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| Names | |||
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| Preferred IUPAC name Pyridine[1] | |||
| Systematic IUPAC name Azabenzene | |||
| Other names Azine Azinine | |||
| Identifiers | |||
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3D model (JSmol) | |||
| ChEBI | |||
| ChEMBL | |||
| ChemSpider |
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| ECHA InfoCard | 100.003.464 | ||
| EC Number |
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| KEGG |
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| UNII | |||
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| Properties | |||
| C5H5N | |||
| Molar mass | 79.102 g·mol−1 | ||
| Appearance | Colorless liquid[2] | ||
| Odor | Nauseating, fish-like[3] | ||
| Density | 0.9819 g/mL (20 °C)[4] | ||
| Melting point | −41.63 °C (−42.93 °F; 231.52 K)[4] | ||
| Boiling point | 115.2 °C (239.4 °F; 388.3 K)[4] | ||
| Miscible[4] | |||
| logP | 0.65[5] | ||
| Vapor pressure | 16 mmHg (20 °C)[3] | ||
| Acidity (pKa) | 5.23 (pyridinium)[6] | ||
| Conjugate acid | Pyridinium | ||
| −48.7·10−6 cm3/mol[7] | |||
| Thermal conductivity | 0.166 W/(m·K)[8] | ||
Refractive index (nD) | 1.5095 (20 °C)[4] | ||
| Viscosity | 0.879 cP (25 °C)[9] | ||
| 2.215 D[10] | |||
| Thermochemistry[11] | |||
| 132.7 J/(mol·K) | |||
Std enthalpy of formation(ΔfH⦵298) | 100.2 kJ/mol | ||
Std enthalpy of combustion(ΔcH⦵298) | −2.782 MJ/mol | ||
| Hazards[15] | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | Low to moderate hazard[13] | ||
| GHS labelling: | |||
  [12] | |||
| Danger | |||
| H225,H302,H312,H315,H319,H332[12] | |||
| P210,P280,P301+P312,P303+P361+P353,P304+P340+P312,P305+P351+P338[12] | |||
| NFPA 704 (fire diamond) | |||
| Flash point | 20 °C (68 °F; 293 K)[16] | ||
| 482 °C (900 °F; 755 K)[16] | |||
| Explosive limits | 1.8–12.4%[3] | ||
Threshold limit value (TLV) | 5 ppm (TWA) | ||
| Lethal dose or concentration (LD, LC): | |||
LD50 (median dose) | 891 mg/kg (rat, oral) 1500 mg/kg (mouse, oral) 1580 mg/kg (rat, oral)[14] | ||
LC50 (median concentration) | 9000 ppm (rat, 1 hr)[14] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) | TWA 5 ppm (15 mg/m3)[3] | ||
REL (Recommended) | TWA 5 ppm (15 mg/m3)[3] | ||
IDLH (Immediate danger) | 1000 ppm[3] | ||
| Related compounds | |||
Relatedamines | Picoline Quinoline | ||
Related compounds | Aniline Pyrimidine Piperidine | ||
| Supplementary data page | |||
| Pyridine (data page) | |||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Pyridine is abasicheterocyclicorganic compound with thechemical formulaC5H5N. It is structurally related tobenzene, with onemethine group(=CH−) replaced by anitrogen atom(=N−). It is a highly flammable, weaklyalkaline, 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 commercial compounds, includingagrochemicals,pharmaceuticals, andvitamins. Historically, pyridine was produced fromcoal tar. As of 2016, it is synthesized on the scale of about 20,000 tons per year worldwide.[2]
Pyridine isdiamagnetic. Itscritical parameters are: pressure 5.63 MPa, temperature 619 K and volume 248 cm3/mol.[18] In the temperature range 340–426 K its vapor pressurep can be described with theAntoine equation
whereT is temperature,A = 4.16272,B = 1371.358 K andC = −58.496 K.[19]
Pyridine ring forms aC5N hexagon. Slight variations of theC−C andC−N distances as well as the bond angles are observed.
Pyridine crystallizes in anorthorhombic crystal system withspace groupPna21 andlattice parametersa = 1752 pm,b = 897 pm,c = 1135 pm, and 16formula units perunit cell (measured at 153 K). For comparison, crystallinebenzene is also orthorhombic, with space groupPbca,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.[17] This difference is partly related to the lowersymmetry 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 groupPbca, lattice parametersa = 1244 pm,b = 1783 pm,c = 679 pm and eight formula units per unit cell (measured at 223 K).[20]
The opticalabsorption spectrum of pyridine inhexane consists of bands at thewavelengths of 195, 251, and 270 nm. With respective extinction coefficients (ε) of 7500, 2000, and 450 L·mol−1·cm−1, these bands are assigned to π → π*, π → π*, and n → π* transitions. The compound displays very lowfluorescence.[21]
The1Hnuclear magnetic resonance (NMR) spectrum shows signals for α-(δ 8.5), γ-(δ7.5) and β-protons (δ7). By contrast, the proton signal for benzene is found at δ7.27. 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 the13C NMR spectra of pyridine and benzene: pyridine shows a triplet atδ(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.[22] Pyridine is conventionally detected by thegas chromatography andmass spectrometry methods.[23]
Pyridine has aconjugated system of sixπ electrons that are delocalized over the ring. The molecule is planar and, thus, follows theHückel criteria for aromatic systems. In contrast to benzene, theelectron density is not evenly distributed over the ring, reflecting the negativeinductive effect of the nitrogen atom. For this reason, pyridine has a dipole moment and a weakerresonant stabilization than benzene (resonance energy 117 kJ/mol in pyridine vs. 150 kJ/mol in benzene).[24]
The ring atoms in the pyridine molecule aresp2-hybridized. The nitrogen is involved in the π-bonding aromatic system using its unhybridized p orbital. Thelone pair is in an sp2 orbital, projecting outward from the ring in the same plane as theσ 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.[25] However, because of the separation of the lone pair from the aromatic ring system, the nitrogen atom cannot exhibit a positivemesomeric effect.
Many analogues of pyridine are known where N is replaced by other heteroatoms from the same column of thePeriodic Table of Elements (see figure below). Substitution of one C–H in pyridine with a second N gives rise to thediazine heterocycles (C4H4N2), with the namespyridazine,pyrimidine, andpyrazine.
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Impure pyridine was undoubtedly prepared by earlyalchemists by heating animal bones and other organic matter,[26] but the earliest documented reference is attributed to the Scottish scientistThomas Anderson.[27][28] In 1849, Anderson examined the contents of theoil obtained through high-temperature heating of animal bones.[28] 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 substancepyridine, afterGreek:πῦρ (pyr) meaningfire. The suffixidine was added in compliance with the chemical nomenclature, as intoluidine, to indicate acyclic compound containing a nitrogen atom.[29][30]
The chemical structure of pyridine was determined decades after its discovery.Wilhelm Körner (1869)[31] andJames Dewar (1871)[32][33] suggested that, in analogy betweenquinoline andnaphthalene, the structure of pyridine is derived frombenzene by substituting one C–H unit with a nitrogen atom.[34][35] The suggestion by Körner and Dewar was later confirmed in an experiment where pyridine was reduced topiperidine withsodium inethanol.[36][37] In 1876,William Ramsay combinedacetylene andhydrogen cyanide into pyridine in ared-hot iron-tube furnace.[38] This was the first synthesis of a heteroaromatic compound.[23][39]
The first major synthesis of pyridine derivatives was described in 1881 byArthur Rudolf Hantzsch.[40] TheHantzsch pyridine synthesis typically uses a 2:1:1 mixture of a β-keto acid (oftenacetoacetate), analdehyde (oftenformaldehyde), andammonia or its salt as the nitrogen donor. First, a doublehydrogenated 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.[41]
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 chemistAleksei Chichibabin invented apyridine synthesis reaction, which was based on inexpensive reagents.[42] This method is still used for the industrial production of pyridine.[2]
Pyridine is not abundant in nature, except for the leaves and roots of belladonna (Atropa belladonna)[43] and in marshmallow (Althaea officinalis).[44] Pyridine derivatives, however, are often part of biomolecules such asalkaloids.
In daily life, trace amounts of pyridine are components of thevolatile organic compounds that are produced in roasting andcanning processes, e.g. in fried chicken,[45]sukiyaki,[46] roasted coffee,[47] potato chips,[48] and friedbacon.[49] Traces of pyridine can be found inBeaufort cheese,[50]vaginal secretions,[51]black tea,[52] saliva of those suffering fromgingivitis,[53] andsunflower honey.[54]
Trace amounts of up to 16 μg/m3 have been detected in tobacco smoke.[23] Minor amounts of pyridine are released into environment from some industrial processes such as steel manufacture,[55] processing ofoil shale,coal gasification,coking plants andincinerators.[23] The atmosphere at oil shale processing plants can contain pyridine concentrations of up to 13 μg/m3,[56] and 53 μg/m3 levels were measured in thegroundwater in the vicinity of a coal gasification plant.[57] According to a study by the USNational Institute for Occupational Safety and Health, about 43,000 Americans work in contact with pyridine.[58]
Pyridine has historically been added to foods to give them a bitter flavour, although this practise is now banned in the U.S.[59][60] It may still be added toethanol to make it unsuitable for drinking.[61]
Historically, pyridine was extracted fromcoal tar or obtained as a byproduct of coalgasification. The process is labor-consuming and inefficient:coal tar contains only about 0.1% pyridine,[62] and therefore a multi-stage purification was required, which further reduced the output. Nowadays, most pyridines are synthesized from ammonia, aldehydes, and nitriles, a few combinations of which are suited for pyridine itself. Variousname reactions are also known, but they are not practiced on scale.[2]
In 1989, 26,000 tonnes of pyridine was produced worldwide. Other major derivatives are2-,3-,4-methylpyridines and5-ethyl-2-methylpyridine. The combined scale of these alkylpyridines matches that of pyridine itself.[2] Among the largest 25 production sites for pyridine, eleven are located in Europe (as of 1999).[23] The major producers of pyridine includeEvonik Industries, Rütgers Chemicals,Jubilant Life Sciences,Imperial Chemical Industries, and Koei Chemical.[2] Pyridine production significantly increased in the early 2000s, with an annual production capacity of 30,000 tonnes in mainland China alone.[63] The US–Chinese joint venture Vertellus is currently the world leader in pyridine production.[64]
TheChichibabin pyridine synthesis was reported in 1924 and the basic approach underpins several industrial routes.[42] In its general form, the reaction involves thecondensation reaction ofaldehydes,ketones,α,β-unsaturated carbonyl compounds, or any combination of the above, inammonia orammonia derivatives. Application of the Chichibabin pyridine synthesis suffer from low yields, often about 30%,[65] however the precursors are inexpensive. In particular, unsubstituted pyridine is produced fromformaldehyde andacetaldehyde. First,acrolein is formed in aKnoevenagel condensation from the acetaldehyde and formaldehyde. The acrolein thencondenses with acetaldehyde and ammonia to givedihydropyridine, which is oxidized to pyridine. This process is carried out in a gas phase at 400–450 °C. Typical catalysts are modified forms ofalumina andsilica. The reaction has been tailored to produce variousmethylpyridines.[2]
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 overvanadium(V) oxide catalyst,[66] by vapor-dealkylation onnickel-based catalyst,[67][68] or hydrodealkylation with asilver- orplatinum-based catalyst.[69] Yields of pyridine up to be 93% can be achieved with the nickel-based catalyst.[2] Pyridine can also be produced by thedecarboxylation ofnicotinic acid withcopper chromite.[70]
Thetrimerization of a part of anitrile molecule and two parts ofacetylene into pyridine is calledBönnemann cyclization. This modification of theReppe synthesis can be activated either by heat or bylight. While thethermal activation requires high pressures and temperatures, the photoinducedcycloaddition proceeds at ambient conditions with CoCp2(cod) (Cp = cyclopentadienyl, cod =1,5-cyclooctadiene) as a catalyst, and can be performed even in water.[71] A series of pyridine derivatives can be produced in this way. When usingacetonitrile as the nitrile, 2-methylpyridine is obtained, which can be dealkylated to pyridine.
TheKröhnke 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 bromomethyl ketones gives the relatedpyridinium salt, wherein themethylene group is highly acidic. This species undergoes aMichael-like addition toα,β-unsaturated carbonyls in the presence ofammonium acetate to undergo ring closure and formation of the targeted substituted pyridine as well as pyridinium bromide.[72]
The Ciamician–Dennstedt rearrangement[73] entails the ring-expansion ofpyrrole withdichlorocarbene to3-chloropyridine.[74][75][76]
In the Gattermann–Skita synthesis,[77] amalonate ester salt reacts with dichloromethylamine.[78]
Other methods include theBoger pyridine synthesis andDiels–Alder reaction of analkene and anoxazole.[79]
Several pyridine derivatives play important roles in biological systems. While its biosynthesis is not fully understood,nicotinic acid (vitamin B3) occurs in somebacteria,fungi, andmammals. Mammals synthesize nicotinic acid through oxidation of theamino acidtryptophan, where an intermediate product, theaniline derivativekynurenine, creates a pyridine derivative,quinolinate and then nicotinic acid. On the contrary, the bacteriaMycobacterium tuberculosis andEscherichia coli produce nicotinic acid by condensation ofglyceraldehyde 3-phosphate andaspartic acid.[80]
The reactivity of pyridine can be distinguished for three chemical groups. Withelectrophiles,electrophilic substitution takes place where pyridine expresses aromatic properties. Withnucleophiles, pyridine reacts at positions 2 and 4 and thus behaves similar toimines andcarbonyls. The reaction with manyLewis 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, formingamine oxides (N-oxides), is also a feature of tertiary amines.[81]
Because of theelectronegativenitrogen in the pyridine ring, pyridine enters less readily intoelectrophilic aromatic substitution reactions than benzene derivatives.[82] Instead, in terms of its reactivity, pyridine resemblesnitrobenzene.[83] That is, pyridine reacts most easily innucleophilic substitution, as evidenced by the ease ofmetalation by strongorganometallic bases.[84][85][contradictory]
The nitrogen center of pyridine features a basiclone pair ofelectrons. This lone pair does not overlap with the aromatic π-system ring, consequently pyridine isbasic, having chemical properties similar to those oftertiary amines.Protonation gives theconjugate acid, apyridinium cation, C5H5NH+. ThepKa of pyridinium is 5.25. The structures of pyridine and pyridinium are almost identical, but the latter isisoelectronic with benzene.[86] Pyridiniump-toluenesulfonate (PPTS) is an illustrative pyridinium salt; it is produced by treating pyridine withp-toluenesulfonic acid.
In addition toprotonation, pyridine undergoes N-centredalkylation,acylation, andN-oxidation.
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.
Directnitration of pyridine is sluggish.[87][88] Pyridine derivatives wherein the nitrogen atom is screened sterically and/or electronically can be obtained by nitration withnitronium tetrafluoroborate (NO2BF4). In this way, 3-nitropyridine can be obtained via the synthesis of 2,6-dibromopyridine followed by nitration and debromination.[89][90]
Sulfonation of pyridine is even more difficult than nitration. However, pyridine-3-sulfonic acid can be obtained. Reaction with the SO3 group also facilitates addition of sulfur to the nitrogen atom, especially in the presence of amercury(II) sulfate catalyst.[84][91]
In contrast to the sluggish nitrations and sulfonations, thebromination andchlorination of pyridine proceed well.[2]
Some electrophilic substitutions on the pyridine are usefully effected usingpyridineN-oxide followed by deoxygenation. Addition of oxygen suppresses further reactions at nitrogen atom and promotes substitution at the 2- and 4-carbons. The oxygen atom can then be removed, e.g., using zinc dust.[92]
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 ahydride ion and elimination-additions with formation of an intermediatearyne configuration, and usually proceed at the 2- or 4-position.[84][85]
The hydride ion is a poor leaving group and direct substitution on the bare pyridine ring occurs in only a few heterocyclic reactions. They include theChichibabin reaction, which yields pyridine derivativesaminated 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.[85][93]
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. Fluorine is the best leaving group for the substitution withorganolithium compounds. The nucleophilic attack compounds may bealkoxides, thiolates,amines, and ammonia (at elevated pressures).[94]
Analogous to benzene, nucleophilic substitutions to pyridine can result in the formation ofpyridyne intermediates as heteroaryne. For this purpose, pyridine derivatives can be eliminated with good leaving groups using strong bases such as sodium andpotassium tert-butoxide. The subsequent addition of a nucleophile to thetriple bond has low selectivity, and the result is a mixture of the two possible adducts.[84]
Pyridine supports a series of radical reactions, which is used in itsdimerization to bipyridines. Radical dimerization of pyridine with elementalsodium orRaney nickel selectively yields4,4'-bipyridine,[95] or2,2'-bipyridine,[96] which are important precursor reagents in the chemical industry. One of thename reactions involving free radicals is theMinisci reaction. It can produce 2-tert-butylpyridine upon reacting pyridine withpivalic acid,silver nitrate andammonium insulfuric acid with a yield of 97%.[84]
Lewis acids easily add to the nitrogen atom of pyridine, forming pyridinium salts. The reaction withalkyl halides leads toalkylation of the nitrogen atom. This creates a positive charge in the ring that increases the reactivity of pyridine to both oxidation and reduction. TheZincke reaction is used for the selective introduction of radicals in pyridinium compounds (it has no relation to the chemical elementzinc).
Oxidation of pyridine occurs at nitrogen to givepyridineN-oxide. The oxidation can be achieved withperacids:[97]
Piperidine is produced byhydrogenation of pyridine with anickel-,cobalt-, orruthenium-based catalyst at elevated temperatures.[98] The hydrogenation of pyridine to piperidine releases 193.8 kJ/mol,[99] which is slightly less than the energy of the hydrogenation ofbenzene (205.3 kJ/mol).[99]
Partially hydrogenated derivatives are obtained under milder conditions. For example, reduction withlithium aluminium hydride yields a mixture of 1,4-dihydropyridine, 1,2-dihydropyridine, and 2,5-dihydropyridine.[100] Selective synthesis of 1,4-dihydropyridine is achieved in the presence of organometallic complexes ofmagnesium andzinc,[101] and (Δ3,4)-tetrahydropyridine is obtained by electrochemical reduction of pyridine.[102]Birch reduction converts pyridine to dihydropyridines.[103]
Pyridine is aLewis base, donating its pair of electrons to a Lewis acid. Its Lewis base properties are discussed in theECW model. Its relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated byC-B plots.[104][105] One example is thesulfur trioxide pyridine complex (melting point 175 °C), which is asulfation agent used to convert alcohols tosulfate esters. Pyridine-borane (C5H5NBH3, melting point 10–11 °C) is a mild reducing agent.
Transition metal pyridine complexes are numerous.[106][107] Typical octahedral complexes have the stoichiometryMCl2(py)4 andMCl3(py)3. Octahedral homoleptic complexes of the typeM(py)+6 are rare or tend to dissociate pyridine. Numerous square planar complexes are known, such asCrabtree's catalyst.[108] The pyridine ligand replaced during the reaction is restored after its completion.
Theη6 coordination mode, as occurs inη6 benzene complexes, is observed only insterically encumbered derivatives that block the nitrogen center.[109]
The main use of pyridine is as a precursor to the herbicidesparaquat anddiquat.[2] The first synthesis step of insecticidechlorpyrifos consists of the chlorination of pyridine. Pyridine is also the starting compound for the preparation ofpyrithione-basedfungicides.[23]Cetylpyridinium and laurylpyridinium, which can be produced from pyridine with aZincke reaction, are used asantiseptic in oral and dental care products.[61] Pyridine is easily attacked by alkylating agents to giveN-alkylpyridinium salts. One example iscetylpyridinium chloride.
It is also used in the textile industry to improve network capacity of cotton.[61]
Pyridine is used as a polar, basic, low-reactive solvent, for example inKnoevenagel condensations.[23][111] It is especially suitable for the dehalogenation, where it acts as the base for theelimination reaction. Inesterifications and acylations, pyridine activates the carboxylicacid chlorides and anhydrides. Even more active in these reactions are the derivatives4-dimethylaminopyridine (DMAP) and 4-(1-pyrrolidinyl) pyridine. Pyridine is also used as a base in somecondensation reactions.[112]
As a base, pyridine can be used as theKarl Fischer reagent, but it is usually replaced by alternatives with a more pleasant odor, such asimidazole.[113]
Pyridinium chlorochromate,pyridinium dichromate, and theCollins reagent (the complex ofchromium(VI) oxide) are used for the oxidation of alcohols.[114]
Pyridine is a toxic, flammable liquid with a strong and unpleasant fishy odour. Itsodour threshold of 0.04 to 20 ppm is close to itsthreshold limit of 5 ppm for adverse effects,[115] thus most (but not all) adults will be able to tell when it is present at harmful levels. Pyridine easily dissolves in water and harms both animals and plants in aquatic systems.[116]
Pyridine has aflash point of 20 °C and is therefore highly flammable. Combustion produces toxic fumes which can includebipyridines,nitrogen oxides, andcarbon monoxide.[12]
Pyridine can cause chemical burns on contact with the skin and its fumes may be irritating to the eyes or upon inhalation.[117] Pyridine depresses thenervous system giving symptoms similar to intoxication with vapor concentrations of above 3600 ppm posing a greater health risk.[2] The effects may have a delayed onset of several hours and include dizziness, headache,lack of coordination, nausea,salivation, and loss of appetite. They may progress into abdominal pain,pulmonary congestion and unconsciousness.[118] The lowest knownlethal dose (LDLo) for the ingestion of pyridine in humans is 500 mg/kg.
Prolonged exposure to pyridine may result in liver, heart and kidney damage.[12][23][119] Evaluations as a possiblecarcinogenic agent showed that there is inadequate evidence in humans for the carcinogenicity of pyridine, although there is sufficient evidence in experimental animals. Therefore,IARC considers pyridine as possibly carcinogenic to humans (Group 2B).[120]
Exposure to pyridine would normally lead to its inhalation and absorption in the lungs and gastrointestinal tract, where it either remains unchanged or ismetabolized. The major products of pyridine metabolism areN-methylpyridiniumhydroxide, which are formed byN-methyltransferases (e.g.,pyridineN-methyltransferase), as well as pyridineN-oxide, and 2-, 3-, and 4-hydroxypyridine, which are generated by the action ofmonooxygenase. In humans, pyridine is metabolized only intoN-methylpyridiniumhydroxide.[12][119]
Pyridine is readily degraded by bacteria to ammonia and carbon dioxide.[121] The unsubstituted pyridine ring degrades more rapidly thanpicoline,lutidine,chloropyridine, oraminopyridines,[122] and a number of pyridine degraders have been shown to overproduceriboflavin in the presence of pyridine.[123] IonizableN-heterocyclic compounds, including pyridine, interact with environmental surfaces (such as soils and sediments) via multiple pH-dependent mechanisms, including partitioning tosoil organic matter,cation exchange, and surface complexation.[124] Suchadsorption to surfaces reduces bioavailability of pyridines for microbial degraders and other organisms, thus slowing degradation rates and reducingecotoxicity.[125]
The systematic name of pyridine, within theHantzsch–Widman nomenclature recommended by theIUPAC, isazinine. 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 ofpyridine.[126] The numbering of the ring atoms in pyridine starts at the nitrogen (see infobox). An allocation of positions by letter of theGreek alphabet (α-γ) and thesubstitution 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 ispyridinyl, wherein the position of the substituted atom is preceded by a number. However, the historical namepyridyl is encouraged by the IUPAC and used instead of the systematic name.[127] Thecationic derivative formed by the addition of anelectrophile to the nitrogen atom is calledpyridinium.
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