James Richard Fromm
Incorporation of an oxygen, a nitrogen, a sulfur, or an atom of a related element into an organic ring structure in place of a carbon atom gives rise to a heterocyclic compound. Since the heterocyclic atom must form more than one bond in order to be incorporated into a ring structure, halogens do not form heterocyclic compounds although they may be substituents on a heterocyclic ring structure. Heterocyclic compounds, like polycyclic ring compounds, are usually known by non-systematic names.
They may be either simple aromatic rings or non-aromatic rings. Some examples are pyridine (C5H5N), pyrimidine (C4H4N2) and dioxane (C4H8O2).
|Three Pyrimidines (C4H4N2) - Cytosine, Thymine, and Uracil|
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.
Heterocyclic chemistry is the chemistry branch dealing exclusively with synthesis, properties and applications of heterocyclics especially vital to drug design.
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 heterocyclics are:
|Aziridine||Ethylene Oxide, Oxirane||Ethylene Sulfide, Thiirane|
Azetidine, oxetane, and thietanethe four-membered rings containing, respectively, nitrogen, oxygen, and sulfur atomsare prepared by nucleophilic displacement reactions such as those used to prepare the corresponding three-membered rings
|Trimethylene Imine, Azetidine||1,3-Epoxypropane, Oxetane||Trimethylene Sulfide, Thietane|
With heterocyclics containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.
|1,2-Diazine, Pyridazine||1,3-Diazine, Pyrimidine||1,4-Diazine, Pyrazine|
1,2-dithiane 1,3-dithiane 1,4-dithiane
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