<unk> s are organic<unk> s 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<unk> s such as cyclopropane, an anaesthetic with explosive properties, and cyclohexane, a solvent, are not<unk> , they are merely cycloalkanes. The suffix '-cyclic' implies a ring structure, while 'hetero' refers to an atom other than carbon, as above.  Many<unk> s, including some amines, are carcinogenic.<unk>  chemistry is the chemistry branch dealing exclusively with synthesis, properties and applications of heterocycles especially vital to drug design.
Heterocyclic compound
Cyclic compound
Polycyclic compound
Biopolymer
Bifunctional catalyst
Diradical
Designer drug
Organic compound