Rationalizing Solvent Effect in Bronsted Acid-Catalyzed C–O Scission: A Thermodynamically Consistent Framework with Assessable Descriptors

Rates of unimolecular C–O bond scission across a broad library of structurally diverse alkanol and ether reactant and chemically distinct solvent (protophilic, protophobic, and protogenic) pairs catalyzed by solid Bro̷nsted acid (Amberlyst 15) span 6 orders of magnitude (338 K) without clear reactivity trends. We capture this apparently complex and unpredictable solvent-induced rate modulation with a simple thermochemical framework, decoupling the solvent and reactant-dependent contributions within the context of excess free energies of the transition states and acid sites. Within this model, we decompose the reaction excess activation free energies, the difference in activation free energies between liquid–solid and vapor–solid interfaces, into the sum of two physically distinct energetic components: (1) the excess adsorption free energy of a probe molecule (methanol), which implicitly captures all solvent–H+ interactions at the interface, that is reactant-independent and experimentally measurable; and (2) an electrostatic correction term, expressed as the product between a regressed transition state polarity parameter and the effective dielectric constant of the solvent, which captures differences in dielectric screening by solvent molecules around transition states polarized to different extents relative to that of the probe molecule. This model predicts all measured excess activation free energies, spanning across 36 kJ mol–1 across all reactant–solvent pairs, within a mean error of 2.3 kJ mol–1 at 338 K. These findings demonstrate that, despite their apparent complexities, solvent effects on intrinsic catalytic rates remain well-governed by fundamental thermodynamic principles.