Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene 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. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula (C6H5)2. It is notable as a starting material for the production of polychlorinated biphenyls (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 compounds such as emulsifiers, optical brighteners, crop protection products, and plastics. Biphenyl is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule consists of two connected phenyl rings. Properties and occurrence Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be isolated from these sources via distillation. 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. In the laboratory, biphenyl can also be synthesized by treating phenylmagnesium bromide with copper(II) salts. It can also be prepared by diazonium salts. When Aniline is treated with NaNO2+dil HCl at 278K, it yields Benzene diazonium chloride. When it is further reacted with Benzene, Biphenyl is formed. It is known as Gomberg Bachmann Reaction. Ph-NH2->[\text{NaNO}_2\text{(aq), HCl}][T\text{=273-278K}] Ph-N2+ ->[\text{Ph-H, Δ}] Ph-Ph Reactions and uses 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. 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. These salts, usually prepared in situ, are versatile reducing agents. Lithium biphenyl offers some advantages relative to the related lithium naphthene. Related to Li/biphenyl is the derivative with two tert-butyl