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Pyridine

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Not to be confused withPyrimidineorPyridyne.

Pyridine
Full structural formula of pyridine
Full structural formula of pyridine
Skeletal formula of pyridine, showing the numbering convention
Skeletal formula of pyridine, showing the numbering convention
Ball-and-stick diagram of pyridine
Ball-and-stick diagram of pyridine
Space-filling model of pyridine
Space-filling model of pyridine
Names
Preferred IUPAC name
Pyridine[1]
Systematic IUPAC name
Azabenzene
Other names
Azine
Azinine
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.003.464Edit this at Wikidata
EC Number
  • 203-809-9
KEGG
UNII
  • InChI=1S/C5H5N/c1-2-4-6-5-3-1/h1-5HcheckY
    Key: JUJWROOIHBZHMG-UHFFFAOYSA-NcheckY
  • InChI=1/C5H5N/c1-2-4-6-5-3-1/h1-5H
    Key: JUJWROOIHBZHMG-UHFFFAOYAY
  • c1ccncc1
Properties
C5H5N
Molar mass 79.102g·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
−48.7·10−6cm3/mol[7]
Thermal conductivity 0.166 W/(m·K)[8]
1.5095 (20 °C)[4]
Viscosity 0.879cP(25 °C)[9]
2.215 D[10]
Thermochemistry[11]
132.7 J/(mol·K)
100.2 kJ/mol
−2.782MJ/mol
Hazards[15]
Occupational safety and health(OHS/OSH):
Main hazards
 Low to moderate hazard[13]
GHSlabelling:
GHS02: FlammableGHS07: Exclamation mark[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)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g. gasolineInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
3
0
Flash point 20 °C (68 °F; 293 K)[16]
482 °C (900 °F; 755 K)[16]
Explosive limits 1.8–12.4%[3]
5 ppm (TWA)
Lethal doseor concentration (LD, LC):
891 mg/kg (rat, oral)
1500 mg/kg (mouse, oral)
1580 mg/kg (rat, oral)[14]
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).

Pyridineis abasicheterocyclicorganic compoundwith thechemical formulaC5H5N.It is structurally related tobenzene,with onemethine group(=CH−)replaced by anitrogenatom(=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, due to the formation of extended, unsaturatedpolymericchains, which show significantelectrical conductivity.[page needed][17]The pyridine ring occurs in many important 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]

Properties[edit]

Internal bond angles and bond distances (pm) for pyridine.[18]

Physical properties[edit]

Crystal structure of pyridine

Pyridine isdiamagnetic.Itscritical parametersare: pressure 5.63 MPa, temperature 619 K and volume 248 cm3/mol.[19]In the temperature range 340–426 °C its vapor pressurepcan be described with theAntoine equation

whereTis temperature,A= 4.16272,B= 1371.358 K andC= −58.496 K.[20]

Structure[edit]

Pyridine ring forms aC5Nhexagon. Slight variations of theC−CandC−Ndistances as well as the bond angles are observed.

Crystallography[edit]

Pyridine crystallizes in anorthorhombic crystal systemwithspace groupPna21andlattice parametersa= 1752pm,b= 897 pm,c= 1135 pm, and 16formula unitsperunit cell(measured at 153 K). For comparison, crystallinebenzeneis 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.[18]This difference is partly related to the lowersymmetryof the individual pyridine molecule (C2vvs D6hfor 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).[21]

Spectroscopy[edit]

The opticalabsorption spectrumof pyridine inhexaneconsists of bands at thewavelengthsof 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.

The1Hnuclear magnetic resonance(NMR) spectrum shows signals for α-(δ8.5), 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 NMRspectra 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 chromatographyandmass spectrometrymethods.[23]

Bonding[edit]

Pyridine with its free electron pair

Pyridine has aconjugatedsystem of sixπ electronsthat are delocalized over the ring. The molecule is planar and, thus, follows theHückel criteriafor aromatic systems. In contrast to benzene, theelectron densityis not evenly distributed over the ring, reflecting the negativeinductive effectof the nitrogen atom. For this reason, pyridine has a dipole moment and a weakerresonant stabilizationthan benzene (resonance energy117 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 pairis in an sp2orbital, 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 thediazineheterocycles (C4H4N2), with the namespyridazine,pyrimidine,andpyrazine.

History[edit]

Thomas Anderson

Impure pyridine was undoubtedly prepared by earlyalchemistsby 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 suffixidinewas added in compliance with the chemical nomenclature, as intoluidine,to indicate acyclic compoundcontaining 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 betweenquinolineandnaphthalene,the structure of pyridine is derived frombenzeneby 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 topiperidinewithsodiuminethanol.[36][37]In 1876,William Ramsaycombinedacetyleneandhydrogen cyanideinto pyridine in ared-hotiron-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 synthesistypically uses a 2:1:1 mixture of a β-keto acid(oftenacetoacetate), analdehyde(oftenformaldehyde), andammoniaor its salt as the nitrogen donor. First, a doublehydrogenatedpyridine is obtained, which is then oxidized to the corresponding pyridine derivative.Emil Knoevenagelshowed that asymmetrically substituted pyridine derivatives can be produced with this process.[41]

Hantzsch pyridine synthesiswith acetoacetate, formaldehyde andammonium acetate,andiron(III) chlorideas the oxidizer.

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 Chichibabininvented apyridine synthesis reaction,which was based on inexpensive reagents.[42]This method is still used for the industrial production of pyridine.[2]

Occurrence[edit]

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 compoundsthat are produced in roasting andcanningprocesses, 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]

Production[edit]

Historically, pyridine was extracted fromcoal taror obtained as a byproduct of coalgasification.The process is labor-consuming and inefficient:coal tarcontains only about 0.1% pyridine,[55]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 reactionsare 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-methylpyridinesand5-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.[56]The US–Chinese joint venture Vertellus is currently the world leader in pyridine production.[57]

Chichibabin synthesis[edit]

TheChichibabin pyridine synthesiswas reported in 1924 and the basic approach underpins several industrial routes.[42]In its general form, the reaction involves thecondensation reactionofaldehydes,ketones,α,β-unsaturated carbonyl compounds,or any combination of the above, inammoniaorammonia derivatives.Application of the Chichibabin pyridine synthesis suffer from low yields, often about 30%,[58]however the precursors are inexpensive. In particular, unsubstituted pyridine is produced fromformaldehydeandacetaldehyde.First,acroleinis formed in aKnoevenagel condensationfrom the acetaldehyde and formaldehyde. The acrolein thencondenseswith 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 ofaluminaandsilica.The reaction has been tailored to produce variousmethylpyridines.[2]

Formation of acrolein from acetaldehyde and formaldehyde
Condensation of pyridine from acrolein and acetaldehyde

Dealkylation and decarboxylation of substituted pyridines[edit]

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) oxidecatalyst,[59]by vapor-dealkylation onnickel-based catalyst,[60][61]or hydrodealkylation with asilver- orplatinum-based catalyst.[62]Yields of pyridine up to be 93% can be achieved with the nickel-based catalyst.[2]Pyridine can also be produced by thedecarboxylationofnicotinic acidwithcopper chromite.[63]

Bönnemann cyclization[edit]

Bönnemann cyclization

Thetrimerizationof a part of anitrilemolecule and two parts ofacetyleneinto pyridine is calledBönnemann cyclization.This modification of theReppe synthesiscan be activated either by heat or bylight.While thethermal activationrequires high pressures and temperatures, the photoinducedcycloadditionproceeds at ambient conditions with CoCp2(cod) (Cp = cyclopentadienyl, cod =1,5-cyclooctadiene) as a catalyst, and can be performed even in water.[64]A series of pyridine derivatives can be produced in this way. When usingacetonitrileas the nitrile, 2-methylpyridine is obtained, which can be dealkylated to pyridine.

Other methods[edit]

TheKröhnke pyridine synthesisprovides 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 relatedpyridiniumsalt, wherein themethylene groupis highly acidic. This species undergoes aMichael-like additiontoα,β-unsaturated carbonylsin the presence ofammonium acetateto undergo ring closure and formation of the targeted substituted pyridine as well as pyridinium bromide.[65]

Figure 1
Figure 1

The Ciamician–Dennstedt rearrangement[66]entails the ring-expansion ofpyrrolewithdichlorocarbeneto3-chloropyridine.[67][68][69]

Ciamician–Dennstedt Rearrangement
Ciamician–Dennstedt Rearrangement

In the Gattermann–Skita synthesis,[70]amalonate estersalt reacts with dichloromethylamine.[71]

Gattermann–Skita synthesis
Gattermann–Skita synthesis

Other methods include theBoger pyridine synthesisandDiels–Alder reactionof analkeneand anoxazole.[72]

Biosynthesis[edit]

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, theanilinederivativekynurenine,creates a pyridine derivative,quinolinateand then nicotinic acid. On the contrary, the bacteriaMycobacterium tuberculosisandEscherichia coliproduce nicotinic acid by condensation ofglyceraldehyde 3-phosphateandaspartic acid.[73]

Reactions[edit]

Because of theelectronegativenitrogenin the pyridine ring, pyridine enters less readily intoelectrophilic aromatic substitutionreactions than benzene derivatives.[74]Instead, in terms of its reactivity, pyridine resemblesnitrobenzene.[75]

Correspondingly pyridine is more prone tonucleophilic substitution,as evidenced by the ease ofmetalationby strongorganometallicbases.[76][77]The reactivity of pyridine can be distinguished for three chemical groups. Withelectrophiles,electrophilic substitutiontakes place where pyridine expresses aromatic properties. Withnucleophiles,pyridine reacts at positions 2 and 4 and thus behaves similar toiminesandcarbonyls.The reaction with manyLewis acidsresults 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.[78]

The nitrogen center of pyridine features a basiclone pairofelectrons.This lone pair does not overlap with the aromatic π-system ring, consequently pyridine isbasic,having chemical properties similar to those oftertiary amines.Protonationgivespyridinium,C5H5NH+.ThepKaof theconjugate acid(the pyridinium cation) is 5.25. The structures of pyridine and pyridinium are almost identical.[79]The pyridinium cation isisoelectronicwith benzene. 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.Pyridine and poly(4-vinyl) pyridine have been shown to form conductingmolecular wireswith remarkable polyenimine structure onUV irradiation,a process which accounts for at least some of the visible light absorption by aged pyridine samples. These wires have been theoretically predicted to be both highly efficient electron donors and acceptors, and yet are resistant to air oxidation.[80]

Electrophilic substitutions[edit]

Owing to the decreased electron density in the aromatic system,electrophilic substitutionsare 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.

substitution in the 2-position
substitution in the 2-position
substitution in the 3-position
substitution in the 3-position
Substitution in 4-position
Substitution in 4-position

Directnitrationof pyridine is sluggish.[81][82]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.[83][84]

Sulfonationof pyridine is even more difficult than nitration. However, pyridine-3-sulfonic acid can be obtained. Reaction with the SO3group also facilitates addition of sulfur to the nitrogen atom, especially in the presence of amercury(II) sulfatecatalyst.[76][85]

In contrast to the sluggish nitrations and sulfonations, thebrominationandchlorinationof pyridine proceed well.[2]

PyridineN-oxide[edit]

Structure of pyridineN-oxide

Oxidation of pyridine occurs at nitrogen to givepyridineN-oxide.The oxidation can be achieved withperacids:[86]

C5H5N + RCO3H → C5H5NO + RCO2H

Some electrophilic substitutions on the pyridine are usefully effected using pyridineN-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.[87]

Nucleophilic substitutions[edit]

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 ahydrideion and elimination-additions with formation of an intermediatearyneconfiguration, and usually proceed at the 2- or 4-position.[76][77]

Nucleophilic substitution in 2-position
Nucleophilic substitution in 2-position
Nucleophilic substitution in 3-position
Nucleophilic substitution in 3-position
Nucleophilic substitution in 4-position
Nucleophilic substitution in 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 withorganolithium compounds.The nucleophilic attack compounds may bealkoxides,thiolates,amines,and ammonia (at elevated pressures).[88]

In general, the hydride ion is a poor leaving group and occurs only in a few heterocyclic reactions. They include theChichibabin reaction,which yields pyridine derivativesaminatedat the 2-position. Here,sodium amideis 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.[77][89]

Analogous to benzene, nucleophilic substitutions to pyridine can result in the formation ofpyridyneintermediates 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 bondhas low selectivity, and the result is a mixture of the two possible adducts.[76]

Radical reactions[edit]

Pyridine supports a series of radical reactions, which is used in itsdimerizationto bipyridines. Radical dimerization of pyridine with elementalsodiumorRaney nickelselectively yields4,4'-bipyridine,[90]or2,2'-bipyridine,[91]which are important precursor reagents in the chemical industry. One of thename reactionsinvolving free radicals is theMinisci reaction.It can produce 2-tert-butylpyridine upon reacting pyridine withpivalic acid,silver nitrateandammoniuminsulfuric acidwith a yield of 97%.[76]

Reactions on the nitrogen atom[edit]

Additions of variousLewis acidsto pyridine

Lewis acidseasily add to the nitrogen atom of pyridine, forming pyridinium salts. The reaction withalkyl halidesleads toalkylationof the nitrogen atom. This creates a positive charge in the ring that increases the reactivity of pyridine to both oxidation and reduction. TheZincke reactionis used for the selective introduction of radicals in pyridinium compounds (it has no relation to the chemical elementzinc).

Hydrogenation and reduction[edit]

Reduction of pyridine to piperidine withRaney nickel

Piperidineis produced byhydrogenationof pyridine with anickel-,cobalt-, orruthenium-based catalyst at elevated temperatures.[92]The hydrogenation of pyridine to piperidine releases 193.8 kJ/mol,[93]which is slightly less than the energy of the hydrogenation ofbenzene(205.3 kJ/mol).[93]

Partially hydrogenated derivatives are obtained under milder conditions. For example, reduction withlithium aluminium hydrideyields a mixture of 1,4-dihydropyridine, 1,2-dihydropyridine, and 2,5-dihydropyridine.[94]Selective synthesis of 1,4-dihydropyridine is achieved in the presence of organometallic complexes ofmagnesiumandzinc,[95]and (Δ3,4)-tetrahydropyridine is obtained by electrochemical reduction of pyridine.[96]Birch reductionconverts pyridine to dihydropyridines.[97]

Lewis basicity and coordination compounds[edit]

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.[98][99]One example is thesulfur trioxide pyridine complex(melting point 175 °C), which is asulfationagent used to convert alcohols tosulfate esters.Pyridine-borane(C5H5NBH3,melting point 10–11 °C) is a mild reducing agent.

structure of theCrabtree's catalyst

Transition metal pyridine complexesare numerous.[100][101] Typical octahedral complexes have the stoichiometryMCl2(py)4andMCl3(py)3.Octahedral homoleptic complexes of the typeM(py)+6are rare or tend to dissociate pyridine. Numerous square planar complexes are known, such asCrabtree's catalyst.[102]The pyridine ligand replaced during the reaction is restored after its completion.

Theη6coordination mode, as occurs inη6benzene complexes, is observed only insterically encumberedderivatives that block the nitrogen center.[103]

Applications[edit]

Pesticides and pharmaceuticals[edit]

The main use of pyridine is as a precursor to the herbicidesparaquatanddiquat.[2]The first synthesis step of insecticidechlorpyrifosconsists of the chlorination of pyridine. Pyridine is also the starting compound for the preparation ofpyrithione-basedfungicides.[23]Cetylpyridiniumand laurylpyridinium, which can be produced from pyridine with aZincke reaction,are used asantisepticin oral and dental care products.[104]Pyridine is easily attacked by alkylating agents to giveN-alkylpyridinium salts. One example iscetylpyridinium chloride.

Synthesis ofparaquat[105]

It is also used in the textile industry to improve network capacity of cotton.[104]

Laboratory use[edit]

Pyridine is used as a polar, basic, low-reactive solvent, for example inKnoevenagel condensations.[23][106]It is especially suitable for the dehalogenation, where it acts as the base for theelimination reaction.Inesterificationsand acylations, pyridine activates the carboxylicacid chloridesand 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.[107]

Elimination reaction with pyridine to form pyridinium

Reagents[edit]

Oxidation of an alcohol to aldehyde with theCollins reagent

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.[108]

Pyridinium chlorochromate,pyridinium dichromate,and theCollins reagent(the complex ofchromium(VI) oxide) are used for the oxidation of alcohols.[109]

Hazards[edit]

Pyridine is a toxic, flammable liquid with a strong and unpleasant fishy odour. Itsodour thresholdof 0.04 to 20 ppm is close to itsthreshold limitof 5 ppm for adverse effects,[110]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.[111]

Fire[edit]

Pyridine has aflash pointof 20 °C and is therefore highly flammable. Combustion produces toxic fumes which can includebipyridines,nitrogen oxides,andcarbon monoxide.[12]

Short-term exposure[edit]

Pyridine can cause chemical burns on contact with the skin and its fumes may be irritating to the eyes or upon inhalation.[112]Pyridine depresses thenervous systemgiving symptoms similar to intoxication with vapor concentrations of above 3600ppmposing 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 congestionand unconsciousness.[113]The lowest knownlethal dose(LDLo) for the ingestion of pyridine in humans is 500 mg/kg.

Long-term exposure[edit]

Prolonged exposure to pyridine may result in liver, heart and kidney damage.[12][23][114]Evaluations as a possiblecarcinogenicagent showed that there is inadequate evidence in humans for the carcinogenicity of pyridine, although there is sufficient evidence in experimental animals. Therefore,IARCconsiders pyridine as possibly carcinogenic to humans (Group 2B).[115]

Occurrence[edit]

Trace amounts of up to 16 μg/m3have been detected in tobacco smoke.[23]Minor amounts of pyridine are released into environment from some industrial processes such as steel manufacture,[116]processing ofoil shale,coal gasification,cokingplants andincinerators.[23]The atmosphere at oil shale processing plants can contain pyridine concentrations of up to 13 μg/m3,[117]and 53 μg/m3levels were measured in thegroundwaterin the vicinity of a coal gasification plant.[118]According to a study by the USNational Institute for Occupational Safety and Health,about 43,000 Americans work in contact with pyridine.[119]

In foods[edit]

Pyridine has historically been added to foods to give them a bitter flavour, although this practise is now banned in the U.S.[120][121]It may still be added toethanolto make it unsuitable for drinking.[104]

Metabolism[edit]

Metabolism of pyridine

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][114]

Environmental fate[edit]

Pyridine is readily degraded by bacteria to ammonia and carbon dioxide.[122]The unsubstituted pyridine ring degrades more rapidly thanpicoline,lutidine,chloropyridine,oraminopyridines,[123]and a number of pyridine degraders have been shown to overproduceriboflavinin the presence of pyridine.[124]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.[125]Suchadsorptionto surfaces reduces bioavailability of pyridines for microbial degraders and other organisms, thus slowing degradation rates and reducingecotoxicity.[126]

Nomenclature[edit]

The systematic name of pyridine, within theHantzsch–Widman nomenclaturerecommended 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.[127]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 patternnomenclature 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 namepyridylis encouraged by the IUPAC and used instead of the systematic name.[128]Thecationicderivative formed by the addition of anelectrophileto the nitrogen atom is calledpyridinium.

See also[edit]

References[edit]

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