A unified view to Brønsted acidity scales: do we need solvated protons?† †Electronic supplementary information (ESI) available: Detailed description of the equilibria, activities, anchoring approach and experiments. See DOI: 10.1039/c7sc01424d Click here for additional data file.

The most comprehensive solvent acidity scale spanning 28 orders of magnitude of acidity was measured in the low-polarity solvent 1,2-dichloroethane (DCE). Its experimental core is linked to the unified acidity scale (pHabs) in an unprecedented and generalized approach only based on experimental values. This enables future measurements of acid strengths and acidity adjustments in low polarity solvents. The scale was cross-validated computationally. The purely experimental and computational data agree very well. The DCE scale includes 87 buffer systems with pH2 abs values between 13.0 and +15.4, i.e. similar to water at hypothetical and extreme pH values of 13.0 to +15.4. Unusually, such high acidities in DCE are not realized via solvated protons, but rather through strongly acidic molecules able to directly donate their proton, even to weak bases dissolved in the solution. Thus, in all examined cases, not a single solvated proton is present in one liter of DCE.