Pyridine

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Pyridine

General
Systematic name	Pyridine
Other names	Azabenzene Azine py
Molecular formula	C₅H₅N
SMILES	C1=NC=CC=C1
Molecular Weight	79.10 g/mol
Appearance	colourless liquid
CAS number	[110-86-1]
Properties
Density and phase	0.9819 g/cm³, liquid
Solubility in water	Miscible
Solubility in ethanol	Miscible
Melting point	−41.6 °C
Boiling point	115.2 °C
Viscosity	0.94 cP at 20 °C
Thermodynamic data
Standard enthalpy of formation Δ_fH°_liquid	101.2 kJ/mol
Standard enthalpy of combustion Δ_cH°_liquid	−2783.2 kJ/mol
Standard molar entropy S°_liquid	? J.K⁻¹.mol⁻¹
Hazards
EU classification	Flammable (F) Harmful (Xn)
NFPA 704	3 2 0
R-phrases	R11, R20, R21, R22
S-phrases	S2, S26, S28
Flash point	20 °C
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Regulatory data	Flash point, RTECS number, etc.
Related compounds
Related amines	Picoline Quinoline
Related compounds	Aniline Pyrimidine
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Pyridine is a chemical compound with the formula C₅ H₅ N. It is a liquid with a distinctively putrid odour. Pyridine is a simple and fundamentally important heterocyclic aromatic organic compound that is structurally related to benzene, wherein one CH group in the six-membered ring replaced by a nitrogen atom. The pyridine ring occurs in many important compounds, includine the nicotinamides.

1 Basicity
2 Pyridine as a solvent
3 Role in chemical synthesis
4 Preparation and occurrence
5 Safety and Environmental
6 Related compounds
7 See also
8 References
9 External links

[edit] Basicity

Pyridine has an equatorial lone pair of electrons at the nitrogen atom. Because this lone pair is not delocalized into the aromatic pi-system, pyridine is basic with chemical properties similar to tertiary amines. The pKa of the conjugate acid is 5.30. Pyridine is protonated by reaction with acids and forms a positively charged aromatic polyatomic ion called pyridinium cation. The bond lengths and bond angles in pyridine and the pyridinium ion are almost identical.^[1] because protonation does not affect the aromatic pi system.

[edit] Pyridine as a solvent

Pyridine is widely used as a versatile solvent being polar but aprotic. It is fully miscible with a very broad range of solvents including hexane and water. The deuterated pyridine, so called pyridine-d₅ is a common solvent for ¹H NMR spectroscopy.

[edit] Role in chemical synthesis

Pyridine is important in industrial organic chemistry as both a fundamental building block and as a solvent and reagent in organic synthesis.^[2] It is used as a solvent in Knoevenagel condensations.

Pyridine-borane, C₅H₅NBH₃ (m.p. 10–11 °C) is a mild reducing agent with improved stability vs NaBH₄ in protic solvents and improved solubility in aprotic organic solvents. Pyridine-sulfur trioxide, C₅H₅NSO₃ (mp 175 °C) is a sulfonation agent used to convert alcohols to sulfonates, which in turn undergo C-O bond scission upon reduction with hydride agents.

It is also a starting material in the synthesis of compounds used as an intermediate in making insecticides, herbicides, pharmaceuticals, food flavorings, dyes, rubber chemicals, adhesives, paints, explosives and disinfectants. Pyridine is also used as a denaturant for antifreeze mixtures, for ethyl alcohol, and for fungicides, and as a dyeing aid for textiles.

Pyridine is a good nucleophile with a donor number of 33.1. It is easily attacked by alkylating agents to give N-alkylpyridinium salts.

[edit] Preparation and occurrence

See also: Hantzsch pyridine synthesis

Pyridine is obtained from crude coal tar or is synthesized from acetaldehyde, formaldehyde and ammonia. Other examples of the pyridine class can be formed by the reaction of 1,5-diketones with ammonium acetate in acetic acid followed by oxidation. Pyridium salts can be obtained in the Zincke reaction.

[edit] Safety and Environmental

Pyridine is toxic with LD₅₀ in rats (oral) of 891 mg kg^–1. It is volatile and can be absorbed through skin. Available data indicates that "exposure to pyridine in drinking-water led to reduction of sperm motility at all dose levels in mice and increased estrous cycle length at the highest dose level in rats". ^[3] Currently its evaluations as a possible carcinogenic agent showed there is inadequate evidence in humans for the carcinogenicity of pyridine, albeit there is limited evidence of carcinogenic effects on animals. ^[3] Effects of an acute pyridine intoxication dizziness, headache, nausea and anorexia. Further symptoms include abdominal pain and pulmonary congestion. ^[3] Though resistant to oxidation, pyridine is readily degraded by bacteria, releasing ammonium and carbon dioxide as terminal degradation products. ^[4]

[edit] Related compounds

Structurally or chemically related compounds are

DMAP is short for 4-dimethylaminopyridine
Bipyridine and viologen are simple polypyridine compounds consisting of two pyridine molecules joined by a single bond
Terpyridine, a molecule of three pyridine rings connected together by two single bonds.
Quinoline and Isoquinoline have pyridine and a benzene ring fused together.
Aniline is a benzene derivative with an attached NH₂ group and NOT a pyridine
Diazines are compounds with one more carbon replaced by nitrogen such as Pyrazine and Pyramidine
Triazines are compounds with two more carbons replaced by nitrogen and a tetrazine has four nitrogen atoms
2,6-Lutidine is a trivial name for 2,6-dimethylpyridine.
Collidine is the trivial name for 2,4,6-trimethylpyridine.
Pyridinium p-toluenesulfonate (PPTS) is a salt formed by proton exchange between pyridine and p-toluenesulfonic acid
2-Chloropyridine is a toxic environmentally significant component of the breakdown of the pesticide imidacloprid.

[edit] See also

Simple aromatic rings
6-Membered aromatic rings with one carbon replaced by another group: borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine, pyrylium salt

[edit] References

^ T. M. Krygowski, H. Szatyowicz, and J. E. Zachara J. Org. Chem. 2005 70(22) 8859 - 8865; DOI:10.1021/jo051354h.
^ Sherman, A. R. “Pyridine” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI:10.1002/047084289.
^ ^a ^b ^c International Agency for Research on Cancer (IARC) (2000-08-22). Pyridine Summary & Evaluation (HTML). IARC Summaries & Evaluations. IPCS INCHEM. Retrieved on 2007-01-17.
^ Sims, G.K. and O'Loughlin, E.J. Degradation of pyridines in the environment. CRC Critical Reviews in Environmental Control. (1989) 19(4):309-340.