An Acidity Scale for Brönsted Acids Including H3PW12O40

The calorimetric titration of H3PW12O40 (H3PW) with a series of bases in CH3CN gives equilibrium constants and enthalpies for the three deprotonation steps. These results show that solutions of H3PW in any solvent whose basicity is equal to or weaker than CH3CN will only furnish one strong proton per molecule of acid for stoichiometric or catalytic reactions. In more strongly basic solvents (S), more S2H+ species are available, but their strength is less being leveled by the solvent basicity. In drying H3PW, it is critical to avoid the weaker acids formed by reducing the material or forming the anhydride. Calorimetric titration of a series of samples subjected to different drying procedures establishes the best procedure for preparing the anhydrous acid. For most donors, the enthalpies of reaction for the first protonation step fit the ECW model. The W term provides a measure of the dissociation energy of the conjugate base in acetonitrile. E A* and C A* permit calculation of the enthalpies for the reaction in acetonitrile of (CH3CN)H+ and H3PW with the >100 bases in the ECW correlation. The C A*/E A* ratio of 2.88 indicates significant covalency in the interactions of this acceptor. This fitting of the enthalpies by using solvation minimized, donor parameters for neutral adducts indicates that the choice of enthalpies in acetonitrile as the solvent provides a temperature-independent, solution, Brönsted acidity scale that permits comparisons with neutral acceptors. The enthalpies of reactions of pyridine with a series of Brönsted acids in acetonitrile solvent is offered, leading to the following acidity order: H3PW12O40 > CF3SO3H > p-CH3C6H4SO3H ≅ H2SO4 > CF3COOH > ClC6H4COOH.