24. Six-membered heterocyclic compounds with one hetero atom (pyridine, pyran, thiopyran): structure. + Biologically active substances containing pyidine ring NAD, vitamin B6, niacin, medicines and alkaloids; vitamin E.

Heterocyclic Compounds: The Building Blocks of Life

• Heterocyclic compounds are ring-shaped molecules where at least one atom in the ring is an element other than carbon. These "heteroatoms" are most commonly nitrogen (N), oxygen (O), or sulfur (S).
• From a biological perspective, nitrogen-containing heterocycles are superstars! Some important examples include:
• Pyrimidine (not Pyron)
• Furan
• Thiophene

Pyridine. Structure and acid-base properties.

• Structure: Pyridine is a basic heterocyclic organic compound with the chemical formula C 5H 5N.

• Before we talk about the Aromaticity of Pyridine we need to know the following terms:
• Cyclic: The molecule has a ring structure formed by covalent bonds.
• p² Hybridization: The participating carbon atoms are likely sp² hybridized, meaning they have three planar sigma bonds and one unhybridized p orbital for pi bonding. Pyridine is Cyclic as its sp2 hybridized.
• Conjugation: The p orbitals of the carbon atoms overlap, allowing for delocalization of pi electrons across the ring. This delocalization is symbolized by the squiggly line (~) between carbon atoms (C-C). Pyridine is conjugated.
• Hückel's Rule (4n + 2 π electrons): The number of pi electrons in the ring must follow this rule for aromaticity. In your example, 4n + 2 = 6, which suggests there are 2 pi electrons (n = 1).
• Acid Base properties: Pyridine is a weak base, cannot accept proton readily because the lone pair of electron is present on sp2 hybridised 'N' and due to the more % of s-character (sp>sp2>sp3) lone pair of electrons are tightly held by the nucleus, less available for donation, therefore a weak base.