Pyridinecarboxylic acids tautomerism

These considerations show that caution must be exercised in applying of the Hammett equation to systems which are known to involve tautomeric equilibria, e.g. the pyridinecarboxylic acids or y-hydroxypyridines. If in such systems the experimental p L "values are linear in a, there is at least a strong suggestion that Krp is insensitive to the nature of the substituents. Some applications of these ideas in the azobenzene series have proved of considerable interest. ... 

The tautomeric form of pyridinecarboxylic acids, neutral, zwitterionic or intermediate between these two forms, in the crystalline state, is of interest, and extensive investigations exist (73CL1121, 73BCJ2020, 73BCJ2292, 73BCJ2372, 73BCJ2669). In the intermediate form, as exemplified by 2-picoIinic acid, the molecules are linked in pairs (37) by two symmetric... 

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