HeteroCyclic System3

6-Membered Heterocyclic Compounds
? With heterocycles containing five C atoms, the unsaturated compounds are frequently more stable because of aromaticity.
? For examples: Pyridine , Quinoline ,


? Structure and Properties
? Pyridine replaces the CH of benzene by a N atom (and a pair of electrons)
? Hybridization = sp2 with similar resonance stabilization energy
? Lone pair of electrons not involved in aromaticity
? Pyridine is a weak base
? Pyridine is ? -electron deficient
? Electrophilic aromatic substitution is difficult
? Nucleophilic aromatic substitution is easy
? Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N
? The pyridine ring occurs in many important compounds, including azines and the vitamins niacin ,sulfadiazine and pyridoxal.
? Pyridine occurs in coal tar (0.1%) and in the distillate from bones (bone oil) and has been produced industrially from these sources.
? According to the resonance theory, pyridine is considered to be hybrid of the following structures:





? Pyridine is colourless liquid.use as solvent and base in organic synthesis .
? It’s boiling point 115.5°C and melting point ?41.6 °C.
? It have characteristic unpleasant odor.
? It is soluble in water and most organic solvent .
? Pyridine is conventionally detect ed by the gas chromatography and mass spectrometry methods.



? The pyridinium ion is a stronger acid than a typical ammonium ion.
? Pyridine reacts like a tertiary amine .
? The nitrogen lone pair is not released into the aromatic system because it is perpendicular to the ? system.
? The nitrogen withdraws electrons by resonance, resulting in an electron-deficient ring system