2.6- Dibromopyridine , reaction with sodium

Pyx idine and mercuric acetate when heated at 175 to 180" C. tor 2 5 hours, and the reaction mixture diluted with 5 to 6 volumes of water, followed by the addition of sodium chloride give 3 5-dichloromercuri pyridine (I.) as a pale brown, amorphous, odourless powder, decomposing at 220" C. Bromine in sodium bromide solution converts it into 3 5-dibromopyridine. The mother-liquors from the dichloromercuri compound yield 3-iodomercuri pyridine (II.) when treated with sodium iodide. The product is a yellow, amorphous powder, M.pt. 68" to 69" C., which forms 3-bromopyridine with bromine in sodium bromide. 

Since sulfonation of pyridine iV-oxide is about as difficult as is that of pyridine itself and takes place at the 3-position,17 it has been assumed18 that, in fuming sulfuric acid, pyridine iV-oxide reacts only in the salt form (3), when the prediction is that substitution at C-3 would take place. It is, however, difficult to account for the fact that bromination, even at 110° in the presence of iron powder, does not occur.17 Bromination in chloroform solution in the presence of acetic anhydride and sodium acetate (when the O-acetate is the the probable substrate) take place readily, however, to give 3,5-dibromopyridine JV-oxide.19 The predicted order of nucleophilic reactivity, on the basis of both atom localization energies and ground-state v-electron density calculations, is 4 > 2 > 3. The same order is predicted for the nucleophilic substitution reactions of the salts of pyridine JV-oxide. In actual practice, iV-alkoxypyridinium derivatives undergo nucleophilic attack preferentially at C-2.20-23 The reaction of some pyridine iV-oxides with phosphorus pentachloride may involve the formation... 

When pyridine 1-oxide reacts with bromine, sodium acetate and acetic anhydride, 3,5-dibromopyridine 1-oxide is formed . / -Nitrobenzoyl chloride and silver nitrate produce small yields of 3-nitro- and 3,5-dinitro-pyridine l-oxide . These reactions probably involve nucleophilic addition, followed by electrophilic substitution, and the first step can be represented as occurring at C(4), though not with any compelling reason ... 

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