Hydroxy-, hydration equilibrium ratios

Triazanaphthalene, although at one time thought to undergo ring-opening under acidic conditions, is now known to form a stable hydrated cation. Examination of the ultraviolet spectra by the stopped-flow technique has shown that the ratio of the hydrated to the anhydrous forms is 95 and 0.0001 for the cation and neutral molecule, respectively, at equilibrium. This substance hydrates in the 1,2-position, Hydration has also been found in the 3-methyl, 3-hydroxy, and 7-amino derivatives, but not in the 2- or 8-hydroxy deriva-tives. ... 

The ratio, at equilibrium, of the hydrated to anhydrous forms (for both neutral species and anions) has been measured for the following 2-hydroxjrpteridine and its 4-, 6-, and 7-methyl and 6,7-dimethyl derivatives 6-hydroxypteridine and its 2-, 4-, and 7-methyl derivatives 2,6-dihydroxypteridine and 2-amino-4,6-dihydroxypteridine. The following showed no evidence of hydration 4- and 7-hydroxy-pteridine 2,4-, 2,7-, 4,7-, and 6,7-dihydroxypteridine and 2-amino-4-hydroxypteridine. The kinetics of the reversible hydration of 2-hydroxypteridine and its C-methyl derivatives (also 2-mercapto-pteridine) have been measured in the pH region 4-12, and all these reactions were found to be acid-base cataljrzed. The amount of the hydrated form in the anions is always smaller than in the neutral species, but it is not always negligible. Thus, the percentages in 2-hydroxy-, 2-hydroxy-6-methyl-, 2-mercapto-, and 2,6-dihydroxypteridine are 12, 9, 19, and 36%, respectively (see also Table VI in ref. 10). 

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