The reaction of an aromatic heterocycle with a proton is called a protonation. One of articles about this theory is 《Pyridine and quinoline derivatives. XLI. Chlorination of pyridine》. Authors are Wibaut, J. P.; Nicolai, J. R..The article about the compound:3,4,5-Trichloropyridinecas:33216-52-3,SMILESS:C1=NC=C(C(=C1Cl)Cl)Cl).Computed Properties of C5H2Cl3N. Through the article, more information about this compound (cas:33216-52-3) is conveyed.
Passing 1.9 moles of C5H5N (I), 0.36 mole N and 2.8 moles Cl through a Cu tube at 270° for 7.25 hr. gives about 0.59 mole of 2-chloropyridine (II) (46% on I consumed) and about 2 g. of 2,6-dichloropyridine (III); there is also present a small quantity of a pyridylpyridinium compound, since hydrolysis yields 2-aminopyridine (IV). At 400° the main product is III. Chlorination in the gas phase takes place very slowly at about 200°; the products are 3,5-di-Cl and 3,4,5-tri-Cl derivatives of I. Chlorination of fused I.HCl at about 170° gives a considerable yield of the 3,5-di-Cl derivative of I, together with a small quantity of the 3,4,5-tri-Cl derivative and the penta-Cl derivative; addition of HgCl2 had no effect upon the type of substitution or yield of products obtained. Heating 6 g. of III with 20% NH4OH a 180-90° for 40 hrs. gives 2.6 g. of 6-chloro-2-aminopyridine, m. 75°. Heating 4 g. I with 20% NH4OH and 2 g. CuSO4 at 180-90° for 20 hrs. gives 1 g. IV.
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Reference:
Bromide – Wikipedia,
bromide – Wiktionary