Biphenyl

Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be isolated from these sources via distillation.[5] It is produced industrially as a byproduct of the dealkylation of toluene to produce benzene:

C₆H₅CH₃ + C₆H₆ → C₆H₅−C₆H₅ + CH₄

The other principal route is by the oxidative dehydrogenation of benzene:

2 C₆H₆ + ​¹⁄₂ O₂ → C₆H₅−C₆H₅ + H₂O

Annually 40,000,000 kg are produced by these routes.[6]

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

Lacking functional groups, biphenyl is fairly non-reactive, which is the basis of its main application. In the laboratory, biphenyl is mainly used as a heat transfer agent as a eutectic mixture with diphenyl ether. This mixture is stable to 400 °C.

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.[6]

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.[7] These salts, usually prepared in situ, are versatile reducing agents.[8] Lithium biphenyl offers some advantages relative to the related lithium naphthenide.[9] Related to Li/biphenyl is the derivative with two tert-butyl groups on the biphenyl.[10]

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°.[11] 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).[12]

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.

It 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).[13]

It is part of the active group in the antibiotic oritavancin.

Substituted biphenyls have many uses. They are prepared by various coupling reactions including the Suzuki-Miyaura reaction and the Ullmann reaction. Polychlorinated biphenyls were once used as cooling and insulating fluids and polybrominated biphenyls are flame retardants. The biphenyl motif also appears in 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 displays (5CB, 7CB, 8OCB and 5CT[14]). 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.

• Naphthalene, where the rings are fused
• Terphenyl, three ringed analog
• Bithiophene

• "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.

• International Chemical Safety Card 0106
• CDC - NIOSH Pocket Guide to Chemical Hazards
• National Pollutant Inventory - Biphenyl
• External MSDS