Acid Ionization Constants of Some N-Benzoylthiourea-Pyrrolidine Derivatives Bearing an Indole Ring by Potentiometric Titration in DMSO–Water

The acid ionization constants (pKa) of N-benzoylthiourea pyrrolidine derivatives bearing indole were calculated using the HYPERQUAD program with data from potentiometric titrations. Potentiometric titrations were performed in the hydroorganic solvent system consisting of DMSO–water (40 : 60). In titrations with NaOH (0.025) as the titrant, HCl (0.1M) was used to ensure compound protonation and NaCl (1.0 M) was utilized to provide ionic strength. Four pKa values were proposed for each compound. pKa1 (3.04–3.46) is associated with the carboxyl group resulting from the hydrolysis of the ester group, pKa2 (6.48–6.92) with the enol group, pKa3 (7.52–7.59) with the thiol group, and the pKa4 value (11.00–11.27) with the amide NH group.