Pyridines bromination

The product distribution can be shifted to favor the 1 -product by use of such milder brominating agents as the pyridine-bromine complex or the tribromide ion, Br3. It is believed that molecular bromine reacts through a cationic intermediate, whereas the less reactive brominating agents involve a process more like the AdgS and-addition mechanism. 

Subsequent kinetic and product distribution data on the reactions of 1,3-butadiene with molecular bromine, pyridine-bromine complex and tetra-n-butylammonium tribromide in chlorinated solvents have shown that pyridine-Br2 and tribromide ion act as independent electrophiles, rather than as sources of molecular bromine75. 

Bromination of pyridine is much easier than chlorination. Vapour phase bromination over pumice or charcoal has been studied extensively (B-67MI20500) and, as with chlorination, orientation varies with change in temperature. At 300 °C, pyridine yields chiefly 3-bromo-and 3,5-dibromo-pyridine (electrophilic attack), whilst at 500 °C 2-bromo- and 2,6-dibromo-pyridine predominate (free radical attack). At intermediate temperatures, mixtures of these products are found. Similarly, bromination of quinoline over pumice at 300 °C affords the 3-bromo product, but at higher temperatures (450 °C) the 2-bromo isomer is obtained (77HC(32-1)319). Mixtures of 3-bromo- and 3,5-dibromo-pyridine may be produced by heating a pyridine-bromine complex at 200 °C, by addition of bromine to pyridine hydrochloride under reflux, and by heating pyridine hydrochloride perbromide at 160-170 °C (B-67MI20500). 

In principle, the photoreactions of CT s are able to offer a great number of photoinitiator systems for radical polymerization. But, so far, this subject has only received little attention, and the current knowledge relative to the photochemistry of such complexes is poor. In addition to the amine complexes mentioned above, chinoline-bromine [124-127], chinoline-chlorine [128], 2-methylpyridine-chlorine [129], pyridine-bromine [130], IV-vinylpyrrolidone-bromine [131], acridone-bro-mine [132], acridone-chlorine [133], benzophenone-S02 [134], isoquinoline-S02 [135, 136], and 2-methylquinoline-S02 [136] combinations are used for radical polymerization of AN, alkyl methacrylates, acrylic and methacrylic acid, and for... 

Charge transfer (CT) complexes, different from aromatic ketone/amine systems, such as quinoline-bromine, pyridine-bromine, tetrahydrofuran-bromine etc. have also been reported to behave as initiators of vinyl polymerization, particularly under photoactivation [65-68]. 

The pyridine-bromine complex or its hydrochloride, heated at 200°, gives 3-bromo- and 3,5-dibromo-pyridinei 7, Later workers Tt obtained more complex results, the 3-bromopyridine probably being contaminated with 3,4-dibromopyridine or other reactive halogen derivatives which subsequently decomposed to polymeric salt-like material. More practicable is the preparation of 3-bromo- and 3,5-dibromo-pyridine by adding bromine to pyridine hydrochloride under reflux (a reaction in which iron and mercuric chloride have been used as catalysts) 3-Bromopyridine in good... 

Haque I, Wood JL (1968) The vibrational spectra and structure of the bis(pyridine)iodine(I), bis(pyridine)bromine(I), bis(Y-picoline)iodine(I) and bis(Y-picoline)bromine(I) cations. J Mol Struct 2 217-238... 

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