Page 55 - Org 3 theoritical book 2024-25

Clinical Pharmacy PharmD - 2024/2025 Level 2 Pharmaceutical Organic Chemistry-3 (PC 305)
SIX-MEMBERED HETEROCYCLES

I- Six-Membered Hetercyclic Rings with One Nitroge Atom

A- PYRIDINE AND ITS DERIVATIVES



' 5 4 3  pyridine
6 2 
' 1
N

• Pyridine is the most important six-membered ring system.

• Pyridine derivatives are present in, e.g., Vit. B 6, nictinamide, dyes, alkaloids and

many drugs.

Structure of pyridine:

• Pyridine is structurally related to benzene.

• The ring nitrogen, as the other C-atoms, is sp hybridized.
2
2
• The electrons in the sp nitrogen atom are arranged as shown in the figure (cf.
Pyrrole).

sp2 orbitals p orbital e - e - e - - e - e - lone pair
e
e - e e - - e -
e - e e - -
N 2e - N 2e -
e -
e - e - e -
sp2 orbital

The arrangement of electrons
in an sp2 nitrogen atom of the
pyridine type The orbital structure of pyridine
Synthesis of pyridine and its derivatives:

1) From 1,5-dicarbonyl compounds & ammonia:

1,5-Dicarbonyl compounds react with ammonia to give 1,4-dihydropyridines which

are easily dehydrogenated to pyridines.
H
H H
H NH 3 [O]
- 2H O N
2
OO N
H

Unsaturated 1,5-dicarbonyl compounds on the other hand react with ammonia
giving pyridines directly, e.g., glutaconic dialdehyde gives pyridine.