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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 (C<sub>5</sub>H<sub>5</sub>N), pyrimidine (C<sub>4</sub>H<sub>4</sub>N<sub>2</sub>) and dioxane (C<sub>4</sub>H<sub>8</sub>O<sub>2</sub>).