Abstract
The ultraviolet (UV) spectra and the pKa and pKsp of 14 different variations of 4-hydroxy-1,3,3a,7-tetraazaindene (TAI) measured in water at 25°C are discussed in this report. The acidity of the tautomeric proton correlated with the inductive electronic effect of the substituent as measured by Hammett' σρ constants; pKa values obtained potentiometrically and spectrophotometrically ranged from 3.4 to 6.5. The solubility products of the 1:1 TAI:Ag salts were measured at equilibrium using a solid-state silver sulfide electrode. The pKsp values ranged from 9.6 to 12.9 and increased with substituent bulk at the 2 position and with electron-withdrawing substituents at the 5 position. Oxidation of an alkylthio substituent at either the 2 or 5 position decreased the pKsp value. A carboxylic acid group at the 2 position yielded a soluble silver complex with log β1 of 7.9; however, carboxylic acid substitution at the 5 position yielded a sparingly soluble salt with pKsp of 10.4. Values of pKa and pKsp were observed to be correlated only for substituents in definite homologous series; however, there was no general correlation within the 14 examples.
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