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.

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.

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)°.

In the first structure determination in the solid state the molecule was described as perfectly planar within the limits of error.

[7][8] 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°.

[10][11] Relative to the free molecule it represents therfore a transition state of inversion stabilized by the intermolecular interactions in a molecular crystal.

[13] 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 a eutectic mixture with diphenyl ether, as a heat transfer agent.

Several solvates of alkali metal salts of biphenyl anion have been characterized by X-ray crystallography.

Some derivatives, as well as related molecules such as BINAP, find application as ligands in asymmetric synthesis.

Research into biphenyl liquid crystal candidates mainly focuses on molecules with highly polar heads (for example cyano or halide groups) and aliphatic tails.