Hubbry Logo
BiphenylBiphenylMain
Open search
Biphenyl
Community hub
Biphenyl
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Biphenyl
Biphenyl
Biphenyl
View on Wikipedia
from Wikipedia
Biphenyl
Skeletal formula
Skeletal formula
Space filling model showing its twisted conformation
Space filling model showing its twisted conformation
sample
sample
Names
Preferred IUPAC name
1,1′-Biphenyl
Other names
  • Biphenyl
  • Phenylbenzene
  • Xenene
Identifiers
3D model (JSmol)
1634058
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.001.967 Edit this at Wikidata
EC Number
  • 202-163-5
E number E230 (preservatives)
3808
KEGG
RTECS number
  • DU8050000
UNII
UN number 3077
  • InChI=1S/C12H10/c1-3-7-11(8-4-1)12-9-5-2-6-10-12/h1-10H checkY
    Key: ZUOUZKKEUPVFJK-UHFFFAOYSA-N checkY
  • InChI=1/C12H10/c1-3-7-11(8-4-1)12-9-5-2-6-10-12/h1-10H
    Key: ZUOUZKKEUPVFJK-UHFFFAOYAV
  • c1ccccc1-c2ccccc2
Properties
C12H10
Molar mass 154.212 g·mol−1
Appearance Colorless to pale-yellow crystals
Odor pleasant[1]
Density 1.04 g/cm3[2]
Melting point 69.2 °C (156.6 °F; 342.3 K)[2]
Boiling point 255 °C (491 °F; 528 K)[2]
4.45 mg/L[2]
Vapor pressure 0.005 mmHg (20°C)[1]
−103.25·10−6 cm3/mol
Hazards
GHS labelling:
GHS07: Exclamation markGHS09: Environmental hazard
Warning
H315, H319, H335, H410
P261, P264, P271, P273, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
1
0
Flash point 113 °C (235 °F; 386 K)[2]
540 °C (1,004 °F; 813 K)[2]
Explosive limits 0.6–5.8%[1]
Lethal dose or concentration (LD, LC):
2400 mg/kg (oral, rabbit)
3280 mg/kg (oral, rat)
1900 mg/kg (oral, mouse)
2400 mg/kg (oral, rat)[3]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 1 mg/m3 (0.2 ppm)[1]
REL (Recommended)
 TWA 1 mg/m3 (0.2 ppm)[1]
IDLH (Immediate danger)
 100 mg/m3[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

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.[5]

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

[edit]

Biphenyl is a solid at room temperature, with a melting point of 69.2 °C (156.6 °F). 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)°.[6] In the room-temperature solid, biphenyl is crystalline with space group P21/c, which does not allow for chiral crystals. 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°.[9] A study from 2021 concludes that in the crystalline phase at 100 K there is no double-well potential associated with the two twisted conformations, but the potential energy is minimized to zero twist.[10][11] Relative to the free molecule it represents therefore a transition state of inversion stabilized by the intermolecular interactions in a molecular crystal.

Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be isolated from these sources via distillation.[12] 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.[13]

In the laboratory, biphenyl can also be synthesized by treating phenylmagnesium bromide with copper(II) salts.

It can also be prepared using diazonium salts. When aniline is treated with NaNO2+dilute HCl at 5 °C, it yields benzene diazonium chloride. When this is further reacted with benzene, biphenyl is formed. This is known as the Gomberg–Bachmann reaction.

Reactions and uses

[edit]

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. This mixture is stable to 400 °C.[13]

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.[13]

Li biphenyl radical

[edit]

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.[14] These salts, usually prepared in situ, are versatile reducing agents.[15] Lithium biphenyl offers some advantages relative to the related lithium naphthene.[16] Related to Li/biphenyl is the derivative with tert-butyl groups on the biphenyl.[17]

Stereochemistry

[edit]

Rotation about the single bond in biphenyl, and especially its ortho-substituted derivatives, is sterically hindered. For this reason, some substituted biphenyls show atropisomerism; that is, the individual C2-symmetric-isomers are optically stable. Some derivatives, as well as related molecules such as BINAP, find application as ligands in asymmetric synthesis. In the case of unsubstituted biphenyl, the equilibrium torsional angle is 44.4° and the torsional barriers are quite small, 6.0 kJ/mol at 0° and 6.5 kJ/mol at 90°.[18] Adding ortho substituents greatly increases the barrier: in the case of the 2,2'-dimethyl derivative, the barrier is 17.4 kcal/mol (72.8 kJ/mol).[19]

Biphenyl compounds

[edit]

Substituted biphenyls have many uses. They are prepared by various coupling reactions including the Suzuki-Miyaura reaction and the Ullmann reaction. Polychlorinated biphenyls were once used as cooling and insulating fluids and polybrominated biphenyls are flame retardants. The biphenyl motif also appears in drugs such as diflunisal and telmisartan. The abbreviation E7 stands for a liquid crystal mixture consisting of several cyanobiphenyls with long aliphatic tails used commercially in liquid crystal displays (5CB, 7CB, 8OCB and 5CT[20]). A variety of benzidine derivatives are used in dyes and polymers. Research into biphenyl liquid crystal candidates mainly focuses on molecules with highly polar heads (for example cyano or halide groups) and aliphatic tails. It is part of the active group in the antibiotic oritavancin.[citation needed]

Safety and bioactivity

[edit]

Biphenyl prevents the growth of molds and fungus, and is therefore used as a preservative (E230, in combination with E231, E232 and E233), particularly in the preservation of citrus fruits during transportation. It is no longer approved as a food additive in the European Union.[citation needed]

Biphenyl is mildly toxic, but can be degraded biologically by conversion into nontoxic compounds. Some bacteria are able to hydroxylate biphenyl and its polychlorinated biphenyls (PCBs).[21]

See also

[edit]

Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Biphenyl

View on Grokipedia
from Grokipedia
Biphenyl, also known as diphenyl, is an with the molecular formula C₁₂H₁₀, consisting of two phenyl rings connected by a single carbon-carbon bond. It appears as a colorless to white crystalline solid with a pleasant, floral and a of approximately 69–71°C, while its boiling point is around 255°C. Biphenyl is practically insoluble in (solubility 4.45 mg/L at 20°C) but readily dissolves in organic solvents such as , , , and . Chemically stable and persistent in the environment, it has a log Kₒw value of 3.16–4.16, indicating moderate and potential for . Naturally occurring in , crude oil, and as a product of incomplete of fuels, biphenyl is also produced industrially by dealkylation of or oxidative dehydrogenation of , as well as via coupling reactions such as the Wurtz-Fittig reaction of aryl halides. Its primary uses include serving as an intermediate in for pharmaceuticals, dyes, and plastics; as a in industrial applications; and historically as a fungistat and for fruits and . Biphenyl derivatives, such as polychlorinated biphenyls (PCBs), have been widely used in electrical insulators and lubricants but are now heavily regulated due to environmental persistence and toxicity concerns. Although biphenyl itself exhibits moderate acute toxicity (e.g., oral LD₅₀ >1,900 mg/kg in rats), with potential for irritation and liver/kidney effects at high exposures, it is classified by the U.S. EPA as Group D (not classifiable as to human carcinogenicity).

Structure and Properties

Molecular Structure

Biphenyl has the molecular formula C₁₂H₁₀ and a molecular weight of 154.21 g/mol. It consists of two phenyl rings (C₆H₅) directly connected by a single carbon-carbon bond between their ipso positions, forming a symmetric biaryl structure. The inter-ring C-C bond length is approximately 1.49 in the crystalline state, longer than a typical aromatic C-C bond (1.39 ) but indicative of partial double-bond character arising from π-conjugation between the rings. This conjugation allows for orbital overlap across the biaryl linkage, extending the aromatic π-system beyond a single ring and influencing electronic properties such as delocalization of π-electrons. In the gas phase, biphenyl adopts a twisted conformation with a of 44.4° between the planes of the two phenyl rings, primarily due to steric hindrance from ortho hydrogens that prevents . This angle varies in other phases: approximately 19° to 32° in solution, reflecting interactions, and nearly 0° (planar on average) in the solid state due to intermolecular forces. The of biphenyl at belongs to the monoclinic space group P2₁/a (phase I), with two molecules per and a layered arrangement where molecules lie in herringbone-packed sheets parallel to the ab plane. Above approximately 40 and up to the near 70°, this phase persists, with molecules exhibiting librational disorder that averages to planarity; at lower temperatures (around 22 , phase III), it transitions to an orthorhombic space group Pa with reduced symmetry.

Physical Properties

Biphenyl appears as colorless to white crystals or a pale-yellow solid, exhibiting a pleasant floral . It has a of 69.2 °C and a of 255 °C at 760 mmHg. The density of the solid is 1.041 g/cm³ at 20 °C relative to 4 °C, while the liquid density is approximately 0.992 g/cm³ at 20 °C. Biphenyl's is low, measuring 0.00893 mmHg at 25 °C, and its is 1.786 J/g at 100 °C. Biphenyl shows limited solubility in water, with a value of 7.48 mg/L at 25 °C, rendering it effectively insoluble for most practical purposes. In contrast, it exhibits high solubility in organic solvents, such as , , , and , where it is miscible. Thermodynamic data indicate stability under standard conditions. The for the solid phase is approximately 98.2 kJ/mol at 298 . The standard of the solid is 209.38 J/mol· at 298 and 1 bar. Direct values for the standard of formation are not widely reported, though derived estimates from related phase equilibria confirm its thermodynamic profile. Optically, biphenyl displays UV absorption maxima at 247 nm (log ε = 4.24) in alcohol, attributed to π-π* transitions, with a of 1.475 at 20 °C.

Chemical Properties

Biphenyl exhibits significant aromatic stability arising from the delocalization of π-electrons across its two connected rings, which allows for extended conjugation despite the partial of the rings in the ground state. This delocalization contributes to a energy approximately twice that of (about 72 kcal/mol total), as the individual ring systems provide additive stabilization with only minor additional resonance from inter-ring overlap. The compound displays high thermal stability, remaining intact up to 400°C under inert atmospheres, which enables its use in applications. Above this temperature, biphenyl decomposes via pathways involving C-C bond cleavage, yielding products such as , , , and higher polyphenyls. Biphenyl's C-H protons are weakly acidic, comparable to due to the aromatic nature limiting . It possesses no basic sites, as the structure lacks lone pairs or electron-donating heteroatoms. The molecule resists solvolysis and , reflecting its chemical inertness and stability in acidic or basic media. Due to its centrosymmetric structure, biphenyl is non-polar, exhibiting a dipole moment of 0 D, which contributes to its low solubility in polar solvents.

Occurrence and Synthesis

Natural Occurrence

Biphenyl is primarily found in natural geological sources, where it constitutes a minor but significant component of -derived materials. It occurs in , a of under high-temperature conditions, at concentrations typically ranging from 0.2% to 1.6% by weight. In crude oil, biphenyl is present at trace levels, up to 0.4 mg/g (0.04%), reflecting its incorporation during the diagenetic transformation of . Similarly, and its condensates contain biphenyl in low concentrations, measured at 3–42 µg/m³ in gas phases, arising from the thermal maturation processes in sedimentary basins. These occurrences stem from the thermal cracking and of larger polycyclic aromatic hydrocarbons during formation, a process that breaks down complex organic precursors under geothermal heat and pressure over geological timescales. High-temperature geological activities, such as volcanic processes and hydrothermal alterations, also contribute to biphenyl's natural distribution by generating polycyclic aromatic hydrocarbons (PAHs) through incomplete combustion or of organic materials in the . For instance, biphenyl has been detected in geothermal fluids and volcanic emissions as part of PAH assemblages formed under extreme conditions. In environmental contexts, natural oil and gas seeps release biphenyl into surrounding soils and sediments, establishing background concentrations typically below 1 µg/g in uncontaminated areas, though levels can vary with seep activity and proximity to reservoirs. These seep-derived inputs represent a baseline natural flux, distinct from anthropogenic . Beyond geological sources, biphenyl appears in trace amounts in biological systems, particularly as a defense . In , especially within the subfamily Pyrinae (e.g., species of apple, , and rowan), biphenyl and related derivatives are synthesized de novo as phytoalexins in response to infection by pathogenic fungi or bacteria, serving as agents. Concentrations in elicited plant tissues can reach up to about 0.1 mg/g dry weight (0.01%) in localized responses, though they are negligible in healthy . Fungal pathways also produce biphenyl metabolites; for example, endophytic fungi associated with like patchouli ( cablin) biosynthesize biphenyl derivatives as secondary metabolites, potentially contributing to ecological interactions. These biogenic occurrences highlight biphenyl's role in natural mechanisms. Biphenyl was first synthesized in 1862 by Rudolph Fittig through the reaction of bromobenzene with metallic sodium, but its natural presence was confirmed shortly thereafter via isolation from coal tar distillates in the 1870s, underscoring its longstanding recognition as a fossil-derived compound.

Synthetic Methods

Biphenyl is produced on an industrial scale primarily through the hydrodealkylation of toluene, in which toluene reacts with hydrogen over a catalyst such as chromium oxide or molybdenum oxide at temperatures of 600–700°C and pressures around 50 bar, generating biphenyl as a significant by-product (typically 5–10% yield based on toluene) alongside benzene and methane. As of 2024, the global market is valued at approximately USD 287 million (implying production on the order of 50,000 metric tons), driven by the demand for benzene as the primary product. An alternative industrial route is the dehydrogenative coupling of benzene, employing nickel or palladium catalysts under oxidative conditions to form biphenyl directly, though this method is less common due to lower selectivity and higher energy requirements. In laboratory settings, biphenyl is commonly synthesized via the Ullmann coupling, a copper-mediated dimerization of aryl halides like in high-boiling solvents such as or at 150–250°C, proceeding through organocopper intermediates to afford the symmetric biaryl. The Gomberg–Bachmann reaction provides another route, involving the base-promoted coupling of benzenediazonium salts with , often using aqueous or phase-transfer conditions to generate aryl radicals that combine. For more versatile synthesis, particularly of substituted variants, the Suzuki–Miyaura cross-coupling employs catalysts to couple phenylboronic acid with aryl halides in aqueous or organic media at mild temperatures (50–100°C), enabling high . Additional laboratory methods include the Wurtz–Fittig coupling, where two equivalents of react with sodium metal in dry ether to form biphenyl via radical intermediates, though this approach suffers from side reactions like reduction products. Biphenyl can also be obtained from the of under pyrolytic conditions (typically 400–600°C in vacuum), where homolytic cleavage of the N=N bond produces phenyl radicals that dimerize. Yields for these methods vary: classical Ullmann coupling typically achieves 70–90% for unsubstituted biphenyl, while modern palladium-catalyzed variants like Suzuki–Miyaura exceed 95% for many substituted analogs, benefiting from improved ligands and bases. Industrially, the hydrodealkylation is favored for its scalability and economic viability, as the co-produced serves as a valuable feedstock for further applications, offsetting costs despite biphenyl's lower yield in the reaction.

Reactions

Electrophilic Substitution

Biphenyl undergoes (EAS) primarily at the 4-position due to the phenyl substituent acting as a weak activator and ortho/para director, which enhances at the ortho and para positions through donation. This directing effect results in favoring the para position over ortho, as the bulky phenyl group imposes steric hindrance at the ortho sites. The phenyl group's ability to delocalize positive charge in the further promotes substitution at these positions compared to meta. The mechanism follows the standard EAS pathway, involving addition of the to form a Wheland (sigma) complex, followed by loss of a proton to restore ; in biphenyl, the phenyl substituent stabilizes the cationic intermediate particularly well at the para position by distributing the positive charge across both rings. Ortho substitution is disfavored not only sterically but also due to less effective charge delocalization in the corresponding intermediate. For multiple substitutions, the initial para product directs further reaction to the symmetric 4,4'-disubstituted biphenyl, as the introduced groups (e.g., nitro or halo) often reinforce para direction. Representative examples illustrate this . using a of nitric and s yields 4-nitrobiphenyl as the major product (approximately 60-70% selectivity), with the ortho forming in lower amounts (ortho:para ratio ≈ 0.6). Sulfonation with fuming predominantly produces 4-biphenylsulfonic acid, which can further sulfonate to 4,4'-disulfonic acid under prolonged reaction. , such as bromination with Br₂ and FeBr₃, also favors 4-bromobiphenyl, reflecting the same directing and steric influences. Historically, EAS on biphenyl provided an early synthetic route to azo dyes; for instance, nitration followed by reduction to 4-aminobiphenyl and subsequent diazotization enabled coupling reactions to form colored azo compounds used in textiles and pigments.

Reduction and Oxidation

Biphenyl can be reduced using dissolving metal conditions in the , where treatment with sodium in liquid yields 1,4-dihydrobiphenyl as the major product, featuring a 1,4-cyclohexadiene ring conjugated to an intact . This partial reduction preserves in one ring while introducing isolated double bonds in the other, facilitating regioselective functionalization in subsequent synthetic steps. The mechanism involves initial electron addition to form a , followed by protonation and a second , with the biphenyl linkage influencing the site of reduction preferentially on unsubstituted rings. Catalytic hydrogenation of biphenyl over supported catalysts, such as on carbon under high pressure and conditions, affords the fully saturated bicyclohexyl (dicyclohexyl). This process typically requires elevated temperatures (around 150–200°C) and pressures (up to 10 MPa) to overcome the stability of the aromatic system, achieving high selectivity to the saturated product without ring cleavage. Selective partial to monohydro derivatives like cyclohexylbenzene is possible under milder conditions with copper-based catalysts, enabling access to intermediates for liquid organic hydrogen carriers. The electrochemical reduction of biphenyl in (DMF) exhibits a standard E∘E^\circ of approximately -2.7 V versus the (SCE) for the one-electron process forming the biphenyl . This highly negative potential reflects the stability of the neutral aromatic system, requiring strong reductants for anion formation; the radical anion is transient and protonates rapidly in protic media. Oxidation of biphenyl proceeds electrochemically via anodic processes to generate the radical cation, with oxidation potentials for biphenyl and its derivatives typically ranging from 1.3 to 1.7 V versus SCE in , depending on substituents that stabilize the cation through delocalization across the biaryl linkage. The radical cation exhibits enhanced stability in the biphenyl system compared to mononuclear analogs due to charge delocalization, though it dimerizes or reacts with nucleophiles under prolonged . Biphenyl demonstrates resistance to mild oxidizing agents, such as or under standard conditions, owing to the stability of its fully aromatic conjugated structure, which lacks oxidizable side chains. It remains air-stable at ambient temperatures but undergoes slow photooxidation in the presence of oxygen and UV light, primarily via addition to form hydroxylated derivatives like 2- or 4-hydroxybiphenyl. These processes find applications in , particularly the selective for preparing partially saturated biphenyl derivatives as versatile building blocks in pharmaceuticals and materials, where the unconjugated moiety allows further stereoselective modifications without affecting the remaining aromatic ring.

Radical Formation

The salt of the biphenyl , [C12H10]⁻ Li+, is generated by dissolving metal in a (THF) solution of biphenyl, producing a deep blue solution characteristic of the delocalized π-electron system. This species functions as a potent one-electron reductant, with a standard of approximately -2.7 V versus the (SCE) in THF, enabling selective reductions under mild conditions. The formation was first reported in the and quickly adopted for organometallic reductions, marking an early application of aromatic radical anions in synthetic chemistry. Electron spin resonance (ESR) spectroscopy of the biphenyl in solution demonstrates extensive delocalization of the across both phenyl rings, with hyperfine constants indicating symmetric distribution and contributing to its relative stability. In aprotic solvents like THF, the radical persists for seconds to minutes before undergoing dimerization or , depending on concentration and effects; lifetimes can extend to hours under rigorously anaerobic and conditions. The can also be produced via electrochemical reduction of biphenyl at potentials around -2.7 V versus SCE, allowing controlled generation for mechanistic studies. Additionally, of biphenyl or its derivatives induces homolytic cleavage of the inter-ring C-C bond in highly excited triplet states, yielding phenyl radicals as transient intermediates observable on timescales. The biphenyl radical anion reduces ketones to ketyl radicals and alkyl halides to alkyl radicals through single-electron transfer pathways, facilitating subsequent coupling or protonation reactions in organic synthesis. It has been employed as a radical initiator in the polymerization of styrene, where the transferred electron generates initiating carbon-centered radicals that propagate chain growth. In contemporary applications, lithium biphenyl solutions serve as anolytes in non-aqueous redox flow batteries and lithium-sulfur cells, leveraging their high solubility, reversible redox behavior, and reducing capacity to enhance energy density and cycle life.

Applications

Industrial Uses

Biphenyl is produced globally on a scale of approximately 40,000 tonnes per year, primarily as a byproduct of processes such as the of or the production of via . This production volume supports its role as a chemical, with the global market valued at around USD 287 million in 2024 and projected to reach USD 304 million in 2025. Industrial-grade biphenyl is typically priced at $5–10 per kg, reflecting its availability from these low-cost feedstocks. A major industrial application of biphenyl is as a component in high-temperature fluids, particularly in the formulation known as Therminol VP-1. This fluid is a eutectic consisting of 73.5% diphenyl oxide and 26.5% biphenyl, offering thermal stability up to 400°C (750°F) in both liquid and vapor phases. It is widely employed in solar thermal power plants, chemical processing, and other systems requiring efficient at elevated temperatures due to its low viscosity and resistance to thermal degradation. Biphenyl has also been used as a preservative under the designation E230, applied to fruits to inhibit mold growth during storage and transport. This antifungal property stems from its ability to form a protective barrier on surfaces. However, its use was banned in the in 2004 due to concerns over residues migrating into and potential risks. Additionally, biphenyl serves as a precursor for incorporated into detergents. These biphenyl-based compounds absorb light and re-emit it as visible blue light, enhancing the perceived whiteness and brightness of fabrics without altering their color. This application leverages biphenyl's conjugated for effective in aqueous environments.

Role in Organic Synthesis

Biphenyl serves as a versatile building block in due to its rigid, conjugated structure and facile functionalization, enabling the construction of complex molecular architectures. Its biaryl framework facilitates π-conjugation, which is essential for electronic communication in target molecules, while the ortho positions allow for steric control in atropisomeric systems. Substituted biphenyls are particularly valuable precursors in the synthesis of pharmaceuticals, where the core motif imparts conformational rigidity and to bioactive compounds. For instance, biphenyl-substituted diaryltriazines have been developed as potent non-nucleoside inhibitors (NNRTIs) for HIV-1 treatment, mimicking the activity of through structural hybridization that enhances binding affinity to the enzyme's allosteric site. These analogs demonstrate values in the low nanomolar range against wild-type HIV-1, highlighting biphenyl's role in optimizing pharmacokinetic properties like metabolic stability. In catalytic applications, biphenyl-derived phosphine ligands have revolutionized cross-coupling reactions by providing steric bulk and electronic tuning for . A prominent example is XPhos (2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl), which enables efficient Buchwald-Hartwig aminations of unactivated aryl and heteroaryl chlorides at , achieving yields often exceeding 90% under mild conditions. This ligand's biphenyl backbone stabilizes the Pd(0) complex through π-stacking interactions, broadening the substrate scope to include challenging N-heterocycles and primary amines. Such advancements have made biphenyl phosphines indispensable for scalable synthesis in , reducing reaction times and catalyst loadings to parts per million levels in some protocols. Biphenyl also functions as a model compound for investigating conjugation effects in polyaromatic systems, owing to its simple yet tunable electronic properties. The molecule's torsional barrier around the inter-ring bond (approximately 2 kcal/mol) allows spectroscopic studies of partial π-overlap, serving as a benchmark for understanding delocalization in larger polycyclic aromatic hydrocarbons (PAHs). This has informed the design of conjugated materials, where biphenyl's conjugation length influences absorption wavelengths and charge transport. Recent developments in the have leveraged twisted biphenyl fluorophores for organic light-emitting diodes (OLEDs), particularly in thermally activated delayed (TADF) emitters. For example, donor-acceptor architectures with twisted biphenyl linkers achieve narrowband blue emission with external quantum efficiencies up to 25%, minimizing concentration through orthogonal donor-acceptor orientations. These materials exhibit quantum yields over 80% in doped films, advancing high-resolution displays. The synthetic utility of biphenyl is further exemplified by its selective functionalization at the 4 and 4' positions, which are activated for and cross-coupling due to para-directing effects. This facilitates iterative Suzuki-Miyaura couplings to construct dendrimers, where biphenyl dendrons with focal points enable stepwise branching with . Such approaches yield well-defined, spherical architectures up to third generation, with peripheral groups tailored for applications like light harvesting or , maintaining monodispersity greater than 95%.

Derivatives

Substituted Biphenyls

Substituted biphenyls encompass a diverse class of compounds where functional groups replace atoms on the biphenyl core, altering its physical and chemical properties for specific uses. A prominent example is 4-hydroxybiphenyl, which exhibits activity due to its phenolic structure, enabling it to scavenge free radicals and inhibit oxidative processes. In contrast, 4-aminobiphenyl serves as an intermediate in dye production but is recognized as a potent , classified by the International Agency for Research on Cancer (IARC) as based on sufficient evidence of human risk from occupational exposures. The incorporation of polar substituents like hydroxy or amino groups enhances the water of these derivatives compared to unsubstituted biphenyl, which is nearly insoluble (approximately 4.5 mg/L at 25°C), with 4-hydroxybiphenyl showing around 700 mg/L at 20°C. Rod-like substituted biphenyls, particularly those bearing cyano and alkyl chains, display thermotropic liquid crystalline behavior, forming nematic phases suitable for electro-optic applications. For instance, the E7 mixture, comprising cyanobiphenyl derivatives such as 4-cyano-4'-pentylbiphenyl and 4-cyano-4'-heptylbiphenyl, exhibits a nematic phase from -10°C to 60°C and is widely employed in twisted nematic liquid crystal displays (LCDs) for its low viscosity and high birefringence. These properties arise from the extended conjugation and rigid core of the biphenyl unit, promoting molecular alignment under electric fields. Ortho-substitution can induce atropisomerism due to restricted rotation, leading to chiral derivatives with potential in asymmetric synthesis, though detailed stereochemistry is addressed elsewhere. The synthesis of monosubstituted and symmetrically disubstituted biphenyls typically relies on transition-metal-catalyzed cross-coupling reactions to ensure and efficiency. The , involving organozinc reagents, is particularly effective for alkyl-substituted biphenyls, offering mild conditions and tolerance for functional groups; for example, coupling of aryl halides with alkylzinc reagents yields 4-alkylbiphenyls in high yields (up to 95%). Other methods, such as Suzuki-Miyaura coupling, complement this for aryl or heteroaryl substitutions, enabling scalable production of symmetric derivatives like 4,4'-dihydroxybiphenyl. Applications of substituted biphenyls span materials, , and sensing technologies. Azo-biphenyl derivatives, such as those in (a disulfonated bis-azo compound), are utilized as dyes for textiles and biological staining due to their intense color and stability from the extended π-system. In , 4-hydroxybiphenyl acts as a to inhibit mold growth on fruits by disrupting fungal , with applications dating to post-harvest treatments. Recent advancements include biphenyl-based electrochemical sensors for ; a 2021 study developed a photoelectrochemical aptasensor using biphenyl-functionalized nanocomposites to detect polychlorinated biphenyls at sub-femtomolar levels in water samples, highlighting their role in trace pollutant analysis.

Polychlorinated Biphenyls

Polychlorinated biphenyls (PCBs) are a class of synthetic derivatives of biphenyl in which one to ten atoms are substituted at various positions on the two phenyl rings, resulting in 209 distinct congeners. These congeners vary in their degree of chlorination and substitution patterns, influencing their physical and chemical behaviors. Commercial PCB formulations, such as Aroclor 1254 produced by , consist of mixtures dominated by penta- and hexachlorinated congeners, with an average content of approximately 54% by weight. PCBs were commercially manufactured from to 1977, with global production totaling about 1.5 million metric tons, primarily by companies like in the United States. These compounds were valued for their use as fluids in electrical equipment, including transformers and capacitors, due to their non-flammable nature and insulating properties. The key properties of PCBs include high chemical and thermal stability, low flammability, and poor in , which contribute to their in the environment. Their lipophilic character, reflected in octanol-water partition coefficients (log Kₒw) typically ranging from 6 to 8 for higher-chlorinated congeners in commercial mixtures, facilitates in fatty tissues of organisms. Production and use of PCBs were phased out following regulatory actions, including a U.S. Environmental Protection Agency ban on manufacturing and most uses in 1979 under the Toxic Substances Control Act. Internationally, the Stockholm Convention on Persistent Organic Pollutants, adopted in 2001, listed PCBs for global elimination, requiring parties to phase out legacy equipment and manage stockpiles. Remediation efforts continue, such as the ongoing cleanup, where over 2.75 million cubic yards of PCB-contaminated sediment have been dredged since 2009 to reduce downstream transport. Recent as of has advanced microbial degradation techniques for PCBs, including optimization of bacterial consortia and anaerobic sludge amendments to enhance dechlorination rates in contaminated soils and sediments. Additionally, studies have explored sustainable alternatives to PCBs in applications, such as bio-based natural fluids (e.g., oil-based) and nanoparticle-enhanced fluids that offer comparable insulating performance with reduced environmental persistence.

Stereochemistry

Atropisomerism

Atropisomerism in biphenyl arises from due to restricted rotation about the central C-C bond connecting the two phenyl rings, resulting from steric hindrance between substituents in the ortho positions. In the unsubstituted biphenyl, the ortho hydrogens cause a low rotational barrier of approximately 2 kcal/mol, allowing rapid interconversion at and preventing the isolation of stable atropisomers. When bulky substituents are present in the 2 and 2' positions, the steric clash significantly increases the rotational barrier to over 20 kcal/mol, enabling the formation of configurationally stable atropisomers that can be isolated as enantiomers. For instance, 2,2'-disubstituted biphenyls with groups such as nitro or carboxy exhibit barriers exceeding this threshold, leading to atropisomerism. A prominent example is 1,1'-bi-2-naphthol (BINOL), an analogous biaryl system with a rotational barrier of about 39 kcal/mol, widely employed as a chiral in asymmetric due to its stable atropisomerism. The presence of atropisomerism is typically detected using dynamic (NMR) spectroscopy, where coalescence temperatures indicate the rotational barrier height by observing the averaging of signals from interconverting enantiomers. (CD) spectroscopy further confirms the enantiomeric nature by measuring differential absorption of left- and right-circularly polarized light, distinguishing the absolute configurations. This phenomenon was first recognized in the through studies of substituted biaryls, including binaphthyl derivatives, where Christie and Kenner reported the isolation of enantiomers from a tetrasubstituted biphenyl diacid in .

Resolution and Applications

The resolution of biphenyl atropisomers typically involves classical methods such as diastereomeric salt formation, where a is reacted with an optically pure chiral resolving agent to form separable diastereomers. For instance, the derived from 6,6'-dimethoxy-2,2'-diiodobiphenyl has been resolved using (R,R)-, allowing isolation of the enantiomers through fractional . Similarly, salts have been employed for the resolution of acidic biphenyl derivatives like 6-carboxy-2'-methoxybiphenyl, yielding optically active products with high enantiomeric purity. These classical approaches rely on the differential solubility of diastereomeric salts and remain valuable for preparative-scale separations despite their dependence on empirical optimization. Chromatographic techniques, particularly chiral high-performance liquid chromatography (HPLC), provide an alternative for resolving biphenyl enantiomers, especially when classical methods are inefficient. Enantiomers of biphenyl dimethyl dicarboxylate derivatives, for example, have been directly separated using HPLC with a chiral stationary phase coated from a tripeptide derivative, achieving baseline resolution for nineteen analogs. This method is particularly effective for polyhalogenated biphenyls, where semipreparative chiral HPLC on columns like Chiralcel OD or OJ enables isolation of individual atropisomers for further study. Chiral HPLC is advantageous for its scalability and ability to handle compounds with moderate rotational barriers, though it requires specialized columns for optimal selectivity. The enantiomeric excess (ee) of resolved biphenyl atropisomers is commonly assessed using polarimetry to measure optical rotation, which correlates with chiral purity when compared to known standards, or via chiral HPLC on columns such as Chiralcel OD to quantify the enantiomer ratio through peak integration. For biphenyl derivatives like axially chiral diols, ee values up to 99% have been confirmed by both techniques, ensuring accurate characterization post-resolution. Polarimetry provides a rapid, non-destructive assessment but is less precise for low ee values, while chiral HPLC offers superior sensitivity and is the gold standard for verifying high-purity samples in synthetic applications. In applications, biphenyl atropisomers serve as chiral ligands in asymmetric catalysis, exemplified by (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), which enables enantioselective Suzuki-Miyaura couplings to produce axially chiral biaryls with up to 97% ee. This ligand's imparts stereocontrol in palladium-catalyzed reactions, contributing to the 2001 awarded to Ryoji Noyori for his pioneering work on chiral catalysts, including BINAP's role in broader asymmetric transformations. Such ligands have facilitated the synthesis of enantioenriched pharmaceuticals and materials, highlighting biphenyl atropisomers' impact on stereoselective synthesis. Pharmaceutical applications leverage biphenyl atropisomers for their biological activity, as seen in diflunisal, a non-steroidal featuring a biphenyl core with restricted rotation leading to atropisomerism. Clinical formulations often use the . Recent spectroscopic studies have confirmed the atropisomeric conformers in diflunisal, underscoring their relevance in where may influence and efficacy. Chiral biphenyl derivatives are also integral to liquid crystal technologies, where they act as dopants to induce helical phases in nematic mixtures. Axially chiral cyanobiphenyls, for instance, exhibit high helical twisting power (HTP) values around 5 × 10^{-3} μm^{-1}, enabling the formation of cholesteric phases with tunable pitch lengths for applications in displays and optical films. These dopants promote spontaneous chiral amplification in nematic hosts, enhancing the selective reflection of circularly polarized light and improving the efficiency of twisted nematic devices. Recent advancements in atropisomer-selective synthesis, particularly for biphenyl scaffolds in , include catalytic methods that achieve high enantioselectivity without resolution steps. These strategies address rotational barriers briefly noted in atropisomerism studies, prioritizing dynamic kinetic resolutions to access stable enantiomers for therapeutic screening.

Safety, Health, and Environmental Effects

Toxicity and Bioactivity

Biphenyl exhibits moderate , with an oral LD50 of approximately 3,280 mg/kg in rats and 2,400 mg/kg in rabbits, indicating low immediate lethality following ingestion. Dermal exposure yields an LD50 of around 2,500 mg/kg in rabbits, while can cause respiratory distress at high concentrations. The compound is a known irritant to , eyes, and the , producing symptoms such as redness, , lacrimation, and coughing upon contact or . The National Institute for Occupational Safety and Health (NIOSH) recommends a time-weighted average exposure limit of 1 mg/m³ (0.2 ppm) to mitigate these effects. Chronic exposure to biphenyl primarily affects the liver, where it induces enzyme activity and can lead to necrosis, cirrhosis, and other hepatotoxic changes in animal models. The U.S. Environmental Protection Agency has established a chronic reference dose of 0.05 mg/kg-day based on liver effects observed in subchronic studies. Regarding carcinogenicity, the EPA classifies biphenyl as Group D (not classifiable as to human carcinogenicity), though limited evidence from animal studies suggests potential for urinary bladder tumors in male rats at high doses. Biphenyl possesses fungistatic and weak bactericidal properties, contributing to its use as an food preservative and agent in applications by altering microbial permeability. In terms of , biphenyl undergoes cytochrome P450-mediated oxidation in the liver to form hydroxylated metabolites, such as 4-hydroxybiphenyl, which are conjugated to water-soluble forms like sulfates for enhanced elimination. These metabolites are primarily excreted via urine, with studies in rats showing rapid clearance where over 90% of an administered dose is eliminated within 96 hours, predominantly through renal pathways.

Environmental Impact and Regulation

Biphenyl demonstrates moderate environmental persistence, primarily degrading through aerobic in with half-lives ranging from 1.5 to 7 days under oxic conditions. Volatilization plays a key role in its atmospheric fate, with an estimated of approximately 1.5 days due to rapid reaction with hydroxyl radicals, while in the further limits its residence time to around 2 days. Under anaerobic conditions in sediments, persistence increases, with half-lives extending to 68 days, potentially leading to longer-term accumulation in aquatic systems. Bioaccumulation of biphenyl occurs in aquatic organisms, with bioconcentration factors (BCF) in such as measured at 1900–2422, indicating substantial uptake from . Ecologically, biphenyl contributes to contamination via runoff from paved surfaces, where it and associated polycyclic aromatic hydrocarbons (PAHs) enter and impair aquatic habitats. Regulatory frameworks address biphenyl to mitigate environmental risks. In the , biphenyl is registered under regulation with annual import volumes exceeding 1000 tonnes, but it faces no specific restrictions beyond general chemical management requirements. In the United States, biphenyl is listed on the TSCA inventory, subjecting it to reporting and testing obligations. Biphenyl undergoes aerobic via pathways involving bacteria like species, which initiate ring cleavage through dioxygenase enzymes, converting it to benign metabolites such as .

References

  1. https://www.t3db.ca/toxins/T3D0389
  2. https://www.epa.gov/sites/default/files/2016-09/documents/biphenyl.pdf
  3. https://www.inchem.org/documents/cicads/cicads/cicad06.htm
  4. https://www.sciencedirect.com/topics/chemistry/biphenyl
  5. https://pubchem.ncbi.nlm.nih.gov/compound/Biphenyl
  6. https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202101490
  7. https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejoc.201201273
  8. https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejoc.201100874
  9. https://webbook.nist.gov/cgi/cbook.cgi?ID=C92524&Mask=2
  10. https://pubs.aip.org/aip/jcp/article-pdf/1/8/606/18787633/606_1_online.pdf
  11. https://www.researchgate.net/publication/293328766_Comparing_the_Thermal_Stability_and_Oxidative_State_of_Mineral_and_Biphenyl_Diphenyl_Oxide_Based_Heat_Transfer_Fluids
  12. https://www.stenutz.eu/chem/solv6.php?name=biphenyl
  13. https://www.chemicalbook.com/ChemicalProductProperty_EN_CB2491271.htm
  14. https://www.sciencedirect.com/science/article/abs/pii/S0304420306000399
  15. https://www.sciencedirect.com/science/article/abs/pii/S004896971833883X
  16. https://www.beilstein-journals.org/bjoc/articles/8/68
  17. https://www.sciencedirect.com/science/article/abs/pii/S0367326X20302902
  18. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0158713
  19. https://link.springer.com/content/pdf/10.1007/978-3-642-73432-8_10
  20. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=91012GZW.TXT
  21. https://www.prosim.net/wp-content/uploads/2019/12/PSPS_EX_EN-HDA-Toluene-Process-Cost.pdf
  22. https://link.springer.com/chapter/10.1007/978-3-642-73432-8_6
  23. https://www.gminsights.com/industry-analysis/global-biphenyl-market
  24. https://pubs.acs.org/doi/10.1021/jacsau.0c00070
  25. https://www.sciencedirect.com/science/article/abs/pii/S0926337317302424
  26. https://pubs.acs.org/doi/10.1021/jo00183a021
  27. https://www.sciencedirect.com/science/article/pii/S2468823123008295
  28. https://sathee.iitk.ac.in/article/chemistry/chemistry-wurtz-fittig-reaction/
  29. https://espace.library.uq.edu.au/view/UQ:665400/UQ665400_OA.pdf
  30. https://www.chemistrysteps.com/is-phenyl-an-ortho-para-or-meta-director/
  31. https://pubs.rsc.org/en/content/articlelanding/1966/j2/j29660000727
  32. https://www.tandfonline.com/doi/abs/10.1080/10426509408021477
  33. https://pubs.acs.org/doi/10.1021/ja01465a038
  34. https://www.nature.com/articles/s40494-019-0343-1
  35. https://www.thieme-connect.com/products/ejournals/html/10.1055/s-0042-1751387
  36. https://www.sciencedirect.com/science/article/abs/pii/S0926860X05002814
  37. https://www.sciencedirect.com/science/article/abs/pii/S1385894724071122
  38. https://www.sciencedirect.com/science/article/abs/pii/S1572665722006762
  39. https://pubs.acs.org/doi/10.1021/ja00810a033
  40. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_%28Organic_Chemistry%29/Reactions/Oxidation_and_Reduction_Reactions/Oxidation_of_Organic_Molecules_by_KMnO4
  41. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1751-1097.1991.tb01980.x
  42. https://research.cm.utexas.edu/nbauld/unit6_anrad.htm
  43. https://pubs.acs.org/doi/10.1021/jacs.3c11655
  44. https://www.sciencedirect.com/science/article/abs/pii/S0022328X00842278
  45. https://www.sciencedirect.com/science/article/abs/pii/S0045653504008422
  46. https://pubmed.ncbi.nlm.nih.gov/17266216/
  47. https://www.sciencedirect.com/science/article/abs/pii/S004040200901374X
  48. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Polymer_Chemistry_%28Schaller%29/02%253A_Synthetic_Methods_in_Polymer_Chemistry/2.09%253A_Radical_Polymerization
  49. https://www.nature.com/articles/ncomms14629
  50. https://www.futuremarketinsights.com/reports/biphenyl-market
  51. https://www.eastman.com/en/products/product-detail/71093459/therminol-vp-1-heat-transfer-fluid
  52. https://www.therminol.com/sites/therminol/files/documents/TF09A_Therminol_VP1.pdf
  53. https://world.openfoodfacts.org/facets/additives/en:e230-biphenyl/1
  54. https://www.sciencedirect.com/science/article/pii/S0160412019329666
  55. https://dirtylabs.com/blogs/the-dirt/what-are-optical-brighteners-and-why-should-you-care
  56. https://pmc.ncbi.nlm.nih.gov/articles/PMC8434544/
  57. https://www.sciencedirect.com/science/article/pii/S2211383519301832
  58. https://pmc.ncbi.nlm.nih.gov/articles/PMC7016291/
  59. https://pubs.acs.org/doi/10.1021/ja037932y
  60. https://pubs.acs.org/doi/10.1021/acs.oprd.2c00051
  61. https://pubs.rsc.org/en/content/articlelanding/2024/cp/d4cp00765d
  62. https://pubs.acs.org/doi/10.1021/acs.chemmater.4c01001
  63. https://pubs.acs.org/doi/10.1021/jacs.9b11578
  64. https://pubs.acs.org/doi/10.1021/ja990430t
  65. https://pmc.ncbi.nlm.nih.gov/articles/PMC17699/
  66. https://www.ncbi.nlm.nih.gov/books/NBK590855/
  67. https://www.fishersci.ca/shop/products/4-phenylphenol-98-thermo-scientific/p-7081027
  68. https://opg.optica.org/josab/abstract.cfm?uri=josab-27-9-1866
  69. https://pubs.rsc.org/en/content/articlehtml/2023/ra/d3ra03531j
  70. https://onlinelibrary.wiley.com/doi/abs/10.1002/adsc.200303235
  71. https://pubmed.ncbi.nlm.nih.gov/7039853/
  72. https://journals.asm.org/doi/10.1128/AEM.67.4.1551-1557.2001
  73. https://pubs.acs.org/doi/10.1021/acsomega.0c02846
  74. https://www.sciencedirect.com/science/article/abs/pii/S2352485515000249
  75. https://www.epa.gov/pcbs/table-aroclors
  76. https://cec.org/files/documents/publications/1705-status-pcb-management-in-north-america-en.pdf
  77. https://p2infohouse.org/ref/22/21814.pdf
  78. https://apps.ecology.wa.gov/publications/documents/1507002.pdf
  79. https://www.eurochlor.org/wp-content/uploads/2019/04/8-11-4-16_marine_ra_pcbs_ddt_dioxin.pdf
  80. https://www.epa.gov/archive/epa/aboutepa/epa-bans-pcb-manufacture-phases-out-uses.html
  81. https://chm.pops.int/TheConvention/Overview
  82. https://www.epa.gov/hudsonriverpcbs/hudson-river-cleanup
  83. https://www.tandfonline.com/doi/abs/10.1080/10643389.2023.2286870
  84. https://enveurope.springeropen.com/articles/10.1186/s12302-025-01197-4
  85. https://www.epri.com/research/products/000000003002027003
  86. https://onlinelibrary.wiley.com/doi/pdf/10.1002/cbic.200300906
  87. https://pubs.acs.org/doi/10.1021/jm200584g
  88. https://pubs.rsc.org/en/content/articlelanding/1922/ct/ct9222100614
  89. https://pubs.acs.org/doi/10.1021/acsomega.9b00619
  90. https://www.sciencedirect.com/science/article/abs/pii/0040403996010891
  91. https://link.springer.com/article/10.1007/BF02490455
  92. https://www.sciencedirect.com/science/article/abs/pii/S0021967302011123
  93. https://pmc.ncbi.nlm.nih.gov/articles/PMC395991/
  94. https://www.sciencedirect.com/science/article/abs/pii/S0957416603005445
  95. https://pubs.acs.org/doi/10.1021/ja005622z
  96. https://www.sciencedirect.com/science/article/pii/S1386142524015026
  97. https://www.beilstein-journals.org/bjoc/articles/5/50
  98. https://onlinelibrary.wiley.com/doi/abs/10.1002/med.22037
  99. https://www.cdc.gov/niosh/npg/npgd0239.html
  100. https://www.epa.gov/iris/substance-information-biphenyl
  101. https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment1.html
  102. https://pubs.usgs.gov/fs/2016/3017/fs20163017.pdf
  103. https://echa.europa.eu/substance-information/-/substanceinfo/100.001.967
  104. https://www.epa.gov/pcbs/learn-about-polychlorinated-biphenyls
  105. https://www.sciencedirect.com/science/article/abs/pii/S0045653502003740
Add your contribution
Related Hubs
User Avatar
No comments yet.