With electrophiles, electrophilic substitution takes place where pyridine expresses aromatic properties.
The reactivity of pyridine can be distinguished for three chemical groups.
Correspondingly pyridine is more prone to nucleophilic substitution, as evidenced by the ease of metalation by strong organometallic bases. It, therefore, enters less readily into electrophilic aromatic substitution reactions than benzene derivatives. Chemical properties īecause of the electronegative nitrogen in the pyridine ring, the molecule is relatively electron deficient. Pyridine is conventionally detected by the gas chromatography and mass spectrometry methods. All shifts are quoted for the solvent-free substances. 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. 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 carbon analog of pyridine, benzene, has only one proton signal at 7.27 ppm. 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 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.
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 A tri hydrate (pyridine♳H 2O) 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). This difference is partly related to the lower symmetry of the individual pyridine molecule (C 2v vs D 6h for benzene). 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. Pyridine crystallizes in an orthorhombic crystal system with space group Pna2 1 and lattice parameters a = 1752 pm, b = 897 pm, c = 1135 pm, and 16 formula units per unit cell (measured at 153 K). Slight variations of the C−C and C−N distances as well as the bond angles are observed. In the temperature range 340–426 ☌ its vapor pressure p can be described with the Antoine equation log 10 p = A − B C + T The critical parameters of pyridine are pressure 6.70 MPa, temperature 620 K and volume 229 cm 3