Heterocyclic Chemistry and Nomenclature

Heterocyclic Chemistry

• Heterocyclic compounds are cyclic compounds where one or more atoms in the ring are heteroatoms (e.g., O, N, S, P).
• They are found in natural compounds (e.g., nucleic acids, hemoglobin, antibiotics) and synthetic drugs (e.g., indomethacin, antipyretic, phenytoin).
• Heterocyclic compounds can be classified as aliphatic or aromatic.
• They can also be classified by the number of cycles:
  • Monocyclic: one ring
  • Bicyclic: two fused rings with at least one heteroatom (e.g., indole)
  • Polycyclic: more than two rings with at least one heteroatom (e.g., carbazole)

Nomenclature of Heterocycles

• IUPAC names are divided into prefix + stem + suffix.
• Prefix: indicates the type, number, and position of heteroatoms.
  • If multiple heteroatoms of the same type, use di, tri, etc.
  • If different heteroatoms, order according to periodic table (decreasing group number, increasing atomic number within the group).
    • Example: oxathiaza (O, then S, then N); dioxaza (2 Os, then N).
• Stem: indicates the ring size, from 3-membered to 10-membered rings.
• Suffix: indicates the degree of unsaturation in the ring.
  • Saturated suffix applies to completely saturated rings.
  • Unsaturated suffix applies to rings with the maximum number of non-cumulated double bonds.
  • Dihydro, tetrahydro, perhydro prefixes are used for rings with a lesser degree of unsaturation.

Numbering of Monocycles

• For monocycles with one heteroatom, numbering starts at the heteroatom to give substituents the lowest possible numbers.
• If multiple different heteroatoms, numbering starts at the heteroatom with higher priority, then proceeds to give other heteroatoms the lowest possible number.

Naming Partially Saturated Rings

• Obligatory saturation: If only one saturated position, it's indicated with a number and "H" (e.g., 1H, 2H, 3H, etc.).
• If fewer than the maximum number of double bonds, use dihydro, tetrahydro, etc., before the normal ring name.

IUPAC and Trivial Names of Common Monocycles

• Lists of three-, four-, five-, and six-membered rings, both saturated and unsaturated, with examples.

Naming of Fused Rings

• Two fused rings:
  • The parent name is the larger heterocyclic ring.
  • The name of the fused benzene ring ("benzo") is the prefix.
  • The side of fusion is indicated by letters (a, b, c...).
• Total numbering: Numbering the complete fused system is necessary to locate saturations and substituents.
• Order of priority for parent ring selection.
• Additional rules for three or more fused rings, including selection criteria when different rings have various heteroatoms or have similar or different sizes.

Explore the fascinating world of heterocyclic compounds, where one or more atoms in cyclic structures are heteroatoms. This quiz covers their classifications, nomenclature according to IUPAC, and examples found in nature and synthetic drugs. Test your knowledge of this important chemistry subject.