Spectrophotometric Determination of Acidity Constants of Some Recently Synthesized Anthraquinones in Methanol + Water (original) (raw)
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The influence of solvent properties on acidity constants of some newly synthesized 9,10anthraquinone and 9-anthrone derivatives was studied in methanol-water mixtures in a composition range of 0.57 to 1.0 methanol mole fraction. The model was established by using both multiple linear regression and target factor analysis. Both methods revealed that the solvent polarity/polarizability parameter π * is a major factor in controlling the acidity behavior of the anthraquinones and anthrones studied in binary methanol-water mixed solvents. A QSPR study was conducted to drive the relationships between the π * coefficient s and the polarity/polarizability of molecules. Both dipole moment and polarizability were found to have a linear relationship with s. The results confirm that, in the dipolar protic solvents used, the dipole-dipole interaction (for neutral molecules) and the ion-dipole interaction (for ionized molecules) are the major factors controlling the acidity behavior of these compounds.
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The acid-base properties of calcon (1-(2-hydroxy-1-naphthylazo)-2--naphthol-4-sulfonic acid) in water and mixed water-organic solvents at 25 °C at an ionic strength of 0.10 M are studied by a multiwavelength spectrophotometric method. The organic solvents used were the amphiprotic (methanol), dipolar aprotic (dimethylsulfoxide), and low basic aprotic (acetonitrile). To evaluate the pH absorbance data, a resolution method based on the combination of soft-and hard-modeling was applied. The acidity constants of all related equilibria were estimated using the whole spectral fitting of the collected data to an established factor analysis model. The data analysis program Datan was applied for determination of the acidity constants. The corresponding pK a values were determined in water and mixed water-organic solvents. Linear relationship between the acidity constants and the mole fraction of the different solvents in the mixtures exist. The effect of solvent properties on acid-base behavior is discussed.
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As constantes de dissociação aparentes (pK a) de quatro 2-hidroxinaftoquinonas, diferentemente substituídas em C-3, foram determinadas em meio aquoso-etanólico (1:1, v/v), utilizando titulações pH-métricas e espectrofotométricas. O isolapachol (pK a <6) mostrou ser mais ácido que seu análogo natural, o lapachol, (pK a >6). Os derivados 3-metilaminados apresentam dois valores de pK a , um relacionado ao grupo enólico e o outro ao sal de amônio, e são zwitteriônicos, em larga extensão, em pH fisiológico. As possíveis conseqüências desses parâmetros frente a suas atividades biológicas são discutidas. The apparent dissociation constants (pK a) of four 2-hydroxynaphthoquinones, differently substituted at C-3, were determined in water:ethanol (1:1, v/v) solutions by pH-metric and hybrid pH-metric/UV titration methods. Isolapachol (pK a < 6) was more acidic than lapachol (pK a > 6). Two pK a values were determined for each of the methylamino-derivatives investigated, the first relating to the enol function and the second to the ammonium salt. It was determined that under physiological pH, these derivatives would be to a large extension, zwitterionic. The possible effects of the measured parameters on the biological activities of the studied compounds are discussed.
Analytica Chimica Acta, 1995
The solvatochromic studies on 9,10-anthraquinone (AQ), and its symmetric dihydroxy derivatives namely l,Sdihydroxyanthraquinone (1,5-DHAQ) and 2,6_dihydroxyanthraquinone (2,6-DHAQ) in aqueous solvent mixtures of methanol, acetonitrile, tetrahydrofuran and n-propanol are reported. Preferential solvation is detected in every case. However, a remarkable variation in the magnitude of the preferential solvation constant KAB is observed when the composition of the mixture is changed. This effect is attributed to the solvent-solvent interaction between components. Moreover, and although stronger dipolar interactions are to be expected between the solute and water, only preferential solvation by the organic solvent is detected in every case. These effects may be explained in terms of self association of water through hydrogen bonding and the "microheterogeneity" of the binary mixtures. Moreover, differences are observed in the solvatochromic behavior of the solutes. In the case of 2,6-DHAQ the so-called synergistic effect is observed and explained as a function of the strong hydrogen bond solute-solvents interactions. Reversed-phase liquid chromatographic (RPLC) studies of AQ and 2,6-DHAQ in aqueous mixtures of methanol, acetonitrile and tetrahydrofuran were also performed. The results were interpreted by the Kamlet-Taft solvatochromic comparison method showing that the solvatochromic results can be correlated with certain success with RPLC data.
Study of natural anthraquinone colorants by EPR and UV/vis spectroscopy
The spectroscopic study was performed on 9,10-anthraquinone derivatives (purpurin, alizarin, carminic acid, and 2-(hydroxymethyl)-9,10-anthraquinone) in dimethylsulfoxide in the presence of triethylamine to bring information on their protonation/deprotonation equilibria in aprotic solvent. An interesting deprotonation effect of 5,5-dimethyl-1-pyrroline N-oxide (a spin trapping agent used in the EPR experiments) was revealed. The quantum chemical calculations enabled the identification of the individual protonated/deprotonated tautomeric forms present in the experimental systems. The UV photoexcitation of hydroxyanthraquinones (HAQ) led to the generation of reactive radical species and singlet oxygen, detected by in situ EPR spectroscopy (spin trapping, nitroxide radical elimination, oxidation of sterically hindered amines). The changes in the electronic absorption spectra upon photoexcitation, linked with the ability of the studied HAQ to generate reactive oxygen species upon exposure, confirm a substantial effect of the substituent character and position on the overall photochemical behavior of the HAQ, significantly influenced by the actual experimental conditions (solvent, pH).
Spectrochimica Acta Part A-molecular and Biomolecular Spectroscopy, 2006
Electronic absorption and excitation spectra of 1-methyl-4-[4'-amino styryl]quinolinium iodide (Q-NH2) were measured in aqueous buffer and micellar solutions. The acid dissociation constants in ground and excited states, pK and pK*, were determined spectrophotometrically and amount for the aqueous solution to 3.24 and 1.19, respectively and the pK and pK* for the (Q-NH2) in sodium dodecyl sulphate are 3.72 and -1.95. The results indicated that the pks of the micellar solution is higher than that of the aqueous solution due to the concentration of protons at the micellar interface which suppress the protonation in the ground state. This reflects the large difference between the excited and the ground state dissociation constants of the micellar solution. The mechanism of the studied reactions is discussed.