Halogenation of pyridine

Halogenation of pyridines is easier than nitration or sulfonation because it can be carried out in non-acidic media and the pyridine-halogen adducts are appreciably dissociated. Dihalogenation can occur since one halogen atom causes little additional deactivation of the ring. The mercuration of pyridines (Section 3.2.1.4.9) probably involves coordination of the pyridine nitrogen to the mercury atom, and such coordination causes less ring deactivation than protonation. 

Vapor phase halogenation of pyridine at high temperatures gives mixtures of 2- and 2,6-dibromopyr-idine (Br2, 500°C or CuBr-Br2, 350°C) and 2- and 2,6-di-chloropyridine (Cl2, 270°C). 

The halogenation of pyridine and some of its derivatives at high temperatures in the gaseous phase has been the object of numerous studies, first by Wibaut and then by den Hertog and their respective... 

Substituent effects on the proton-losing step would then be responsible for the isomer-distribution pattern. A similar picture could be drawn for the high temperature gas-phase halogenation of pyridines. 

Halogenation of pyridine occurs with elemental chlorine or bromine at high temperature. 3-Halo- and 3,5-dihalopyridines are formed at ca. 300°C as a result of an ionic S Ar process ... 

In a kinetic study of the halogenation of pyridine, quinoline, isoquinoline, 2-picoline, and 2,6-lutidine with ICl, linear Eyring plots are obtained. Dipole-moment calculations indicate that the charge-transfer state is the transition state. ... 

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