Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene[4] 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.[5]
Biphenyl is a solid at room temperature, with a melting point of 69.2 °C (156.6 °F). 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°. In the room-temperature solid, biphenyl is crystalline with space group P21/c, which does not allow for chiral crystals. Rather than there being a double-well potential entailing the two twisted conformations, the potential energy is minimized at zero twist.[6][7]
Annually 40,000,000 kg are produced by these routes.[9]
In the laboratory, biphenyl can also be synthesized by treating phenylmagnesium bromide with copper(II) salts.
It can also be prepared using diazonium salts. When aniline is treated with NaNO2+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.
Reactions and uses
Lacking functional groups, biphenyl is fairly non-reactive, which is the basis of its main application: its use as a heat transfer agent as a eutectic mixture with diphenyl ether. This mixture is stable to 400 °C.[9]
Biphenyl does undergo sulfonation which, followed by base hydrolysis, produces p-hydroxybiphenyl and p,p′-dihydroxybiphenyl, which are useful fungicides. In other substitution reactions, it undergoes halogenation. Polychlorinated biphenyls were once popular pesticides.[9]
Li biphenyl radical
Lithium biphenyl contains the radical anion, which is highly reducing (-3.1 V vs Fc+/0). Several solvates of alkali metal salts of biphenyl anion have been characterized by X-ray crystallography.[10] These salts, usually prepared in situ, are versatile reducing agents.[11] Lithium biphenyl offers some advantages relative to the related lithium naphthene.[12] Related to Li/biphenyl is the derivative with tert-butyl groups on the biphenyl.[13]
Stereochemistry
Rotation about the single bond in biphenyl, and especially its ortho-substituted derivatives, is sterically hindered. For this reason, some substituted biphenyls show atropisomerism; that is, the individual C2-symmetric-isomers are optically stable. Some derivatives, as well as related molecules such as BINAP, find application as ligands 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°.[14] 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).[15]
Biphenyl prevents the growth of molds and fungus, and is therefore used as a preservative (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 biphenyls (PCBs).[17]
^"Diphenyl". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH). 4 December 2014. Retrieved 17 March 2015.
^Adams, N. G., and D. M. Richardson, 1953. Isolation and Identification of Biphenyls from West Edmond Crude Oil. Analytical Chemistry 25 (7): 1073-1074 doi:10.1021/ac60079a020
^ abcKarl Griesbaum, Arno Behr, Dieter Biedenkapp, Heinz-Werner Voges, Dorothea Garbe, Christian Paetz, Gerd Collin, Dieter Mayer, Hartmut Höke "Hydrocarbons" in Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinheim. doi:10.1002/14356007.a13_227
^Castillo, Maximiliano; Metta-Magaña, Alejandro J.; Fortier, Skye (2016). "Isolation of gravimetrically quantifiable alkali metal arenides using 18-crown-6". New Journal of Chemistry. 40 (3): 1923–1926. doi:10.1039/C5NJ02841H.
^Rieke, Reuben D.; Wu, Tse-Chong; Rieke, Loretta I. (1995). "Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol". Org. Synth. 72: 147. doi:10.15227/orgsyn.072.0147.
^Mudryk, Boguslaw; Cohen, Theodore (1995). "1,3-Diols from Lithium β-Lithioalkoxides Generated by The Reductive Lithiation of Epoxides: 2,5-Dimethyl-2,4-hexanediol". Org. Synth. 72: 173. doi:10.15227/orgsyn.072.0173.
^Mikael P. Johansson and Jeppe Olsen (2008). "Torsional Barriers and Equilibrium Angle of Biphenyl: Reconciling Theory with Experiment". J. Chem. Theory Comput. 4 (9): 1460–1471. doi:10.1021/ct800182e. PMID26621432.
^B. Testa (1982). "The geometry of molecules: basic principles and nomenclatures". In Christoph Tamm (ed.). Stereochemistry. Elsevier. p. 18.
^Mouquinho, Ana; Saavedra, Mara; Maiau, Alexandre; Petrova, Krasimira; Barros, M. Teresa; Figueirinhas, J. L.; Sotomayor, João (30 June 2011). "Films Based on New Methacrylate Monomers: Synthesis, Characterisation and Electro-Optical Properties". Molecular Crystals and Liquid Crystals. 542 (1): 132/[654]–140/[662]. doi:10.1080/15421406.2011.570154. S2CID97514765.
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