Heterocyclic Rings

Introduction

• Heterocyclic Rings: Rings that contain atoms of at least two different elements as members of its ring(s).
• Heteroatom: Any atom other than carbon and hydrogen within the ring structure.
• Examples: Pyridine, Pyrimidine.

Basic Concepts

• Benzene: Hydrocarbon ring composed of carbon and hydrogen only.
• Pyridine: Nitrogen replaces one carbon in benzene, making it a heterocyclic ring with a nitrogen heteroatom.
• Pyrimidine: Contains two nitrogen atoms in the ring structure.

Nomenclature

Prefixes Based on Heteroatoms

• Nitrogen (N): aza-
• Oxygen (O): oxa-
• Sulfur (S): thia-
• Phosphorus (P): phosph-

Ring Size and Saturation Suffixes

• 3-membered rings:
  • Unsaturated: -irene (e.g., azirene)
  • Saturated with Nitrogen: -iridine (e.g., aziridine)
• 4-membered rings:
  • Saturated: -etidine
• 5-membered rings:
  • Unsaturated: -ole (e.g., pyrrole)
  • Saturated with Nitrogen: -olidine (e.g., pyrrolidine)
• 6-membered rings and above: Generally, no specific suffix for saturated rings.
• 7-membered unsaturated: -epine
• 8-membered unsaturated: -ocine
• 9-membered unsaturated: -onine
• 10-membered unsaturated: -ecine

Examples of Heterocyclic Rings

Common 3-membered Rings

• With Nitrogen (N): azirene (unsaturated), aziridine (saturated)
• With Oxygen (O): oxirane (unsaturated), oxirane (saturated form called oxirane)
• With Sulfur (S): thiirane (unsaturated)

Common 4-membered Rings

• With Nitrogen (N): azetidine (saturated)
• With Oxygen (O): oxetane (saturated)

Common 5-membered Rings

• With Nitrogen (N): pyrrole (unsaturated), pyrrolidine (saturated)
• With Oxygen (O): furan (unsaturated), tetrahydrofuran (THF, saturated)
• With Sulfur (S): thiophene (unsaturated)

5-membered Rings with Two Heteroatoms

• Two Nitrogens (N): pyrazole (1,2-positions), imidazole (1,3-positions)
• Oxygen and Nitrogen: oxazole (1,2-positions, isoxazole), (1,3-positions, oxazole)
• Sulfur and Nitrogen: thiazole (1,2-positions, isothiazole), (1,3-positions, thiazole)

5-membered Rings with Three Heteroatoms

• Sulfur and Two Nitrogens: e.g., 1,2,5-thiadiazole

Summary

• Heterocyclic rings vary based on the heteroatoms involved and whether they are saturated or unsaturated.
• Prefixes indicate the type of heteroatom, while suffixes (based on ring size and saturation) complete the ring's name.
• Common heterocyclic rings often have specific names based on their prevalence and historical nomenclature, such as furan and thiophene.