Biphenyl
[CAS 92-52-4]

Identification

• Classification: Organic raw materials >> Hydrocarbon compounds and their derivatives >> Aromatic hydrocarbon
• Name: Biphenyl
• Synonyms: Diphenyl; 1,1'-Biphenyl; 1,1'-Diphenyl; Phenylbenzene
• Molecular Formula: C₁₂H₁₀
• Molecular Weight: 154.21
• CAS Registry Number: 92-52-4
• EC Number: 202-163-5
• SMILES: C1=CC=C(C=C1)C2=CC=CC=C2

Properties

• Density: 1.0±0.1 g/cm³ Calc.^*, 0.99 g/mL (Expl.)
• Melting point: 68 - 70 °C (Expl.)
• Boiling point: 258.0±7.0 °C 760 mmHg (Calc.)^*, 255 °C (Expl.)
• Flash point: 105.5±8.9 °C (Calc.)^*, 112.8 °C (Expl.)
• Solubility: water: insoluble, a,cohol / ether: soluble (Expl.)
• Index of refraction: 1.571 (Calc.)^*, 1.588 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07;GHS09 Warning
• Risk Statements: H315-H319-H335-H400-H410
• Safety Statements: P261-P264-P264+P265-P271-P273-P280-P302+P352-P304+P340-P305+P351+P338-P319-P321-P332+P317-P337+P317-P362+P364-P391-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Acute toxicity	Acute Tox.	2	H330
Acute toxicity	Acute Tox.	3	H331
Aspiration hazard	Asp. Tox.	1	H304
Eye irritation	Eye Irrit.	2A	H319

• Transport Information: UN 3077

chemBlink Chemical Story

Biphenyl appears to be one of the simplest possible ways to join two benzene rings: connect them with a single carbon-carbon bond. Yet the molecule immediately reveals a conflict between two fundamental forces in organic chemistry. Conjugation favors the rings becoming coplanar so their pi systems can overlap, while steric repulsion between the ortho hydrogens pushes the rings out of plane. The resulting twist made biphenyl a model for understanding molecular conformation.

Rotation around the central bond is possible in unsubstituted biphenyl, so the molecule does not exist as isolable left- and right-handed forms under ordinary conditions. Add sufficiently bulky substituents near that bond, however, and rotation can become slow enough that the twisted conformations are configurationally stable. This is atropisomerism - chirality created not by a conventional asymmetric carbon but by restricted rotation. Substituted biphenyls therefore became foundational examples in stereochemistry and later important motifs in ligands, catalysts, pharmaceuticals, and natural products.

Biphenyl also has a practical thermal history. Mixed with diphenyl ether, it forms a low-melting eutectic heat-transfer fluid that can circulate at temperatures where water would require high pressure. Commercial diphenyl ether/biphenyl mixtures became classic thermal media for industrial heat-transfer systems. Here the important chemistry is collective: mixing two aromatic solids depresses the melting point and creates a fluid with a broader useful operating range than either component alone.

The molecule also reminds us that chemical families must be distinguished carefully. Biphenyl itself is not the same as polychlorinated biphenyls, the persistent pollutants known as PCBs. PCBs are chlorinated derivatives whose environmental persistence, bioaccumulation, and toxicology depend strongly on chlorination pattern. The parent hydrocarbon shares their two-ring skeleton but not their regulatory identity or full hazard profile.

Biphenyl matters because a single bond between two benzene rings opens several major ideas at once: competition between conjugation and steric crowding, conformational motion, atropisomeric chirality, eutectic thermal engineering, and the profound effects of substitution on environmental behavior. Few molecules show so clearly that "two benzene rings joined together" is not the end of the structural story but the beginning of it.

References:
Velez C, Khayet M, Ortiz de Zarate JM. Journal of Chemical Thermodynamics. 2012;50:80-88. DOI: 10.1016/j.jct.2012.02.001.
NTP Chemical Effects in Biological Systems. Biphenyl, CAS 92-52-4. DOI: 10.22427/NTP-DATA-DTXSID4020161.
Oki M. Topics in Stereochemistry. Restricted rotation and atropisomerism literature.

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