Hydroxypyridines ionization constants

DE Metzler, EE SneU. Spectra and ionization constants of the vitamin B-6 group and related 3-hydroxypyridine derivatives. J Am Chem Soc 77 2431-2437, 1955. 

The individual ionization constants for pairs of tautomers and the constant for the equilibrium between them can be deduced from the experimentally determined ionization constants in another way. It will be seen (p. 156) that the effect of substituents on the basic strength of the pyridine nitrogen atom can be represented by the Hammett equation. Accordingly, the ionization constant for the NM form (T b) can be estimated from the Hammett substituent constant of the group — A H. Combined with the experimentally determined Ki this leads to estimates of the remaining quantities. In this way Jaffe 2 deduced that nicotinic and isonicotinic acids exist predominantly as zwitterions in aqueous solution. Similarly, Bryson s obtained the values pi B 5-0 and Kt = 0 78 for 3-hydroxypyridine (cf. Tables 5,10 and 5.12), Hammett correlations have been used in evaluating the tautomeric equilibria in a number of 2-arylsulphonamidopyridines . ... 

By the use of ionization constants it has been concluded that 2- and 4-aminopyridine 1-oxide exist predominantly as such in aqueous solution, whilst 4-hydroxypyridine 1-oxide is in equilibrium with a comparable amount of l-hydroxypyrid-4-one. Application of the method to the pyridine 1-oxides is more complicated than in the pyridine series, and it fails with 2-hydroxypyridine 1-oxide . ... 

X-Ray structural data (p. 120), dipole moments (p. 123), spectra (pp. 139, 144), ionization constants, the tautomerism of compounds in these groups (pp. 154-5) and the behaviour of hydroxypyridines towards alkylating (p. 183) and acylating reagents (p. 196) have been discussed. Some of these compounds are described in Table 6.14. 

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