Acid-base properties of hydroxyquinolines in aqueous solution: Effect of hydroxyl group position on thermodynamic protonation parameters

The acid-base properties of 2-hydroxyquinoline (2-HQ), 4-hydroxyquinoline (4-HQ), 6-hydroxyquinoline (6-HQ), and 8-hydroxyquinoline (8-HQ) were investigated in this work by UV–Vis spectrophotometry, ISE-H+ potentiometry (glass electrode) and Isothermal Titration Calorimetry (ITC) in KCl(aq) at I = 0.2 mol·dm−3, and T = 298.15 K. Potentiometric titrations were also performed at different temperatures (288.15 ≤ T/K ≤ 318.15) to derive, together with direct ITC measurements, the corresponding protonation enthalpy and entropy changes. The analysis of the results obtained using various techniques allowed for a comprehensive characterization of the thermodynamic profile and chemical speciation of the studied hydroxyquinolines. Most importantly, it enabled the evaluation of how the position of the hydroxyl group influences the stability and driving forces involved in the protonation/deprotonation processes of both the quinolinic nitrogen and the hydroxyl groups on the pyridine or benzene ring of the hydroxyquinolines.