2,2'-Bipyridine (366-18-7) Physical and Chemical Properties
2,2'-Bipyridine
A bidentate nitrogen-containing ligand frequently used as an analytical reagent and coordination ligand in synthetic, materials and analytical chemistry workflows.
| CAS Number | 366-18-7 |
| Family | Pyridines |
| Typical Form | White crystalline solid |
| Common Grades | ACS Grade, BP, EP, JP, Reagent Grade, USP |
2,2'-Bipyridine is an aromatic nitrogen heterocycle of the bipyridine structural class consisting of two pyridine rings joined by a single bond at their 2-positions. The connectivity produces a ligand capable of chelation through the two ring nitrogen atoms; in its neutral form the molecule is essentially neutral and behaves as a bidentate N,N-donor that can adopt near-planar or mildly twisted conformations depending on steric and coordination context. Electronic structure is dominated by the pyridyl π-systems with two basic lone pairs on the ring nitrogens; conjugation across the inter-ring bond leads to modest electron delocalization and a low overall polar surface area for a bidentate heterocycle.
Acid–base and solvation behaviour reflect pyridine-like basicity: the conjugate acid has a reported \(\mathrm{p}K_a\) of 4.33, indicating that under neutral to basic conditions the free ligand will be largely unprotonated and available for metal coordination. The compound has modest lipophilicity (reported experimental log \(K_{ow}\) = 1.50; computed XLogP3 = 1.7) and a topological polar surface area of 25.8, consistent with moderate aqueous solubility but good solubility in common organic solvents (alcohols, ethers, aromatic hydrocarbons). It lacks hydrolyzable functional groups and is not prone to hydrolysis; degradation in the environment or in solution proceeds mainly by slow oxidation (reaction with photochemically generated hydroxyl radicals) rather than by hydrolysis.
2,2'-Bipyridine is widely used industrially and in research as a metal-chelating ligand and analytical reagent. It functions as an iron(II) chelator and a colorimetric indicator/reagent for photometric determination of several metal ions, and it is an important intermediate in specialty chemical manufacture. Common commercial grades reported for this substance include: ACS Grade, BP, EP, JP, Reagent Grade, USP.
Basic Physical Properties
Density
No experimentally established value for this property is available in the current data context.
Melting Point
Reported melting point values: \(\,72\,^\circ\mathrm{C}\) and \(\,70\,^\circ\mathrm{C}\). These are consistent with a crystalline white solid that sublimes or melts in the \(70\)–\(72\,^\circ\mathrm{C}\) range depending on sample history and purity.
Boiling Point
Reported boiling points: \(\,273.5\,^\circ\mathrm{C}\) and \(\,272\)–\(\,273\,^\circ\mathrm{C}\). The high boiling point reflects the aromatic, conjugated structure and low volatility relative to low-molecular-weight solvents.
Vapor Pressure
Reported vapor pressure (experimental): 0.000013 \(\mathrm{mmHg}\). An additional atmospheric partitioning estimate cited in physicochemical summaries corresponds to \(1.3\times 10^{-5}\) \(\mathrm{mmHg}\), indicating that the compound will partition between vapor and particulate phases under ambient conditions but has low volatility.
Flash Point
Reported flash point: \(\,121\,^\circ\mathrm{C}\). The flash point, together with the low vapor pressure, implies limited flammability in bulk form but combustible behaviour at elevated temperatures or when dispersed as fine particles.
Chemical Properties
Solubility and Phase Behavior
Experimental solubility notes indicate the substance is a white crystalline solid that is "very soluble" in alcohol, ether, and benzene and "soluble in oxygenated solvents." Reported water solubility values appear as: In water, 5.93X10+3 \(\mathrm{mg}\,\mathrm{L}^{-1}\) at \(\,25\,^\circ\mathrm{C}\), and as "Solubility in water, g/100ml: 6.4". These nominal values indicate appreciable aqueous solubility for an aromatic heterocycle, consistent with modest lipophilicity (log \(K_{ow}\) ≈ 1.50–1.7) and the presence of two hydrogen-bond-accepting pyridyl nitrogens (H-bond acceptor count = 2, donor count = 0). In practical terms the ligand is readily dissolved in common polar organic solvents and can be dissolved in water to concentrations useful for analytical and coordination chemistry applications; solvent choice will influence the degree of aggregation, protonation state, and coordination behaviour.
Phase behaviour: crystalline solid at ambient conditions (white crystals reported, also described as white or yellow crystalline solid), melts in the low 70s \(\,^\circ\mathrm{C}\), and boils above \(270\,^\circ\mathrm{C}\). Finely divided material presents dust and dust-explosion considerations when dispersed in air.
Reactivity and Stability
2,2'-Bipyridine is chemically stable toward hydrolysis due to absence of hydrolyzable substituents; it undergoes slow oxidative degradation (e.g., by photochemically produced hydroxyl radicals in aqueous solution; aqueous half-life on the order of months has been reported under radical-rich conditions). On heating to decomposition it emits toxic nitrogen oxide species. It is combustible and should be kept away from strong oxidants; drying and powder handling increase the risk of dust dispersion and potential dust explosion. As a bidentate N,N-donor ligand it forms stable coordination complexes with many transition metals (e.g., bis- and tris-chelate species), and metal binding can markedly change redox properties and photophysical behaviour.
Acid–base reactivity: reported \(\mathrm{p}K_a\) of the conjugate acid is 4.33, reflecting typical pyridine-level basicity; protonation occurs at pyridyl nitrogens and suppresses metal binding until deprotonation or exchange occurs.
Thermodynamic Data
Standard Enthalpies and Heat Capacity
No experimentally established value for this property is available in the current data context.
Molecular Parameters
Molecular Weight and Formula
Molecular formula: C10H8N2.
Molecular weight: 156.18.
Additional computed molecular identifiers: Exact mass / monoisotopic mass = 156.068748264; heavy atom count = 12; formal charge = 0; complexity = 120.
LogP and Polarity
Reported partition coefficients: experimental log \(K_{ow}\) = 1.50 (reported as "log Kow= 1.50"); computed XLogP3 = 1.7. Topological polar surface area (TPSA) = 25.8. Hydrogen-bond donor count = 0; hydrogen-bond acceptor count = 2. These values indicate moderate lipophilicity with sufficient polarity for measurable aqueous solubility and good compatibility with polar organic media.
Structural Features
2,2'-Bipyridine is a symmetric bidentate ligand formed by two pyridine rings joined at their 2-positions. The N atoms at each ring provide lone pairs for coordination; chelation typically produces five-membered metal chelate rings. Conformational flexibility around the inter-ring bond allows near-planar or twisted arrangements, which influence crystal packing and coordination geometry in complexes. Extensive crystallographic data are available for the substance (CCDC number cited in structural records: 177724), and numerous crystal structure entries have been reported for free ligand and metal complexes. The SMILES, InChI and InChIKey are provided below in the Identifiers section for unambiguous structural representation.
Identifiers and Synonyms
Registry Numbers and Codes
CAS: 366-18-7
EC (EINECS) number: 206-674-4
UNII: 551W113ZEP
NSC Number: 615009 (also reported as 1550 in some records)
ChEBI: CHEBI:30351
ChEMBL: CHEMBL39879
SMILES: C1=CC=NC(=C1)C2=CC=CC=N2
InChI: InChI=1S/C10H8N2/c1-3-7-11-9(5-1)10-6-2-4-8-12-10/h1-8H
InChIKey: ROFVEXUMMXZLPA-UHFFFAOYSA-N
Synonyms and Structural Names
Common synonyms reported in nomenclature records include: 2,2'-Dipyridyl; 2,2'-Bipyridyl; 2,2-Bipyridine; 2-(2-Pyridyl)pyridine; alpha,alpha-Dipyridyl; Bipyridyl; Bipy; 2-pyridin-2-ylpyridine. Analytical and reagent grade descriptions (e.g., "2,2'-Bipyridine, ACS") are commonly used in supplier and specification contexts.
Industrial and Commercial Applications
Representative Uses and Industry Sectors
2,2'-Bipyridine is employed as an iron(II) chelator and as an indicator/reagent for the photometric determination of metals such as Ag, Cd, Cu, Fe, and Mo. It serves as an intermediate in the manufacture of certain herbicide derivatives (for example, in production routes related to diquat). It is a widely used coordination ligand in homogeneous catalysis, synthetic inorganic chemistry, materials synthesis, and in research applications where tunable metal–ligand assemblies are required.
Occupational exposure can occur in manufacturing and handling of reagent or intermediate; use sectors include basic organic chemical manufacturing and specialty chemical production.
Role in Synthesis or Formulations
Functionally the compound is a bidentate ligand used to form stable metal complexes (e.g., tris(bipyridine) complexes) that are exploited for redox chemistry, photophysical studies, and as precursors to heterogeneous catalysts after immobilization. In analytical chemistry it is used as a colorimetric reagent for iron and other transition-metal assays. In formulations it is used primarily as a reagent or intermediate rather than as an end-use commodity additive.
Safety and Handling Overview
Acute and Occupational Toxicity
Toxicity data indicate acute toxicity by oral and dermal routes. Reported non-human toxicity values include LD50 (rat, oral) values such as \(256\,\mathrm{mg}\,\mathrm{kg}^{-1}\) and \(100\,\mathrm{mg}\,\mathrm{kg}^{-1}\) in separate reports, LD50 (rat, sc) \(155\,\mathrm{mg}\,\mathrm{kg}^{-1}\), and LD50 (rat, ip) \(150\,\mathrm{mg}\,\mathrm{kg}^{-1}\). Health hazard classifications reported in hazard summaries include acute toxicity (oral and dermal), eye irritation, and specific target-organ toxicity on single or repeated exposure; adverse effects cited include irritation of skin, eyes and respiratory tract, central nervous system effects and potential hepatotoxicity in occupational settings. Animal data indicate neurobehavioral effects at higher doses and some reports of mutagenicity in bacterial assays.
First-aid and acute measures summarized from safety guidance include: move exposed persons to fresh air for inhalation exposure; flush skin with water and soap for dermal exposure; irrigate eyes with water for several minutes for ocular exposure; do not induce emesis for ingestion and seek medical attention. In emergency care, supportive measures (oxygen, monitoring for pulmonary edema, seizure control) are described for severe exposures.
Storage and Handling Considerations
Store separated from strong oxidants and sources of ignition. Avoid generation and dispersion of dust; finely divided material can form explosive mixtures with air. Use dust-control measures and appropriate ventilation; avoid inhalation and contact with skin and eyes. Recommended personal protective equipment generally includes chemical-resistant gloves, eye protection, and suitable respiratory protection where airborne concentrations may exceed occupational limits or where dust may be generated. In fire situations, combustion can produce irritating or toxic nitrogen oxides; recommended extinguishing media include water spray, alcohol-resistant foam, dry powder, or carbon dioxide.
For spill response: use particulate respirator if dust is generated, sweep into covered containers, dampen to minimize dust if appropriate, and dispose of according to local environmental regulations. For detailed hazard, transport and regulatory information and for product-specific emergency procedures consult the product Safety Data Sheet (SDS) and applicable local legislation.
