Assessment of the substituent constants of the 3-thioxo-1,2-dithiol-4- and -5-yl groups through pKa values measurements of 4 and 5-(hydroxy- or amino-phenyl)-1,2-dithiole-3-thiones in water at 298 K

The UV-VIS spectrophotometric assessment of the electronic substituent constants of the 3-thioxo-1,2-dithiol-4- and -5-yl and 3-oxo-1,2-dithiol-4- and -5-yl groups through pK(a) measurements in water of 4 and 5-(hydroxy or amino-phenyl)-1,2-dithiole-3-thiones and corresponding dithiolones has been performed. Linear free-enthalpy relationships based on pK(a) values of phenols and anilinium ions indicated a strong withdrawing effect for the 3-thioxo-1,2-dithiol-5-yl group by both an inductive and a resonance effect [sigma(p-) = 0.761 sigma(m) = 0.44 (anilinium ions relationship) and sigma(p-) = 0.97, sigma(m) = 0.45 (phenols relationship)]. The corresponding 3-oxo group was slightly less attractive than the 3-thioxo one [sigma(p-) = 0.64 (anilinium ions) and sigma(p-) = 0.78 (phenols)]. The 3-thioxo-1,2-dithiol-4-yl group and also its oxo analogue had only a weak withdrawing effect by an inductive effect (sigma(p) congruent to sigma(p-) congruent to 0.2) indicating a lack of conjugation between the dithiole and phenyl nuclei in 4-phenyldithiolethiones. The original 1,2-dithiole-3-thiones and 1,2-dithiol-3-ones used in correlations are described.