BASICITY MEASUREMENTS IN AQUEOUS-ORGANIC SOLVENTS .1. 20-PERCENT ETHANOL-AQUEOUS SULFURIC-ACID SYSTEM

The acidity functions Ho, Hazo, Hdip, HGf, and HAhave been compared in the solvent 20% ethanol-aqueous sulfuric acid. They are all linearly related according to the (HX + log CH+)-(1-Φe)(Ho + log CH+) relationship. The thermodynamic pKxH+values can be estimated for every base, even for those whose acidity function is unknown, by applying also in this solvent the Hunnett-Olsen equation. The protonation data of dimethyl ether and dimethyl sulfide show that the pKxH+ value is not sufficient to define the protonation equilibrium of weak bases. In fact, the inversion of basicity order that occurs in going from dilute (Me2O > Me2S) to more concentrated (Me2S > Me2S)acid solutions can be accounted for only through the solvation parameter 4Φe. Comparison of the Φe values shows that the differentiation among acidity functions occurs because of differences in onium ion solvation energies, the number of hydrogen bonding sites making only a second-order contribution. For the same compound, the protonation parameters are in the mixed solvent only slightly different (pKCxH+ more negative,Φe more positive) from those evaluated in aqueous acid solutions. © 1979, American Chemical Society. All rights reserved.