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Title

MEDIUM EFFECT STUDIES ON THE ACID DISSOCIATION CONSTANTS OF 4-MEO-4-H-1-ME-THIOXANTHONE

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Abstract

 THE ACIDITY CONSTANTS OF (4-METHOXY PHENYL DIAZENYL) -4-HYDROXY-1-METHYL-THIOXANTHEN (4-MEO-4-H-1-ME-TX) IN WATER–ORGANIC MIXTURES AT 25°C WERE DETERMINED SPECTROPHOTOMETRICALLY. THE ORGANIC SOLVENTS CHOSEN FOR THE STUDY WERE ETHANOL (ETOH), ACETONITRILE (MECN), AND DIMETHYL SULFOXIDE (DMSO).THE PROCEDURE USED WAS THE PHTITRATION AT A CONSTANT IONIC STRENGTH 0.1 M KNO3. FOUR TITRATIONS OF KNOWN CONCENTRATION OF THE THIOXANTHONE WAS APPLIED IN 20–50% W/W MIXTURES OF WATER–ORGANIC, USING STANDARDIZED HCL AND NAOH. THE ACIDITY CONSTANTS OF THE 4-MEO-4-H-1-ME-TX WERE CALCULATED IN TERMS OF SOLVENT CHARACTERISTICS USING DATAN PROGRAM. THE IONIZATION CONSTANTS OF THE THIOXANTHONE DEPEND ON BOTH THE PROPORTION AND NATURE OF THE ORGANIC COSOLVENT. MEDIUM EFFECT SUCH AS HYDROGEN BONDING, SOLVENT BASICITY, DISPERSIVE FORCES, AND PROTON-SOLVENT INTERACTIONS PLAY VITAL ROLES IN THE IONIZATION PROCESS OF ACIDS IN THE PRESENCE OF ORGANIC SOLVENTS [1, 2]. HYDROGEN-BONDING INTERACTIONS OF THE CONJUGATE BASE WITH SOLVENT MOLECULES AS WELL AS THE SOLVENT BASICITY CONTRIBUTE THE MAJOR EFFECTS ON THE IONIZATION PROCESS [3]. THE RESULTS HAVE SHOWN THAT THE PKA VALUES ARE INFLUENCED BY THE PERCENTAGES AND KIND OF SOLVENTS THAT WERE APPLED IN WATER–ORGANIC MIXTURES. THE ACIDITY CONSTANTS OF THE THIS COMPOUND IN THE MEDIUM INCREASE BY INCREASING THE AMOUNT OF ORGANIC SOLVENTS.THE AMOUNTS OF THE VARIATIONS ARE FULLY DEPENDING ON THE EXTENT OF THE DIFFERENCES OF PHYSICAL PARAMETERS (DONOR NUMBER AND DIELECTRIC CONSTANT) OF THE ADDED ORGANIC SOLVENTS WITH WATER (TABLE 1). THE PKA VALUES OF THE THIOXANTHONE INCREASE IN THE VARIOUS SOLVENTS ACCORDING TO THE FOLLOWING SEQUENCE: ETOH<MECN<DMSO.

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