Pyridine, aromaticity bond lengths

Comparison of the bond lengths with those of benzene, naphthalene and pyridine suggests that whilst there is certainly aromatic character some bond fixation occurs in the naphthyridines. It is probable that canonical structure (1) (of, for example, 1,5-naphthyridine) and to a lesser extent structures (7) and (8) are the major contributors to the ground state, but the charged structures (9) and (10) must also be included in order to account for the chemical properties of the naphthyridines. 

As with pyridine, the nitrogen atoms of quinoline and isoquinoline each bear a lone pair of electrons not involved in aromatic bonding which can be protonated, alkylated, or complexed to Lewis acids. This chapter should be read in conjunction with the chapter on pyridines as several points discussed at length there are also relevant to the chemistry of quinoline and isoquinoline. 

Both C5H5P and C5H5As have planar structures with C—C and C—E bond lengths indicative of aromatic delocalization of the ir electrons. However, the scope of their aromatic organic chemistry is not nearly as extensive as that of pyridine. Phosphinine and its derivatives form both type complexes, for example, 10-XXXVI and 10-XXXVII. 

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