Lec 3 | Six-membered ring

Structure of Pyridine

• Pyridine is classified as aromatic on the basis of its properties.
• It is flat, with bond angles of 120°.
• The four carbon-carbon bonds are of the same length, and so are the two carbon-nitrogen bonds.
• It resists addition and undergoes electrophilic substitution.
• Its heat of combustion indicates a resonance energy of 23 kcal/mole.
• Pyridine can be considered a hybrid of the Kekul structures I and II.
• The circle represents the aromatic sextet.

Electronic Configuration of Pyridine

• The nitrogen atom, like each of the carbon atoms, is bonded to other members of the ring by the use of sp2 orbitals.
• The nitrogen atom provides one electron for the π cloud.
• The third sp2 orbital of each carbon atom is used to form a bond to hydrogen.
• The third sp2 orbital of nitrogen contains a pair of electrons, which are available for sharing with acids.
• This electronic configuration makes pyridine a much stronger base than pyrrole.

Pyridine Molecule

• One electron is in each p orbital.
• Two electrons are in the sp2 orbital of nitrogen.
• The p orbitals overlap to form π clouds above and below the plane of the ring.
• Two unshared electrons are still in the sp2 orbital of nitrogen.

Source of Pyridine Compounds

• Pyridine is found in coal tar.
• Along with it are found a number of methylpyridines, the most important of which are the monomethyl compounds, known as picolines.
• Oxidation of the picolines yields the pyridinecarboxylic acids.
• The 3-isomer (nicotinic acid or niacin) is a vitamin.
• The 4-isomer (Isonicotinic acid) has been used, in the form of its hydrazide, in the treatment of tuberculosis.

Synthesis of Pyridine Derivatives

• The increasing demand for certain pyridine derivatives has led to the development of syntheses involving ring closure.