Kinetic Approach for Assessing Absolute Hydrogen Potential (pHabs) 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 (pHabs) using the electrophilic Car-D/Car-H substitution of phenol-d6 as a kinetic probe, applicable for wide-ranging solvents with diverse properties, categorized here according to their ability to solvate and stabilize sulfuric acid (H2SO4) and its conjugate base (HSO4-) as polar HSO4- stabilizers, polar HSO4- destabilizers, midpolar solvents, bridger solvents, and nonpolar solvents. In midpolar solvents, bridger solvents, and nonpolar solvents, H2SO4 molecules form a cluster, whereas in polar HSO4- destabilizer solvents, HSO4- anions undergo homoassociation. Despite the distinct differences in solvation properties, the electrophilic Car-D/Car-H substitution rate constants, measured at dilute H2SO4 concentrations and normalized to 1 mol L-1, exhibit a log-linear correlation with the solvent-modulated chemical potential of H+, spanning ∼20 pHabs units and corresponding to a >37,000,000-fold variation in the normalized rate constants at 333 K. The correlation gives estimated pHabs values for both the nonaqueous and aqueous systems, notably in agreement between the estimated pKa of H2SO4 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.