The stability of the proton in water + co-solvent mixtures

Abstract

The Hammett acidity function H 0 gives an indication of the ability of a neutral molecule to be protonated and can be used to compare the relative stabilities of protons in solvents consisting of neutral molecules. The rise in H 0 when an organic, oxygen-containing base is introduced into water containing a mineral acid, accompanied by a decrease in conductivity, conforms to the existence of a solvent-sorting equilibrium around the solvated proton together with competing equilibria involving the protonation of the Hammett-indicator base by two different proton solvates. The proton solvate containing a co-solvent molecule is more stable than that without it, and the ionic mobility of the proton involved with the former is less than that in the latter. Free energies of transfer of the proton from water into the mixture Δ G t ⊖(H +) calculated using this solvent-sorting method and the reference ion method (Δ G t ⊖(Ph 4As +) = Δ G t ⊖(BPh 4 −) are compared with the rises in H 0 in water-rich media: those using the latter method are shown to be inconsistent with the rise in H 0 for some alcohols. From the extent of the conversion of the aqua-proton into the proton solvate containing the co-solvent, the factors influencing the proton-co-solvent solvate are discussed for a range of solvent compositions. The principal effect over the whole range of water-rich compositions is found to be the ability of substituent groups to supply or withdraw electron density from the basic site in the co-solvent molecule.