Editing Organic chemistry (section)

===Heterocyclic compounds===
{{Main|Heterocyclic compound}}
The characteristics of the cyclic hydrocarbons are again altered if heteroatoms are present, which can exist as either substituents attached externally to the ring (exocyclic) or as a member of the ring itself (endocyclic). In the case of the latter, the ring is termed a [[heterocycle]].  [[Pyridine]] and [[furan]] are examples of aromatic heterocycles while [[piperidine]] and [[tetrahydrofuran]] are the corresponding [[alicyclic]] heterocycles. The heteroatom of heterocyclic molecules is generally oxygen, sulfur, or nitrogen, with the latter being particularly common in biochemical systems.

Heterocycles are commonly found in a wide range of products including aniline dyes and medicines. Additionally, they are prevalent in a wide range of biochemical compounds such as [[alkaloids]], vitamins, steroids, and nucleic acids (e.g. DNA, RNA).

Rings can fuse with other rings on an edge to give [[polycyclic compound]]s. The [[purine]] nucleoside bases are notable polycyclic aromatic heterocycles.  Rings can also fuse on a "corner" such that one atom (almost always carbon) has two bonds going to one ring and two to another.  Such compounds are termed ''[[spiro compound|spiro]]'' and are important in several [[natural product]]s.