Organic Chemistry, Heterocyclic Compounds, Lec.3.pdf

Organic Chemistry III Heterocyclic Compounds Six-membered ring | Lec.3 Organic Chemistry | Heterocyclic Compounds Contents : Structure of pyridine 3 Source of pyridine compounds 8 Organic Chemistry | Heterocyclic Compounds Structure of pyridine:Of the six-membered aromatic heterocycles, we shall tak...

Organic Chemistry III Heterocyclic Compounds Six-membered ring | Lec.3 Organic Chemistry | Heterocyclic Compounds Contents : Structure of pyridine 3 Source of pyridine compounds 8 Organic Chemistry | Heterocyclic Compounds Structure of pyridine:Of the six-membered aromatic heterocycles, we shall take up only one, 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. Organic Chemistry | Heterocyclic Compounds Pyridine can be considered a hybrid of the Kekul structures I and II. We shall represent it as structure III, in which the circle represents the aromatic sextet:- Organic Chemistry | Heterocyclic Compounds In electronic configuration, the nitrogen of pyridine is considerably different from the nitrogen of pyrrole. In pyridine the nitrogen atom, like each of the carbon atoms, is bonded to other members of the ring by the use of sp2 orbitals, and provides one electron for the π cloud. The third sp2 orbital of each carbon atom is used to form a bond to hydrogen. Organic Chemistry | Heterocyclic Compounds The third sp2 orbital of nitrogen simply contains a pair of electrons, which are available for sharing with acids. Because of this electronic configuration, the nitrogen atom makes pyridine a much stronger base than pyrrole, and affects the reactivity of the ring in a quite different way, as we shall see:- Organic Chemistry | Heterocyclic Compounds Pyridine molecule, (a) One electron in each p orbital; two electrons in sp2 orbital of nitrogen, (b) The p orbitals overlap to form π clouds above and below plane of ring; two unshared electrons still in sp2 orbital of nitrogen. Source Of Pyridine Compounds Organic Chemistry | Heterocyclic Compounds 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:- Organic Chemistry | Heterocyclic Compounds The 3-isomer (nicotinic acid or niaciri) is a vitamin. The 4-isomer (Isonicotinic acid) has been used, in the form of its hydrazide, in the treatment of tuberculosis:- Organic Chemistry | Heterocyclic Compounds The increasing demand for certain pyridine derivatives has led to the development of syntheses involving ring closure. For example:-