Editing Pyridine (section)

==Properties==
[[File:PyridineXray.svg|class=skin-invert-image|thumb|142px|left|Internal bond angles and bond distances (in [[picometer|pm]]) for pyridine.<ref name=cox/>]]

===Physical properties===
[[File:Kristallstruktur Pyridin.png|thumb|left|Crystal structure of pyridine]]

Pyridine is [[diamagnetism|diamagnetic]]. Its [[critical point (thermodynamics)|critical parameters]] are: pressure 5.63&nbsp;MPa, temperature 619&nbsp;K and volume 248&nbsp;cm<sup>3</sup>/mol.<ref>[[#Haynes|Haynes]], p.&nbsp;6.80</ref> In the temperature range 340–426&nbsp;°C its vapor pressure ''p'' can be described with the [[Antoine equation]]
:<math>\log_{10} p = A-\frac{B}{C+T}</math>
where ''T'' is temperature, ''A''&nbsp;=&nbsp;4.16272, ''B''&nbsp;=&nbsp;1371.358&nbsp;K and ''C''&nbsp;=&nbsp;−58.496&nbsp;K.<ref>{{cite journal|last1=McCullough|first1=J. P.|last2=Douslin|first2=D. R.|last3=Messerly|first3=J. F.|last4=Hossenlopp|first4=I. A.|last5=Kincheloe|first5=T. C.|last6=Waddington|first6=Guy|title=Pyridine: Experimental and Calculated Chemical Thermodynamic Properties between 0 and 1500&nbsp;K.; a Revised Vibrational Assignment|journal=Journal of the American Chemical Society|volume=79|pages=4289|year=1957|doi=10.1021/ja01573a014|issue=16}}</ref>

===Structure===
Pyridine ring forms a {{chem2|C5N}} hexagon.  Slight variations of the {{chem2|C\sC}} and {{chem2|C\sN}} distances as well as the bond angles are observed.

===Crystallography===
Pyridine crystallizes in an [[orthorhombic crystal system]] with [[space group]] ''Pna2<sub>1</sub>'' and [[lattice parameters]] ''a''&nbsp;=&nbsp;1752&nbsp;[[picometer|pm]], ''b''&nbsp;=&nbsp;897&nbsp;pm, ''c''&nbsp;=&nbsp;1135&nbsp;pm, and 16 [[formula unit]]s per [[unit cell]] (measured at 153&nbsp;K). For comparison, crystalline [[benzene]] is also orthorhombic, with space group ''Pbca'', ''a''&nbsp;=&nbsp;729.2&nbsp;pm, ''b''&nbsp;=&nbsp;947.1&nbsp;pm, ''c''&nbsp;=&nbsp;674.2&nbsp;pm (at 78&nbsp;K), but the number of molecules per cell is only 4.<ref name=cox>{{cite journal|last1=Cox|first1=E.|title=Crystal Structure of Benzene|journal=Reviews of Modern Physics|volume=30|issue=1|pages=159–162|year=1958|doi=10.1103/RevModPhys.30.159|bibcode=1958RvMP...30..159C}}</ref> This difference is partly related to the lower [[Molecular symmetry|symmetry]] of the individual pyridine molecule (C<sub>2v</sub> vs D<sub>6h</sub> for benzene). A tri[[hydrate]] (pyridine·3H<sub>2</sub>O) is known; it also crystallizes in an orthorhombic system in the space group ''Pbca'', lattice parameters ''a''&nbsp;=&nbsp;1244&nbsp;pm, ''b''&nbsp;=&nbsp;1783&nbsp;pm, ''c''&nbsp;=&nbsp;679&nbsp;pm and eight formula units per unit cell (measured at 223&nbsp;K).<ref name=str>{{cite journal|last1=Mootz|first1=D.|title=Crystal structures of pyridine and pyridine trihydrate|journal=The Journal of Chemical Physics|volume=75|pages=1517–1522|year=1981|doi=10.1063/1.442204|issue=3|bibcode = 1981JChPh..75.1517M }}</ref>

===Spectroscopy===
The optical [[Absorption Spectrum|absorption spectrum]] of pyridine in [[hexane]] consists of bands at the [[wavelength]]s of 195, 251, and 270&nbsp;nm.  With respective extinction coefficients (''ε'') of 7500, 2000, and 450&nbsp;L·mol<sup>−1</sup>·cm<sup>−1</sup>, these bands are assigned to π&nbsp;→&nbsp;π*, π&nbsp;→&nbsp;π*, and n&nbsp;→&nbsp;π*  transitions. The compound displays very low [[fluorescence]].<ref>{{cite journal |last1=Varras |first1=Panayiotis C. |last2=Gritzapis |first2=Panagiotis S. |last3=Fylaktakidou |first3=Konstantina C. |title=An explanation of the very low fluorescence and phosphorescence in pyridine: a CASSCF/CASMP2 study |journal=Molecular Physics |date=17 January 2018 |volume=116 |issue=2 |pages=154–170 |doi=10.1080/00268976.2017.1371800|url=https://figshare.com/articles/journal_contribution/5414566 }}</ref>

The <sup>1</sup>H [[nuclear magnetic resonance]] (NMR) spectrum shows signals for α-([[chemical shift|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 the [[13C NMR|<sup>13</sup>C&nbsp;NMR]] spectra of pyridine and benzene: pyridine shows a triplet at ''δ''(α-C)&nbsp;=&nbsp;150&nbsp;ppm, δ(β-C)&nbsp;=&nbsp;124&nbsp;ppm and δ(γ-C)&nbsp;=&nbsp;136&nbsp;ppm, whereas benzene has a single line at 129&nbsp;ppm. All shifts are quoted for the solvent-free substances.<ref>[[#Joule|Joule]], p.&nbsp;16</ref> Pyridine is conventionally detected by the [[gas chromatography]] and [[mass spectrometry]] methods.<ref name=osha>{{cite book |url=http://monographs.iarc.fr/ENG/Monographs/vol77/mono77-21.pdf |title=Pyridine |work=IARC Monographs 77 |publisher=OSHA |location=Washington DC |date=1985 |access-date=7 January 2011 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304083832/http://monographs.iarc.fr/ENG/Monographs/vol77/mono77-21.pdf |url-status=live }}</ref>

===Bonding===
[[File:Pyridine-2D-Skeletal.png|class=skin-invert-image|upright=.5|thumb|left|Pyridine with its free electron pair]]
Pyridine has a [[conjugation (organic chemistry)|conjugated]] system of six [[Pi bond|π electrons]] that are delocalized over the ring. The molecule is planar and, thus, follows the [[Hückel rule|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 [[Resonance (chemistry)|resonant stabilization]] than benzene ([[Resonance (chemistry)#Resonance energy|resonance energy]] 117&nbsp;kJ/mol in pyridine vs. 150&nbsp;kJ/mol in benzene).<ref>[[#Joule|Joule]], p.&nbsp;7</ref>

The ring atoms in the pyridine molecule are [[orbital hybridisation#sp2 hybrids|sp<sup>2</sup>-hybridized]]. The nitrogen is involved in the π-bonding aromatic system using its unhybridized p orbital. The [[lone pair]] is in an sp<sup>2</sup> orbital, projecting outward from the ring in the same plane as the [[σ bond]]s. 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.<ref>{{cite book|last=Sundberg|first=Francis A. Carey; Richard J.|title=Advanced Organic Chemistry : Part A: Structure and Mechanisms|year=2007|publisher=Springer US|location=Berlin|isbn=978-0-387-68346-1|edition=5.|page=794}}</ref> 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 from the same column of the [[Periodic Table of Elements]] (see figure below). Substitution of one C–H in pyridine with a second N gives rise to the [[diazine]] heterocycles (C<sub>4</sub>H<sub>4</sub>N<sub>2</sub>), with the names [[pyridazine]], [[pyrimidine]], and [[pyrazine]].

<div><ul> 
<li style="display: inline-block;">[[File:Bond lengths of group 15 heterobenzenes and benzene.svg|class=skin-invert-image|thumb|center|upright=2|Bond lengths and angles of benzene, pyridine, [[phosphorine]], [[arsabenzene]], [[stibabenzene]], and [[bismabenzene]]]]</li>
<li style="display: inline-block;">[[File:Pyridine-orbitals.svg|class=skin-invert-image|thumb|center|upright=.5|Atomic orbitals in pyridine]]</li>
<li style="display: inline-block;">[[File:Pyridine-10.png|class=skin-invert-image|thumb|center|upright=2.4|Resonance structures of pyridine]]</li>
<li style="display: inline-block;">[[File:Pyridinium-orbitals.svg|class=skin-invert-image|thumb|center|upright=.57|Atomic orbitals in protonated pyridine]]</li>
</ul></div>