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Aromatic heterocycles 1: structures and reactions

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With one nitrogen atom. this is pyridine and the drug sulfapyridine is an example. aromaticity survives when parts of benzene's ring are replaced. 10 pages
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IntroductionBenzene is aromatic because it has six electrons in a cyclic conjugated system. We know it is aromat-ic because it is exceptionally stable and it has a ring current and hence large chemical shifts in theproton NMR spectrum as well as a special chemistry involving substitution rather than addition withelectrophiles. This chapter and the next are about the very large number of other aromatic systems inwhich one or more atoms in the benzene ring are replaced by heteroatoms such as N, O, and S. Thereare thousands of these systems with five- and six-membered rings, and we will examine just a few.Our subject is aromatic heterocycles and it is important that we treat it seriously because most—probably about two-thirds of—organic compounds belong to this class, and they number amongthem some of the most significant compounds for human beings. If we think only of drugs we candefine the history of medicine by heterocycles. Even in the sixteenth century quinine was used to pre-vent an...