Activation Energies Reveal Unusual Temperature Sensitivity in Mechanochemical Reactions

Mechanochemistry is gaining momentum as a solvent-efficient alternative to classical synthesis. Its widespread appeal rests not only on the avoidance of bulk solvents but also on the notion that it typically proceeds without external heating. This view has fostered a long-standing assumption—reflected even in the IUPAC definition—that temperature plays only a minor role in mechanochemical transformations. In this work, we challenge this assumption by systematically determining activation energies (Ea) of organic reactions under ball milling, resonant acoustic mixing (RAM), and solution conditions. Using the Diels–Alder reaction between anthracene and maleic anhydride as a benchmark, we found significantly higher Ea values in the solid state (~25–29 kcal·mol-1) than in solution (~15 kcal·mol-1). A similar trend was observed for the Knoevenagel condensation, indicating that elevated activation barriers are a general feature of mechanochemical systems. Progressive addition of solvent revealed a continuum from neat grinding to solution behavior, with Ea decreasing systematically as liquid-assisted grinding (LAG) content increased. These findings demonstrate, for the first time, that mechanochemical reactions are not less, but more strongly influenced by temperature than solution reactions. By linking this unusual temperature sensitivity to the decisive yet poorly understood role of LAG, our study provides a new conceptual framework for interpreting mechanochemical reactivity and clarifies how solid-state and solution chemistry are connected.