View source for Biphenyl

{{For|the compound including hydroxyl groups|Bisphenol}}
{{Redirect|Lemonene|the monoterpene in citrus oil|Limonene}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 443423597
| ImageFile = Bifenyl.svg
| ImageName = Skeletal formula
| ImageFile1 = Biphenyl-3D-vdW.png
| ImageSize1 = 200px
| ImageName1 = Space filling model showing its twisted conformation
| ImageFile2 = BiphenylSample.jpg
| ImageSize2 = 200px
| ImageName2 = sample
| PIN = 1,1′-Biphenyl <!-- Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) -->
| OtherNames = {{Unbulleted list|Biphenyl|Phenylbenzene|Xenene}}
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 6828
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = C06588
| Gmelin = 3808
| Beilstein = 1634058
| EINECS = 202-163-5
| RTECS = DU8050000
| UNNumber = 3077
| 3DMet = B00985
| InChI = 1/C12H10/c1-3-7-11(8-4-1)12-9-5-2-6-10-12/h1-10H
| InChIKey = ZUOUZKKEUPVFJK-UHFFFAOYAV
| ChEMBL_Ref = {{ebicite|correct|EBI}}
| ChEMBL = 14092
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C12H10/c1-3-7-11(8-4-1)12-9-5-2-6-10-12/h1-10H
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = ZUOUZKKEUPVFJK-UHFFFAOYSA-N
| CASNo = 92-52-4
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 2L9GJK6MGN
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 17097
| SMILES = c1ccccc1-c2ccccc2
| CASNo_Ref = {{cascite|correct|CAS}}
| PubChem = 7095
 }}
|Section2={{Chembox Properties
| C=12 | H=10
| Density=1.04 g/cm<sup>3</sup><ref name=GESTIS>{{GESTIS|ZVG=13450}}</ref>
| Appearance = Colorless to pale-yellow crystals
| Odor = pleasant<ref name=PGCH/>
| MeltingPtC=69.2
| MeltingPt_ref = <ref name=GESTIS/>
| BoilingPtC=255
| BoilingPt_ref = <ref name=GESTIS/>
| Solubility=4.45 mg/L<ref name=GESTIS/>
| VaporPressure = 0.005 mmHg (20°C)<ref name=PGCH/>
| MagSus = −103.25·10<sup>−6</sup> cm<sup>3</sup>/mol
 }}
|Section4={{Chembox Hazards
| NFPA-H=1
| NFPA-F=1
| NFPA-R=0
| GHSPictograms = {{GHS07}}{{GHS09}}
| GHSSignalWord = Warning
| HPhrases = {{H-phrases|315|319|335|410}}
| PPhrases = {{P-phrases|261|264|271|273|280|302+352|304+340|305+351+338|312|321|332+313|337+313|362|391|403+233|405|501}}
| FlashPtC=113
| FlashPt_ref=<ref name=GESTIS/>
| AutoignitionPtC=540
| AutoignitionPt_ref =<ref name=GESTIS/>
| PEL = TWA 1 mg/m<sup>3</sup> (0.2 ppm)<ref name=PGCH>{{PGCH|0239}}</ref>
| ExploLimits = 0.6–5.8%<ref name=PGCH/>
| IDLH = 100 mg/m<sup>3</sup><ref name=PGCH/>
| LD50 = 2400 mg/kg (oral, rabbit)<br/>3280 mg/kg (oral, rat)<br/>1900 mg/kg (oral, mouse)<br/>2400 mg/kg (oral, rat)<ref>{{cite web |url = https://www.cdc.gov/niosh/idlh/92524.html |title = Diphenyl |work = Immediately Dangerous to Life or Health Concentrations (IDLH) |publisher = National Institute for Occupational Safety and Health (NIOSH) |date = 4 December 2014 |access-date = 17 March 2015}}</ref>
| REL = TWA 1 mg/m<sup>3</sup> (0.2 ppm)<ref name=PGCH/>
  }}
}}

'''Biphenyl''' (also known as '''diphenyl''', '''phenylbenzene''', '''1,1′-biphenyl''', '''lemonene'''<ref>{{cite web |title=Biphenyl |url=https://webbook.nist.gov/cgi/cbook.cgi?ID=92-52-4 |website=NIST Chemistry WebBook |publisher=US [[National Institute of Standards and Technology]]}} An obscure name, according to {{cite web |title=Limonene |url=https://www.acs.org/molecule-of-the-week/archive/l/limonene.html |website=Molecule of the Week Archive |publisher=[[American Chemical Society]] |date=Nov 1, 2021}}</ref> or '''BP''') is an [[organic compound]] that forms colorless  crystals.  Particularly in older literature, compounds containing the [[functional group]] consisting of biphenyl less one hydrogen (the site at which it is attached) may use the prefixes '''xenyl''' or '''diphenylyl'''.<ref>{{cite web|url=https://archive.org/stream/beilsteinshandb01gesegoog/beilsteinshandb01gesegoog_djvu.txt|title=Beilsteins Handbuch der organischen Chemie, Volume 5}}</ref>

It has a distinctively pleasant smell. Biphenyl is an [[aromatic hydrocarbon]] with a [[molecular formula]] (C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>. It is notable as a starting material for the production of [[polychlorinated biphenyl]]s (PCBs), which were once widely used as [[dielectric]] fluids and [[heat transfer]] agents.

Biphenyl is also an intermediate for the production of a host of other [[organic compound]]s such as [[emulsifier]]s, [[optical brightener]]s, [[fungicide|crop protection]] products, and [[plastic]]s. Biphenyl is [[solubility|insoluble]] in water, but soluble in typical [[organic solvent]]s. The biphenyl molecule consists of two connected [[phenyl ring]]s.

==Properties and occurrence==
Biphenyl is a solid at room temperature, with a melting point of {{convert|69.2|C}}. In the gas phase the molecule exists in two [[enantiomorphic]] twisted forms with an angle between the planes of the two rings of 44.4(2)°.<ref>{{Cite journal |last=Almenningen |first=Arne |last2=Bastiansen |first2=Otto |last3=Fernholt |first3=Liv |last4=Cyvin |first4=Bjørg N. |last5=Cyvin |first5=Sven J. |last6=Samdal |first6=Svein |date=1985-04-01 |title=Structure and barrier of internal rotation of biphenyl derivatives in the gaseous state: Part 1. The molecular structure and normal coordinate analysis of normal biphenyl and pedeuterated biphenyl |url=https://linkinghub.elsevier.com/retrieve/pii/0022286085850419 |journal=Journal of Molecular Structure |volume=128 |issue=1 |pages=59–76 |doi=10.1016/0022-2860(85)85041-9 |issn=0022-2860}}</ref> In the room-temperature solid, biphenyl is crystalline with [[space group]] P2{{sub|1}}/c, which does not allow for [[chiral]] crystals. In the first structure determination in the solid state the molecule was described as perfectly planar within the limits of error.<ref>{{Cite journal |last=Hargreaves |first=A. |last2=Rizvi |first2=S. H. |date=1962-04-10 |title=The crystal and molecular structure of biphenyl |url=https://journals.iucr.org/paper?S0365110X62000894 |journal=Acta Crystallographica |language=en |volume=15 |issue=4 |pages=365–373 |doi=10.1107/S0365110X62000894 |issn=0365-110X}}</ref><ref>{{Cite journal |last=Trotter |first=J. |date=1961-11-10 |title=The crystal and molecular structure of biphenyl |url=https://journals.iucr.org/paper?S0365110X6100334X |journal=Acta Crystallographica |language=en |volume=14 |issue=11 |pages=1135–1140 |doi=10.1107/S0365110X6100334X |issn=0365-110X}}</ref> The planar arrangement in the crystalline phase I is assumed to result from dynamic disorder, i.e. it represents an average of two twisted structures. The lower temperature solid phases II and III have been described as modulated structures with the biphenyl molecules being locked in one of the two non-planar conformations with torsion angles ranging from 9 to 21°.<ref>{{Cite journal |last=Charbonneau |first=G. P. |last2=Delugeard |first2=Y. |date=1977-05-15 |title=Biphenyl: three-dimensional data and new refinement at 293 K |url=https://journals.iucr.org/paper?S0567740877006566 |journal=Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry |language=en |volume=33 |issue=5 |pages=1586–1588 |doi=10.1107/S0567740877006566 |issn=0567-7408}}</ref> A study from 2021 concludes that in the crystalline phase at 100 K there is no [[double-well potential]] associated with the two twisted conformations, but the potential energy is minimized to zero twist.<ref>{{cite journal |last1=Dr. Bruno Landeros-Rivera, Jesús Hernández-Trujillo |title=Control of Molecular Conformation and Crystal Packing of Biphenyl Derivatives |journal=[[Chemistry Europe]] |date=Dec 15, 2021 |volume=87 |doi=10.1002/cplu.202100492 |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/epdf/10.1002/cplu.202100492}}</ref><ref>{{Cite journal |last1=Landeros-Rivera |first1=Bruno |last2=Jancik |first2=Vojtech |last3=Moreno-Esparza |first3=Rafael |last4=Martínez Otero |first4=Diego |last5=Hernández-Trujillo |first5=Jesús |date=May 27, 2021 |title=Non-Covalent Interactions in the Biphenyl Crystal: Is the Planar Conformer a Transition State? |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202101490 |journal=[[Chemistry Europe]] |language=en |volume=27 |issue=46 |pages=11912–11918 |doi=10.1002/chem.202101490 |pmid=34043851 |issn=0947-6539}}</ref> Relative to the free molecule it represents therefore a transition state of inversion stabilized by the intermolecular interactions in a molecular crystal.

Biphenyl occurs naturally in [[coal tar]], [[crude oil]], and [[natural gas]] and can be isolated from these sources via [[distillation]].<ref>{{cite journal | doi = 10.1021/ac60079a020 | title = Isolation and Identification of Biphenyls from West Edmond Crude Oil | date = 1953 | last1 = Adams | first1 = N. G. | last2 = Richardson | first2 = D. M. | journal = Analytical Chemistry | volume = 25 | issue = 7 | pages = 1073–1074 }}</ref>  It is produced industrially as a byproduct of  the dealkylation of [[toluene]] to produce [[methane]]:
:{{chem2|C6H5CH3 + C6H6 -> C6H5\sC6H5 + CH4}}
The other principal route is by the [[Scholl reaction|oxidative dehydrogenation]] of [[benzene]]:
:{{chem2|2 C6H6 + ½ O2 -> C6H5\sC6H5 + H2O}}
Annually 40,000,000&nbsp;kg are produced by these routes.<ref name=Ullmanns>{{citation | doi = 10.1002/14356007.a13_227 | chapter = Hydrocarbons | title = Ullmann's Encyclopedia of Industrial Chemistry | date = 2000 | last1 = Griesbaum | first1 = Karl | last2 = Behr | first2 = Arno | last3 = Biedenkapp | first3 = Dieter | last4 = Voges | first4 = Heinz-Werner | last5 = Garbe | first5 = Dorothea | last6 = Paetz | first6 = Christian | last7 = Collin | first7 = Gerd | last8 = Mayer | first8 = Dieter | last9 = Höke | first9 = Hartmut | isbn = 978-3-527-30385-4 }}</ref>

In the laboratory, biphenyl can also be synthesized by treating [[phenylmagnesium bromide]] with copper(II) salts.

It can also be prepared using [[Diazonium compound|diazonium salts]]. When [[aniline]] is treated with NaNO<sub>2</sub>+dilute HCl at 5°C, it yields benzene diazonium chloride. When this is further reacted with benzene, biphenyl is formed. This is known as the [[Gomberg–Bachmann reaction]].

<chem>Ph-NH2->[\text{NaNO}_2\text{(aq), HCl}][T\text{=273-278K}]  Ph-N2+ ->[\text{Ph-H, Δ}] Ph-Ph  </chem>

==Reactions and uses==
Lacking functional groups, biphenyl is fairly non-reactive, which is the basis of its main application: in a [[eutectic mixture]] with [[diphenyl ether]], as a heat transfer agent. This mixture is stable to 400&nbsp;°C.<ref name=Ullmanns/>

Biphenyl does undergo [[sulfonation]] which, followed by base hydrolysis, produces ''p''-hydroxybiphenyl and ''p'',''p''′-dihydroxybiphenyl, which are useful fungicides. In other [[Electrophilic aromatic substitution|substitution]] reactions, it undergoes halogenation.  [[Polychlorinated biphenyl]]s were once popular pesticides.<ref name=Ullmanns/>

===Li biphenyl radical===
Lithium biphenyl contains the [[radical anion]], which is highly reducing (-3.1 V vs Fc<sup>+/0</sup>).  Several solvates of alkali metal salts of biphenyl anion have been characterized by [[X-ray crystallography]].<ref>{{cite journal |doi=10.1039/C5NJ02841H|title=Isolation of gravimetrically quantifiable alkali metal arenides using 18-crown-6|year=2016|last1=Castillo|first1=Maximiliano|last2=Metta-Magaña|first2=Alejandro J.|last3=Fortier|first3=Skye|journal=New Journal of Chemistry|volume=40|issue=3|pages=1923–1926}}</ref> These salts, usually prepared in situ, are versatile reducing agents.<ref>{{cite journal|title=Regio- and Stereoselective Carboxylation of Allylic Barium Reagents: (E)-4,8-dimethyl-3,7-nonadienoic Acid
|first1=Akira |last1=Yanagisawa |first2=Katsutaka |last2=Yasue |first3=Hisashi |last3=Yamamoto
|journal=Org. Synth.|year=1997|volume=74|pages=178
|doi=10.15227/orgsyn.074.0178}}</ref> Lithium biphenyl offers some advantages relative to the related [[sodium naphthalene|lithium naphthene]].<ref>{{cite journal|title=Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol
|first1=Reuben D. |last1=Rieke|first2=Tse-Chong |last2=Wu|first3=Loretta I.|last3=Rieke|journal=Org. Synth.|year=1995|volume=72|page=147|doi=10.15227/orgsyn.072.0147}}</ref>  Related to Li/biphenyl is the derivative with ''tert''-butyl groups on the biphenyl.<ref>{{cite journal|title=1,3-Diols from Lithium β-Lithioalkoxides Generated by The Reductive Lithiation of Epoxides: 2,5-Dimethyl-2,4-hexanediol
|first1=Boguslaw |last1=Mudryk|first2=Theodore |last2=Cohen|journal=Org. Synth.|year=1995|volume=72|pages=173|doi=10.15227/orgsyn.072.0173}}</ref>

==Stereochemistry==
Rotation about the single bond in biphenyl, and especially its [[Arene substitution patterns|ortho-substituted]] derivatives, is [[steric hindrance|sterically hindered]]. For this reason, some substituted biphenyls show [[atropisomerism]]; that is, the individual C<sub>2</sub>-[[symmetry group|symmetric]]-isomers are [[optical rotation|optically stable]]. Some derivatives, as well as related molecules such as [[BINAP]], find application as [[ligand]]s in [[asymmetric synthesis]]. In the case of unsubstituted biphenyl, the equilibrium torsional angle is 44.4° and the torsional barriers are quite small, 6.0 kJ/mol at 0° and 6.5 kJ/mol at 90°.<ref>{{cite journal |journal=J. Chem. Theory Comput. |doi=10.1021/ct800182e |title=Torsional Barriers and Equilibrium Angle of Biphenyl: Reconciling Theory with Experiment |year=2008 |author=Mikael P. Johansson and Jeppe Olsen |volume=4 |pages=1460–1471 |issue=9|pmid=26621432 }}</ref> Adding ortho substituents greatly increases the barrier: in the case of the 2,2'-dimethyl derivative, the barrier is 17.4 kcal/mol (72.8 kJ/mol).<ref>{{cite book |title=Stereochemistry |editor=Christoph Tamm |author=B. Testa |pages=18 |publisher=Elsevier |year=1982 |chapter=The geometry of molecules: basic principles and nomenclatures}}</ref>

==Biphenyl compounds==
Substituted biphenyls have many uses.  They are prepared by various [[coupling reaction]]s including the [[Suzuki-Miyaura reaction]] and the [[Ullmann reaction]]. [[Polychlorinated biphenyl]]s were once used as cooling and insulating fluids and [[polybrominated biphenyl]]s are [[flame retardant]]s. The biphenyl motif also appears in [[pharmaceuticals|drugs]] such as [[diflunisal]] and [[telmisartan]]. The abbreviation '''E7''' stands for a [[liquid crystal]] mixture consisting of several cyanobiphenyls with long aliphatic tails used commercially in [[liquid crystal display]]s ([[5CB]], [[7CB]], [[8OCB]] and [[5CT]]<ref>{{cite journal |last1=Mouquinho |first1=Ana |last2=Saavedra |first2=Mara |last3=Maiau |first3=Alexandre |last4=Petrova |first4=Krasimira |last5=Barros |first5=M. Teresa |last6=Figueirinhas |first6=J. L. |last7=Sotomayor |first7=João |title=Films Based on New Methacrylate Monomers: Synthesis, Characterisation and Electro-Optical Properties |journal=Molecular Crystals and Liquid Crystals |date=30 June 2011 |volume=542 |issue=1 |pages=132/[654]–140/[662] |doi=10.1080/15421406.2011.570154|s2cid=97514765 }}</ref>). A variety of [[benzidine]] derivatives are used in dyes and polymers. Research into biphenyl liquid crystal candidates mainly focuses on molecules with highly polar heads (for example cyano or halide groups) and aliphatic tails. It is part of the active group in the antibiotic [[oritavancin]].

==Safety and bioactivity==
Biphenyl prevents the growth of [[Mold (fungus)|mold]]s and [[fungus]], and is therefore used as a [[preservative]] ([[E number|E230]], in combination with E231, E232 and E233), particularly in the preservation of [[citrus]] fruits during transportation. It is no longer approved as a food additive in the European Union.

Biphenyl is mildly toxic, but can be degraded biologically by conversion into nontoxic compounds. Some [[bacteria]] are able to hydroxylate biphenyl and its [[polychlorinated biphenyl]]s (PCBs).<ref>{{cite web|url=http://www.genome.jp/dbget-bin/show_pathway?sco00621+SCO6442|title=''Biphenyl degradation - Streptomyces coelicolor'', at GenomeNet Database|work=genome.jp}}</ref>

==See also==
* [[Naphthalene]], where the rings are fused
* [[Terphenyl]], three ringed analog
* [[Bithiophene]]
* [[Polypyrrole]]
* [[Organic Moderated Reactor Experiment|OMRE]], experimental [[Organic nuclear reactor|organic]] [[nuclear reactor]] that tested biphenyl as reactor coolant

==Notes==
{{Reflist}}

==References==
* "Isolation and Identification of Biphenyls from West Edmond Crude Oil". N. G. Adams and D. M. Richardson. ''Analytical Chemistry'' 1953 25 (7), 1073–1074.
* ''Biphenyl (1,1-Biphenyl)''. Wiley/VCH, Weinheim (1991), {{ISBN|3-527-28277-7}}.

==External links==
*{{ICSC|0106|01}}
* [https://www.cdc.gov/niosh/npg/npgd0239.html CDC - NIOSH Pocket Guide to Chemical Hazards]
* [https://web.archive.org/web/20060304144243/http://www.npi.gov.au/database/substance-info/profiles/14.html National Pollutant Inventory - Biphenyl]
* [https://web.archive.org/web/20111217024943/http://msds.chem.ox.ac.uk/BI/biphenyl.html External MSDS]

{{Citrus}}
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[[Category:Biphenyls| ]]
[[Category:Preservatives]]
[[Category:Phenyl compounds]]
[[Category:Nuclear reactor coolants]]