Pyridines mesomeric structures

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

When the imidazole ring is considered to be something resembling a pyrrole-pyridine combination (1) it would appear that any electrophilic attack should take place preferably at C-5 (pyrrole-or, pyridine-j8). Such a model, though, fails to take account of the tautomeric equivalence of C-4 and C-5 (Section 4.06.5.1). The overall reactivities of imidazole and benzimidazole can be inferred from sets of resonance structures in which the dipolar contributors have finite importance (Section 4.06.2) or by mesomeric structures such as (2). These predict electrophilic attack in imidazole at N-3 or any ring carbon atom, nucleophilic attack at C-2 or C-1, and also the amphoteric nature of the molecule. In benzimidazole the acidic and basic properties, the preference for nucleophilic attack at C-2 and the tendency for electrophiles to react at the fused benzene ring can be readily rationalized. 

Structure of pyridine resonance contributors (mesomeric structures)... 

Molecules having a high dipole moment or with mesomeric structures, such as dimethylsulphoxide and pyridine-Af-oxide... 

Molecules having a high dipole moment or with mesomeric structures, such as dimethylsulphoxide and pyridine-A -oxide Molecules combining two or more of the preceding characteristics, such as ammonium acetate, A -methylacetamide, and the potassium salt of picolinic acid-A -oxide. 

The fact that the equilibrium for aminopyridine 1-oxides is displaced further in favor of the amino form than is the equilibrium for aminopyridines is in accord with the mesomerism of these compounds. The stabilization of the amino forms (e.g., 241) by structures of type 240 is more effective than the corresponding stabilization in the pyridine series since the negative charge is associated with the oxygen atom. The stabilization of the imino form (e.g., 242) by structures of type 243 is less than in the pyridine series because of the adverse inductive effect of the oxygen atom. ... 

Pyrolysis of the 2-(2-azidobenzoyl)pyridines 334 (R and R = H or Br) at llO C gives the benzisoxazoles 335, which rearrange to the mesomeric betaines 338 in good yield at higher temperatures (215°C). The yellow, crystalline betaines (v q 1640 cm ) are stable compounds whose structure is supported by an X-ray crystallographic study of the bromo derivative 338 (R = H, = Br). The following reactions of the parent system (338 R = R2 = H) have been reported (Scheme 13) (i) Nitration gives the 2-nitro... 

Figure 3.11. Mesomeric Kekuli structures of various systems (a) pyridine, (b) pyrazoie, (c) pyrrole and their graphs S, 0 mi M.

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