162|issue=1|at=pp.&nbsp;77–124|date=1872|language=German|doi=10.1002/jlac.18721620110|url=https://upload.wikimedia.org/wikipedia/commons/5/5d/Kekule_-_Ueber_einige_Condensationsproducte_des_Aldehyds.pdf}}</ref>
|- style="background-color:white"
| [[File:Historic Benzene Formulae Armstrong (original).png|130px]]
| [[File:Historic Benzene Formula Baeyer 1888 (original).png|150px]]
| [[File:Historic Benzene Formula Thiele 1899 (original).png|90px]]
|- align="center" style="vertical-align:top"
| By [[Henry Edward Armstrong]] (1887)<ref>{{cite journal | author = Armstrong Henry E | year = 1887 | title = An explanation of the laws which govern substitution in the case of benzenoid compounds | url = https://books.google.com/books?id=4QbzAAAAMAAJ&pg=PA258 | journal = Journal of the Chemical Society | volume = 51 | pages = 258–268 [264] | doi = 10.1039/ct8875100258 | access-date = 2015-06-27 | archive-date = 2015-10-22 | archive-url = https://web.archive.org/web/20151022073257/https://books.google.com/books?id=4QbzAAAAMAAJ&pg=PA258 | url-status = live| url-access = subscription }}</ref><ref>In his 1890 paper, Armstrong represented benzene nuclei within polycyclic benzenoids by placing inside the benzene nuclei a letter "C", an abbreviation of the word "centric". Centric affinities (i.e., bonds) acted within a designated cycle of carbon atoms.  From p. 102:  " ... benzene, according to this view, may be represented by a double ring, in fact."  See: 
*  {{cite journal|last1=Armstrong|first1=H.E.|title=The structure of cycloid hydrocarbons|journal=[[Proceedings of the Chemical Society]]|date=1890|volume=6|pages=101–105|url=https://babel.hathitrust.org/cgi/pt?id=pst.000052368141;view=1up;seq=319|ref=none|access-date=2018-02-17|archive-date=2021-11-16|archive-url=https://web.archive.org/web/20211116024104/https://babel.hathitrust.org/cgi/pt?id=pst.000052368141;view=1up;seq=319|url-status=live}}
The use of a circle to denote a benzene nucleus first appeared in:
*  {{cite journal|last1=Armit|first1=James Wilson|last2=Robinson|first2=Robert|title=Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases|journal=Journal of the Chemical Society, Transactions|date=1925|volume=127|pages=1604–1618|doi=10.1039/ct9252701604|ref=none}}
A history of the determination of benzene's structure is recounted in:
*  {{cite journal|last1=Balaban|first1=Alexandru T.|last2=Schleyer|first2=Paul v. R.|last3=Rzepa|first3=Henry S.|title=Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons|journal=Chemical Reviews|date=2005|volume=105|issue=10|pages=3436–3447|doi=10.1021/cr0300946|pmid=16218557|doi-access=free|ref=none}}</ref>
| By [[Adolf von Baeyer]] (1888)<ref>{{citation|surname1=Adolf Baeyer|periodical=[[Justus Liebigs Annalen der Chemie]]|title=Über die Constitution des Benzols|volume=245|issue=1–2|at=pp.&nbsp;103–190|date=1888|language=German|doi=10.1002/jlac.18882450110|url=https://zenodo.org/record/1851261 }}</ref>
| By [[Johannes Thiele (chemist)|Friedrich Karl Johannes Thiele]] (1899)<ref>Thiele, Johannes (1899) "Zur Kenntnis der ungesättigten Verbindungen" (On our knowledge of unsaturated compounds), ''Justus Liebig's Annalen der Chemie'''''306''':  87–142;  see:  [https://books.google.com/books?id=NYw8AAAAIAAJ&pg=RA1-PA125 "VIII. Die aromatischen Verbindungen.  Das Benzol." (VIII.  The aromatic compounds.  Benzene.),  pp. 125–129.] {{Webarchive|url=https://web.archive.org/web/20200310154749/https://books.google.com/books?id=NYw8AAAAIAAJ&pg=RA1-PA125 |date=2020-03-10}} See further:  Thiele (1901) [https://babel.hathitrust.org/cgi/pt?id=hvd.hx3kjj;view=1up;seq=543 "Zur Kenntnis der ungesättigen Verbindungen,"] {{Webarchive|url=https://web.archive.org/web/20211226183108/https://babel.hathitrust.org/cgi/pt?id=hvd.hx3kjj;view=1up;seq=543 |date=2021-12-26 }} ''Justus Liebig's Annalen der Chemie'', '''319''': 129–143.</ref>
|}

The [[empirical formula]] for benzene was long known, but its highly [[Saturated and unsaturated compounds|polyunsaturated]] structure, with just one [[hydrogen]] atom for each [[carbon]] atom, was challenging to determine. [[Archibald Scott Couper]] in 1858 and [[Johann Josef Loschmidt]] in 1861<ref>{{cite book |first=J. |last=Loschmidt |title=Chemische Studien |location=Vienna, Austria-Hungary |publisher=Carl Gerold's Sohn |year=1861 |url=https://books.google.com/books?id=ksw5AAAAcAAJ&pg=PA30 |language=de |pages=30, 65 |access-date=2015-06-27 |archive-date=2016-05-07 |archive-url=https://web.archive.org/web/20160507144736/https://books.google.com/books?id=ksw5AAAAcAAJ&pg=PA30 |url-status=live }}</ref> suggested possible structures that contained multiple double bonds or multiple rings, but in these years very little was known about aromatic chemistry, and so chemists were unable to adduce appropriate evidence to favor any particular formula.

But many chemists had begun to work on aromatic substances, especially in Germany, and relevant data was coming fast. In 1865, the German chemist [[Friedrich August Kekulé]] published a paper in French (for he was then teaching in Francophone Belgium) suggesting that the structure contained a ring of six carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German on the same subject.<ref name="Kekulé1865">{{cite journal|author=Kekulé, F. A.|title=Sur la constitution des substances aromatiques|url=https://gallica.bnf.fr/ark:/12148/bpt6k281952v/f102|journal=Bulletin de la Société Chimique de Paris|volume=3|year=1865|pages=98–110|author-link=Friedrich August Kekulé von Stradonitz|access-date=2015-06-27|archive-date=2015-11-14|archive-url=https://web.archive.org/web/20151114102723/https://books.google.com/books?id=bFsSAAAAYAAJ&pg=PA98|url-status=live}} On p. 100, Kekulé suggests that the carbon atoms of benzene could form a "chaîne fermée" (closed chain).</ref><ref>{{cite journal|author=Kekulé, F. A.|title=Untersuchungen über aromatische Verbindungen (Investigations of aromatic compounds)|url=https://books.google.com/books?id=pNryAAAAMAAJ&pg=PA129|doi=10.1002/jlac.18661370202|journal=Liebigs Annalen der Chemie und Pharmacie|volume=137|year=1866|pages=129–36|issue=2|access-date=2021-12-25|archive-date=2021-12-26|archive-url=https://web.archive.org/web/20211226183108/https://books.google.com/books?id=pNryAAAAMAAJ&pg=PA129|url-status=live}}</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 disubstituted derivative—now understood to correspond to the ortho, meta, and para patterns of [[Arene substitution pattern|arene substitution]]—to argue in support of his proposed structure.<ref>{{cite book |url=https://books.google.com/books?id=U5iHmP_dTLwC&pg=PA186 |last=Rocke |first=A. J. |year=2010 |title=Image and Reality: Kekule, Kopp, and the Scientific Imagination |publisher=University of Chicago Press |pages=186–227 |isbn=978-0226723358 |access-date=2020-05-15 |archive-date=2020-07-27 |archive-url=https://web.archive.org/web/20200727092623/https://books.google.com/books?id=U5iHmP_dTLwC&pg=PA186 |url-status=live }}</ref> Kekulé's symmetrical ring could explain these curious facts, as well as benzene's 1:1 carbon-hydrogen ratio.

<gallery mode="packed">
Historic Benzene Formulae Kekulé (original).png|Kekulé's 1872 modification of his 1865 theory, illustrating rapid alternation of double bonds<ref group=note>Critics pointed out a  problem with Kekulé's original (1865) structure for benzene:  Whenever benzene underwent substitution at the ortho position, two distinguishable isomers should have resulted, depending on whether a double bond or a single bond existed between the carbon atoms to which the substituents were attached; however, no such isomers were observed.  In 1872, Kekulé suggested that benzene had two complementary structures and that these forms rapidly interconverted, so that if there were a double bond between any pair of carbon atoms at one instant, that double bond would become a single bond at the next instant (and vice versa). To provide a mechanism for the conversion process, Kekulé proposed that the valency of an atom is determined by the frequency with which it collided with its neighbors in a molecule. As the carbon atoms in the benzene ring collided with each other, each carbon atom would collide twice with one neighbor during a given interval and then twice with its other neighbor during the next interval. Thus, a double bond would exist with one neighbor during the first interval and with the other neighbor during the next interval. Therefore, between the carbon atoms of benzene there were no fixed (i.e., constant) and distinct single or double bonds; instead, the bonds between the carbon atoms were identical. See [http://de.wikipedia.org/w/index.php?title=Datei:Kekule_-_Ueber_einige_Condensationsproducte_des_Aldehyds.pdf&page=10 pages 86–89] {{Webarchive|url=https://web.archive.org/web/20200320235654/https://de.wikipedia.org/w/index.php?title=Datei%3AKekule_-_Ueber_einige_Condensationsproducte_des_Aldehyds.pdf&page=10 |date=2020-03-20 }} of Auguste Kekulé (1872) "Ueber einige Condensationsprodukte des Aldehyds" (On some condensation products of aldehydes), ''Liebig's Annalen der Chemie und Pharmacie'', '''162'''(1): 77–124, 309–320.  From p. 89:  ''"Das einfachste Mittel aller Stöße eines Kohlenstoffatoms ergiebt sich aus der Summe der Stöße der beiden ersten Zeiteinheiten, die sich dann periodisch wiederholen.  ... man sieht daher, daß jedes Kohlenstoffatom mit den beiden anderen, ... daß diese Verschiedenheit nur eine scheinbare, aber keine wirkliche ist."'' (The simplest average of all the collisions of a carbon atom [in benzene] comes from the sum of the collisions during the first two units of time, which then periodically repeat.  ... thus one sees that each carbon atom collides equally often with the two others against which it bumps, [and] thus stands in exactly the same relation with its two neighbors. The usual structural formula for benzene expresses, of course, only the collisions that occur during ''one'' unit of time, thus during one phase, and so one is led to the view [that] doubly substituted derivatives [of benzene] must be different at positions 1,2 and 1,6 [of the benzene ring]. If the idea [that was] just presented—or a similar one—can be regarded as correct, then [it] follows therefrom that this difference [between the bonds at positions 1,2 and 1,6] is only an apparent [one], not a real [one].)</ref>
</gallery>
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 biting its own tail (a symbol in ancient cultures known as the [[ouroboros]]).<ref>{{cite book | last = Read | first = John | title = From alchemy to chemistry | url = https://archive.org/details/fromalchemytoche00read_063 | url-access = limited | year = 1995 | publisher = Dover Publications | location = New York | isbn = 9780486286907 | pages = [https://archive.org/details/fromalchemytoche00read_063/page/n210 179]–180}}</ref> This vision, he said, came to him after years of studying the nature of carbon-carbon bonds. This was seven years after he had solved the problem of how carbon atoms could bond to up to four other atoms at the same time. Curiously, a similar, humorous depiction of benzene had appeared in 1886 in a pamphlet entitled ''Berichte der Durstigen Chemischen Gesellschaft'' (Journal of the Thirsty Chemical Society), a parody of the ''Berichte der Deutschen Chemischen Gesellschaft'', only the parody had monkeys seizing each other in a circle, rather than snakes as in Kekulé's anecdote.<ref>English translation {{cite journal|doi=10.1021/ed042p266|journal=Journal of Chemical Education|volume=42|year=1965|pages=266–67|title=Kekule's benzene ring theory: A subject for lighthearted banter|last1=Wilcox|first1=David H.|last2=Greenbaum|first2=Frederick R.|issue=5|bibcode = 1965JChEd..42..266W}}</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/> Kekulé's 1890 speech<ref>{{cite journal| author=Kekulé, F. A.| 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| access-date=2007-03-12| archive-date=2007-05-16| archive-url=https://web.archive.org/web/20070516101338/http://gallica.bnf.fr/ark:/12148/bpt6k90720c/f1304.chemindefer| url-status=live}}</ref> in which this anecdote appeared has been translated into English.<ref>{{cite journal|author = Benfey O. T.|year = 1958|title = August Kekulé and the Birth of the Structural Theory of Organic Chemistry in 1858|journal = Journal of Chemical Education|volume = 35|issue = 1|pages = 21–23|doi = 10.1021/ed035p21 |bibcode = 1958JChEd..35...21B }}</ref> If the anecdote is the memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862.<ref>{{cite journal | author = Gillis Jean | year = 1966 | title = Auguste Kekulé et son oeuvre, réalisée à Gand de 1858 à 1867 | journal = Mémoires de la Classe des Sciences - Académie Royale des Sciences, des Lettres et des Beaux-arts de Belgique | volume = 37 | issue = 1| pages = 1–40 }}</ref>

In 1929, the cyclic nature of benzene was finally confirmed by the [[Crystallography|crystallographer]] [[Kathleen Lonsdale]] using [[X-ray diffraction]] methods.<ref>{{cite journal |author= Lonsdale, K. |title=The Structure of the Benzene Ring in Hexamethylbenzene |journal=[[Proceedings of the Royal Society]] |volume=123A |issue=792 |pages=494–515 |year=1929 |author-link= Kathleen Lonsdale|doi=10.1098/rspa.1929.0081 |doi-access=free |bibcode=1929RSPSA.123..494L}}</ref><ref>{{cite journal |author=Lonsdale, K. |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 |bibcode=1931RSPSA.133..536L |doi=10.1098/rspa.1931.0166 |issue=822 |doi-access=free |access-date=2007-03-12 |archive-date=2021-03-19 |archive-url=https://web.archive.org/web/20210319094750/https://gallica.bnf.fr/ark:/12148/bpt6k56226p/f558.table |url-status=live}}</ref> Using large crystals of [[hexamethylbenzene]], a benzene derivative with the same core of six carbon atoms, Lonsdale obtained diffraction patterns. Through calculating more than thirty parameters, Lonsdale demonstrated that the benzene ring could not be anything but a flat hexagon, and provided accurate distances for all carbon-carbon bonds in the molecule.<ref>{{Cite web |last=Ramos-Figueroa |first=Josseline |date=2021-05-21 |title=Meet Kathleen Lonsdale, the physicist and prison reformer who cracked benzene's code |url=https://massivesci.com/articles/kathleen-lonsdale-benzene-xray-crystallography/ |access-date=2022-03-01 |website=Massive Science}}</ref>

===Nomenclature===
The German chemist [[Wilhelm Körner]] suggested the prefixes ortho-, meta-, para- to distinguish di-substituted benzene derivatives in 1867; however, he did not use the prefixes to distinguish the relative positions of the substituents on a benzene ring.<ref>Wilhelm Körner (1867)  [https://www.biodiversitylibrary.org/item/28090#page/176/mode/1up "Faits pour servir à la détermination du lieu chimique dans la série aromatique"] {{Webarchive|url=https://web.archive.org/web/20170707092456/http://www.biodiversitylibrary.org/item/28090#page/176/mode/1up |date=2017-07-07 }} (Facts to be used in determining chemical location in the aromatic series), ''Bulletins de l'Académie royale des sciences, des lettres et des beaux-arts de Belgique'', 2nd series, '''24''' : 166–185; see especially p. 169. From p. 169: ''"On distingue facilement ces trois séries, dans lesquelles les dérivés bihydroxyliques ont leurs terms correspondants, par les préfixes ortho-, para- et mêta-."''  (One easily distinguishes these three series – in which the dihydroxy derivatives have their corresponding terms – by the prefixes ortho-, para- and meta-.)</ref><ref>Hermann von Fehling, ed., ''Neues Handwörterbuch der Chemie'' [New concise dictionary of chemistry] (Braunschweig, Germany:  Friedrich Vieweg und Sohn, 1874), vol. 1, [https://books.google.com/books?id=X_gfAQAAMAAJ&pg=PA1142 p. 1142.]</ref> It was the German chemist [[Carl Gräbe]] who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a di-substituted aromatic ring (viz, naphthalene).<ref>Graebe (1869) [http://babel.hathitrust.org/cgi/pt?id=uva.x002457978;view=1up;seq=32 "Ueber die Constitution des Naphthalins"] {{Webarchive|url=https://archive.today/20151128145356/http://babel.hathitrust.org/cgi/pt?id=uva.x002457978;view=1up;seq=32 |date=2015-11-28}} (On the structure of naphthalene), ''Annalen der Chemie und Pharmacie'', '''149''' : 20–28; see especially p. 26.</ref> In 1870, the German chemist [[Viktor Meyer]] first applied Gräbe's nomenclature to benzene.<ref>Victor Meyer (1870) [http://babel.hathitrust.org/cgi/pt?id=njp.32101044011672;view=1up;seq=649 "Untersuchungen über die Constitution der zweifach-substituirten Benzole"] {{Webarchive|url=https://archive.today/20151023152601/http://babel.hathitrust.org/cgi/pt?id=njp.32101044011672;view=1up;seq=649 |date=2015-10-23 }} (Investigations into the structure of di-substituted benzenes), ''Annalen der Chemie und Pharmacie'', '''156''' : 265–301; see especially pp. 299–300.</ref>

===Early applications===
In 1903, [[Ludwig Roselius]] popularized the use of benzene to [[decaffeination|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 [[Penetrating oil|liquid wrench]], several [[paint stripper]]s, [[rubber cement]]s, 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.<ref>{{Cite journal|last1=Williams|first1=P.R.D.|last2=Knutsen|first2=J.S.|last3=Atkinson|first3=C.|last4=Madl|first4=A.K.|last5=Paustenbach|first5=D.J.|date=2007|title=Airborne Concentrations of Benzene Associated with the Historical Use of Some Formulations of Liquid Wrench|journal=Journal of Occupational and Environmental Hygiene|volume=4|issue=8|pages=547–561|doi=10.1080/15459620701446642|pmid=17558801|bibcode=2007JOEH....4..547W |s2cid=32311057}}</ref>

===Occurrence===
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]], much of benzene was obtained as a by-product of [[coke (fuel)|coke]] production (or "coke-oven light oil") for the [[steel]] industry. However, in the 1950s, increased demand for benzene, especially from the growing [[polymer]]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.<ref name=Ullmann/> Benzene has been detected on [[Mars]].<ref name="NYT-20180607">{{cite news |url=https://www.nytimes.com/2018/06/07/science/mars-nasa-life.html |title=Life on Mars? Rover's Latest Discovery Puts It 'On the Table' |work=[[The New York Times]] |last=Chang |first=Kenneth |date=June 7, 2018 |access-date=June 8, 2018 |quote=The identification of organic molecules in rocks on the red planet does not necessarily point to life there, past or present, but does indicate that some of the building blocks were present. |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528075820/https://www.nytimes.com/2018/06/07/science/mars-nasa-life.html |url-status=live }}</ref><ref name="SCI-20180608a">{{cite journal |title=Organic molecules on Mars |journal=[[Science (journal)|Science]] |last=ten Kate |first=Inge Loes |volume=360 |issue=6393 |pages=1068–1069 |date=June 8, 2018 |doi=10.1126/science.aat2662 |pmid=29880670 |bibcode=2018Sci...360.1068T|hdl=1874/366378 |s2cid=46952468 |hdl-access=free }}</ref><ref name="SCI-20180608c">{{cite journal |title=Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars |journal=[[Science (journal)|Science]] |first1=Jennifer L. |last1=Eigenbrode |first2=Roger E. |last2=Summons |first3=Andrew |last3=Steele |first4=Caroline |last4=Freissinet |first5=Maëva |last5=Millan |first6=Rafael |last6=Navarro-González |first7=Brad |last7=Sutter |first8=Amy C. |last8=McAdam |first9=Heather B. |last9=Franz |first10=Daniel P. |last10=Glavin |first11=Paul D. |last11=Archer |first12=Paul R. |last12=Mahaffy |first13=Pamela G. |last13=Conrad |first14=Joel A. |last14=Hurowitz |first15=John P. |last15=Grotzinger |first16=Sanjeev |last16=Gupta |first17=Doug W. |last17=Ming |first18=Dawn Y. |last18=Sumner |first19=Cyril |last19=Szopa |first20=Charles |last20=Malespin |first21=Arnaud |last21=Buch |first22=Patrice |last22=Coll |display-authors=1 |volume=360 |issue=6393 |pages=1096–1101 |date=June 8, 2018 |doi=10.1126/science.aas9185 |pmid=29880683 |bibcode=2018Sci...360.1096E |s2cid=46983230 |url=https://authors.library.caltech.edu/86910/2/aas9185-Eigenbrode-SM.pdf |doi-access=free |access-date=January 4, 2021 |archive-date=August 25, 2021 |archive-url=https://web.archive.org/web/20210825114549/https://authors.library.caltech.edu/86910/2/aas9185-Eigenbrode-SM.pdf |url-status=live }}</ref>

==Structure==
{{Main|Aromaticity}}
[[File:Benzene Representations.svg|thumb|400px|The various representations of benzene.]]
[[X-ray diffraction]] shows that all six carbon-carbon bonds in benzene are of the same length, at 140 [[picometre]]s (pm).<ref>{{Cite journal|last1=Bacon|first1=G. E.|last2=Curry|first2=N.|last3=Wilson|first3=S.|date=May 12, 1964|title=A Crystallographic Study of Solid Benzene by Neutron Diffraction|journal=[[Proceedings of the Royal Society of London, Series A]]|volume=279|issue=1376|issn=2053-9169|url=https://royalsocietypublishing.org/doi/10.1098/rspa.1964.0092|pages=98–110|doi=10.1098/rspa.1964.0092 |jstor=2414835|bibcode=1964RSPSA.279...98B|s2cid=94432897|url-access=subscription}}</ref> The C–C [[bond length]]s are greater than a double bond (135 pm) but shorter than a single bond (147 pm).<!--147pm is the sp2–sp2 single bond length without conjugation --> This intermediate distance is caused by electron [[Delocalized electron|delocalization]]: the electrons for C=C bonding are distributed equally between each of the six carbon atoms. Benzene has 6 hydrogen atoms, fewer than the corresponding parent [[alkane]], [[hexane]], which has 14. Benzene and [[cyclohexane]] have a similar structure; only the ring of delocalized electrons and the loss of one hydrogen per carbon distinguishes it from cyclohexane. 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|vauthors=Moran D, Simmonett AC, Leach FE, Allen WD, Schleyer PV, Schaefer HF |journal = Journal of the American Chemical Society|volume = 128|issue = 29|pages = 9342–3| bibcode=2006JAChS.128.9342M }}</ref> The molecular orbital description involves the formation of three delocalized [[pi bond|π orbitals]] spanning all six carbon atoms, while the valence bond description involves a superposition of [[Resonance (chemistry)|resonance structures]].<ref>{{cite journal|last1=Cooper|first1=David L.|last2=Gerratt|first2=Joseph|last3=Raimondi|first3=Mario|title=The electronic structure of the benzene molecule|journal=Nature|volume=323|pages=699–701|year=1986|doi=10.1038/323699a0|issue=6090|bibcode = 1986Natur.323..699C |s2cid=24349360}}</ref><ref>{{cite journal|last1=Pauling|first1=Linus|title=Electronic structure of the benzene molecule|journal=Nature|volume=325|page=396|year=1987|doi=10.1038/325396d0|issue=6103|bibcode = 1987Natur.325..396P |s2cid=4261220|doi-access=free}}</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|page=492|year=1987|doi=10.1038/329492a0|issue=6139|bibcode = 1987Natur.329..492M |s2cid=45218186|doi-access=free}}</ref><ref>{{cite journal|last1=Harcourt|first1=Richard D.|title=The electronic structure of the benzene molecule|journal=Nature|volume=329|pages=491–492|year=1987|doi=10.1038/329491b0|issue=6139|bibcode = 1987Natur.329..491H |s2cid=4268597}}</ref> It is likely that this stability 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.

Derivatives of benzene occur sufficiently often as a component of organic molecules, so much so that the [[Unicode]] Consortium has allocated a 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=https://www.fileformat.info/info/unicode/char/232c/index.htm |title=Unicode Character 'BENZENE RING' (U+232C) |access-date=2009-01-16 |website=fileformat.info |archive-date=2009-02-19 |archive-url=https://web.archive.org/web/20090219001516/http://www.fileformat.info/info/unicode/char/232c/index.htm |url-status=live }}</ref> and U+23E3 (⏣) for a delocalized version.<ref>{{cite web |url=https://www.fileformat.info/info/unicode/char/23e3/index.htm |title=Unicode Character 'BENZENE RING WITH CIRCLE' (U+23E3) |access-date=2009-01-16 |website=fileformat.info |archive-date=2009-07-03 |archive-url=https://web.archive.org/web/20090703232339/http://www.fileformat.info/info/unicode/char/23e3/index.htm |url-status=live }}</ref>

==Benzene derivatives==
{{Main|Aromatic hydrocarbons|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 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]], [[anthracene]], [[phenanthrene]], and [[pyrene]]. The limit of the fusion process is the hydrogen-free allotrope of carbon, [[graphite]].

In [[heterocyclic chemistry|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]], 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]], or [[pyrazine]].<ref>{{cite web | url = https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/heterocy.htm | title = Heterocyclic Chemistry: Heterocyclic Compounds | publisher = [[Michigan State University]], Department of Chemistry | access-date = 2016-07-08 | archive-date = 2016-04-06 | archive-url = https://web.archive.org/web/20160406215111/https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/heterocy.htm | url-status = live }}</ref>

==Production==
Four chemical processes contribute to industrial benzene production: [[catalytic reforming]], [[toluene]] hydrodealkylation, toluene disproportionation, and [[steam cracking]] etc. According to the [[ATSDR]] Toxicological Profile for benzene, between 1978 and 1981, catalytic reformates accounted for approximately 44–50% of the total U.S. benzene production.<ref name=Ullmann>{{cite encyclopedia|author=Hillis O. Folkins|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH|place=Weinheim|year=2005|doi=10.1002/14356007.a03_475|isbn=978-3527306732|chapter=Benzene}}</ref>

===Catalytic reforming===
In catalytic reforming, a mixture of [[hydrocarbon]]s with boiling points between 60 and 200&nbsp;°C is blended with [[hydrogen]] gas and then exposed to a [[bifunctional]] [[platinum(II) chloride|platinum chloride]] or [[rhenium]] chloride [[catalyst]] at 500–525&nbsp;°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. Recovery of the aromatics, commonly referred to as [[BTX (chemistry)|BTX]] (benzene, toluene and xylene isomers), involves such extraction and distillation steps.

In similar fashion 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 500–650&nbsp;°C and 20–60 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]]:

:{{chem2 | C6H5CH3 + H2 -> C6H6 + CH4 }}

This irreversible reaction is accompanied by an equilibrium side reaction that produces
[[biphenyl]] (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) extraction processes.

===Toluene disproportionation===
Toluene [[disproportionation]] ('''TDP''') is the conversion of toluene to benzene and [[xylene]].<ref>{{Cite journal |last1=Ali |first1=M. A. |last2=Haji |first2=Shaker |last3=Al-Khayyat |first3=Mohammed |last4=Abutaleb |first4=Ahmed |last5=Ahmed |first5=Shakeel |date=2022-12-01 |title=Kinetic modeling and thermodynamic analysis of toluene disproportionation reaction over ZSM-5 based catalysts |journal=Catalysis Communications |volume=172 |pages=106541 |doi=10.1016/j.catcom.2022.106541 |issn=1566-7367|doi-access=free }}</ref>

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% ''p''-xylene. In some systems, even the benzene-to-xylenes ratio is modified to favor xylenes.

===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 routed through an extraction process to recover [[BTX (chemistry)|BTX]] aromatics (benzene, toluene and xylenes).

=== Other methods ===
Although of no commercial significance, many other routes to benzene exist. [[Phenol]] and [[halobenzene]]s can be reduced with metals. [[Benzoic acid]] and its salts undergo [[decarboxylation]] to benzene.  The reaction of the [[diazonium compound]] derived from [[aniline]] with [[hypophosphorus acid]] gives benzene. [[Alkyne trimerisation]] of [[acetylene]] gives benzene. Complete [[decarboxylation]] of [[mellitic acid]] gives benzene.

==Uses==
Benzene is used mainly as an intermediate to make other chemicals, above all [[ethylbenzene]] (and other [[alkylbenzenes]]), [[cumene]], [[cyclohexane]], and [[nitrobenzene]]. In 1988 it was reported that two-thirds of all chemicals on the [[American Chemical Society]]'s lists contained at least one benzene ring.<ref name=nyt>{{citation|url=https://www.nytimes.com/1988/08/16/science/a-pervasive-molecule-is-captured-in-a-photograph.html|title=A pervasive molecule is captured in a photograph|newspaper=[[The New York Times]]|date=August 16, 1988|first=Malcolm W.|last=Browne|access-date=August 13, 2021|archive-date=August 12, 2021|archive-url=https://web.archive.org/web/20210812234904/https://www.nytimes.com/1988/08/16/science/a-pervasive-molecule-is-captured-in-a-photograph.html|url-status=live}}</ref> 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]]. 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 around 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 [[rubber]]s, [[lubricant]]s, [[dye]]s, [[detergent]]s, [[drug]]s, [[explosive]]s, and [[pesticide]]s. 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.<ref name="Ceresana">{{cite web|url=http://www.ceresana.com/en/market-studies/chemicals/benzene/|title=Market Study: Benzene (2nd edition), Ceresana, August 2014|publisher=ceresana.com|access-date=2015-02-10|archive-date=2017-12-21|archive-url=https://web.archive.org/web/20171221002731/http://www.ceresana.com/en/market-studies/chemicals/benzene/|url-status=live}}</ref>

[[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.<ref name="CeresanaB">{{cite web|url=http://www.ceresana.com/en/market-studies/chemicals/toluene/|title=Market Study: Toluene, Ceresana, January 2015|publisher=ceresana.com|access-date=2015-02-10|archive-url=https://web.archive.org/web/20170429112325/http://www.ceresana.com/en/market-studies/chemicals/toluene/|archive-date=2017-04-29|url-status=dead}}</ref>

<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 [[#Uses|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>

===Component of gasoline===
As a [[gasoline]] (petrol) additive, benzene increases the [[octane rating]] and reduces [[Engine knocking|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 entering the [[groundwater]] has 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]] introduced new regulations in 2011 that lowered the benzene content in gasoline to 0.62%.<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 |page=15853 |access-date=2008-06-27 |date=2006-03-29 |url-status=dead |archive-url=https://web.archive.org/web/20081205233108/http://www.epa.gov/EPA-AIR/2006/March/Day-29/a2315b.htm |archive-date=2008-12-05}}</ref>

In some European languages, the word for petroleum or gasoline is an exact cognate of "benzene". For instance all [[North Germanic languages|Scandinavian]] languages refer to gasoline as 'bensin'. In [[Catalan language|Catalan]] the word 'benzina' can be used for gasoline, though now it is relatively rare.

==Reactions==
The most common reactions of benzene involve substitution of a proton by other groups.<ref name="StranksEtAl1970">{{Cite book| last = Stranks| first = D. R.| author2=M. L. Heffernan|author3=K. C. Lee Dow|author4=P. T. McTigue|author5=G. R. A. Withers| title = Chemistry: A structural view| publisher = Melbourne University Press| year = 1970| location = [[Carlton, Victoria]]| page = 347| isbn = 978-0-522-83988-3}}</ref> [[Electrophilic aromatic substitution]] is a general method of derivatizing benzene. Benzene is sufficiently [[nucleophile|nucleophilic]] that it undergoes substitution by [[acylium]] ions and alkyl [[carbocation]]s to give substituted derivatives.
:[[File:OChem-Mech-ElectrophilicAromaticSubstitution-General.png|thumb|center|450px|Electrophilic aromatic substitution of benzene]]

The most widely practiced example of this reaction is the [[ethylation]] of benzene.
::[[File:EtC6H5route.png|320 px]]
Approximately 24,700,000 tons were produced in 1999.<ref>{{cite book |doi=10.1002/14356007.a10_035.pub2|chapter=Ethylbenzene|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2005|last1=Welch|first1=Vincent A.|last2=Fallon|first2=Kevin J.|last3=Gelbke|first3=Heinz-Peter|isbn=3527306730}}</ref> 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]].

[[File:Benzen acylowany.svg|thumb|center|300px|Friedel-Crafts acylation of benzene by acetyl chloride]]

===Sulfonation, chlorination, nitration===
Using electrophilic aromatic substitution, many functional groups are introduced onto the benzene framework.  [[Aromatic sulfonation|Sulfonation]] of benzene involves the use of [[oleum]], a mixture of sulfuric acid with [[sulfur trioxide]]. Sulfonated benzene derivatives are useful [[detergent]]s.  In [[nitration]], benzene reacts with nitronium ions (NO<sub>2</sub><sup>+</sup>), 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 Lewis acid catalyst such as aluminium tri-chloride.

===Hydrogenation===
Via [[hydrogenation]], benzene and its derivatives convert to [[cyclohexane]] and derivatives. This reaction is achieved by the use of high pressures of [[hydrogen]] in the presence of [[heterogeneous catalyst]]s, such as finely divided [[nickel]]. Whereas alkenes can be hydrogenated near room temperatures, benzene and related compounds are more reluctant substrates, requiring temperatures >100&nbsp;°C. This reaction is practiced on a large scale industrially. In the absence of the catalyst, benzene is impervious to hydrogen.  Hydrogenation cannot be stopped to give cyclohexene or cyclohexadienes as these are superior substrates. [[Birch reduction]], a non catalytic process, however selectively hydrogenates benzene to the diene.

===Metal complexes===
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 [[(Benzene)ruthenium dichloride dimer|[RuCl<sub>2</sub>(C<sub>6</sub>H<sub>6</sub>)]<sub>2</sub>]].

==Health effects==
[[File:Benzol.JPG|A bottle of benzene. The warnings show benzene is a toxic and flammable liquid.|thumb]]
Benzene is classified as a [[carcinogen]], which 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.<ref>Kasper, Dennis L.''et al.'' (2004) ''Harrison's Principles of Internal Medicine'', 16th ed., McGraw-Hill Professional, p. 618, {{ISBN|0071402357}}.</ref><ref>[http://www.merckmanuals.com/home/print/sec13/ch159/ch159a.html Merck Manual, Home Edition] {{Webarchive|url=https://web.archive.org/web/20110929032122/http://www.merckmanuals.com/home/print/sec13/ch159/ch159a.html |date=2011-09-29 }}, "Overview of Leukemia".</ref><ref name="Bard 2014 Jun 16;9(6)">{{cite journal|last=Bard|first=D|title=Traffic-related air pollution and the onset of myocardial infarction: disclosing benzene as a trigger? A small-area case-crossover study |journal=PLOS ONE|year=2014|volume=9|issue=6|pages=6|doi=10.1371/journal.pone.0100307|pmid=24932584|bibcode=2014PLoSO...9j0307B|pmc=4059738|doi-access=free}}</ref>  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).<ref name="Smith 2010 133–48">{{cite journal|last=Smith|first=Martyn T.|title=Advances in understanding benzene health effects and susceptibility|journal=Annu Rev Public Health|year=2010|volume=31|pages=133–48|doi=10.1146/annurev.publhealth.012809.103646|pmid=20070208|pmc=4360999}}</ref>

Carcinogenic activity of benzene was discovered by Swedish pharmacologist {{ill|Carl Gustaf Santesson (farmakolog)|lt=C. G. Santesson|sv}} in 1897 on female workers of a tire-making factory.<ref>{{cite news |url=https://montrealgazette.com/opinion/columnists/the-right-chemistry-a-tale-of-a-snake-snaring-its-tail |title=The Right Chemistry: A tale of benzene poisoning and a snake snaring its own tail |work=Montreal Gazette |last=Schwarcz |first=Joe |date=30 March 2024 |access-date=10 June 2025}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=b0v183YksKkC&pg=PA336 |title=Archiv für Hygiene |date=1897 |publisher=Oldenbourg |language=de}}</ref> The [[American Petroleum Institute]] (API) stated in 1948 that "it is generally considered that the only absolutely safe concentration for benzene is zero".<ref>[https://web.archive.org/web/20030310145140/http://hobsonlaw.com/benzene_pages/pdffile.pdf American Petroleum Institute, API Toxicological Review, Benzene, September 1948], Agency for Toxic Substances and Disease Registry, Department of Health and Human Services</ref> There is no safe exposure level; even tiny amounts can cause harm.<ref>{{Cite journal|last=Smith|first=Martyn T.|date=2010-01-01|title=Advances in Understanding Benzene Health Effects and Susceptibility|journal=Annual Review of Public Health|volume=31|issue=1|pages=133–148|doi=10.1146/annurev.publhealth.012809.103646| doi-access=free|pmc=4360999|pmid=20070208}}</ref>  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 caused by benzene.<ref>WHO. [[International Agency for Research on Cancer]], [http://monographs.iarc.fr/ENG/Monographs/suppl7/suppl7.pdf IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs] {{webarchive |url=https://web.archive.org/web/20080306134819/http://monographs.iarc.fr/ENG/Monographs/suppl7/suppl7.pdf |date=2008-03-06 }}, Volumes 1 to 42, Supplement 7</ref> IARC rated benzene as "known to be carcinogenic to humans" ([[IARC group 1 Carcinogens|Group 1]]).

As benzene is ubiquitous in gasoline and hydrocarbon fuels that are in use everywhere, human exposure to benzene is a global health problem. Benzene targets the liver, kidney, lung, heart and brain and can cause [[DNA]] strand breaks and [[chromosomal]] damage, hence is [[Teratology#Teratogenesis|teratogenic]] and [[mutagen]]ic. 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.<ref>{{cite journal| journal =Int J Occup Environ Health| year =2007|volume =13| issue =2| pages =213–21| title =Benzene-induced cancers: abridged history and occupational health impact| author =Huff J| pmid=17718179 | pmc =3363002| doi=10.1179/oeh.2007.13.2.213}}</ref><ref>{{cite journal| journal =J Environ Biol |year =2005 | volume =26|issue =2|pages =157–68|title =Biochemical toxicity of benzene |author =Rana SV |author2= Verma Y |pmid =16161967}}</ref>

==Exposure to benzene==
According to the [[Agency for Toxic Substances and Disease Registry]] (ATSDR) (2007), benzene is both a synthetically made and naturally occurring chemical from processes that include: volcanic eruptions, wild fires, synthesis of chemicals such as [[phenol]], production of [[synthetic fiber]]s, and fabrication of [[rubber]]s, [[lubricant]]s, [[pesticide]]s, medications, and [[dye]]s. 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 test|urine]], [[Blood test|blood]], and [[breath test]]s; however, all of these have their limitations because benzene is rapidly metabolized in the human body.<ref>Agency for Toxic Substances and Disease Registry. (2007) [http://www.atsdr.cdc.gov/toxprofiles/tp3.pdf Benzene: Patient information sheet.] {{Webarchive|url=https://web.archive.org/web/20141215113112/http://www.atsdr.cdc.gov/toxprofiles/tp3.pdf |date=2014-12-15 }}</ref>

Exposure to benzene may lead progressively to aplastic [[anemia]], [[leukaemia]], and [[multiple myeloma]].<ref>{{Cite journal |pmid = 1854646|pmc = 1035396|year = 1991|last1 = Yardley-Jones|first1 = A.|title = The toxicity of benzene and its metabolism and molecular pathology in human risk assessment|journal = British Journal of Industrial Medicine|volume = 48|issue = 7|pages = 437–44|last2 = Anderson|first2 = D.|last3 = Parke|first3 = D. V.|doi=10.1136/oem.48.7.437}}</ref>

[[Occupational Safety and Health Administration|OSHA]] regulates levels of benzene in the workplace.<ref>[https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=10042&p_table=STANDARDS Occupational Safety and Health Standards, Toxic and Hazardous Substances, 1910.1028] {{Webarchive|url=https://web.archive.org/web/20150629202425/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=10042&p_table=STANDARDS |date=2015-06-29 }}. Osha.gov. Retrieved on 2011-11-23.</ref> 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]], [[National Institute for Occupational Safety and Health|NIOSH]] recommends that all workers wear special [[Self-contained breathing apparatus|breathing equipment]] when they are likely to be exposed to benzene at levels exceeding the recommended (8-hour) exposure limit of 0.1 ppm.<ref>[http://www.atsdr.cdc.gov/PHS/PHS.asp?id=37&tid=14 Public Health Statement for Benzene, Agency for Toxic Substances and Disease Registry. (August 2007). Benzene: Patient information sheet] {{Webarchive|url=https://web.archive.org/web/20100528032504/http://www.atsdr.cdc.gov/PHS/PHS.asp?id=37&tid=14 |date=2010-05-28 }}. Atsdr.cdc.gov (2011-03-03). Retrieved on 2011-11-23.</ref>

===Benzene exposure limits===
The [[United States Environmental Protection Agency]] has set a [[maximum contaminant level]] for benzene in [[drinking water]] at 0.005&nbsp;mg/L (5 ppb), as promulgated via the U.S. National Primary Drinking Water Regulations.<ref>{{cite web|url=https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations#:~:text=Benzene|title=Drinking Water Contaminants {{!}} Organic Chemicals {{!}} Benzene|archive-url=https://web.archive.org/web/20150202094944/http://water.epa.gov/drink/contaminants/index.cfm#:~:text=Benzene|archive-date=February 2, 2015|publisher=[[United States Environmental Protection Agency]] (EPA)|access-date=April 17, 2014}}</ref> 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&nbsp;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.<ref name="OSHASampling2015">{{cite web|url=https://www.osha.gov/dts/chemicalsampling/data/CH_220100.html|title=Chemical Sampling Information Benzene|archive-url=https://web.archive.org/web/20150629200917/https://www.osha.gov/dts/chemicalsampling/data/CH_220100.html|archive-date=June 29, 2015|publisher=[[Occupational Safety and Health Administration]] (OSHA)|access-date=November 23, 2011}}</ref> 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.<ref>{{cite web|url=http://www.atsdr.cdc.gov/csem/benzene/standards_regulations.html|archive-url=https://web.archive.org/web/20100610004540/http://www.atsdr.cdc.gov/csem/benzene/standards_regulations.html|archive-date=June 10, 2010|title=Benzene Toxicity: Standards and Regulations|publisher=[[Agency for Toxic Substances and Disease Registry]] (ATSDR); Environmental Medicine & Environmental Health Education – CSEM|date=June 30, 2000|access-date=October 9, 2010}}</ref>

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.<ref name="NIOSH2004"/> 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 [[Self-contained breathing apparatus|breathing apparatus]] providing maximum worker protection is permitted.<ref name="NIOSH2004">{{cite web|url=https://www.cdc.gov/niosh/docs/2005-100/pdfs/05-100.pdf|title=NIOSH respirator selection logic|archive-url=https://web.archive.org/web/20170829074637/https://www.cdc.gov/niosh/docs/2005-100/pdfs/05-100.pdf|archive-date=August 29, 2017|date=October 2004|location=Cincinnati, Ohio|publisher=U.S. [[Department of Health and Human Services]] (DHHS), Public Health Service, [[Centers for Disease Control]] (CDC), [[National Institute for Occupational Safety and Health]] (NIOSH)}} publication No. 2005-100.</ref><ref>{{cite web|url=https://www.cdc.gov/niosh/idlh/idlhintr.html#CNU|title=Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH): Introduction|archive-url=https://archive.today/20120722214619/http://www.cdc.gov/niosh/idlh/idlhintr.html%23CNU|archive-date=July 22, 2012|publisher=[[Centers for Disease Control]] (CDC)|access-date=November 23, 2011}}</ref> In September 1995, NIOSH issued a new policy for developing [[recommended exposure limit]]s (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.<ref>{{cite web |url=https://www.atsdr.cdc.gov/PHS/PHS.asp?id=37&tid=14 |title=Public Health Statement for Benzene |work=Agency for Toxic Substances and Disease Registry |publisher=U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health |via=Atsdr.cdc.gov |date=August 2007 |access-date=2011-11-23 |archive-date=2012-01-20 |archive-url=https://web.archive.org/web/20120120145724/http://www.atsdr.cdc.gov/phs/phs.asp?id=37&tid=14 |url-status=live }}</ref> 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.<ref name="OSHASampling2015" />

===Toxicology===
====Biomarkers of exposure====
Several tests can determine exposure to benzene. Benzene itself can be measured in breath, blood or urine, but such testing is usually limited to the first 24 hours post-exposure due to the relatively rapid removal of the chemical by exhalation or biotransformation. Most people in developed countries have measureable baseline levels of benzene and other aromatic petroleum hydrocarbons in their blood. In the body, benzene is enzymatically converted to a series of oxidation products including [[muconic acid]], [[phenylmercapturic acid]], [[phenol]], [[catechol]], [[hydroquinone]] and [[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|doi=10.1093/clinchem/40.7.1401|url=http://www.clinchem.org/cgi/reprint/40/7/1401.pdf|access-date=2010-08-25|archive-date=2011-11-22|archive-url=https://web.archive.org/web/20111122101015/http://www.clinchem.org/cgi/reprint/40/7/1401.pdf|url-status=live}}</ref><ref>{{cite journal|vauthors=Fustinoni S, Buratti M, Campo L, Colombi A, Consonni D, Pesatori AC, Bonzini M, Farmer P, Garte S, Valerio F, Merlo DF, Bertazzi PA |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|bibcode=2005CBI...153..253F }}</ref><ref>ACGIH (2009). ''2009 TLVs and BEIs''. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio.</ref><ref>Baselt, R. (2008) ''Disposition of Toxic Drugs and Chemicals in Man'', 8th edition, Biomedical Publications, Foster City, CA, pp. 144–148, {{ISBN|0962652377}}.</ref>

====Biotransformations====
Even if it is not a common substrate for metabolism, benzene can be oxidized by both [[bacteria]] and [[eukaryote]]s. 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 [[citric acid 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 or DT-diaphorase or [[NAD(P)H dehydrogenase (quinone 1)]]), 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 [[Type II topoisomerase|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]].<ref name="Smith 2010 133–48"/><ref>{{cite journal|last=Snyder|first=R|author2=Hedli, C.C.|title=An overview of benzene metabolism|journal=Environ Health Perspect|year=1996|volume=104|pages=1165–1171|pmid=9118888|pmc=1469747|issue=Suppl 6|doi=10.1289/ehp.96104s61165}}</ref>

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.<ref>{{cite journal|last=Dougherty|first=D|author2=Garte, S |author3=Barchowsky, A |author4=Zmuda, J |author5= Taioli, E |title=NQO1, MPO, CYP2E1, GSTT1 and STM1 polymorphisms and biological effects of benzene exposure—a literature review|journal=Toxicology Letters|year=2008|volume=182|pages=7–17|doi=10.1016/j.toxlet.2008.09.008|pmid=18848868|issue=1–3}}</ref>

====Molecular toxicology====
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.<ref name=r3>{{Cite journal|vauthors=Fracasso ME, Doria D, Bartolucci GB, Carrieri M, Lovreglio P, Ballini A, Soleo L, Tranfo G, Manno 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|year=2010|pages=22–8|issue=1}}</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|last1=Eastmond|first1=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|doi=10.1016/0165-1218(94)90028-0}}</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|last1=Garte|first1=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|s2cid=36885673}}</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.

===Routes of exposure===
====Inhalation====
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.<ref>[https://www.atsdr.cdc.gov/toxfaqs/tfacts3.pdf ToxFAQs for Benzene], Agency for Toxic Substances and Disease Registry, Department of Health and Human Services</ref> 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] {{Webarchive|url=https://web.archive.org/web/20100528032457/http://www.atsdr.cdc.gov/toxguides/toxguide-3.pdf?id=39&tid=14 |date=2010-05-28}}, Agency for Toxic Substances and Disease Registry, Department of Health and Human Services</ref>  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.<ref>[http://www.atsdr.cdc.gov/ToxProfiles/tp3-c1-b.pdf Public Health Statement. Benzene] {{Webarchive|url=https://web.archive.org/web/20100527105042/http://www.atsdr.cdc.gov/toxprofiles/tp3-c1-b.pdf |date=2010-05-27 }}, Division of Toxicology and Environmental Medicine, August 2007</ref>

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&nbsp;mg/m<sup>3</sup> 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.<ref>[http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rn+71-43-2 Benzene, CASRN: 71-43-2] {{Webarchive|url=https://web.archive.org/web/20111022211013/http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rn+71-43-2 |date=2011-10-22}}. Hazardous Substances Data Bank, U.S. National Library of Medicine. National Institutes of Health.</ref>

====Exposure from soft drinks====
{{Main|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 [[United Kingdom]] 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.<ref>[https://www.cbsnews.com/news/fda-too-much-benzene-in-some-drinks/ "FDA: Too Much Benzene In Some Drinks"], ''CBS News'', May 19, 2006. Retrieved July 11, 2006.</ref>

====Contamination of water supply====
In 2005, the water supply to the city of [[Harbin]] in China with a population of almost nine million people, was cut off because of a major benzene exposure.<ref name="Guardian-25Nov2005">{{cite news |title=100 tonnes of pollutants spilled into Chinese river |url=https://www.theguardian.com/news/2005/nov/25/china.internationalnews |access-date=7 January 2020 |work=[[The Guardian]] |date=25 November 2005 |archive-date=10 March 2020 |archive-url=https://web.archive.org/web/20200310211730/https://www.theguardian.com/news/2005/nov/25/china.internationalnews |url-status=live }}</ref> Benzene leaked into the [[Songhua River]], which supplies drinking water to the city, after [[Jilin chemical plant explosions 2005|an explosion at a China National Petroleum Corporation (CNPC) factory]] in the city of Jilin on 13 November 2005.

When plastic water pipes are subject to high heat, the water may be contaminated with benzene.<ref>{{Cite journal|doi = 10.1039/D0EW00836B|title = Drinking water contamination from the thermal degradation of plastics: Implications for wildfire and structure fire response|year = 2021|last1 = Isaacson|first1 = Kristofer P.|last2 = Proctor|first2 = Caitlin R.|last3 = Wang|first3 = Q. Erica|last4 = Edwards|first4 = Ethan Y.|last5 = Noh|first5 = Yoorae|last6 = Shah|first6 = Amisha D.|last7 = Whelton|first7 = Andrew J.|journal = Environmental Science: Water Research & Technology|volume = 7|issue = 2|pages = 274–284|doi-access = free}}</ref>

====Genocide====
[[Nazi Germany]]  used benzene administered via [[Injection (medicine)|injection]] as one of their many methods for killing.<ref>{{cite web |url=http://auschwitz.org/en/history/camp-hospitals/selections-and-lethal-injections/ |title=Selections and lethal injections |work=[[Auschwitz-Birkenau State Museum]] |access-date=May 15, 2020 |archive-date=May 9, 2020 |archive-url=https://web.archive.org/web/20200509105620/http://auschwitz.org/en/history/camp-hospitals/selections-and-lethal-injections |url-status=live}}</ref><ref>{{cite news |url=https://www.haaretz.com/world-news/europe/.premium-austria-s-former-nazi-labor-camp-that-s-billed-as-a-tourist-site-1.7195524 |title=A Former Nazi Labor Camp in Austria, Now Billed as a Tourist Site |newspaper=Haaretz |date=May 3, 2019 |access-date=May 11, 2019 |archive-date=May 11, 2019 |archive-url=https://web.archive.org/web/20190511075141/https://www.haaretz.com/world-news/europe/.premium-austria-s-former-nazi-labor-camp-that-s-billed-as-a-tourist-site-1.7195524 |url-status=live}}</ref>

==See also==
{{Portal|Environment|Medicine}}
* [[BTEX]]
* [[1,2,3-Cyclohexatriene]]
* ''[[Industrial Union Department v. American Petroleum Institute]]''
* Six-membered aromatic rings with one carbon replaced by another element: [[borabenzene]], [[silabenzene]], [[germabenzene]], [[stannabenzene]], [[pyridine]], [[phosphorine]], [[arsabenzene]], [[bismabenzene]], [[pyrylium salt|pyrylium]], [[thiopyrylium]], [[selenopyrylium]], [[telluropyrylium]]

== Explanatory notes ==
{{reflist|group=note}}

==References==
{{reflist}}

== External links ==
{{Commons and category|Benzene|Benzene}}
{{Wiktionary|benzene}}
{{wikiquote}}
{{Scholia|chemical}}
* [http://www.periodicvideos.com/videos/mv_benzene.htm Benzene] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://www.inchem.org/documents/icsc/icsc/eics0015.htm International Chemical Safety Card 0015]
* [http://www.epa.gov/iris/subst/0276.htm USEPA Summary of Benzene Toxicity]
* [https://www.cdc.gov/niosh/npg/npgd0049.html NIOSH Pocket Guide to Chemical Hazards]
* {{PubChem|241|Benzene}}
* [https://web.archive.org/web/20070301122020/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 Recording] of Sir John Cadogan giving a lecture on Benzene at the Royal Institution, 22nd September 1991
* [https://web.archive.org/web/20070224171424/http://ntp.niehs.nih.gov/ntp/roc/eleventh/profiles/s019benz.pdf Substance profile]
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs+hsdb:@term+@DOCNO+35 NLM Hazardous Substances Databank – Benzene]

{{Annulenes}}
{{Cycloalkenes}}
{{Hydrocarbons}}
{{Functional groups}}
{{GABAA receptor positive modulators}}
{{Molecules detected in outer space}}
{{Authority control}}

[[Category:Benzene| ]]
[[Category:Annulenes]]
[[Category:Aromatic hydrocarbons]]
[[Category:Aromatic solvents]]
[[Category:Carcinogens]]
[[Category:Commodity chemicals]]
[[Category:GABAA receptor positive allosteric modulators]]
[[Category:Hydrocarbon solvents]]
[[Category:IARC Group 1 carcinogens]]
[[Category:Immunotoxins]]
[[Category:Mutagens]]
[[Category:Chemical hazards]]
[[Category:Petrochemicals]]
[[Category:Simple aromatic rings]]
[[Category:Six-membered rings]]
[[Category:Soil contamination]]
[[Category:Sweet-smelling chemicals]]
[[Category:Teratogens]]