Heterocycies aromatic 6-ring

X = 0, NH, S benzo[c]heterocycies aromaticity is iost in the heterocyciic ring but enhanced in the benzenoid ring... 

We use the same disconnections as we used for saturated heterocycies in Chapter 29, but our task is sier because the thermodynamic stability of the aromatic rings allows short cuts. This chapters introduces few new principles and the synth is of the carbon frameworks can be useful revision. 

A heterocycle is a compound with a ring that has more than one kind of atom. Nitrogen, oxygen, and sulfur are often found along with carbon in heterocyclic rings. Saturated heterocyclic amines, ethers, and sulfides usually have the same chemistry as their open-chain analogs, but unsaturated heterocycies such as pyrrole, furan, and thiophene are aromatic. All three are unusually stable, and all three undergo aromatic substitution on reaction with electrophiles. 

Section 21.9 Aromatic Five-Membered-Ring Heterocycies... 

The saturated heterocyclic rings are shown in black, and names for the most important ring types are given some (like piperidine, morphoiine) you wiil need to remember others (tetrahydrofuran, pyrrolidine) are more obviously derived from the names for aromatic heterocycies you met in the last chapter. Some of these compounds (nicotine, coniine, cocaine) are plant products falling into the class called alkaloids which are discussed in Chapter 42. Another important class of saturated heterocycies, sugars, will also appear in Chapter 42. 

Aromatic heterocycies are extremely important, and that is where we will focus most of our attention. We begin with an important six-membered ring aromatic nitrogen heterocycle, pyridine. 

---