Black Sheep of the Family Heterocyclic Aromatic Compounds

Black Sheep of the Family Heterocyclic Aromatic Compounds 

Up to this point, this chapter has discussed aromatic systems composed exclusively of rings of carbon atoms. But aromatic systems can contain heteroatoms, which in this case means any atoms in the ring other than carbon. 

Heteroatoms (such as O, S, N) that are double-bonded to other atoms in a ring can t donate lone-pair electrons to the pi system because their p-electrons are already involved in the double bond. Single-bonded heteroatoms can donate a single lone-pair to the pi system but not two, because one lone-pair must be in an unhybridized p orbital orthogonal (at 90 degrees) to the sp2 ring plane. 

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