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Aromatic compounds which contain heteroatoms (e.g. O, N, S) as part of the cyclic conjugated π system are called heteroaromatics.

Study Tip:
Each heteroatom can only make ONE contribution to the π system. This is because there can ONLY be ONE p orbital in the valence shell with the appropriate geometry to interact with the other p orbitals of the π system.  There are three possible options:
  1. The heteroatom is part of a π bond (e.g. pyridine)
  2. The heteroatom contributes a lone pair to the π system (e.g. furan)
  3. or it contributes an empty p orbital to the π system 
Hence, in modes 1 and 2, the heteratom contributes 2 electrons to the π system and in mode 3 it contributes none.

Several of the simpler and more common heteroaromatic systems are shown below.
In each case consider role of heteroatom in the π system :  is it part of a π bond or does it contribute a lone pair to the π system ?
Check the πsystem link to see.

* Purine is a precursor to the DNA bases Adenine and Guanine.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary