Activity coefficients, acid–base properties and weak Na+ ion pair formation of some resorcinol derivatives

Setschenow coefficients of resorcinol (1,3-benzenediol), 2-methylresorcinol (2-methyl-1,3-benzenediol) and 4-chlororesorcinol (4-chloro-1,3-benzenediol) were determined in NaCl(aq) (0–3 mol L−1) at 25 °C by distribution (2-methyl-1-propanol/water) measurements. Moreover, the protonation constants in NaCl(aq) and in Me4NCl(aq) were determined potentiometrically, at different salt concentrations (0–3 mol L−1). The dependence on ionic strength of the protonation constants was modelled by Debye-Hückel, SIT (Specific ion Interaction Theory) and Pitzer approaches. From distribution measurements at different ionic strengths Setschenow coefficients were determined and, therefore, the activity coefficients of the neutral species. By the dependence on ionic strength of the protonation constants the activity coefficients of charged species can be calculated. The differences between protonation constants of the resorcinols in NaCl(aq) and Me4NCl(aq) (NaCl(aq) ≪ Me4NCl(aq)) were also interpreted in terms of weak complexes formation. Enthalpy changes for the protonation of these compounds were determined by calorimetric titrations in NaCl(aq) at different salt concentrations (0–3 mol L−1) at 25 °C. The strength of proton ligand interactions is similar to that of phenols and very different from that of another class of O-ligands, namely carboxylates. This difference comes from the enthalpic contribution, whilst the entropic contribution to the stability is very similar for the two O-ligands classes.In general it was found that the resorcinols here studied behave in a similar way and can be considered a homogeneous class of O-ligands.