pKa values of 3-thioxo-1,2-dithiole-4- and -5-carboxylic acids and of 3-oxo-1,2-dithiole-4- and -5-carboxylic acids in aqueous solution at 298 K

Abstract

The pK_a values in aqueous solution of some 3-thioxo-1,2-dithiole-5-, 3-oxo-1,2-dithiole-5-, 3-thioxo-1,2-dithiole-4- and 3-oxo-1,2-dithiole-4-carboxylic acids have been determined both by UV–VIS spectrophotometry and by potentiometry at 298 K. The 3-thioxo-1,2-dithiole- and 3-oxo-1,2-dithiole-5-carboxylic acids exhibit very low pK_a values of the order of 1.20. The 1,2-dithiole-3-thione- and 1,2-dithiole-3-one-4-carboxylic acids exhibit pK_a values in the range 1.8–3.2. The pK_a values of 5-alkyl-4-carboxylic acids are 0.6 pH units lower than those of the corresponding unsubstituted dithiolone and dithiolethione acids. Thermodynamic ionization parameters determined by thermometric titrimetry show that: (i) for the 5-carboxylic acids, the strength is the result of an entropic effect; (ii) for the 4-carboxylic acids, the unexpected pK_a values are essentially of enthalpic origin.