Heterocyclic compound

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Pyridine a simple heterocyclic compound

Heterocyclic compounds are organic compounds that contain a ring structure containing atoms in addition to carbon, such as sulfur, oxygen or nitrogen, as part of the ring. They may be either simple aromatic rings or non-aromatic rings. Some examples are pyridine (C₅H₅N), pyrimidine (C₄H₄N₂) and dioxane (C₄H₈O₂).

Note that compounds such as cyclopropane, an anaesthetic with explosive properties, and cyclohexane, a solvent, are not heterocyclic, they are merely cycloalkanes. The suffix '-cyclic' implies a ring structure, while 'hetero' refers to an atom other than carbon, as above.

Heterocyclic chemistry is the chemistry branch dealing exclusively with synthesis, properties and applications of heterocycles especially vital to drug design.

3-membered rings

Heterocycles with three atoms in the ring are more reactive because of ring strain. Those containing one heteroatom are generally stable. Those with two heteroatoms are more likely to occur as reactive intermediates. Common 3-membered heterocycles are:

• One N: Aziridine
• One O: Ethylene oxide, generally known as epoxides. The systematic name is oxirane. The unsaturated ring system is an oxirene (generally unstable).
• One S: Thiirane, generally known as an episulfide.

4-membered rings

• One N: Azetidine
• One O: Oxetane

5-membered rings

With heterocycles containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.

• With one heteroatom:
  • With N: Pyrrole (aromatic), dihydropyrrole and tetrahydropyrrole
  • With O: Furan (aromatic), dihydrofuran and tetrahydrofuran
  • With S: Thiophene (aromatic), dihydrothiophene and tetrahydrothiophene
  • With As: Arsole and analogues
• With two heteroatoms:
  • One N and one or more of N,S,O: the azoles
  • Two S: Dithiolane

6-membered rings

• With one heteroatom:
  • With N: Pyridine, Piperidine
  • With O: Pyran
  • With S: Thiine, also known as thiane and thiapyrane
• With two heteroatoms:
  • Two N: Pyridazine, Pyrimidine, and Pyrazine are the 1,2-, 1,3-, and 1,4-isomers, respectively.
  • Two N: Piperazine
  • One N and one O: Oxazine
  • One N and one S: Thiazine
  • Two S: Dithiane
  • Two O: Dioxane