2- Phenylpyridine 1-oxide, nitration

Hands and Katritzky (58JCSI754) compared the nitration of the phenyl and benzylpyridines and their /V-oxides. The results are shown in Table VIII. The tentative conclusion was that 2-phenylpyridine and its N-oxide nitrated as the free base, but that benzylpyridines may be nitrated as conjugate acids. However, later work has shown that, in fact, both compounds react as their conjugate acids [68JCS(B)862]. Further work has shown that the same result occurs with 4-phenyl and 4-benzylpyridines [71 JCS(B)712]. 

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

The partial rate factor for nitration of pyridine-N-oxide in the 4 position was estimated as 4x 10"6 which is, therefore, close to that found for the 3 position of pyridine, and 2-phenylpyridine-N-oxide was evaluated as 2xl0-4 times less reactive than benzene from rate measurements in 74.7-78.6 wt. % acid at 25 °C. 

The catalytic hydrogenation (p. 263), their behaviour on oxidation (p. 265) and the nitration of the phenylpyridines and their derivatives (p. 175) have been discussed. The interesting intramolecular cyclizations (70)-(71) are brought about by hydrobromic acid. ... 

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