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

The acid ionization constants (pK_a) 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 pK_a values were proposed for each compound. pK_a1 (3.04–3.46) is associated with the carboxyl group resulting from the hydrolysis of the ester group, pK_a2 (6.48–6.92) with the enol group, pK_a3 (7.52–7.59) with the thiol group, and the pK_a4 value (11.00–11.27) with the amide NH group.