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Diphenyl compounds, also known as biphenyls, are organic compounds consisting of two phenyl rings connected by a single carbon-carbon bond. The most common and simplest example is biphenyl itself, with the chemical formula (C6H5)2. The phenyl rings can be substituted with various other atoms or functional groups, leading to a vast array of diphenyl derivatives. These derivatives exhibit a wide range of physical and chemical properties depending on the nature and position of the substituents.

Diphenyl compounds find applications in numerous fields. For instance, polychlorinated biphenyls (PCBs), a class of chlorinated diphenyls, were historically used as coolants and insulating fluids in electrical equipment due to their stability and non-flammability. However, their persistence in the environment and toxicity have led to widespread bans. Unsubstituted biphenyl is used as a starter material in the synthesis of other organic compounds and as a component in heat transfer fluids. Certain substituted diphenyls are employed as optical brighteners in textiles and paper, absorbing ultraviolet light and emitting visible blue light to make materials appear whiter.

The synthesis of diphenyl compounds can be achieved through various organic reactions, including coupling reactions such as the Ullmann reaction or Suzuki coupling, which involve forming the carbon-carbon bond between two phenyl rings. The study of diphenyl compounds is important in organic chemistry for understanding structure-property relationships and developing new materials and chemical processes. Their environmental impact, particularly for persistent derivatives, is also a significant area of research.