Britannica
Sources used in this answer
- heterocyclic compound - Major classes of heterocyclic compounds
- heterocyclic compound - Rings with seven or more members
- heterocyclic compound - General aspects of heterocyclic compounds
- heterocyclic compound - Three-membered rings
- heterocyclic compound - The nature of heteroaromaticity
- heterocyclic compound - Four-membered rings
How are heterocyclic compounds classified by ring size?
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heterocyclic compound
There are many different sulfur-containing heterocycles. One of the best known is thiophene, C4H4S, derivatives of which occur as plant pigments and as other natural products such as biotin.
In chemistry, heterocyclic compounds are classified by ring size because rings of a given size typically share common structural and chemical features. This classification is organized as follows:1
- Three-membered rings: These include compounds such as aziridine, oxirane, and thiirane. Due to their small size, they are geometrically strained, making them highly reactive and prone to ring-opening reactions.12
- Four-membered rings: Examples include azetidine, oxetane, and thietane. Like three-membered rings, they are strained and reactive, though they generally form and react less readily than their three-membered counterparts.13
- Five- and six-membered rings: These are the most common and stable heterocycles. Their size allows for the development of aromatic character, and they are readily formed. Common examples include pyridine, pyrrole, furan, and thiophene.14
- Seven-membered rings and larger: These compounds are stable but less readily formed and less extensively investigated than five- or six-membered rings. As size increases, they often become nonplanar to reduce ring strain. This category includes crown ethers and porphyrins, which are known for their ability to bind specific ions and molecules.15