biphenyls

Biphenyls are a family of chemical compounds that consist of two benzene rings joined by a single carbon–carbon bond. The parent compound, biphenyl (C12H10), features two phenyl rings connected at the 1-1' positions and exhibits rotational freedom around the central bond. In many unsubstituted forms, steric repulsion between ortho hydrogens causes the rings to twist relative to one another, giving a nonplanar structure with a dihedral angle commonly around 40–45 degrees in the solid state; bulky or ortho substituents can increase this twist or lock the rings into even more distant arrangements.

Substituted biphenyls are widespread and can be designated by the positions of their substituents (ortho, meta, para). Ortho substitution often reduces conjugation, alters physical properties, and can lead to atropisomerism, where hindered rotation around the central bond yields isolable enantiomers for sufficiently bulky or highly substituted biaryls.

Biphenyls are typically prepared by biaryl coupling reactions. Classic routes include Ullmann-type couplings and modern transitions-metal-catalyzed cross-couplings such as Suzuki-Miyaura coupling of phenylboronic acids with aryl halides. Other methods include oxidative or direct biaryl formations from simple arenes. They are generally nonpolar and have limited water solubility, with properties strongly influenced by substituents.

Uses span chemical synthesis, liquid crystal materials, dyes, and pharmaceutical or polymer chemistry. The core structure serves as a versatile scaffold for more complex architectures. Some halogenated biphenyls, notably polychlorinated biphenyls, are environmentally persistent and toxic in certain contexts.