Biphenyl

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]

Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be isolated from these sources via distillation.[8] It is produced industrially as a byproduct of the dealkylation of toluene to produce methane:

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.[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.

${\displaystyle {\ce {Ph-NH2->[{\text{NaNO}}_{2}{\text{(aq), HCl}}][T{\text{=273-278K}}]Ph-N2+->[{\text{Ph-H, Δ}}]Ph-Ph}}}$

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]

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]

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[16]). 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.

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]

• 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