The acid dissociation constants of four hydroxy azo pyrazolopyrimidine
derivatives were determined in
aqueous−organic solvent mixtures. The organic solvents are
methanol, ethanol, acetone, and dimethylformamide. The results obtained are discussed in terms of the
solvent characteristics. The ionization
constants of the dyes in question depend largely on both the proportion
and the nature of the organic
cosolvent. Hydrogen-bonding interactions of the conjugate base
with solvent molecules as well as the
solvent basicity contribute the major effects on the ionization
process. The effect of molecular structure
of the azo compound on the pK
a value is
discussed.
The effect of different organic solvent + water mixtures on the acid dissociation constants of protonated
5-amino-4-(arylazo)-3-methyl-1-phenylpyrazole derivatives were studied. The organic solvents used are
methanol, ethanol, acetone, and dimethylformamide. The results obtained were discussed in terms of
the solvent characteristics.The pK
BH
+
values of the dyes in question were determined and found to depend
on both the amount and nature of the organic cosolvent. The decrease of the pK
BH
+
values as the amount
of organic solvent in the medium is increased can be accounted for in terms of the high stabilization of
the protonated form of the compounds by dispersion forces rather than by hydrogen bonding. Also the
effect of solvent polarizability and hydrogen-bonding interaction on the pK
BH
+
values and thus on the
spectra of the charge-transfer band observed have been discussed. The absolute pK
BH
+
value for each
compound despite the presence of more than one basicity center has also been discussed.
The acid dissociation constants of some new 8-[(aryl)azo]-2,4-dimethylpyrazolo[1,5-a]pyrimidine-7(6H)-one derivatives were determined in aqueous-organic solvent mixtures. The organic solvents are methanol, ethanol, acetone, and dimethylformamide. The results obtained were discussed in terms of the solvent characteristics. The ionization constants of the title dyes depend largely on both the ratio and the nature of the organic cosolvent. Hydrogen-bonding interactions of the conjugate base with solvent molecules as well as the solvent basicity contribute the major effects on the ionization process. It has been concluded that the pK a values increase by increasing the amount of organic cosolvent in the medium. The effect of molecular structure of the azo compound on the pK a value is discussed.
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