Kinetic Approach for Assessing Absolute Hydrogen Potential (pH abs ) across Diverse Solvents

The catalytic reactivity of the proton (H + ) is often governed by its chemical potential, yet quantifying this potential across nonaqueous and mixed-solvent systems remains a fundamental challenge. Here, we present a framework for assessing the absolute potential of hydrogen (pH abs ) using the electrophilic C ar –D/C ar –H substitution of phenol-d 6 as a kinetic probe, applicable for wide-ranging solvents with diverse properties, categorized here according to their ability to solvate and stabilize sulfuric acid (H 2 SO 4 ) and its conjugate base (HSO 4 – ) as polar HSO 4 – stabilizers, polar HSO 4 – destabilizers, midpolar solvents, bridger solvents, and nonpolar solvents. In midpolar solvents, bridger solvents, and nonpolar solvents, H 2 SO 4 molecules form a cluster, whereas in polar HSO 4 – destabilizer solvents, HSO 4 – anions undergo homoassociation. Despite the distinct differences in solvation properties, the electrophilic C ar –D/C ar –H substitution rate constants, measured at dilute H 2 SO 4 concentrations and normalized to 1 mol L –1, exhibit a log–linear correlation with the solvent-modulated chemical potential of H +, spanning ∼20 pH abs units and corresponding to a > 37,000,000-fold variation in the normalized rate constants at 333 K. The correlation gives estimated pH abs values for both the nonaqueous and aqueous systems, notably in agreement between the estimated p K a of H 2 SO 4 in methanol (1.4) versus that obtained from electrochemical measurements (1.5). This framework lays the foundation for the prediction of proton reactivity in thermal catalysis.