PYRIDINE

PYRIDINE

CAS Number: 110-86-1
EC Number: 203-809-9
Chemical formula: C5H5N
Systematic IUPAC name: Azabenzene
Molar mass: 79.102 g

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N.
Pyridine is structurally related to benzene, with one methine group (=CH−) replaced by a nitrogen atom.
Pyridine is a highly flammable, weakly alkaline, water-miscible liquid with a distinctive, unpleasant fish-like smell.
Pyridine is colorless, but older or impure samples can appear yellow.
The pyridine ring occurs in many important compounds, including agrochemicals, pharmaceuticals, and vitamins. Historically, pyridine was produced from coal tar.
As of 2016, it is synthesized on the scale of about 20,000 tons per year worldwide.

Physical properties
Crystal structure of pyridine
The molecular electric dipole moment is 2.2 debyes.
Pyridine is diamagnetic and has a diamagnetic susceptibility of −48.7 × 10−6 cm3·mol−1.
The standard enthalpy of formation is 100.2 kJ·mol−1 in the liquid phase and 140.4 kJ·mol−1 in the gas phase.
At 25 °C pyridine has a viscosity of 0.88 mPa/s and thermal conductivity of 0.166 W·m−1·K−1.
The enthalpy of vaporization is 35.09 kJ·mol−1 at the boiling point and normal pressure.
The enthalpy of fusion is 8.28 kJ·mol−1 at the melting point.

The critical parameters of pyridine are pressure 6.70 MPa, temperature 620 K and volume 229 cm3·mol−1.
In the temperature range 340–426 °C its vapor pressure p can be described with the Antoine equation

Structure
Pyridine ring forms a C
5N hexagon. Slight variations of the C−C and C−N distances as well as the bond angles are observed.

Crystallography
Pyridine crystallizes in an orthorhombic crystal system with space group Pna21 and lattice parameters a = 1752 pm, b = 897 pm, c = 1135 pm, and 16 formula units per unit cell (measured at 153 K).
For comparison, crystalline benzene is also orthorhombic, with space group Pbca, a = 729.2 pm, b = 947.1 pm, c = 674.2 pm (at 78 K), but the number of molecules per cell is only 4.
This difference is partly related to the lower symmetry of the individual pyridine molecule (C2v vs D6h for benzene).
A trihydrate (pyridine·3H2O) is known; it also crystallizes in an orthorhombic system in the space group Pbca, lattice parameters a = 1244 pm, b = 1783 pm, c = 679 pm and eight formula units per unit cell (measured at 223 K).[20]

Spectroscopy
The optical absorption spectrum of pyridine in hexane contains three bands at the wavelengths of 195 nm (π → π* transition, molar absorptivity ε = 7500 L·mol−1·cm−1), 251 nm (π → π* transition, ε = 2000 L·mol−1·cm−1) and 270 nm (n → π* transition, ε = 450 L·mol−1·cm−1).
The 1H nuclear magnetic resonance (NMR) spectrum of pyridine contains three signals with the integral intensity ratio of 2:1:2 that correspond to the three chemically different protons in the molecule. These signals originate from the α-protons (positions 2 and 6, chemical shift 8.5 ppm), γ-proton (position 4, 7.5 ppm) and β-protons (positions 3 and 5, 7.1 ppm).
The carbon analog of pyridine, benzene, has only one proton signal at 7.27 ppm.
The larger chemical shifts of the α- and γ-protons in comparison to benzene result from the lower electron density in the α- and γ-positions, which can be derived from the resonance structures.
The situation is rather similar for the 13C NMR spectra of pyridine and benzene: pyridine shows a triplet at δ(α-C) = 150 ppm, δ(β-C) = 124 ppm and δ(γ-C) = 136 ppm, whereas benzene has a single line at 129 ppm.
All shifts are quoted for the solvent-free substances.
Pyridine is conventionally detected by the gas chromatography and mass spectrometry methods.

Chemical properties
Because of the electronegative nitrogen in the pyridine ring, the molecule is relatively electron deficient.
It, therefore, enters less readily into electrophilic aromatic substitution reactions than benzene derivatives. Correspondingly pyridine is more prone to nucleophilic substitution, as evidenced by the ease of metalation by strong organometallic bases.
The reactivity of pyridine can be distinguished for three chemical groups.
With electrophiles, electrophilic substitution takes place where pyridine expresses aromatic properties.
With nucleophiles, pyridine reacts at positions 2 and 4 and thus behaves similar to imines and carbonyls.
The reaction with many Lewis acids results in the addition to the nitrogen atom of pyridine, which is similar to the reactivity of tertiary amines.
The ability of pyridine and its derivatives to oxidize, forming amine oxides (N-oxides), is also a feature of tertiary amines.

The nitrogen center of pyridine features a basic lone pair of electrons.
This lone pair does not overlap with the aromatic π-system ring, consequently pyridine is basic, having chemical properties similar to those of tertiary amines.
Protonation gives pyridinium, C5H5NH+.
The pKa of the conjugate acid (the pyridinium cation) is 5.25.
The structures of pyridine and pyridinium are almost identical.
The pyridinium cation is isoelectronic with benzene.
Pyridinium p-toluenesulfonate (PPTS) is an illustrative pyridinium salt; it is produced by treating pyridine with p-toluenesulfonic acid.
In addition to protonation, pyridine undergoes N-centred alkylation, acylation, and N-oxidation.

Bonding
Pyridine with its free electron pair
Pyridine has a conjugated system of six π electrons that are delocalized over the ring.
The molecule is planar and, thus, follows the Hückel criteria for aromatic systems.
In contrast to benzene, the electron density is not evenly distributed over the ring, reflecting the negative inductive effect of the nitrogen atom.
For this reason, pyridine has a dipole moment and a weaker resonant stabilization than benzene (resonance energy 117 kJ·mol−1 in pyridine vs. 150 kJ·mol−1 in benzene).

The ring atoms in the pyridine molecule are sp2-hybridized.
The nitrogen is involved in the π-bonding aromatic system using its unhybridized p orbital.
The lone pair is in an sp2 orbital, projecting outward from the ring in the same plane as the σ bonds.
As a result, the lone pair does not contribute to the aromatic system but importantly influences the chemical properties of pyridine, as it easily supports bond formation via an electrophilic attack. However, because of the separation of the lone pair from the aromatic ring system, the nitrogen atom cannot exhibit a positive mesomeric effect.

Many analogues of pyridine are known where N is replaced by other heteroatoms (see figure below). Substitution of one C–H in pyridine with a second N gives rise to the diazine heterocycles (C4H4N2), with the names pyridazine, pyrimidine, and pyrazine.

History

Thomas Anderson
Impure pyridine was undoubtedly prepared by early alchemists by heating animal bones and other organic matter, but the earliest documented reference is attributed to the Scottish scientist Thomas Anderson.
In 1849, Anderson examined the contents of the oil obtained through high-temperature heating of animal bones.
Among other substances, he separated from the oil a colorless liquid with unpleasant odor, from which he isolated pure pyridine two years later.
He described it as highly soluble in water, readily soluble in concentrated acids and salts upon heating, and only slightly soluble in oils.

Owing to its flammability, Anderson named the new substance pyridine, after Greek: πῦρ (pyr) meaning fire.
The suffix idine was added in compliance with the chemical nomenclature, as in toluidine, to indicate a cyclic compound containing a nitrogen atom.

The chemical structure of pyridine was determined decades after its discovery. Wilhelm Körner (1869) and James Dewar (1871) suggested that, in analogy between quinoline and naphthalene, the structure of pyridine is derived from benzene by substituting one C–H unit with a nitrogen atom.
The suggestion by Körner and Dewar was later confirmed in an experiment where pyridine was reduced to piperidine with sodium in ethanol.
In 1876, William Ramsay combined acetylene and hydrogen cyanide into pyridine in a red-hot iron-tube furnace.
This was the first synthesis of a heteroaromatic compound.

The first major synthesis of pyridine derivatives was described in 1881 by Arthur Rudolf Hantzsch.
The Hantzsch pyridine synthesis typically uses a 2:1:1 mixture of a β-keto acid (often acetoacetate), an aldehyde (often formaldehyde), and ammonia or its salt as the nitrogen donor.
First, a double hydrogenated pyridine is obtained, which is then oxidized to the corresponding pyridine derivative.
Emil Knoevenagel showed that asymmetrically-substituted pyridine derivatives can be produced with this process.

The contemporary methods of pyridine production had a low yield, and the increasing demand for the new compound urged to search for more efficient routes.
A breakthrough came in 1924 when the Russian chemist Aleksei Chichibabin invented a pyridine synthesis reaction, which was based on inexpensive reagents.
This method is still used for the industrial production of pyridine.

Occurrence
Pyridine is not abundant in nature, except for the leaves and roots of belladonna (Atropa belladonna) and in marshmallow (Althaea officinalis).
Pyridine derivatives, however, are often part of biomolecules such as alkaloids.

In daily life, trace amounts of pyridine are components of the volatile organic compounds that are produced in roasting and canning processes, e.g. in fried chicken, sukiyaki, roasted coffee, potato chips, and fried bacon. Traces of pyridine can be found in Beaufort cheese, vaginal secretions, black tea, saliva of those suffering from gingivitis, and sunflower honey.

Production
Historically, pyridine was extracted from coal tar or obtained as a byproduct of coal gasification. The process was labor-consuming and inefficient: coal tar contains only about 0.1% pyridine, and therefore a multi-stage purification was required, which further reduced the output. Nowadays, most pyridine is produced synthetically using various name reactions, and the major ones are discussed below.

In 1989, 26,000 tonnes of pyridine was produced worldwide.
Among the largest 25 production sites for pyridine, eleven are located in Europe (as of 1999).
The major producers of pyridine include Evonik Industries, Rütgers Chemicals, Jubilant Life Sciences, Imperial Chemical Industries, and Koei Chemical.
Pyridine production significantly increased in the early 2000s, with an annual production capacity of 30,000 tonnes in mainland China alone.
The US–Chinese joint venture Vertellus is currently the world leader in pyridine production.

Chichibabin synthesis
The Chichibabin pyridine synthesis was reported in 1924 and is still in use in industry.
In its general form, the reaction can be described as a condensation reaction of aldehydes, ketones, α,β-unsaturated carbonyl compounds, or any combination of the above, in ammonia or ammonia derivatives.
In particular, unsubstituted pyridine is produced from formaldehyde and acetaldehyde, which are inexpensive and widely available.
First, acrolein is formed in a Knoevenagel condensation from the acetaldehyde and formaldehyde. The acrolein is then condensed with acetaldehyde and ammonia to give dihydropyridine, which is oxidized with a solid-state catalyst to pyridine.
This process is carried out in a gas phase at 400–450 °C. The product consists of a mixture of pyridine, simple methylated pyridines (picolines and lutidines); its composition depends on the catalyst used and can be adapted to the needs of the manufacturer.
The catalyst is usually a transition metal salt such as cadmium(II) fluoride or manganese(II) fluoride, but cobalt and thallium compounds can also be used. The recovered pyridine is separated from byproducts in a multistage process.

Practical application of the traditional Chichibabin pyridine synthesis are limited by its consistently low yield, typically about 20%. This low yield, together with the high prevalence of byproducts, render unmodified forms of Chichibabin's method unpopular.

Dealkylation of alkylpyridines
Pyridine can be prepared by dealkylation of alkylated pyridines, which are obtained as byproducts in the syntheses of other pyridines.
The oxidative dealkylation is carried out either using air over vanadium(V) oxide catalyst, by vapor-dealkylation on nickel-based catalyst, or hydrodealkylation with a silver- or platinum-based catalyst.
Yields of pyridine up to be 93% can be achieved with the nickel-based catalyst.

Bönnemann cyclization
The trimerization of a part of a nitrile molecule and two parts of acetylene into pyridine is called Bönnemann cyclization.
This modification of the Reppe synthesis can be activated either by heat or by light.
While the thermal activation requires high pressures and temperatures, the photoinduced cycloaddition proceeds at ambient conditions with CoCp2(cod) (Cp = cyclopentadienyl, cod = 1,5-cyclooctadiene) as a catalyst, and can be performed even in water.
A series of pyridine derivatives can be produced in this way.
When using acetonitrile as the nitrile, 2-methylpyridine is obtained, which can be dealkylated to pyridine.

Other methods
The Kröhnke pyridine synthesis provides a fairly general method for generating substituted pyridines using pyridine itself as a reagent which does not become incorporated into the final product.
The reaction of pyridine with α-bromoesters give the related pyridinium salt, wherein the methylene group is highly acidic.
This species undergoes a Michael-like addition to α,β-unsaturated carbonyls in the presence of ammonium acetate to undergo ring closure and formation of the targeted substituted pyridine as well as pyridinium bromide.

Biosynthesis
Several pyridine derivatives play important roles in biological systems.
While its biosynthesis is not fully understood, nicotinic acid (vitamin B3) occurs in some bacteria, fungi, and mammals.
Mammals synthesize nicotinic acid through oxidation of the amino acid tryptophan, where an intermediate product, aniline, creates a pyridine derivative, kynurenine.
On the contrary, the bacteria Mycobacterium tuberculosis and Escherichia coli produce nicotinic acid by condensation of glyceraldehyde 3-phosphate and aspartic acid.

Reactions
Despite the structural and bonding commonalities of benzene and pyridine, their reactivity differ significantly.
Instead, in terms of its reactivity, pyridine more closely resembles nitrobenzene.

Electrophilic substitutions
Owing to the decreased electron density in the aromatic system, electrophilic substitutions are suppressed in pyridine and its derivatives.
Friedel–Crafts alkylation or acylation, usually fail for pyridine because they lead only to the addition at the nitrogen atom.
Substitutions usually occur at the 3-position, which is the most electron-rich carbon atom in the ring and is, therefore, more susceptible to an electrophilic addition.

Nucleophilic substitutions
In contrast to benzene ring, pyridine efficiently supports several nucleophilic substitutions. The reason for this is relatively lower electron density of the carbon atoms of the ring.
These reactions include substitutions with elimination of a hydride ion and elimination-additions with formation of an intermediate aryne configuration, and usually proceed at the 2- or 4-position.

Many nucleophilic substitutions occur more easily not with bare pyridine but with pyridine modified with bromine, chlorine, fluorine, or sulfonic acid fragments that then become a leaving group.
So fluorine is the best leaving group for the substitution with organolithium compounds.
The nucleophilic attack compounds may be alkoxides, thiolates, amines, and ammonia (at elevated pressures).

In general, the hydride ion is a poor leaving group and occurs only in a few heterocyclic reactions.
They include the Chichibabin reaction, which yields pyridine derivatives aminated at the 2-position. Here, sodium amide is used as the nucleophile yielding 2-aminopyridine.
The hydride ion released in this reaction combines with a proton of an available amino group, forming a hydrogen molecule.

Analogous to benzene, nucleophilic substitutions to pyridine can result in the formation of pyridyne intermediates as heteroaryne.
For this purpose, pyridine derivatives can be eliminated with good leaving groups using strong bases such as sodium and potassium tert-butoxide.
The subsequent addition of a nucleophile to the triple bond has low selectivity, and the result is a mixture of the two possible adducts.

Radical reactions
Pyridine supports a series of radical reactions, which is used in its dimerization to bipyridines. Radical dimerization of pyridine with elemental sodium or Raney nickel selectively yields 4,4'-bipyridine, or 2,2'-bipyridine, which are important precursor reagents in the chemical industry.
One of the name reactions involving free radicals is the Minisci reaction.
It can produce 2-tert-butylpyridine upon reacting pyridine with pivalic acid, silver nitrate and ammonium in sulfuric acid with a yield of 97%.

Reactions on the nitrogen atom
Lewis acids easily add to the nitrogen atom of pyridine, forming pyridinium salts.
The reaction with alkyl halides leads to alkylation of the nitrogen atom.
This creates a positive charge in the ring that increases the reactivity of pyridine to both oxidation and reduction.
The Zincke reaction is used for the selective introduction of radicals in pyridinium compounds (it has no relation to the chemical element zinc).

Hydrogenation and reduction
Piperidine is produced by hydrogenation of pyridine with a nickel-, cobalt-, or ruthenium-based catalyst at elevated temperatures.
The hydrogenation of pyridine to piperidine releases 193.8 kJ·mol−1, which is slightly less than the energy of the hydrogenation of benzene (205.3 kJ·mol−1).

Partially hydrogenated derivatives are obtained under milder conditions.
For example, reduction with lithium aluminium hydride yields a mixture of 1,4-dihydropyridine, 1,2-dihydropyridine, and 2,5-dihydropyridine.
Selective synthesis of 1,4-dihydropyridine is achieved in the presence of organometallic complexes of magnesium and zinc, and (Δ3,4)-tetrahydropyridine is obtained by electrochemical reduction of pyridine.

Lewis basicity and coordination compounds
Pyridine is a Lewis base, donating its pair of electrons to a Lewis acid.
Pyridine Lewis base properties are discussed in the ECW model.
Pyridine relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated by C-B plots.
One example is the sulfur trioxide pyridine complex (melting point 175 °C), which is a sulfation agent used to convert alcohols to sulfate esters.
Pyridine-borane (C5H5NBH3, melting point 10–11 °C) is a mild reducing agent.

Transition metal pyridine complexes are numerous.[93][94] Typical octahedral complexes have the stoichiometry MCl2(py)4 and MCl3(py)3.
Octahedral homoleptic complexes of the type M(py)6+ are rare or tend to dissociate pyridine.
Numerous square planar complexes are known, such as Crabtree's catalyst.
The pyridine ligand replaced during the reaction is restored after its completion.

The η6 coordination mode, as occurs in η6 benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center.

Applications
Pesticides
The main use of pyridine is as a precursor to the herbicides paraquat and diquat.
The first synthesis step of insecticide chlorpyrifos consists of the chlorination of pyridine. Pyridine is also the starting compound for the preparation of pyrithione-based fungicides.
Cetylpyridinium and laurylpyridinium, which can be produced from pyridine with a Zincke reaction, are used as antiseptic in oral and dental care products.
Pyridine is easily attacked by alkylating agents to give N-alkylpyridinium salts.
One example is cetylpyridinium chloride.

Solvent
Pyridine is used as a polar, basic, low-reactive solvent, for example in Knoevenagel condensations.
It is especially suitable for the dehalogenation, where it acts as the base of the elimination reaction and bonds the resulting hydrogen halide to form a pyridinium salt.
In esterifications and acylations, pyridine activates the carboxylic acid halides or anhydrides.
Even more active in these reactions are the pyridine derivatives 4-dimethylaminopyridine (DMAP) and 4-(1-pyrrolidinyl) pyridine.
Pyridine is also used as a base in condensation reactions.

Specialty reagents based on pyridine
As a base, pyridine can be used as the Karl Fischer reagent, but it is usually replaced by alternatives with a more pleasant odor, such as imidazole.

Pyridinium chlorochromate, pyridinium dichromate, and the Collins reagent (the complex of chromium(VI) oxide are used for the oxidation of alcohols.

Nomenclature
The systematic name of pyridine, within the Hantzsch–Widman nomenclature recommended by the IUPAC, is azinine.
However, systematic names for simple compounds are used very rarely; instead, heterocyclic nomenclature follows historically established common names.
IUPAC discourages the use of azinine/azine in favor of pyridine.
The numbering of the ring atoms in pyridine starts at the nitrogen (see infobox).
An allocation of positions by letter of the Greek alphabet (α-γ) and the substitution pattern nomenclature common for homoaromatic systems (ortho, meta, para) are used sometimes.
Here α (ortho), β (meta), and γ (para) refer to the 2, 3, and 4 position, respectively.
The systematic name for the pyridine derivatives is pyridinyl, wherein the position of the substituted atom is preceded by a number.
However, the historical name pyridyl is encouraged by the IUPAC and used instead of the systematic name.
The cationic derivative formed by the addition of an electrophile to the nitrogen atom is called pyridinium.

Appearance: Colorless liquid
Odor: Nauseating, fish-like
Density: 0.9819 g/mL
Melting point: −41.6 °C (−42.9 °F; 231.6 K)
Boiling point: 115.2 °C (239.4 °F; 388.3 K)
Solubility in water: Miscible
log P: 0.73
Vapor pressure: 16 mmHg (20 °C)
Basicity (pKb): 8.77
Conjugate acid: Pyridinium
Refractive index (nD): 1.5093
Viscosity: 0.88 cP 25℃
Dipole moment: 2.2 D
Flash point: 21 °C (70 °F; 294 K)
Explosive limits: 1.8–12.4%
Hydrogen Bond Donor Count: 0
Hydrogen Bond Acceptor Count:1
Rotatable Bond Count: 0
Exact Mass: 79.042199164
Monoisotopic Mass: 79.042199164
Topological Polar Surface Area: 12.9 Ų
Heavy Atom Count: 6
Formal Charge: 0
Complexity: 30.9)
Isotope Atom Count: 0
Defined Atom Stereocenter Count: 0
Undefined Atom Stereocenter Count: 0
Defined Bond Stereocenter Count: 0
Undefined Bond Stereocenter Count: 0
Covalently-Bonded Unit Count: 1
Compound Is Canonicalized: Yes

Widespread uses by professional workers
Pyridine is used in the following products: laboratory chemicals and pH regulators and water treatment products.
Pyridine is used in the following areas: scientific research and development and health services.
Other release to the environment of this substance is likely to occur from: indoor use (e.g. machine wash liquids/detergents, automotive care products, paints and coating or adhesives, fragrances and air fresheners).

Formulation or re-packing
Pyridine is used in the following products: laboratory chemicals and pH regulators and water treatment products.
Release to the environment of Pyridine can occur from industrial use: formulation of mixtures and formulation in materials.

Uses at industrial sites
Pyridine is used in the following products: pharmaceuticals, pH regulators and water treatment products, plant protection products and laboratory chemicals.
Pyridine has an industrial use resulting in manufacture of another substance (use of intermediates).
Pyridine is used in the following areas: municipal supply (e.g. electricity, steam, gas, water) and sewage treatment.
Pyridine is used for the manufacture of: chemicals and textile, leather or fur.
Release to the environment of this substance can occur from industrial use: in processing aids at industrial sites, as an intermediate step in further manufacturing of another substance (use of intermediates) and of substances in closed systems with minimal release.

Pyridine is a colorless liquid with an unpleasant smell.
Pyridine can be made from crude coal tar or from other chemicals.
Pyridine is used to dissolve other substances.
Pyridine is also used to make many different products such as medicines, vitamins, food flavorings, paints, dyes, rubber products, adhesives, insecticides, and herbicides.
Pyridine can also be formed from the breakdown of many natural materials in the environment.

Pyridine is an azaarene comprising a benzene core in which one -CH group is replaced by a nitrogen atom.
Pyridine is the parent compound of the class pyridines.
Pyridine has a role as an environmental contaminant.
Pyridine is a mancude organic heteromonocyclic parent, a monocyclic heteroarene, an azaarene and a member of pyridines.

Pyridine, any of a class of organic compounds of the aromatic heterocyclic series characterized by a six-membered ring structure composed of five carbon atoms and one nitrogen atom.
The simplest member of the pyridine family is pyridine itself, a compound with molecular formula C5H5N.

Pyridine is used as a solvent and is added to ethyl alcohol to make it unfit for drinking.
Pyridine is converted to such products as sulfapyridine, a drug active against bacterial and viral infections; pyribenzamine and pyrilamine, used as antihistaminic drugs; piperidine, used in rubber processing and as a chemical raw material; and water repellents, bactericides, and herbicides.
Compounds not made from pyridine but containing its ring structure include niacin and pyridoxal, both B vitamins; isoniazid, an antitubercular drug; and nicotine and several other nitrogenous plant products.

Pyridine occurs in coal tar, its principal source before development of a synthesis based on acetaldehyde and ammonia. The pure substance is a colourless, flammable, weakly alkaline, water-soluble liquid with an unpleasant odour; it boils at 115.5° C 

Pyridine is a sequencing-grade preparation with high purity and consistent absorptivity properties for use in HPLC and mass spectrometry methods.

Features of Pyridine:
Water-miscible organic solvent for a variety of molecular and protein biology methods
High purity, low water, and no peroxides or aldehydes
Ninhydrin test ensures low amines
UV-absorption test confirms suitability for HPLC detection
Liquid reagent packaged by mass in amber glass bottles

Pierce pyridine is specially purified and each lot is tested to the highest specifications to ensure the integrity of your data, maximize sensitivity in your assay and to prolong the life of your equipment.

Pyridine is a clear liquid with an odor that is sour, putrid, and fish-like.
Pyridine is a relatively simple heterocyclic aromatic organic compound that is structurally related to benzene, with one CH group in the six-membered ring replaced by a nitrogen atom.
Pyridine is obtained from crude coal tar or is synthesized from acetaldehyde, formaldehyde and ammonia.
Pyridine is often used as a denaturant for antifreeze mixtures, for ethyl alcohol, for fungicides, and as a dyeing aid for textiles.
Pyridine is a harmful substance if inhaled, ingested or absorbed through the skin.
In particular, it is known to reduce male fertility and is considered carcinogenic.
Common symptoms of acute exposure to pyridine include: headache, coughing, asthmatic breathing, laryngitis, nausea and vomiting.

Belongs to the class of organic compounds known as pyridines and derivatives.
Pyridines and derivatives are compounds containing a pyridine ring, which is a six-member aromatic heterocycle which consists of one nitrogen atom and five carbon atoms.

Although pyridine is used in many laboratories and chemical industries as a solvent for anhydrous mineral salts, in synthesis of organic compounds and in analytic procedures, only a few instances of its toxic effect on man have been recorded.

The unfortunate outcome of an experiment in the treatment of epilepsy by the use of pyridine afforded us an opportunity to observe the development of its toxic effect.
Since the effect of pyridine on the human organism has been incompletely studied and knowledge concerning its action is sketchy, we concluded that our observations were worthy of description.

In searching for an anticonvulsant which does not produce a narcotic side effect, we studied the effect of pyridine on convulsions produced by the intravenous injection into rabbits of minimal convulsant doses of metrazol.

Expanded porphyrins have received considerable attention due to their unique optical, electrochemical and coordination properties.
Here, we report benzene- and pyridine-incorporated octaphyrins(1.1.0.0.1.1.0.0), which are synthesized through Suzuki-Miyaura coupling of α,α′-diboryltripyrrane with m-dibromobenzene and 2,6-dibromopyridine, respectively, and subsequent oxidation with 2,3-dicyano-5,6-dichlorobenzoquinone. Both octaphyrins are nonaromatic and take on dumbbell structures.
Upon treatment with Pd(OOCCH3)2, the benzene-incorporated one gives a Ci symmetric NNNC coordinated bis-PdII complex but the pyridine incorporated one gives Ci and Cs symmetric NNNC coordinated bis-PdII complexes along with an NNNN coordinated bis-PdII complex bearing a transannular C–C bond between the pyrrole α-positions.
In addition, these two pyridine-containing NNNC PdII complexes undergo trifluoroacetic acid-induced clean interconversion.

Pyridine is a colorless liquid with a foul odor and several hazardous properties.
In the late 1840s, physician/chemist Thomas Anderson at the University of Edinburgh produced several liquids by heating animal bones to high temperatures.
One of these turned out to be pyridine, which he named after the Greek word pyr (fire).

About 20 years later, chemists Wilhelm (aka Guglielmo) Körner at the University of Milan (Italy) and James Dewar at the University of Cambridge (UK), working separately, elucidated the structure of pyridine.
The two chemists were friends, but they disagreed about which of them was the first to conceive of the structure.
An interesting account of this episode is given by Alan J. Rocke at Case Western Reserve University (Cleveland) in a 1988 article.

In 1881, German chemist Arthur Rudolf Hantzsch at the University of Leipzig (Germany) developed a cumbersome, low-yielding synthesis of pyridine.
Later, in 1924, Russian chemist Aleksei Chichibabin came up with an improved method, which is still in use today: a reaction between formaldehyde, acetaldehyde, and ammonia over a transition-metal fluoride catalyst to give dihydropyridine, followed by high-temperature catalytic oxidation to pyridine.
Several other methods are also used, including the oxidative dealkylation of alkylpyridines.

Pyridine’s structure is isoelectronic with that of benzene, but its properties are quite different. Pyridine is completely miscible with water, whereas benzene is only slightly soluble.
Like all hydrocarbons, benzene is neutral (in the acid–base sense), but because of its nitrogen atom, pyridine is a weak base.

In industry and in the lab, pyridine is used as a reaction solvent, particularly when its basicity is useful, and as a starting material for synthesizing some herbicides, fungicides, and antiseptics.
Current worldwide pyridine production is ≈20,000 t/year, valued at about US$600 million.

The uses of Pyridine are:
In the chemical industries as an important raw material.
As an antiseptic in dental care products.
As a solvent that is suitable for dehalogenation.
In pharmaceuticals.
For antifreeze mixtures as a denaturant.
As a sulfonating agent.
As a reducing agent.
In dyes and paints.
As a disinfectant.
In coordination chemistry as a ligand.

Synonyms:
Azabenzene
110-86-1
Azine
Pyridin
Piridina
Pirydyna
RCRA waste number U196
NCI-C55301
CCRIS 2926
HSDB 118
pyridine-ring
pyridine-
CHEBI:16227
CP 32
UNII-NH9L3PP67S
EINECS 203-809-9
UN1282
NSC 406123
Pyridine, homopolymer
MFCD00011732
NH9L3PP67S
pyridine fraction
Pyridine [UN1282] [Flammable liquid]
Pyridine, 99+%, extra pure
Pyridine, 99+%, for analysis
Caswell No. 717
Pyridine, 99+%, ACS reagent
Pyridine, 99+%, for spectroscopy
FEMA Number 2966
Pyridine, ACS reagent, >=99.0%
Pyridine, ReagentPlus(R), >=99%
25013-01-8
FEMA No. 2966
Pyridine, 99.5%, Extra Dry, AcroSeal(R)
Pyridine, 99.8%, for biochemistry, AcroSeal(R)
RCRA waste no. U196
EPA Pesticide Chemical Code 069202
Pyridine, 99+%, extra pure, nonaqueous titration grade
hydropyridine
polypyridine
Pyridine, 99.5%, Extra Dry over Molecular Sieve, AcroSeal(R)
AI3-01240
N-pyridine
pyridine solution
[pyridine]
2-pyridine
4-pyridine
3-Pyridine
Pyridine anhydrous
Azabenzene; Azine
0PY
Pyridine, anhydrous
Pyridine, for HPLC
Pyridine, 99%
Pyridine, ACS reagent
Pyridine, >=99%
Pyridine, HPLC Grade
DSSTox_CID_1924
1-Hydro-3-dehydropyridine
bmse000432
Epitope ID:140099
WLN: T6NJ
EC 203-809-9
Pyridine, p.a., 99%
DSSTox_RID_76405
NCIOpen2_002809
NCIOpen2_007786
NCIOpen2_007866
NCIOpen2_007986
NCIOpen2_007999
DSSTox_GSID_21924
Pyridine, LR, >=99%
Pyridine, analytical standard
ACMC-20a483
Pyridine, anhydrous, 99.8%
CHEMBL266158
DTXSID9021924
Pyridine, AR, >=99.5%
Pyridine, >=99.5% (GC)
ZINC895354
BCP23599
Pyridine, for HPLC, >=99.9%
Tox21_200960
ANW-41400
ANW-56401
BDBM50176909
NSC141574
NSC406123
STL264195
1,4-Dihydropyridine-4-ylideneradical
AKOS000120998
MCULE-4028120692
NSC-141574
NSC-406123
Pyridine, biotech. grade, >=99.9%
UN 1282
NCGC00091476-01
NCGC00091476-02
NCGC00258513-01
Pyridine, SAJ first grade, >=99.0%
BP-13452
CAS-110-86-1
NCI60_006101
Pyridine, JIS special grade, >=99.5%
Pyridine, p.a., ACS reagent, 99.0%
Pyridine, purification grade, >=99.75%
Pyridine, spectrophotometric grade, >=99%
FT-0654216
FT-0658212
FT-0658683
Q0034
X4374
C00747
A802257
Q210385
J-002482
F0001-0227
Pyridine, puriss. p.a., ACS reagent, >=99.8% (GC)
Pyridine, suitable for hydroxyl value determination, >=99.5%
Pyridine, Pharmaceutical Secondary Standard; Certified Reference Material
Pyridine, puriss. p.a., ACS reagent, reag. Ph. Eur., >=99.5% (GC)
Pyridine, puriss., absolute, over molecular sieve (H2O <=0.005%), >=99.8% (GC)
Pyridine, puriss., Reag. Ph. Eur., dried, >=99.5% (GC), <=0.0075% water