Solvation of the proton and the trifluoromethanesulfonate ion in eight solvents

Abstract

The enthalpies of solution of a sterically hindered aromatic diamine, 1,8-bis(dimethylamino)naphthalene (proton sponge, PS) and of its trifluoromethanesulfonate salt, and of trifluoromethanesulfonic acid have been determined in water, methanol (MeOH), ethanol (EtOH), pyridine (Py), dimethyl sulfoxide (Me2SO), dimethylformamide (DMF), acetonitrile (MeCN), and sulfolane (Sul). Single ion enthalpies of transfer for CF3SO3− and H+, from Me2SO to the other solvents, have been calculated using two extrathermodynamic assumptions, namely the tetraphenylarsonium–tetraphenylborate one and the recently proposed assumption based on the equality of transfers of uncharged PS and its conjugated cation PSH+. While both sets of transfer enthalpies using each assumption, for CF3SO3− and H+, differ by some 12 kJ mol−1 for water and MeOH, the values are in satisfactory agreement for Py, DMF, Me2SO, and Sul. These results support the validity of both extrathermodynamic assumptions for the nonprotogenic solvents, thus putting on firm ground the solvent basicity scale based on the enthalpy of transfer of H+ (in kJ mol−1): Py(−63) > DMF(−32) > Me2SO(−27) > EtOH(−1) ~ H2O(0) ~ MeOH(+2) > PC(+44) > MeCN(+55) > Sul(+68). These values correlate well with the enthalpies of complex formation with boron trifluoride. Key words: solvation, proton, trifluoromethanesulfonate ion, proton sponge, extrathermodynamic assumption.