Protonation of water in propylene carbonate

The protonation of water in propylene carbonate in the presence of perchloric and trifluoromethanesulfonic acids (HClO4 and HSO3CF3) has been investigated using voltammetry, conductimetry and potentiometry. Three protonated forms of water exist in these solutions: (H2O)H+ or H3O+, (H2O)2H+ or H5O2+ and (H2O)3H+ or H7O3+. The 1–4 complex (H2O)4H+ or H9O4+ seems not to be formed in detectable amount in this aprotic solvent. The hydrates of perchloric acid are extensively dissociated, contrary to those of trifluoromethanesulfonic acid. The determination of the pK of the complexes H3O+, H5O2+, H7O3+, has been achieved from solutions of perchloric acid and of its hydrates by potentiometry with hydrogen and glass electrodes. The distribution of the proton between the different species present in the medium has been plotted as a function of total concentration of water. These results obtained in propylene carbonate are compared to those in other dipolar aprotic solvents.