5-Membered Rings
With heterocycles containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.
Five-membered rings with one heteroatom:
| Heteroatom | Saturated | Unsaturated |
|---|---|---|
| Nitrogen | Pyrrolidine (Azolidine is not used) | Pyrrole (Azole is not used) |
| Oxygen | Tetrahydrofuran(Oxolane is rare) | Furan (Oxole is not used) |
| Sulfur | Thiolane | Thiophene (Thiole is not used) |
| Boron | Borolane | Borole |
| Phosphorus | Phospholane | Phosphole |
| Arsenic | Arsolane | Arsole |
| Antimony | Stibolane | Stibole |
| Bismuth | Bismolane | Bismole |
| Silicon | Silolane | Silole |
| Tin | Stannolane | Stannole |
The 5-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are collectively called the azoles. Thiazoles and isothiazoles contain a sulfur and a nitrogen atom in the ring. Dithiolanes have two sulfur atoms.
| Heteroatom | Saturated | Unsaturated (and partially unsaturated) |
|---|---|---|
| Nitrogen/nitrogen | Imidazolidine Pyrazolidine |
Imidazole (Imidazoline) Pyrazole (Pyrazoline) |
| Nitrogen/oxygen | Oxazolidine Isoxazolidine |
Oxazole (Oxazoline) Isoxazole |
| Nitrogen/sulfur | Thiazolidine Isothiazolidine |
Thiazole (Thiazoline) Isothiazole |
| Oxygen/oxygen | Dioxolane | |
| Sulfur/sulfur | Dithiolane |
A large group of 5-membered ring compounds with three heteroatoms also exists. One example is dithiazoles that contain two sulfur and a nitrogen atom.
| Heteroatom | Saturated | Unsaturated |
|---|---|---|
| 3 × Nitrogen | Triazoles | |
| 2 × Nitrogen / 1 × oxygen | Furazan Oxadiazole |
|
| 2 × Nitrogen / 1 × sulfur | Thiadiazole | |
| 1 × Nitrogen / 2 × sulfur | Dithiazole |
Five-member ring compounds with four heteroatoms:
| Heteroatom | Saturated | Unsaturated |
|---|---|---|
| 4 × Nitrogen | Tetrazole |
With 5-heteroatoms, the compound may be considered inorganic rather than heterocyclic. Pentazole is the all nitrogen heteroatom unsaturated compound.
Read more about this topic: Heterocyclic Compound
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