ABSTRACT
This chapter describes the structures of aromatic heterocycles and gives a brief summary of some physical properties. Benzene is described as a resonance hybrid of the two extreme forms which correspond, in terms of orbital interactions, to the two possible spin-coupled pairings of adjacent p-electrons structures. The degree of aromaticity depends on the contribution which dipolar structures, with a 'complete' pyridinium ring make to the overall structure. Five equivalent contributing structures, show each carbon atom to be equivalent and hence to carry one fifth of the negative charge. The UV spectra of the simple five-membered heteroaromatic systems all show just one medium-to-strong low-wavelength band with no fine structure. Electrophilic addition to the pyridine nitrogen generates pyridinium ions, the simplest being 1H-pyridinium formed by addition of a proton. 1H-Pyridinium is actually isoelectronic with benzene, the only difference being the nuclear charge of nitrogen, which makes the system, as a whole, positively charged.
